Dissertations / Theses on the topic 'Drought recovery'

Climate extremes, in particular droughts are significant driving forces towards riverine and terrestrial ecosystems disturbance. Drought impacts on stream ecosystems include losses that can either be direct (e.g. destruction of habitat for aquatic species) or indirect (e.g. deterioration of water quality, soil quality, and increased chance of wildfires). This study investigates hydrological and agricultural droughts and their recovery durations. For the riverine ecosystems, this study combines hydrologic drought and water quality changes during droughts and represents a multi-stage framework to detect and characterize hydrological droughts, while considering water quality parameters. Hydrological droughts are categorized into three stages of growth, persistence, retreat, and water quality variables (i.e., water temperature, dissolved oxygen concentration, and turbidity) are utilized to further investigate drought recovery. The framework is applied to 400 streamflow gauges across the Contiguous United States (CONUS) over the study period of 1950-2016. The framework is assessed and validated based on three drought events declared by the state of California in 2002, 2008, and for the 2012 US drought, which affected most of the nation. Results reveal the duration, frequency, and severity of historical droughts in various regions, additionally, duration of each stage of drought (i.e., growth, persistence, and retreat) is also assessed and the spatial patterns are diagnosed across the CONUS. Varied drought recovery durations are perceived for different water quality variables, and in general, it takes about two more months for water quality variables to recover from a drought, following the hydrological drought termination. For the terrestrial ecosystem, this study evaluated drought impacts on gross primary productivity (GPP), evapotranspiration (ET), and water use efficiency (WUE = GPP/ET) of different terrestrial ecosystems over the CONUS, as well as the drought-recovery during the period of 2000 to 2014. The response of WUE to drought showed large differences in various regions and biomes. WUE for arid ecosystems typically showed a positive response (increase) to drought, whereas WUE for humid ecosystems showed both positive and negative response to drought. The results revealed that WUE is correlated with drought severity, and for more severe droughts, WUE changes more significantly. Furthermore, terrestrial drought recovery shows a positive correlation with drought severity and in regions that experienced more severe drought episodes, ecosystem requires longer period to recover.

The effect of drought on freshwater ecosystems is a growing environmental concern. This study aimed to determine the effect of the supra-seasonal 2010-2012 drought on the macroinvertebrate communities and functional feeding groups (FFG) of chalk streams. Three rivers were sampled during and after the drought, three sites were analysed on each river, a perennial control site, a dewatering site (shallow during drought) and a rewetted site (dry during drought). All wetted sites were sampled for macroinvertebrates, algae, velocity and depth monthly. In addition an in-situ experiment investigated grazing rates in dewatering, rewetted and perennial sites and a mesocosm experiment investigated the effect of reduced water velocity and temporary drying on grazing function and macroinvertebrate mortality. Macroinvertebrate data were analysed for structural and functional changes in communities and algal data were analysed for changes of biomass both during drought and recovery. Both found rapid recovery post-drought with high incidence of resilience in the macroinvertebrate communities. Functional recovery was determined by investigating grazing rates in dewatering, rewetted and perennial sites. Algal and macroinvertebrate biomass, and grazing function recovered within a month of rewetting. Despite a rapid recovery of overall biomass and grazing function, FFG results showed delayed effects, and four months following drought differences were still present. Mesocosm experiments showed that macroinvertebrate mortality increased due to dewatering for some species, however slower velocity had little effect. Thus, this study showed that ecological function can return despite changes to macroinvertebrate community structure, indicating that functional redundancy promotes the resilience of chalk stream communities to drought.

Severe drought plays a critical role in altering the magnitude and interannual variability of the net terrestrial carbon sink. Drought events immediately decrease net primary production (NPP), and drought length and magnitude tend to enhance this negative impact. However, satellite and in-situ measurements have also indicated that ecosystem recovery from extreme drought can extend several years beyond the return to normal climate conditions. If an ecosystem’s drought recovery time exceeds the time interval between successive droughts, these legacy effects may reinforce the impact of future drought. Since the frequency and severity of extreme climate events are expected to increase with climate change, both the immediate and prolonged impact of drought may contribute to amplified climate warming by decreasing the strength of the land carbon sink. However, it is unknown whether terrestrial biosphere models capture the impact of drought legacy effects on carbon stocks and cycling. Using a suite of twelve land surface models from the Multi-scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP), we assessed model ability to simulate drought legacy effects by analyzing the modeled NPP response to drought events across forested regions of the US and Europe. We found that modeled drought legacy effects last about one year (2% reduction in NPP), with complete NPP recovery in the second post-drought year. Since observations suggest that legacy effects extend up to four years post-drought, with a 9% growth reduction in the first post-drought year, models appear to underestimate both the timescales and magnitude of drought legacy effects. We further explored vegetation sensitivity to climate anomalies through global, time-lagged correlation analysis of NPP and climatic water deficit. Regional differences in the lag time between climate anomaly and NPP response are prevalent, but low sensitivities (correlations) characterize the entire region. Significant correlations coincided with characteristic lag times of 0 to 6 months, indicating relatively immediate NPP response to moisture anomalies. Model ability to accurately simulate vegetation’s response to drought and sensitivity to climate anomalies is necessary in order to produce reliable forecasts of land carbon sink strength and, consequently, to predict the rate at which climate change will progress in the future. Thus, the discrepancies between observed and simulated vegetation recovery from drought points to a potential critical model deficiency.

The importance of several factors in limiting recovery from defoliation was investigated in field-grown plants of Agropyron desertorum and Agropyron spicatum exposed to drought , natural or irrigated conditions. Leaf extension rate, components of leaf area production, number of metabolically active axillary buds and carbohydrate availability were examined on the same plants immediately after defoliation and/or in the following spring from 1984 until 1986. The diurnal course of leaf growth did not relate to turgor pressure in the expanded portion of leaf laminae. Rather growth was apparently associated with air temperature. Leaf extension rate was lower under drought than under better moisture levels during 1984 to 1986. For both species, reduced growth rates and shorter growth periods resulted in reduced tiller height, leaf number and leaf size under drought compared with natural or irrigated conditions in 1985 and 1986, but not in 1984. As a result, leaf area and/or yields were also lower under drought in 1985 and 1986, and lowest under drought plus defoliation in 1986. Production of daughter tillers immediately after defoliation was also lowest under drought. Regrowth capacity of both species was not limited by axillary bud number, size or viability immediately after defoliation under any water level in 1986. In early spring, however, tiller number and growth were lower on clipped than on unclipped plants of both species under drought and irrigated conditions in 1986, and under all water levels in 1987; this resulted in considerably reduced photosynthetic canopies on clipped plants. Crown and root total nonstructural carbohydrate (TNC) pools were higher under drought than under better moisture levels in A. desertorum and A. spicatum in early spring 1986. These high pools of TNC apparently enhanced the production of etiolated regrowth in both species when meristematic limitations did not exist in early spring. The productive potential of both Agropyron species will probably not be affected following a late and severe defoliation under drought. However, vegetative growth and/or productivity, and probably the persistence of these species in the community, will be reduced after two or more years of late and heavy defoliations under drought.

Doctor of Philosophy
Department of Horticulture, Forestry, and Recreation Resources
Dale J. Bremer
Jack Fry
Increasing water scarcity may result in greater irrigation restrictions for turfgrass. Drought tolerance and recovery of Kentucky bluegrasses (Poa. pratensis L.) (KBG) were evaluated during and after 88 and 60 day dry downs in 2010 and 2011, respectively, under a rainout shelter. Changes in green coverage were evaluated with digital images. Green coverage declined slowest during dry downs and increased fastest during recoveries in the cultivar ‘Apollo’, indicating it had superior drought tolerance. Electrolyte leakage, photosynthesis, and leaf water potential were evaluated in 7 KBG cultivars during and after the dry downs. Soil moisture at 5 and 20 cm was measured. There were generally no differences in physiological parameters among cultivars during or after dry down. The highest reduction in soil moisture at 5 and 20 cm was in Apollo, suggesting it had a better developed root system for mining water from the profile during drought. Weed prevention and turfgrass establishment of ‘Legacy’ buffalograss (Buchloe dactyloides [Nutt.] Engelm.) and ‘Chisholm’ zoysiagrass (Zoysia japonica Steud.) grown on turf reinforcement mats (TRM) was evaluated. ‘Chisholm’ zoysiagrass stolons grew under the TRM; as such, use of TRM for this cultivar is not practical. Buffalograss had 90% or greater coverage when established on TRM in 2010 and 65% or greater coverage in 2011; coverage was similar to that in oxadiazon-treated plots at the end of each year. ‘Legacy’ buffalograss plugs were established on TRM over plastic for 3 weeks, stored in TRM under tree shade for 7, 14, or 21 days, and evaluated for establishment after storage. In 2010, plugs on mats stored for 7 days had similar coverage to the control, but in 2011 displayed similar coverage to plugs stored on TRM for 14 or 21 day treatments. Green leaf are index (LAI) is an important indicator of turfgrass performance, but its measurement is time consuming and destructive. Measurements using hyperspectral radiometry were compared with destructive measurements of LAI. Results suggest spectral radiometry has potential to accurately predict LAI. The robustness of prediction models varied over the growing season. Finding one model to predict LAI across and entire growing season still seems unrealistic.

Rice (Oryza sativa L.) is one of the main staple foods of the world. With the increase of population and the deficit of irrigated water, the increase in rice production that is predicted will be dependent on areas prone to drought. Root depth is important for plant growth and survival during drought because of its role in facilitating water uptake from deep soil layers. By advances in genomics, the plant root systems can be linked to quantitative trait locus (QTL) information to achieve a most beneficial design of root system architecture. There is a demand to develop and validate techniques that permit estimation of the root system. Therefore, two techniques (root penetration ability to non-woven fabric and a buried herbicide method at depth 30 cm) were used in this study to assess root traits in a total of 36 rice cultivars. The results from these screens were assessed with root traits measured on the same cultivars in the rhizotrons and hydroponic experiment. Correlations between these methods showed that herbicide score at day 35 was most strongly related to traits of the rhizotron experiment, especially number of roots passed 50 cm at 35 days, root angle at day 21, root thickness, water use and % root mass. Using all of these traits obtained in the rhizotron in a best subset regression suggests that up to 71% of the variation in herbicide score can be explained. These data strongly imply that symptoms are related to root development and transpiration demand and are therefore ideal for assessing water extraction by roots at depth. The herbicide method has been applied to screen root depth in 138 recombinant inbred lines derived from crossing between two subspecies Indica Bala and Japonica Azucena. Analysis of mapping population revealed two putative QTLs on chromosome 6 near marker RZ682 and one on chromosome 7 near marker RG351 and two significant QTLs near marker G1010b and G1073 for root depth on chromosome 8. The same method has been used to evaluate root depth in 325 rice diversity panel allowing high resolution genetic mapping. Using efficient mixed model (statistical analyses for genome wide association), four associations were detected, two on chromosome 1 and one on each of chromosome 4 and 6. In addition, the same rice diversity panel has been evaluated for drought avoidance by assessing leaf rolling (as one plant mechanism against drought stress) and drought recovery. A total of three significant associations, on chromosomes 4, 6 and 7 were identified for leaf rolling while only one significant association on chromosome 2 for drought recovery was found. Positional candidate genes underneath QTLs were examined bioinformatically and through the literature revealing several interesting genes which may offer potential for developing drought resistant rice cultivars. Therefore, developing a cost effective high-throughput system that can measure traits related to drought avoidance and drought recovery on a large number of plants would aid genetic studies in breeding and gene identification.

Doutoramento em Biologia
Eucalyptus plantations are among the most productive forest stands worldwide. In Portugal and Spain, they are widely used for pulp production and as an energy crop. However, the region’s Mediterranean climate, with increasingly severe summer drought, negatively affects eucalypt growth and increases mortality. The aim of this doctoral thesis was to unravel drought tolerance in Eucalyptus globulus by investigating and interconnecting information on the processes mediating water deficit and rehydration, from gene and molecular regulation to physiological responses and plant performance, using two different genotypes and different stress trials. The thesis disclosed herein is presented in a series of research papers (chapters 2, 3, 4 and 5), preceded by a general introduction (chapter 1) and closed with concluding remarks (chapter 6). Chapter 2 describes a greenhouse trial and a slowly imposed water deficit, and is divided into three subchapters. Two genotypes (AL-18 and AL-10) were submitted to a 3-week water stress period at two different intensities (18% and 25% of field capacity), followed by 1 week of rewatering. Recovery was assessed 1 day and 1 week after rehydration. Several phytohormones were monitored in leaves, xylem sap and roots, 2 h, 4 h, 24 h, and 168 h after rewatering. Water deficit reduced height, biomass, water potential, and gas exchange. Contrarily, the levels of pigments, chlorophyll fluorescence parameters and MDA increased. ABA and ABA-GE levels increased, and JA content decreased in leaves and increased in xylem sap. During recovery, most of the physiological and biochemical responses of stressed plants were reversed. Comparative proteome (using difference gel electrophoresis) and metabolome (using gas chromatography–mass spectrometry) analyses enabled the separation and isolation of 2031 peptide spots, 217 of which were identified, and the detection of 121 polar metabolites. The analysis of the resilient clone AL-18, which presented a response network very distinct from the responsive clone AL-10, reinforced the role of specific photosynthetic and defence-related proteins as key players in mediating drought tolerance and revealed new players: glutamine synthetase, malate dehydrogenase and isoflavone reductase-like protein. Chapter 3 regards a climate chamber trial and a sudden water shortage, and is divided in two subchapters. The relative expression of 12 transcripts was analysed by quantitative PCR in two clones with different degrees of tolerance (AL-18 and AL-13) 7 and 11 days after water withholding and rehydration (2 h and 3 days after rewatering). Sudden water shortage was more detrimental to the plants than when slowly imposed, with heavier outcomes in clone AL-13, including plant death. Potential molecular indicators linked to enhanced water stress tolerance in Eucalyptus globulus were identified: rubisco activase (RCA), ferredoxin-NADP(H) oxireductase (FNR), mitochondrial malate dehydrogenase (mMDH), peroxisomal catalase (CAT) and isoflavone reductase (IFR). Afterwards, several biochemical markers of oxidative stress and DNA methylation patterns were quantified in the leaves of AL-18. The alterations detected using global and specific indicators reflected the parallel induction of redox and complex DNA methylation changes occurring during stress imposition and relief. Chapter 4 reports a field trial: the previously identified set of indicators for selection of water stress tolerance was tested in field-grown AL-18 and AL-13. Some of the plants were irrigated (IR), and others were left under environmental conditions of reduced rainfall (NI) during six and a half weeks prior to rewatering. Clone AL-18 showed few fluctuations in the conditions tested, and the alterations found in clone AL-13 highlighted an induction of photosynthetic and photorespiration metabolism after artificial rehydration. The results corroborated that responses to field conditions cannot be extrapolated from a stress applied individually in the context of developing selection markers. Chapter 5 describes a climate chamber trial that tested the isolated and combined effect of drought and heat. Physiological, biochemical and metabolomic alterations were monitored in AL-18 after a 5-day of consistent drought and/or 4 h at 40ºC. Testing drought-stressed plants subject to a heat shock revealed a decrease in gas exchange, Ψpd and JA, no alterations in electrolyte leakage, MDA, starch and pigments and increased glutathione pool in relation to control. The induction of cinnamate was a novel response triggered only by the combined stress. These results highlighted that the combination of drought and heat provides significant protection from more detrimental effects of drought-stressed eucalypts, confirming that combined stresses alter plant metabolism in a novel manner that cannot be extrapolated by the sum of the different stresses applied individually. This thesis describes a number of biological responses that enable E. globulus to thrive under conditions of water deficit and provides useful information of pathways to be explored in order to find suitable markers of abiotic stress tolerance in this species. Despite that, a bigger challenge remains and consists of the need to focus our studies in more realistic, field-like experiments, at least in the context of finding suitable selection markers in the climate change era.
As plantações de eucalipto estão entre as mais produtivas do mundo inteiro. Em Portugal e Espanha, são amplamente utilizadas na produção de polpa e como fonte de energia. No entanto, o clima mediterrânico da região, com secas de verão cada vez mais severas, afeta negativamente o crescimento do eucalipto e aumenta a sua mortalidade. Esta tese de doutoramento tem como objetivo desvendar a tolerância à seca da espécie Eucalyptus globulus, investigando e interligando informação dos processos que medeiam o défice hídrico e a reidratação, desde a regulação genética e molecular até às respostas fisiológicas e desempenho da planta, utilizando dois genótipos distintos e diferentes ensaios experimentais. Esta tese está estruturada sob a forma de estudos científicos (capítulos 2, 3, 4 e 5), precedidos por uma introdução geral (capítulo 1), e termina com as notas finais (capítulo 6). O capítulo 2 descreve um ensaio de estufa e um défice hídrico imposto lentamente, e está dividido em 3 subcapítulos. Dois genótipos (AL-18 e AL-10) foram sujeitos a um período de stress hídrico de 3 semanas com duas intensidades diferentes (18% e 25% da capacidade de campo), seguido de uma semana de reidratação. A recuperação foi avaliada um dia e uma semana depois da reidratação. Várias fitohormonas foram monitorizadas nas folhas, seiva xilémica e raízes, 2 h, 4 h, 24 h e 168 h depois da reidratação. A falta de água reduziu a altura, a biomassa, o potencial hídrico e as trocas gasosas. Pelo contrário, os níveis de pigmentos, parâmetros da fluorescência da clorofila e MDA aumentaram. Os níveis de ABA e de ABA-GE aumentaram, enquanto o JA diminuiu nas folhas e aumentou na seiva xilémica. Durante a recuperação, a maioria das alterações fisiológicas e bioquímicas provocadas pelo stress reverteram. Análises comparativas do proteoma (analisado por eletroforese em gel diferencial) e do metaboloma (analisado por cromatografia gasosa com espetrometria de massa) permitiram a separação de 2031 pontos peptídicos, dos quais 217 foram identificados, e a deteção de 121 metabolitos polares. A análise do clone resiliente AL-18, que apresentou uma rede de resposta bem distinta do clone responsivo AL-10, reforçou o papel de proteínas específicas da fotossíntese e relacionadas com a defesa como intermediários chave na tolerância à seca e revelou novos intermediários: glutamina sintetase, malato desidrogenase e isoflavona redutase. O capítulo 3 diz respeito a um ensaio em câmara climática e a uma rápida escassez de água, e está dividido em 2 subcapítulos. A expressão relativa de 12 transcritos foi analisada por PCR quantitativo em dois clones com diferentes graus de tolerância (AL-18 e AL-13) depois de 7 e 11 dias sem qualquer rega e posterior reidratação. A rápida escassez de água foi mais prejudicial para as plantas do que o défice hídrico imposto lentamente, com maior visibilidade no clone AL-13 que revelou morte de algumas plantas. Indicadores moleculares potencialmente ligados a uma tolerância aumentada foram identificados: rubisco ativase (RCA), ferredoxina-NADP(H) oxidorredutase (FNR), malato desidrogenase mitocondrial (mMDH) catalase peroxissomal (CAT) e isoflavona redutase (IFR). De seguida, vários marcadores bioquímicos de stress oxidativo e padrões de metilação do DNA foram quantificados nas folhas do clone AL-18. As alterações detetadas utilizando indicadores globais e específicos refletiram a indução de complexas modificações redox e de metilação do DNA, que ocorrem paralelamente durante a imposição e interrupção do stress. O capítulo 4 reporta um ensaio de campo: o conjunto de indicadores de seleção de tolerância hídrica identificado anteriormente foi testado em AL-18 e AL-13 plantados no campo. Algumas das plantas foram regadas artificialmente (IR) e outras foram deixadas nas condições ambientais de precipitação reduzida (NI) durante seis semanas e meia antes de voltar a regar. O clone AL-18 mostrou pouca variação nas condições testadas, e as alterações encontradas no clone AL-13 realçaram a indução do metabolismo fotossintético e fotorespiratório após a reidratação artificial. Estes resultados mostraram que as respostas das plantas no campo não podem ser extrapoladas a partir do estudo de um stress aplicado individualmente, particularmente no contexto de encontrar marcadores de seleção. O capítulo 5 descreve um ensaio em câmara climática que testou o efeito isolado e combinado de seca e calor. Alterações fisiológicas, bioquímicas e metabolómicas foram monitorizadas no clone AL-18 após 5 dias de seca consistente e/ou 4 h a 40ºC. Testar plantas em stress hídrico sujeitas a um choque térmico revelou uma diminuição das trocas gasosas, do potencial hídrico e do JA, nenhum efeito a nível da perda de eletrólitos, MDA, amido e pigmentos e um aumento na glutationa, em comparação com condições controlo. O stress combinado induziu também a produção do cinamato, uma resposta nova. Estes resultados realçam que a combinação de seca e calor fornece uma proteção significante contra os efeitos mais prejudiciais da seca isolada em eucalipto, confirmando que o stress combinado altera o metabolismo das plantas de uma forma nova que não pode ser extrapolada pela soma dos diferentes stresses aplicados individualmente. Esta tese descreve um conjunto de respostas biológicas que permitem ao eucalipto manter-se em condições de défice hídrico e revela informação útil de várias vias metabólicas a serem exploradas de modo a encontrar marcadores de tolerância ao stress abiótico apropriados. Apesar disso, um desafio maior permanece. Consiste na necessidade de focarmos os nossos estudos em experiências mais realistas, que mimetizem as condições de campo, pelo menos no contexto de encontrarmos marcadores de seleção ajustados a uma era de alterações climáticas.

The swift parrot (Lathamus discolor, Psittacidae) is an endangered, austral migrant that inhabits forests and woodlands of south-eastern Australia. With a small population size (2500 birds), broad winter distribution (1 250 000 km2) and often cryptic nature, the swift parrot is a challenging species to study. In autumn they migrate north from their Tasmanian breeding grounds in search of suitable food resources throughout their winter range on mainland Australia. They are therefore dependent on a combination of suitable wintering, migration and breeding habitats. Although they spend a large proportion of their lives within winter habitats, the spatial and temporal dynamics of habitat use in this part of their range is poorly understood. This thesis aims to provide a greater understanding of large-scale winter habitat use by swift parrots, in both historic and current contexts, and provide a basis for future conservation management. ¶ Swift parrots, or red-shouldered paroquets as they were previously known, were among the first Australian birds to be scientifically described and illustrated following European settlement in 1788. However, within 60 years of settlement, habitats throughout the range of the species were being impacted upon. An important aspect of this habitat loss is the speed and spatial extent with which it occurred throughout the parrots’ broad distribution. Although the most extensive habitat loss in some areas occurred during colonial times, habitats continue to be lost as a result of various land management practices. Such impacts are also likely to be exacerbated by the ongoing cumulative impact with rapid climate change. As a result the swift parrot is an endangered species and is the subject of an ongoing national recovery program, to which this thesis contributes. ¶ Conserving habitat for the swift parrot, and other wide-ranging fauna species, is challenging since impacts in one area tend to be dismissed based on the assumption that there is sufficient habitat in other areas. These conservation challenges are discussed in regard to the national swift parrot recovery program. Although recovery program implementation for this species has been successful in identifying and protecting some important habitats, there are still many gaps in our knowledge that need to be addressed through a continuing and adaptive recovery effort, including an understanding of variable habitat use throughout their winter range. ¶ A study of swift parrot winter habitat use was therefore conducted at 53 sites across New South Wales over five years (2001-2005). Swift parrots used a diversity of winter foraging habitats in coastal and/or western slopes regions of New South Wales each year, including several habitats that occur in endangered ecological communities. Landscapes containing winter foraging habitat included scattered trees, remnant vegetation and continuous forests, and swift parrots foraged extensively on lerp and nectar from a diversity of tree species within these. The occurrence of swift parrots at foraging sites was primarily associated with the abundance of lerp, nectar and non-aggressive competitors. Although swift parrot abundance fluctuated significantly between years and regions, over half of all foraging sites were used repeatedly, highlighting their likely importance for conservation. ¶ Patterns of habitat use throughout the species’ winter range were also studied across five states/territories using volunteer data from 4140 surveys. These surveys were conducted by up to 300 volunteers twice a year, for seven years (1998-2004) with swift parrots detected in 19% of surveys. As a result, this study provided the first demonstration of large-scale drought related movements by a migratory population throughout their winter range. It also demonstrated the dynamic spatial and temporal patterns of winter habitat use, including repeated use of sites, by an austral migrant. Four regions in central Victoria were used most consistently, although the birds also visited other regions each year. ¶ During drought swift parrot abundance was significantly correlated with rainfall, whereby most of the population either concentrated in a few regions or migrated longer distances (up to 1000km) to drought refuges in wetter coastal areas. However, swift parrot abundance was not associated with specific climate variables during years of average to high rainfall throughout most of their range. Instead they appeared to prefer habitats within particular regions. Importantly this study emphasises that conservation measures need to be implemented throughout the distribution of migratory species, including drought refuge habitats and areas outside conservation reserves.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico
FundaÃÃo Cearense de Apoio ao Desenvolvimento Cientifico e TecnolÃgico
As membranas biolÃgicas sÃo alvos preferenciais dos efeitos deletÃrios do estresse hÃdrico, induzidos por aÃÃo de enzimas hidrolÃticas e espÃcies reativas do oxigÃnio (ERO), ambas estimuladas durante o estresse. A biossÃntese dos lipÃdeos dos cloroplastos pode ser importante para a tolerÃncia ao estresse hÃdrico e para a recuperaÃÃo apÃs reidrataÃÃo. Nesse trabalho nos estudamos o metabolismo dos cloroplastos, monogalactosil-diacilglicerol (MGDG), digalactosil-diacilglicerol (DGDG), sulfoquinovosil-diacilglicerol (SQDG), phosphatidil-glicerol (PG), no Ãmbito do dÃficit hÃdrico e da reidrataÃÃo apÃs o fim do estresse. Com este intuito, nos medimos o teor dos lipÃdeos da folhas, acompanhamos a incorporaÃÃo do precursor 14C-acetato nos lipÃdeos e analisamos a expressÃo dos genes codificadores das enzimas de sÃntese chave dos lipÃdeos (MGD1, MGD2, DGD1, DGD2, SQD2 e PGP1) durante o estresse hÃdrico e apÃs a reidrataÃÃo. Visando de uma melhor compreensÃo da relaÃÃo entre o metabolismo destes lipÃdeos e a tolerÃncia a seca, nos trabalhamos com duas cultivares de Vigna unguiculata L. Walp, uma tolerante (cv. EPACE) e outra sensÃvel (cv. 1183) a seca. Por meio de varredura diferencial de um biblioteca de cDNA de V.unguiculata, foram obtidas as seqÃÃncias completas dos cDNA dos genes VuMGD1, VuMGD2, VuDGD1, VuDGD2, VuSQD2 e VuPGP1. Os resultados mostram que em condiÃÃes de estresse hÃdrico a cultivar tolerante, alÃm de preservar seu teor de lipÃdeos durante a seca, à igualmente capaz de estimular a biossÃntese do DGDG aumentando significativamente a relaÃÃo DGDG:MGDG de suas membranas. NÃs sugerimos que o DGDG acumulado em condiÃÃo de seca à transportado para as membranas externas ao cloroplasto e que isso contribui para a tolerÃncia à seca. Os efeitos da desidrataÃÃo celular sobre as membranas tÃm conseqÃÃncias diretas sobre a capacidade das plantas a se recuperarem apÃs reidrataÃÃo. 48 horas apÃs a rega, a cv. sensÃvel 1183 nÃo à capaz de se recuperar em termos de teor de galactolipÃdeos e incorporaÃÃo do precursor. Na cv. tolerante, no entanto, o teor de DGDG permanece elevado, mesmo apÃs a reidrataÃÃo. Em conclusÃo, nossos resultados sugerem a importÃncia dos lipÃdeos membranares na tolerÃncia/sensibilidade das plantas ao dÃficit hÃdrico, em particular o balanÃo entre as classes lipÃdicas de propriedades fÃsico-quÃmicas diferentes (SQDG versus PG e DGDG versus MGDG) que poderiam afetar a estrutura e o funcionamento das membranas.
Membranes are main targets of degradation by reactive oxygen species and hydrolytic activities induced by drought. Chloroplasts lipid biosynthesis, especially galactolipids monogalactosyl-diacylglycerol (MGDG) and digalactosyl-diacylglycerol (DGDG) are important for plant tolerance to water deficit and for recovery after rehydration. In this thesis, we studied the metabolism of the chloroplast membrane lipids, MGDG, DGDG, sulphoquinovosyl-diacylglycerol (SQDG), phosphatidyl-glycerol (PG) under drought and during recovery from drought. Aiming this, we measured leaf lipids content, followed 14C-acÃtate incorporation and expression of genes coding for chloroplast membrane lipid synthases (MGD1, MGD2, DGD1, DGD2, SQD2 and PGP1) during drought and recovery. In order to better understand the relationship between drought tolerance and lipid metabolism, two cultivars of Vigna unguiculata L. Walp, one drought tolerant (cv. EPACE) the other drought susceptible (cv. 1183) were compared. The cDNA complete sequences for VuMGD1, VuMGD2, VuDGD1, VuDGD2, VuSQD2 and VuPGP1 were obtained from screening of a V.unguiculata cDNA library. The results showed that under water stress conditions, the tolerant cultivar, besides its ability to preserve its lipids pool despites drought, is able to strongly stimulate the DGDG biosynthesis, increasing the DGDG:MGDG ratio in its membranes. We suggest that DGDG accumulated under drought condition, when phosphate is deficient, is exported for extrachloroplastic membranes, and thus contributes to plant drought tolerance. Effects of loss of water on cell membranes have direct consequences on plant capacity to recover from stress. 48 hours after rewatering, the susceptible cv. 1183 was not able to fully recover from a moderate stress in terms of leaf galactolipid content and acetate incorporation into MGDG. In EPACE-1, MGDG leaf content remained unchanged after rehydration and DGDG remained higher than in the control plants. In conclusion, our results highlight the importance of membrane lipids in plant adaptation to water deficit and in their capacity to recover from stress. Of particular importance is the balance between lipid classes with various physico-chemical properties (SQDG versus PG, DGDG versus MGDG), since they most likely have a profound influence on membrane structure and function.

Les membranes cellulaires sont des cibles préférentielles de la dégradation induite par les espèces réactives de l’oxygène produites durant la sécheresse et par la stimulation d’activités hydrolytiques. La biosynthèse des lipides des chloroplastes peut être importante pour la tolérance à la sécheresse ainsi que pour la reprise après réhydratation. Dans ce travail nous avons étudié le métabolisme des lipides des membranes chloroplastiques, monogalactosyldiacylglycerol (MGDG), digalactosyl-diacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG), phosphatidyl-glycerol (PG), dans le cadre de la sécheresse et de la reprise après la fin de la contrainte hydrique. Dans ce but, nous avons mesuré la teneur des lipides des feuilles, suivi l’incorporation du précurseur 14C-acétate dans les lipides et analysé l’expression des gènes codant les enzymes de biosynthèse des lipides (MGD1, MGD2, DGD1, DGD2, SQD2 et PGP1) durant le stress hydrique et après réhydratation. Afin de mieux comprendre le rapport entre le métabolisme de ces lipides et la tolérance à la sécheresse, nous avons travaillé sur deux cultivars de Vigna unguiculata L. Walp, un tolérant (cv. EPACE) et l’autre sensible (cv. 1183) à la sécheresse. Les séquences complètes des ADNc des gènes VuMGD1, VuMGD2, VuDGD1, VuDGD2, VuSQD2 et VuPGP1 ont été obtenues par le criblage d’une banque d’ADNc de V.unguiculata. Les résultats montrent qu’en condition de stress hydrique le cultivar tolérant, en plus de préserver la teneur en lipides, est capable de stimuler la biosynthèse du DGDG augmentant significativement le rapport DGDG:MGDG de ces membranes. Nous suggérons que le DGDG accumulé en sécheresse est exporté vers les membranes extrachloroplastiques et que cela contribue à la tolérance à la contrainte hydrique. Les effets de la perte d’eau cellulaire sur les membranes ont des conséquences directes sur la capacité des plantes à reprendre après réhydratation. 48 heures après réarrosage, le cv. sensible 1183 n’est pas capable de récupérer en termes de teneurs en galactolipides et incorporation de précurseur. Chez le cv. tolérant, par contre, la teneur en DGDG demeure élevé, même après réhydratation. En conclusion, nos résultats suggèrent l’importance des lipides membranaires dans la tolérance/sensibilité des plantes au déficit hydrique, en particulier la balance entre des classes lipidiques de propriétés physico-chimiques différentes (SQDG versus PG et DGDG versus MGDG) qui pourraient affecter la structure et le fonctionnement des membranes
Membranes are main targets of degradation by reactive oxygen species and hydrolytic activities induced by drought. Chloroplasts lipid biosynthesis, especially galactolipids monogalactosyl-diacylglycerol (MGDG) and digalactosyl-diacylglycerol (DGDG) are important for plant tolerance to water deficit and for recovery after rehydration. In this thesis, we studied the metabolism of the chloroplast membrane lipids, MGDG, DGDG, sulphoquinovosyl-diacylglycerol (SQDG), phosphatidyl-glycerol (PG) under drought and during recovery from drought. Aiming this, we measured leaf lipids content, followed 14Cacétate incorporation and expression of genes coding for chloroplast membrane lipid synthases (MGD1, MGD2, DGD1, DGD2, SQD2 and PGP1) during drought and recovery. In order to better understand the relationship between drought tolerance and lipid metabolism, two cultivars of Vigna unguiculata L. Walp, one drought tolerant (cv. EPACE) the other drought susceptible (cv. 1183) were compared. The cDNA complete sequences for VuMGD1, VuMGD2, VuDGD1, VuDGD2, VuSQD2 and VuPGP1 were obtained from screening of a V.unguiculata cDNA library. The results showed that under water stress conditions, the tolerant cultivar, besides its ability to preserve its lipids pool despites drought, is able to strongly stimulate the DGDG biosynthesis, increasing the DGDG:MGDG ratio in its membranes. We suggest that DGDG accumulated under drought condition, when phosphate is deficient, is exported for extrachloroplastic membranes, and thus contributes to plant drought tolerance. Effects of loss of water on cell membranes have direct consequences on plant capacity to recover from stress. 48 hours after rewatering, the susceptible cv. 1183 was not able to fully recover from a moderate stress in terms of leaf galactolipid content and acetate incorporation into MGDG. In EPACE-1, MGDG leaf content remained unchanged after rehydration and DGDG remained higher than in the control plants. In conclusion, our results highlight the importance of membrane lipids in plant adaptation to water deficit and in their capacity to recover from stress. Of particular importance is the balance between lipid classes with various physico-chemical properties (SQDG versus PG, DGDG versus MGDG), since they most likely have a profound influence on membrane structure and function

Les cultures françaises de Lavande et Lavandin sont soumises à un sévère dépérissement, résultant de l’interaction entre la maladie du Stolbur et des sécheresses de plus en plus intenses et fréquentes. L’amélioration de la résistance des lavanderaies est un défi pour sauvegarder cette filière à fort impact économique, touristique et social dans son territoire. Un projet associant des partenaires de la recherche et de la filière lavandicole vise à augmenter la résistance des cultures par des nouvelles pratiques. Dans ce cadre, les objectifs de la thèse ont été i) d’identifier des indicateurs physiologiques de l’état de stress, et en particulier un seuil physiologique de non-récupération, suite à des sécheresses sévères chez la Lavande et le Lavandin et ii) évaluer l’effet de la pratique agro-écologique de la culture en inter-rang sur la réponse du Lavandin à la sécheresse estivale. Par ailleurs, une étude sur les prévisions climatiques focalisée sur les départements de production a révélé une augmentation des températures estivales et automnales et une diminution des précipitations automnales à l’horizon proche (2021-2050), confortant l’intérêt de nos travaux pour la filière. Pour suivre en continu l’état physiologique des plants lors d’une sécheresse, des dendromètres de type PépiPIAF, qui mesurent les variations du diamètre des tiges, ont été utilisés sur ces espèces. Deux indices liés à l’intensité de la sécheresse et aux dégâts engendrés ont ainsi été identifiés : le pourcentage de perte de diamètre (PLD) et la perte de capacité de réhydratation (PLRC). Les plants ne récupéraient pas d’une sécheresse (PLRC = 100%) lorsque le PLD avait atteint une valeur maximale d’environ 21%. Ces résultats signifient que l’incapacité à récupérer survient lorsque les réservoirs élastiques d’eau sont vides et démontrent ainsi leur importance lors de sécheresses intenses. Lors des déshydratations, des mesures acoustiques et de fuites d’électrolytes ont montré que cette incapacité était associée à des dégâts cellulaires importants, et non à un haut niveau d’embolie. Ces deux indices développés en conditions contrôlées ont aussi permis de diagnostiquer l’état de stress de lavandins en champ. Les dendromètres sur les lavandins ont révélé une contrainte hydrique exercée par un couvert végétal en inter-rang pendant la période de croissance et de sécheresse estivale. Le stress hydrique fut plus précoce avec ce couvert, mais l’intensité de la sécheresse identique à celle avec un sol nu en inter-rang. Outre l’importance de ces travaux pour la recherche fondamentale, en particulier sur les mécanismes de mortalité des plantes, ces résultats ouvrent des perspectives sur l’anticipation des mortalités en lavanderaie, voire sur le pilotage de l’irrigation de ces cultures, pour un contrôle raisonné de la contrainte hydrique nécessaire à la production d’huiles essentielles, mais dommageables pour les plants selon son intensité et la période
French Lavender and Lavandin crops are subject to severe dieback, resulting from the interaction between Stolbur's disease and increasingly intense and frequent droughts. Improving the resistance of Lavender plantations is a challenge to safeguard this sector with a strong economic, tourist and social impact on its territory. A project involving partners from research and the Lavender sector aims to increase crop resistance through new practices. Within this framework, the objectives of the thesis were i) to identify physiological indicators of the state of stress, and in particular a physiological threshold of non-recovery, following severe droughts in Lavender and Lavandin and ii) to evaluate the effect of the agro-ecological practice of inter-row cultivation on the response of Lavandin to summer drought. In addition, a study on climate forecasting focused on production departments revealed an increase in summer and autumn temperatures and a decrease in autumn rainfall on the near horizon (2021-2050), confirming the interest of our work for the sector. To continuously monitor the physiological condition of the plants during a drought, PépiPIAF-type dendrometers, which measure variations in stem diameter, were used on these species. Two indices related to the intensity of the drought and the damage caused were thus identified: percentage loss of diameter (PLD) and loss of rehydration capacity (PLRC). Plants did not recover from a drought (PLRC = 100%) when the PLD had reached a maximum value of about 21%. These results mean that the inability to recover occurs when the elastic water storage are empty, thus demonstrating their importance during intense droughts. During dehydration, acoustic measurements and electrolyte leakage showed that this inability was associated with significant cell damage and not a high level of embolism. These two indices developed under controlled conditions also made it possible to diagnose the stress state of lavandins in the field. Dendrometers on lavandins revealed water stress exerted by an inter-crop vegetation cover during the growing season and summer drought. The water stress was earlier with this cover but the intensity of the drought was identical to that with bare soil. In addition to the importance of this work in fundamental research, in particular on the mechanisms of plant mortality, these results open perspectives on the anticipation of mortality in Lavender plantations, and even on the management of irrigation of these crops, for a reasoned control of the hydric stress necessary for the production of essential oils but harmful to the plants according to its intensity and the period

This thesis examined vegetation change over the last 43 years in Australia's largest contiguous alpine area, the Kosciuszko alpine zone in south-eastern Australia. Using historical and current data about the state of the most common vegetation community, tall alpine herbfield, this thesis addressed the questions: (1) what were the patterns of change at the species/genera and life form levels during this time period; (2) what were the patterns of recovery, if recovery occurred, from anthropogenic disturbances such as livestock grazing or trampling by tourists; (3) what impacts did natural disturbances such as drought have on the vegetation and how does it compare to anthropogenic disturbances; and (4) What are the benefits, limitations and management considerations when using long-term data for assessing vegetation changes at the species/genera, life form and community levels? The Kosciuszko alpine zone has important economic, cultural and ecological values. It is of great scientific and biological importance, maintaining an assemblage of vegetation communities found nowhere else in the world. It is one of the few alpine regions in the world with deep loamy soils, and contains endemic flora and fauna and some of the few periglacial and glacial features in Australia. The area also forms the core of the Australian mainland's most important water catchment and is a popular tourist destination, offering a range of recreational opportunities. The vegetation of the Kosciuszko alpine zone is recovering from impacts of livestock grazing and is increasingly exposed to pressures from tourism and anthropogenic climate change. At the same time, natural disturbances such as drought and fire can influence the distribution, composition and diversity of plants. Thus, there is a need for detailed environmental data on this area in order to: (1) better understand ecological relationships; (2) understand existing and potential effects of recreational and management pressures on the region; (3) provide data against which future changes can be assessed; and (4) provide better information on many features of this area, including vegetation, for interpretation, education and management. The research in this thesis utilised three types of ecological information: (1) scientific long-term datasets; (2) photographic records; and (3) a comparison of disturbed and undisturbed vegetation. This research analysed data from one of the longest ongoing monitoring programs in the Australian Alps established by Alec Costin and Dane Wimbush in 1959. Permanent plots (6 transects and 30 photoquadrats) were established at two locations that differed in the time since grazing and have been repeatedly surveyed. Plots near Mt Kosciuszko had not been grazed for 15 years and had nearly complete vegetation cover in 1959, while plots near Mt Gungartan showed extensive impacts of grazing and associated activities which only ceased in 1958. Some transect data from 1959 to 1978 have been analysed by the original researchers. The research presented in this thesis extends this monitoring program with data from additional surveys in 1990, 1999 and 2002 and applies current methods of statistical evaluation, such as ordination techniques, to the whole data set for the first time. Results indicated that the recovery from livestock grazing and the effects of drought have been the main factors affecting vegetation. Recovery from livestock grazing at the three transects at Gungartan was slow and involved: (1) increasing genera diversity; (2) increasing vegetation cover; (3) decreasing amounts of bare ground; and (4) a directional change over time in species composition. Patterns of colonisation and species succession were also documented. In 2002, 44 years after the cessation of grazing, transects near Mt Gungartan had similar vegetation cover and genera diversity to the transects near Mt Kosciuszko, but cover by exposed rock remained higher. A drought in the 1960s resulted in a temporary increase of litter and a shift in the proportional cover of life forms, as grasses died and herb cover increased at both locations. Proportions of cover for life forms reverted to pre-drought levels within a few years. The results also highlighted the spatial variability of tall alpine herbfield. The photoquadrats were surveyed in the years 1959, 1964, 1968, 1978 and 2001 and are analysed for the first time in this thesis. After comparing a range of methods, visual assessment using a 130 point grid was found to be the most suitable technique to measure vegetation cover and genera diversity. At the 18 quadrats near Mt Gungartan, there was a pattern of increasing vegetation cover as bare areas were colonised by native cudweeds and the naturalized herb Acetosella vulgaris. Revegetation from within bare areas largely occurred by herb species, while graminoids and shrub species predominately colonised bare ground by lateral expansion from the edges, eventually replacing the colonising herbs. At the 12 quadrats near Mt Kosciuszko, vegetation cover was almost complete in all years surveyed except 1968, which was at the end of a six year drought. Similar to the results from the transect study, the drought caused an increase in litter at both locations as graminoid cover declined. Initially herb cover increased, potentially as a result of decreased competition from the graminoids and a nutrient spike from decaying litter, but as the drought became more severe, herb cover also declined. Graminoid cover rapidly recovered after the drought, reaching pre-drought levels by 1978, and was at similar levels in 2001. Herb cover continued to decline after peaking in 1964. The photoquadrat study also documented the longevity and growth rates of several species indicating that many taxa may persist for several decades. It further provided insights into replacement patterns amongst life forms. In addition to assessing vegetation change following livestock grazing and drought at the long-term plots, recovery from tourism impacts was examined by comparing vegetation and soils on a closed walking track, with that of adjacent undisturbed tall alpine herbfield at a series of 22 paired quadrats. Fifteen years after the track was closed there was limited success in restoration. Over a quarter of the closed track was still bare ground with non-native species the dominant vegetation. Plant species composition differed and vegetation height, soil nutrients and soil moisture were lower on the track which had a higher compaction level than adjacent natural vegetation. The results presented in this thesis highlight that tall alpine herbfield is characterised by nearly entire vegetation cover which is dominated by graminoids, followed by herbs and shrubs in the absence of disturbance by livestock grazing, trampling or drought. The studies also showed that under quot;average" conditions, the relative cover of herbs and graminoids remained fairly stable even though there can be considerable cycling between them. Spatial variability in terms of taxa composition was high. The only common introduced species in unrehabilitated sites was Acetosella vulgaris, which was effective at colonising bare ground but was eventually replaced by other native species. However, in areas actively rehabilitated, such as on the closed track, non-native species introduced during revegetation efforts still persist with high cover 15 years after their introduction. Monitoring of vegetation change is also important at the landscape scale. This thesis provides a review of the potential use, the limitations and the benefits of aerial photography to examine vegetation change in the Kosciuszko alpine zone. Numerous aerial photography runs have been flown over the area since the 1930s for government agencies, industry and the military. Some of these records have been used to map vegetation communities and eroding areas at a point in time. Other studies compared different types and scales of photographs, highlighting in particular the benefits and potential of large scale colour aerial photography to map alpine vegetation. However, despite their potential to assess vegetation change over time, a temporal comparison of vegetation in the Kosciuszko alpine zone from aerial photographs has not been completed to this date. Historical photographs may not be easy to locate or access and difficulties with vegetation classification may restrict the practicality of using historical aerial photographs to assess vegetation change. Despite these issues, aerial photography may provide a very useful and efficient tool to assess changes over time when applied appropriately, even in alpine environments. The development of digital classification techniques, the application of statistical measures of error to both methodology and data, and the application of geographic information systems are likely to further improve the practicality of historical aerial photographs for the detection of vegetation change and assist in overcoming some of the limitations. The results presented in this thesis highlight the need for limiting disturbance, for ongoing rehabilitation of disturbed areas and for long-term monitoring in the Kosciuszko alpine zone. The results contribute to our understanding of how vegetation may change in the future and may be affected by new land use activities and climate change. This type of information, which otherwise would require the establishment of long-term studies and years of monitoring, can assist land managers of this and other important protected areas. The study highlights how the use and expansion of already existing datasets to gather ecological information can save considerable money and time, providing valuable data for current and emerging issues.

En Europe, les prairies permanentes représentent l'une des principales formes d'utilisation des terres. La durabilité des services rendus par ces écosystèmes est étroitement liée à la structure et au fonctionnement des communautés végétales qui les composent. La stabilité des processus écosystémiques, sous l'impact des fluctuations environnementales, va alors dépendre des mécanismes de résistance et de récupération propres à ces communautés. Les risques d'apparition d'extrêmes climatiques liés à l'évolution du climat, rendent nécessaire une adaptation des agro-écosystèmes à cette fluctuation climatique accrue. Cela implique de mieux comprendre les mécanismes par lesquels la variabilité du climat influe sur le fonctionnement et la structure des communautés végétales, et comment les pratiques agricoles impactent ces réponses. Cette thèse à pour objectif d'évaluer la vulnérabilité des prairies permanentes à un extrême climatique combinant une canicule et une sécheresse, dans un contexte de changement climatique. Afin d'identifier et de caractériser les mécanismes intrinsèques de la résistance à la sécheresse des communautés végétales, une démarche expérimentale mobilisant les concepts de l'écologie fonctionnelle a été développée. Les effets de la variabilité climatique sur la structure (diversité, composition) et le fonctionnement des communautés végétales ont d'abord été analysés in situ pendant 3 ans, après simulation d'une canicule-sécheresse dans les conditions climatiques actuelles et sous climat modifié (réchauffement et réduction des précipitations). Pour évaluer l'influence de la gestion sur la réponse des communautés végétales aux perturbations climatiques, deux fréquences de fauches ont été appliquées. Ce travail présente l'originalité d'avoir étudié conjointement le fonctionnement aérien et souterrain de la végétation. Sont considérées la production de biomasse, la démographie et la durée de vie des racines, afin de déterminer le rôle du système racinaire dans la résistance et la récupération des communautés végétales après une canicule sécheresse. Nous montrons un effet direct de l'extrême climatique sur la sénescence aérienne et la croissance racinaire dans les deux régimes de fauches. En réponse à un déficit hydrique modéré, la stratégie d'évitement au stress par le maintien de la croissance racinaire a été favorisée par des fauches non fréquentes. A l'automne suivant, les précipitations ont permis un reverdissement rapide de la couverture végétale et la production de nouvelles racines. Une baisse significative de la production de biomasse aérienne annuelle sous climat modifié, et suite à l'application de l'extrême climatique, a été observée pendant les 3 années d'expérimentation. En revanche, la production racinaire n'a pas été significativement affectée. Néanmoins une acclimatation de la croissance racinaire a été mise en évidence un an après l'extrême, en particulier dans les communautés de fauches non fréquemment. Ces changements de fonctionnement aériens et souterrains peuvent s'expliquer en partie par des modifications de la composition botanique. De plus, en réponse aux perturbations climatiques, la mortalité racinaire a été interrompue. Ces résultats inattendus ont donc entrainé une augmentation de la durée de vie des racines et suggèrent un effet négatif sur le turnover racinaire et les cycles biogéochimiques. Une expérimentation complémentaire à l'étude in situ a été menée en conditions semicontrôlées pour déterminer les mécanismes intrinsèques de la résistance à la sécheresse de la prairie permanente. Des traits morphologiques et physiologiques, aériens et souterrains, ont été mesurés sur des monocultures de sept populations prairiales en conditions optimales, puis en conditions de sécheresse afin de caractériser leurs stratégies de survie. Les résultats ont mis en évidence le rôle du système racinaire dans la survie des plantes. (...)

Dissertação de Mestrado em Ecologia apresentada à Faculdade de Ciências e Tecnologia
O ritmo acelerado das mudanças climáticas pode superar a capacidade das populações de se adaptarem em regiões onde as secas podem aumentar tanto em duração quanto em gravidade. Em um cronograma de longo prazo, declínio da floresta generalizados podem levar a reduções na produtividade primária líquida dos ecossistemas florestais, perda de biodiversidade e mudanças em sua composição e comunidades florestais. A mortalidade de plantas devido a períodos mais longos de secas e temperaturas mais elevadas tornou-se um importante foco de atenção recentemente, com vários relatos destacando episódios de mortalidade graves em todo o mundo, portanto a pesquisa sobre a resposta das plantas ao stress hídrico está se tornando cada vez mais importante. Embora exista uma rica literatura sobre as respostas das plantas ao stress hídrico, nossa compreensão actual das causas da mortalidade das árvores ainda é limitada. Prever como as florestas responderão às futuras mudanças climáticas depende de um melhor entendimento da reacção à seca, em nível de espécie, por meio de mecanismos fisiológicos.Vários ecossistemas podem já estar respondendo às mudanças climáticas. Entre outros, a capacidade de recuperação após o distúrbio depende do impacto e frequência de episódios de seca no passado. Sob as actuais projecções de aquecimento global, os ecossistemas terrestres poderiam levar mais tempo para se recuperar após a seca, o que poderia aumentar a vulnerabilidade desses sistemas à seca. Assim, identificar os mecanismos subjacentes que controlam a resiliência das plantas ao longo do tempo é essencial para entender melhor a capacidade de recuperação dos ecossistemas.O principal objetivo desta pesquisa foi avaliar os diferentes componentes da resiliência à seca em mudas de Pinus pinaster Aiton para entender a capacidade de lidar com o estresse, a curto e a longo prazo. A fotossíntese, a condutância estomática, a transpiração, o teor relativo de água, o potencial hídrico e o potencial hídrico do meio dia, a eficiência no uso e a altura do crescimento foram mensurados em mudas de Pinus pinaster submetidas a estresse hídrico e após a reidratação. O comportamento das plantas foi avaliado principalmente através da medição da assimilação de CO2, condutância estomática, crescimento de plantas e desempenho hidráulico de mudas de Pinus pinaster Aiton durante a seca e após a rega. Os objetivos específicos foram: 1) identificar se a recuperação e a resiliência são condicionadas pelo nível de estresse induzido pela seca; 2) avaliar se a resiliência e recuperação diferem a curto e longo prazo; 3) avaliar se, após a reidratação, a quantidade de água recebida tem impacto na resiliência e velocidade de recuperação em cada variável medida. Durante o stress hídrico, todas as variáveis ​​apresentaram valores menores, quando comparadas com o período pré-seca. Após a rega, as variáveis ​​hidráulicas aumentaram logo após, somente após 27 dias as mudas atingiram os mais altos níveis de fotossíntese, transpiração e condutância estomática. Estes resultados indicam que a limitação hidráulica foi o processo que rege a recuperação da troca de gás da seca. Depois de reabastecer todos os parâmetros, recupere para valores mais altos em comparação com o período de seca. No entanto, em geral, a resiliência foi baixa no tempo da experiência, com a maioria dos parâmetros não alcançando valores similares aos anteriores à seca.
The rapid pace of climate change may outstrip the capacity of populations to adapt in regions where droughts are predicted to increase in both duration and severity. In a long-term schedule, widespread forest declines could lead to reductions in net primary productivity of forest ecosystems, loss in biodiversity and changes on its composition and woodland communities. Mortality of plants because of longer droughts and higher temperatures events has become a major focus of attention recently, with various reports highlighting severe mortality episodes around the globe, thus research on plant response to water stress is becoming increasingly important. Even though exists a rich literature on plant responses to water stress, our current understanding of the causes of tree mortality is still limited. Predicting how forests will respond to future climate changes hinges on a better understanding in the reaction to drought, at a species level, through physiological mechanisms.Several ecosystems may already be responding to climate change. Among others, recovery capacity after disturbance depends on the impact and frequency of past drought episodes. Under current global warming projections, terrestrial ecosystems could take longer to recover after drought, which could increase the vulnerability of these systems to drought. Thus, identifying the underlying mechanisms that control plants’ resilience over time is essential to better understand ecosystems capacity to recover. The main goal of this research was to evaluate the different components of drought resilience in Pinus pinaster Aiton saplings to understand this species’ ability to cope with stress, in short and long-term periods. Photosynthesis, stomatal conductance, transpiration, relative water content, leaf predawn and midday water potential, water use efficiency and growth parameters (height and diameter) were measured in Pinus pinaster saplings under water stress, and after rehydration. Plants’ behavior was mainly assessed through the measurement of CO2 assimilation, stomatal conductance, plant growth and hydraulic performance of Pinus pinaster Aiton saplings during drought and after re-watering. The specific aims were: 1) to identify whether recovery and resilience are conditioned by the level of drought induced stress; 2) to evaluate whether resilience and recovery differ at a short and long-term period; 3) to assess if, after rehydration, the amount of water received has an impact on resilience and recovery speed in each measured variable. During water stress, all variables showed lower values, compared with the pre-drought period. After re-watering, hydraulic variables increased soon after, only after 27 days did saplings reach the highest levels of photosynthesis, transpiration, and stomatal conductance. These results indicate that hydraulic limitation was the process governing gas-exchange recovery from drought. After rewatering all the parameters recover to higher values compared with the drought period. However, in general, the resilience was low within the time of the experience, with most of the parameters not attaining similar values to the ones before the drought.