Academic literature on the topic 'Drought-Induced tree mortality'

Large trees in the tropics are reportedly more vulnerable to droughts than their smaller neighbours. This pattern is of interest due to what it portends for forest structure, timber production, carbon sequestration and multiple other values given that intensified El Niño Southern Oscillation (ENSO) events are expected to increase the frequency and intensity of droughts in the Amazon region. What remains unclear is what characteristics of large trees render them especially vulnerable to drought-induced mortality and how this vulnerability changes with forest degradation. Using a large-scale, long-term silvicultural experiment in a transitional Amazonian forest in Bolivia, we disentangle the effects of stem diameter, tree height, crown exposure and logging-induced degradation on risks of drought-induced mortality during the 2004/2005 ENSO event. Overall, tree mortality increased in response to drought in both logged and unlogged plots. Tree height was a much stronger predictor of mortality than stem diameter. In unlogged plots, tree height but not crown exposure was positively associated with drought-induced mortality, whereas in logged plots, neither tree height nor crown exposure was associated with drought-induced mortality. Our results suggest that, at the scale of a site, hydraulic factors related to tree height, not air humidity, are a cause of elevated drought-induced mortality of large trees in unlogged plots. This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications'.

Understanding the vulnerability of trees to drought-induced mortality is key to predicting the fate of forests in a future climate with more frequent and intense droughts, although the underlying mechanisms are difficult to study in adult trees. Here, we explored the dynamic changes of water relations and limits of hydraulic function in dying adults of Norway spruce (Picea abies L.) during the progression of the record-breaking 2018 Central European drought. In trees on the trajectory to drought-induced mortality, we observed rapid, nonlinear declines of xylem pressure that commenced at the early onset of xylem cavitation and caused a complete loss of xylem hydraulic conductance within a very short time. We also observed severe depletions of nonstructural carbohydrates, though carbon starvation could be ruled out as the cause of the observed tree death, as both dying and surviving trees showed these metabolic limitations. Our observations provide striking field-based evidence for fast dehydration and hydraulic collapse as the cause of drought-induced mortality in adult Norway spruce. The nonlinear decline of tree water relations suggests that considering the temporal dynamics of dehydration is critical for predicting tree death. The collapse of the hydraulic system within a short time demonstrates that trees can rapidly be pushed out of the zone of hydraulic safety during the progression of a severe drought. In summary, our findings point toward a higher mortality risk for Norway spruce than previously assumed, which is in line with current reports of unprecedented levels of drought-induced mortality in this major European tree species.

Drought-induced tree mortality, which rapidly alters forest ecosystem composition, structure, and function, as well as the feedbacks between the biosphere and climate, has occurred worldwide over the past few decades, and is expected to increase pervasively as climate change progresses. The objectives of this review are to (1) highlight the likely ecological consequences of drought-induced tree mortality, (2) synthesize the hypotheses related to drought-induced tree mortality, (3) discuss the implications of current knowledge for modeling tree mortality processes under climate change, and (4) highlight future research needs. First, we emphasize the likely ecological consequences of tree mortality from ecosystem to biome to continental scales. We then document and criticize multiple non-exclusive tree mortality hypotheses (e.g., carbon starvation — carbon supply is less than carbon demand; and hydraulic failure — desiccation from failed water transport) from a more comprehensive ecological perspective. Next, we extend a forest decline concept model, Manion’s framework, by considering new emerging environmental conditions, for a more thorough understanding of the effects of climate change on forest decline. We find that an increase in drought frequency and (or) climate-change-type droughts may trigger increased background tree mortality rates and severe forest dieback events, accelerating species turnover and ecological regime shifts. The contribution of CO2 fertilization, rising temperature within the optimal growth range, and increased nitrogen deposition may defer or reduce this trend in tree mortality, but such contributions will vary between locations, species, and tree sizes. Multiple hypotheses proposed for drought-induced tree mortality are discussed, but coupling carbon and water cycles could help resolve the debate. The absence of a physiological understanding of tree mortality mechanisms limits the predictive ability of current models from stand-level process-based models to dynamic global vegetation models. We thus suggest that long-term observations, experiments, and models should be tightly interwoven during the research process to better forecast future climate changes and evaluate their impacts on forests.

Research Highlights: We compared annually resolved records of tree-ring width and stable isotope of dead and surviving Fokienia hodginsii (Dunn) Henry et Thomas trees. We provide new insights into the relationships and sensitivity of tree growth to past and current climate, and explored the underlying mechanism of drought-induced mortality in F. hodginsii. Background and Objectives: Drought-induced tree decline and mortality are increasing in many regions around the world. Despite the high number of studies that have explored drought-induced decline, species-specific responses to drought still makes it difficult to apply general responses to specific species. The endangered conifer species, Fokienia hodginsii, has experienced multiple drought-induced mortality events in recent years. Our objective was to investigate the historical and current responses to drought of this species. Materials and Methods: We used annually resolved ring-width and δ13C chronologies to investigate tree growth and stand physiological responses to climate change and elevated CO2 concentration (Ca) in both dead and living trees between 1960 and 2015. Leaf intercellular CO2 concentration (Ci), Ci/Ca and intrinsic water-use efficiency (iWUE) were derived from δ13C. Results: δ13C were positively correlated with mean vapor pressure deficit and PDSI from previous October to current May, while ring widths were more sensitive to climatic conditions from previous June to September. Moreover, the relationships between iWUE, basal area increment (BAI), and Ci/Ca changed over time. From 1960s to early 1980s, BAI and iWUE maintained a constant relationship with increasing atmospheric CO2 concentration. After the mid-1980s, we observed a decrease in tree growth, increase in the frequency of missing rings, and an unprecedented increase in sensitivity of 13C and radial growth to drought, likely related to increasingly dry conditions. Conclusions: We show that the recent increase in water stress is likely the main trigger for the unprecedented decline in radial growth and spike in mortality of F. hodginsii, which may have resulted from diminished carbon fixation and water availability. Given that the drought severity and frequency in the region is expected to increase in the future, our results call for effective mitigation strategies to maintain this endangered tree species.

AbstractInsect defoliation contributes to tree mortality under drought conditions. Defoliation-induced alterations to the vascular transport structure may increase tree vulnerability to drought; however, this has been rarely studied. To evaluate the response of tree vascular function following defoliation, 2-year-old balsam poplar were manually defoliated, and both physiological and anatomical measurements were made after allowing for re-foliation. Hydraulic conductivity measurements showed that defoliated trees had both increased vulnerability to embolism and decreased water transport efficiency, likely due to misshapen xylem vessels. Anatomical measurements revealed novel insights into defoliation-induced alterations to the phloem. Phloem sieve tube diameter was reduced in the stems of defoliated trees, suggesting reduced transport capability. In addition, phloem fibers were absent, or reduced in number, in stems, shoot tips and petioles of new leaves, potentially reducing the stability of the vascular tissue. Results from this study suggest that the defoliation leads to trees with increased risk for vascular dysfunction and drought-induced mortality through alterations in the vascular structure, and highlights a route through which carbon limitation can influence hydraulic dysfunction.

Abstract Forests are sensitive to droughts, which increase the mortality rate of tree species. Various processes have been proposed to underlie drought-induced tree mortality, including hydraulic failure, carbon starvation and increased susceptibility to natural enemies. To give insights into these processes, we assessed the metabolic effects of a mortality-inducing drought on seedlings of Pinus sylvestris L. (Scots Pine), a widespread and important Eurasian species. We found divergence over time in the foliar metabolic composition of droughted vs well-watered seedlings, with the former showing increased abundance of aromatic amino acids and decreases in secondary metabolism associated with defence. We observed no significant differences amongst provenances in these effects: seedlings from drought-prone areas showed the same foliar metabolic changes under drought as seedlings from moist environments, although morphological effects of drought varied by provenance. Overall, our results demonstrate how severe drought prior to death may target particular primary and secondary metabolic pathways, weakening defences against natural enemies and contributing to the risk of drought-induced mortality in P. sylvestris.

The Three-North Shelter Forest (TNSF) is a critical ecological barrier against sandstorms in northern China, but has shown extensive decline and death in Populus simonii Carr. in the last decade. We investigated the characteristics—tree-ring width, basal area increment (BAI), carbon isotope signature (13Ccor), and intrinsic water-use efficiency (iWUE)—of now-dead, dieback, and non-dieback trees in TNSF shelterbelts of Zhangbei County. Results from the three groups were compared to understand the long-term process of preceding drought-induced death and to identify potential early-warning proxies of drought-triggered damage. The diameter at breast height (DBH) was found to decrease with the severity of dieback, showing an inverse relationship. In all three groups, both tree-ring width and BAI showed quadratic relationships with age, and peaks earlier in the now-dead and dieback groups than in the non-dieback group. The tree-ring width and BAI became significantly lower in the now-dead and dieback groups than in the non-dieback group from 17 to 26 years before death, thus, these parameters can serve as early-warning signals for future drought-induced death. The now-dead and dieback groups had significantly higher δ13Ccor and iWUEs than the non-dieback group at 7–16 years prior to the mortality, indicating a more conservative water-use strategy under drought stress compared with non-dieback trees, possibly at the cost of canopy defoliation and long-term shoot dieback. The iWUE became significantly higher in the now-dead group than in the dieback group at 0–7 years before death, about 10 years later than the divergence of BAI. After the iWUE became significantly different among the groups, the now-dead trees showed lower growth and died over the next few years. This indicates that, for the TNSF shelterbelts studied, an abrupt iWUE increase can be used as a warning signal for acceleration of impending drought-induced tree death. In general, we found that long-term drought decreased growth and increased iWUE of poplar tree. Successive droughts could drive dieback and now-dead trees to their physiological limits of drought tolerance, potentially leading to decline and mortality episodes.

Drought-induced tree mortality is an emerging global phenomenon that appears related to climate change and rising temperatures in particular, and may be an early indication of vegetation change. However, vegetation response to climate change is uncertain, particularly for future novel climates. Notably, no current models of vegetation change attempt to mechanistically predict plant mortality, and in particular, mortality of trees, which exerts strong influences on ecological function. Resolving uncertainties surrounding the physiological mechanism and temperatures sensitivity of tree mortality is a current challenge in global change ecology. The objectives of this dissertation were to 1) consider tree mortality consequences for earth system processes related to carbon, water, and energy exchange that include climate regulation; 2) explore tree mortality effects on the water cycle by developing hypotheses and research needs; 3) quantify the temperature sensitivity of drought-induced tree mortality and gain insight into the physiological mechanism of mortality; 4) quantify the relationships among temperature, stored carbohydrate resources, and gas exchange to further elucidate physiological tree mortality mechanisms; and 5) quantify the sensitivity of two species of pine seedlings to progressively elevated temperatures and relate mortality to the effect of temperature on carbon metabolism. Major findings of this dissertation relate to the temperature sensitivity, physiological mechanism, and implications of tree mortality. Assessment of the potential consequences of tree mortality for earth system processes documented the contrasting influences of tree mortality on the terrestrial C cycle and land-surface energy exchange, the balance of which will determine the net effects on climate regulation (Appendix A). Following a survey of the ecohydrology literature, thresholds for tree mortality to cause watershed changes were hypothesized at ~20% loss of canopy cover, ~500 mm of annual precipitation, and whether flows are snowmelt dominated (Appendix B). Elevated temperature (~+4°C) accelerated tree mortality by 28% during experimental drought, a difference related to cumulative respiration dynamics in piñon pine (Appendix C). Stored carbohydrate resources were declined during lethal drought but were not entirely depleted prior to mortality (Appendix D). Seedlings exhibited progressive declines in time-to mortality with increased temperatures, a response related to C metabolism (Appendix E).

Les plantes estan exposades a diversos estressos ambientals incloent la sequera i temperatures extremes els quals poden limitar el seu creixement i supervivència. La disponibilitat d'aigua es considera el principal factor limitant per a la productivitat vegetal. Les plantes presenten una sèrie d'estratègies per fer front a la sequera i mantenir un balanç hídric adequat entre les quals s'inclouen modificacions de l'àrea foliar, control estomàtic, canvis en l'assignació de biomassa, modificacions del balanç de carboni font/embornal, i la resistència a l'embolisme del xilema. Tot i així, la mortalitat forestal induïda per sequera és un fenomen generalitzat, amb grans implicacions a nivell d'ecosistema, i s'espera que incrementi degut a l'augment dels episodis de sequera com a resultat de les condicions de canvi climàtic actuals. Entendre com la complexa xarxa de trets que presenten les plantes implicats en la resistència a la sequera determina la seva supervivència tant a nivell d'espècie com d'individu és fonamental per avaluar la vulnerabilitat de la vegetació actual als canvis del clima, i l’impacte potencial en el serveis ecosistèmics. Al 2008, McDowell et al. varen sintetitzar els mecanismes de mortalitat induïda per sequera en un marc hidràulic coherent i senzill. La seva hipòtesi incloïa dos mecanismes fisiològics, no excloents, com a principals causants de la mortalitat d'arbres induïda per sequera: la fallida hidràulica i l’exhauriment de carboni. La fallida hidràulica és el punt en que el transport d'aigua de tota la planta queda bloquejat per cavitació com a resultat de tensions crítiques al xilema. L'exhauriment de carboni és la situació en que el subministrament de carboni provinent de la fotosíntesi, d'estocs de carboni o d'autofàgia no satisfà les necessitats metabòliques mínimes. En aquest marc, la preponderància d'un o altre mecanisme depèn de la intensitat i duració de la sequera, així com de la capacitat de les plantes per regular el seu potencial hídric (Ψw). Les especies isohídriques serien més vulnerables a l'exhauriment de carboni degut a un tancament estomàtic més ràpid per tal de mantenir el Ψw relativament constant (i evitar l'embolisme), mentre que les especies anisohídriques serien més susceptibles a la fallida hidràulica a mesura que el sòl s'asseca ja que operen amb marges de seguretat hidràulica més estrets degut als seus Ψw més negatius. El marc previ es centra en el comportament estomàtic sense tenir en compte la plètora de trets que també intervenen en resposta a la sequera. A més a més, els estomes responen a altres factors a banda del Ψw, i és per això que assumir que la regulació iso/anisohídrica del Ψw és capaç d’explicar completament el comportament estomàtic pot ser enganyós. Per aquest motiu, els principals objectius d’aquesta tesi foren: (2) determinar si les diferències en la regulació estomàtica entre especies estan relacionades amb comportaments iso/anisohídrics i com s’associen aquests als distints mecanismes de mortalitat en condicions de sequera, escalfament o ambdós factors; (2) testar les assumpcions que relacionen comportaments anisohídrics amb majors conductàncies estomàtiques i marges de seguretat hidràulica més amplis; i (3) comprendre com i en quina mesura expliquen els trets morfològics i fisiològics, així com la seva plasticitat, el temps fins la mort en resposta a la sequera dins d’espècie. Per abordar els objectius (1) i (2) vàrem estudiar dos sistemes models amb contrastada vulnerabilitat a l’embolisme entre espècies: la formació boscosa piñon-juniper i l’alzinar Mediterrani. En ambdós casos vàrem comparar les respostes a la sequera entre espècies isohídriques (Pinus edulis i Quercus ilex) i espècies anisohídriques (Juniperus monosperma i Phillyrea latifolia), fent èmfasi en la regulació estomàtica i l’economia de l’aigua i el carboni. En aquestes especies, observem que un comportament més anisohídric no es tradueix necessàriament amb menor sensitivitat estomàtica al Ψw i, per tant, amb major taxa d’embolisme. De la mateixa manera, una major regulació del Ψw (comportament isohídric) no s’associa amb un tancament estomàtic més ràpid en condicions de sequera ni tampoc amb majors limitacions de carboni. Ambdós estudis desafien les idees previes i adverteixen de la confusió que pot generar l’associació directa de la iso/anisohidria amb un comportament estomàtic contrastat i els mecanismes de mortalitat. A nivell d’individu en Pinus sylvestris (3), mantenir activa l’adquisició de carboni i els estocs de carboni per sobre d’un nivell crític fou clau per perllongar la supervivència front a una sequera extrema, fins i tot a costa de majors pèrdues d’aigua. Una completa integració de l’economia del carboni i l’aigua és el repte per poder avançar en el coneixement de les respostes de les plantes a la sequera i els mecanismes de mortalitat.
Plants are exposed to several environmental stressors including drought and extreme temperatures that can limit their growth and survival. Water availability is considered the main limiting factor for plant productivity. Plants display a plethora of strategies to cope with drought and maintain an adequate water balance, including modifications of the leaf area, stomatal control, changes in biomass allocation, modifications of source/sink carbon balance, and resistance to xylem embolism. Despite this, drought-induced forest mortality is a widespread phenomenon with potentially large ecosystem-level implications and is expected to increase due to increasing drought events as a result of ongoing climate change. Understanding how the complex network of traits involved in drought resistance determine species’ or individuals’ to survive drought is critical to assess the vulnerability of current vegetation to changes in climate and the potential impacts on ecosystem functioning and services. In 2008, McDowell et al. summarized drought-induced mortality mechanisms in a coherent and simple hydraulic framework. They hypothesized two main, non-exclusive physiological mechanisms leading to plant death under drought: hydraulic failure and carbon starvation. Hydraulic failure is the point at which whole-plant water transport becomes blocked due to excessive cavitation resulting from critical tensions in the xylem. Carbon starvation is the situation in which carbon supply from photosynthesis, carbon stocks or autophagy fails to meet the minimum metabolic needs. According to this framework, the preponderance of one or the other mechanism depends on the drought intensity and duration and plants' ability to regulate their water potential (Ψw). Isohydric species might be more vulnerable to carbon starvation due to earlier stomatal closure to maintain relatively constant Ψw (and avoid embolism), while anisohydric species would be more susceptible to hydraulic failure as soil dries as they operate with narrow hydraulic safety margins due to their lower Ψw. The previous framework is centered on stomatal behavior, regardless of the plethora of traits involved in plant drought responses. In addition, stomata respond to several factors besides Ψw, hence assuming that iso/anisohydric regulation of Ψw is able to fully explain stomatal behavior may be misleading. For these reasons, the main objectives in this thesis were to: (1) determine if differences in stomatal regulation between species relate to iso/anisohydric behaviors and how these are associated to different mortality mechanisms under drought, warming or both; (2) test the assumptions that relate anisohydric behaviors with higher stomatal conductances and longer periods of carbon uptake under drought, and isohydric behaviors with stronger stomatal control and wider hydraulic safety margins; and (3) understand how morphological and physiological traits and their plasticity in response to drought explain, and to what extent, time until death within species. To address targets (1) and (2) we studied two reference models with contrasted drought-vulnerability between species: piñon-juniper and holm oak systems. In both cases, we compared drought responses between isohydric (Pinus edulis and Quercus ilex) and anisohydric species (Juniperus mosperma and Phillyrea latifolia), emphasizing stomatal regulation and carbon and water economies. In these species, we provided evidence that more anisohydric behavior is not necessarily related with looser stomatal responses to Ψw and, thus, with higher levels of xylem embolism. Likewise, stronger regulation of Ψw (isohydric behavior) was neither associated with earlier stomatal closure under drought nor with higher carbon constrains. Both studies challenge widespread notions and warn against linking iso/anisohydry with contrasted stomatal behaviors and mortality mechanisms. At the tree level (3), sustaining carbon uptake and carbon stocks above some critical level was the key factor prolonging survival under extreme drought, even at expenses of higher water losses. Fully integrating carbon and water economies is the key challenge to advance our understanding of drought responses and mortality mechanisms in plants.

Les sécheresses ont eu un impact récurrent sur les forêts tropicales amazoniennes, amenuisant la capacité de puits de carbone de la biomasse forestière. La plupart des modèles globaux de surface terrestre utilisés pour les évaluations du budget mondial du carbone et les projections climatiques futures, n'intègrent pas la mortalité des arbres induite par la sécheresse. Leurs prévisions de la dynamique de la biomasse sont donc sujettes à de grandes incertitudes. Les faiblesses des modèlesglobaux sont liés à : (1) l’absence de la représentation explicite du transport hydraulique; (2) le manque d'équations basées sur les processus à travers la description de la façon dont une altération du système de transport hydraulique des arbres conduit à la mortalité ; (3) le manque de représentation de la mortalité à travers les tailles des arbres. Tout d'abord, j'ai implémenté une architecture hydraulique mécaniste qui a été conçue par E. Joetzjer, et un module de mortalité des arbres que j'ai conçu dans l'ORCHIDEE-CAN-NHA. Notre modèle a produit des taux annuels de mortalité des arbres comparables à ceux observés et a capturé la dynamique de la biomasse. Ce travail fournit une base pour des recherches ultérieures sur l'assimilationdes données d'observation expérimentales afin de paramétrer la mortalité des arbres induite par la défaillance hydraulique.Deuxièmement, j'ai appliqué ORCHIDEE-CAN-NHA sur la forêt tropicale intacte de l'Amazonie. Le modèle a reproduit la sensibilité à la sécheresse de la croissance et de la mortalité de la biomasse aérienne (AGB) observée sur des réseaux de placettes d'inventaire forestier dans les forêts intactes d'Amazonie pour les deux récentes méga-sécheresses de 2005 et 2010. Dans le modèle, même si le changement climatique, avec des sécheresses devenant plus sévères, a eu tendance à intensifier la mortalité des arbres, l'augmentation de la concentration de CO2 a contribué à atténuer la perte de carbone due à la mortalité en supprimant la transpiration. Enfin, j'ai utilisé le modèle ORCHIDEE-CAN-NHA afin de simuler le futur du stockage ducarbone dans la biomasse en Amazonie. La plupart des modèles climatiques (ISIMIP-2) projettent néanmoins de manière cohérente une tendance plus sèche dans le nord-est de l'Amazonie. La simulation forcée par le modèle climatique HadGEM dans le scénario RCP8.5 montre un assèchement plus prononcé dans l'est et le nord-est de l'Amazonie, avec un point d'intersection où le puits de carbone se transforme en source de carbone dans le bouclier guyanais et le centre-est de l'Amazonie, au milieu du 21e siècle. Cette étude permet de prédire l'évolution future de la dynamique de la biomasse de la forêtamazonienne avec un modèle amélioré basé sur les processus, capable de reproduire la mortalité induitepar le changement climatique. Dans les sections conclusion et perspectives, des développements futurs et des priorités de recherche sont proposés, qui amélioreraient la fiabilité et les performances du modèle basé sur les processus présentés dans cette thèse, permettant de mieux capturer les mécanismes qui contrôlent l'évolution de la dynamique de la biomasse forestière face à des risques de sécheresse plus fréquents
Droughts have recurrently impacted the Amazon rainforests, undermining the forest biomass carbon sink capacity due to a quicker increase of biomass mortality compared to growth. Most global land surface models used for assessments of the Global Carbon Budget and future climate projections have not incorporated drought-induced tree mortality. Their prediction of biomass dynamics are therefore subject to large uncertainties, as a result of (1) lack of explicit simulation of hydraulic transportin the continuum from soil to leaves; (2) lack of process-based equations connecting the impairment of the hydraulic transport system of trees to mortality; (3) lack of representation of mortality across trees sizes. To address these critical research gaps, I improved plant hydraulic representation in ORCHIDEECAN. This model was re-calibrated and evaluated over rainforests in Amazon basin, and applied to simulate the future evolution of biomass dynamics facing droughts. Firstly, I implemented a mechanistic hydraulic architecture that was designed by E. Joetzjer, and a hydraulic-failure related tree mortality module that I designed into ORCHIDEE-CAN. The model was calibrated against the world’s longest running drought manipulation experiment of Caxiuana in the eastern Amazon. Our model produced comparable annual tree mortality rates than the observation andcaptured biomass dynamics. This work provides a basis for further research in assimilating experimental observation data to parameterize the hydraulic failure induced tree mortality. Secondly, I applied ORCHIDEE-CAN-NHA over the Amazon intact rainforest. The model reproduced the drought sensitivity of aboveground biomass (AGB) growth and mortality observed atnetworks of forest inventory plots across Amazon intact forests for the two recent mega-droughts of 2005 and 2010. We predicted a more negative sensitivity of the net biomass carbon sink to water deficits for the recent 2015/16 El Nino, which was the most severe drought in the historical record. In the model, even if climate change with droughts becoming more severe tended to intensify tree mortality, increased CO2 concentration contributed to attenuate the C loss due to mortality by suppressing transpiration.Lastly, I used the ORCHIDEE-CAN-NHA model for future simulations of biomass carbondynamics. Most climate models (ISIMIP2 program) consistently predict a drier trend in northeastern Amazon. The simulation forced by the HadGEM climate model in the RCP8.5 scenario shows the most pronounced drying in eastern and northeastern Amazon, with a cross-over point at which the carbon sink turned to a carbon source in the Guiana Shield and East-central Amazon in the middle of the 21st century. This study sheds light on predicting the future evolution of Amazon rainforest biomass dynamics with an improved process-based model able to reproduce climate-change induced mortality.In the conclusion and outlook sections, future developments and research priorities are proposed, which would improve the reliability and performances of the process-based model presented in this dissertation, allowing to better capture mechanisms that control the evolution of forest biomass dynamics in the face of more frequent drought risks

Worldwide forest die-off events have been observed in a number of forest biomes due to severe droughts, rising global temperatures and associated increased vapour pressure deficit (VPD). If drought duration or severity increases with rising temperatures and increased VPD, all forest biomes may be increasingly vulnerable to drought-induced mortality. Despite the importance of forests in the biosphere and the significant potential consequences of forest die-offs, the mechanisms underpinning drought-induced tree mortality are poorly understood. In the context of climate change, elevated temperature has often been reported to exacerbate drought stress and accelerate the time-to-mortality in plants exposed to prolonged drought, while elevated [CO2] has been proposed as a mitigating factor because it can reduce stomatal conductance (gs) and thereby reduce water loss. Rarely have these three environmental factors (elevated [CO2], elevated temperature, and drought) been studied in combination to generate a more complete assessment of the wide-ranging, long-term effects of climate change on trees. Therefore, my PhD thesis was designed to investigate the main and interactive effects of elevated [CO2] and temperature on tree response to drought and subsequent mortality in four species representing different taxa and functional groups: Eucalyptus globulus Labill. (relatively isohydric, angiosperm), Eucalyptus radiata Sieber ex DC (relatively anisohydric, angiosperm), Pinus radiata D. Don (relatively isohydric, gymnosperm) and Callitris rhomboidea R. Br (relatively anisohydric, gymnosperm). My goal was to use these tree species to generate improved understanding of tree physiological responses to drought and its interactions with elevated [CO2] and temperature. This PhD research addressed the main and interactive effects of elevated [CO2] and temperature on tree response to drought and drought-induced tree mortality, by linking water relations and carbon dynamics in four tree species representing different taxa (angiosperms and gymnosperms) and functional groups (relatively isohydric and anisohydric). The study confirmed that hydraulic failure was the dominant mechanism underpinning tree mortality during severe droughts regardless of species or stomatal response strategy. Increasing temperature (ambient + 4 °C) and consequent higher VPD exacerbated drought stress and led to more rapid mortality through hydraulic failure in most species in this study. Rising [CO2] (ambient + 240 μl l-1) ameliorated moderate drought stress in E. globulus, but the positive effects of rising [CO2] were eliminated by increasing drought intensity. Further, elevated [CO2] did not ameliorate drought stress in E. radiata, P. radiata and C. rhomboidea or delay the time-to-mortality. These results suggest that elevated [CO2] may not ameliorate drought or temperature stress in these tree species, particularly when drought is prolonged and severe. Elevated [CO2] partially offset the negative effects of elevated temperature during moderate drought stress in E. globulus, but did not ameliorate drought response to elevated temperature in the other three species in this study. This study suggests that rising temperatures and associated higher VPD may be the predominant contributing factors to drought-induced mortality. Global forests maintain very narrow hydraulic safety margins and these findings raise concern that under future climate scenarios, characterised by rising temperatures and changing drought frequency and intensity, forests will be increasingly vulnerable to large scale mortality events with associated changes in the cycle of mass and energy with ecosystems and the provision of vital ecosystem services.

Moisture stored in live and dead vegetation acts as a regulator on fire behaviour and area burned. Climate change is altering the distribution of live and dead fuels in forests through drought and insect-induced mortality and simultaneously making dead fuels more flammable because of decreasing fuel moisture. These system changes, both of which are driven by increasing temperature, have the potential to increase the heat flux from combustion, contributing to an increased risk of fires in affected areas becoming plume-dominated. In the southern Sierra Nevada of California and the Rocky Mountains in Colorado, drought and insect outbreaks have increased tree mortality rates, increasing the proportion of biomass that is in dead versus live fuel pools. We sought to determine the contribution that high rates of mortality could have onpotential changes in energy release (energy release component and fire radiant energy) for mixed-conifer forests in the southern Sierra Nevada and lodgepole pine forests in the Colorado Rocky Mountains, the site of two large wildfires during the 2020 fire season. We found substantial increases in dead fuels and substantial decreases in fuel moisture during 2020, which increased the potential fire radiative energy. Our results demonstrate that climate-driven tree mortality and increasing temperatures that lead to lower fuel moisture are increasing the amount of energy stored in biomass that is available for combustion.

Globally, anthropogenic climate change is one of the greatest threats to resources in protected areas. This report examines historical and projected climate change across the Greater Grand Canyon Landscape (GGCL), including Grand Canyon National Park. Grand Canyon National Park warmed significantly from 1895-2020 (annual mean increase of 1.89? F/century), with temperatures increasing at a faster rate from 1970-2020 (6.31? F/century). Warming occurred at all elevations and seasons across the GGCL, but rates differed spatially. Average annual total precipitation within Grand Canyon National Park did not change significantly over either period examined (1895-2020; 1970-2020). A variety of changes in the region of Grand Canyon National Park have been detected and attributed, at least in part, to anthropogenic climate change, including reduced soil moisture (and associated drought), reduced Colorado River flow, doubling of the area burned by wildfire across the western United States, reduced regeneration of low-elevation ponderosa pine and Douglas-fir as well as pinyon pine and juniper populations, northward shifts in many bird species distributions and declines of bird species occupancy in the Mojave Desert, and reduced bumble bee species richness and abundance (key pollinators). To help managers understand and plan around a range of plausible future climates, we present two plausible but contrasting climate futures for the Greater Grand Canyon Landscape, characterized at mid-century (2040-2069) and late-century (2070-2099). Examining multiple plausible futures avoids over-optimizing management strategies for a single projected future that may not occur. Overarching patterns that emerged from both climate futures include additional warming (average, as well as extreme temperatures), seasonal increases in extreme precipitation events, fewer freezing days and days with snow, and higher moisture deficit (a correlate with landscape dryness, conditions conducive to fire, and vegetation stress). The selected climate futures differed in terms of 1) the degree of warming, 2) whether winter precipitation increases or decreases, 3) whether annual precipitation increases or stays similar, 4) whether drought conditions increase or decrease, and 5) whether runoff increases or decreases. Runoff is projected to occur earlier under both climate futures and is projected to exhibit a more episodic pattern. Based on a literature review, projected changes to the physical, ecological, and cultural resource domains of the region resulting from anthropogenic climate change include: ? Increasing drought risk and aridification ? Reduced Colorado River flow ? Reduced groundwater infiltration ? Decreasing runoff (from snow or rain) in the spring, summer, and fall, and increasing runoff in the winter ? Increasing occurrence of large fires ? Increasing invasive grasses in the Mojave Desert ecosystems west of the park, providing more fuel for wildfire ? Exacerbated post-fire erosion and sediment in Grand Canyon watersheds ? Increased episodes of drought-induced tree mortality ? Upslope shifts of the elevational zones of pinyon-juniper woodland, ponderosa pine forest, and spruce-fir forest, as well as increases in non-forest areas and aboveground biomass declines ? Reduced abundance of riparian vegetation that tolerates water inundation ? Increasing invasive plant distribution and abundance, favoring their establishment and productivity ? Colonization of the GGCL by some bird species and extirpation of others ? Increasing non-native fish populations relative to native fishes ? Declining butterfly populations ? Increasing temperatures will increase visitation, especially during winter and shoulder seasons ? Exacerbation of existing threats to archeological resources, cultural landscapes, and historic structures, as well as emergent vulnerabilities related to climate change One goal of this work is to support the Resource Stewardship Strategy (RSS) process that Grand Canyon National Park plans to undertake. We anticipate that connecting the climate changes described here to the climate sensitivities of resources within the park will play a critical role in setting goals and strategies during development of the RSS, as well as proactively adapting to anticipated changes.