Drought's legacy: multiyear hydraulic deterioration underlies widespread aspen forest die‐off and portends increased future risk

Forest mortality constitutes a major uncertainty in projections of climate impacts on terrestrial ecosystems and carbon‐cycle feedbacks. Recent drought‐induced, widespread forest die‐offs highlight that climate change could accelerate forest mortality with its diverse and potentially severe consequences for the global carbon cycle, ecosystem services, and biodiversity. How trees die during drought over multiple years remains largely unknown and precludes mechanistic modeling and prediction of forest die‐off with climate change. Here, we examine the physiological basis of a recent multiyear widespread die‐off of trembling aspen (Populus tremuloides) across much of western North America. Using observations from both native trees while they are dying and a rainfall exclusion experiment on mature trees, we measure hydraulic performance over multiple seasons and years and assess pathways of accumulated hydraulic damage. We test whether accumulated hydraulic damage can predict the probability of tree survival over 2 years. We find that hydraulic damage persisted and increased in dying trees over multiple years and exhibited few signs of repair. This accumulated hydraulic deterioration is largely mediated by increased vulnerability to cavitation, a process known as cavitation fatigue. Furthermore, this hydraulic damage predicts the probability of interyear stem mortality. Contrary to the expectation that surviving trees have weathered severe drought, the hydraulic deterioration demonstrated here reveals that surviving regions of these forests are actually more vulnerable to future droughts due to accumulated xylem damage. As the most widespread tree species in North America, increasing vulnerability to drought in these forests has important ramifications for ecosystem stability, biodiversity, and ecosystem carbon balance. Our results provide a foundation for incorporating accumulated drought impacts into climate–vegetation models. Finally, our findings highlight the critical role of drought stress accumulation and repair of stress‐induced damage for avoiding plant mortality, presenting a dynamic and contingent framework for drought impacts on forest ecosystems.     

Embolism recovery strategies and nocturnal water loss across species influenced by biogeographic origin

Drought‐induced tree mortality is expected to increase in future climates with the potential for significant consequences to global carbon, water, and energy cycles. Xylem embolism can accumulate to lethal levels during drought, but species that can refill embolized xylem and recover hydraulic function may be able to avoid mortality. Yet the potential controls of embolism recovery, including cross‐biome patterns and plant traits such as nonstructural carbohydrates (NSCs), hydraulic traits, and nocturnal stomatal conductance, are unknown. We exposed eight plant species, originating from mesic (tropical and temperate) and semi‐arid environments, to drought under ambient and elevated CO₂ levels, and assessed recovery from embolism following rewatering. We found a positive association between xylem recovery and NSCs, and, surprisingly, a positive relationship between xylem recovery and nocturnal stomatal conductance. Arid‐zone species exhibited greater embolism recovery than mesic zone species. Our results indicate that nighttime stomatal conductance often assumed to be a wasteful use of water, may in fact be a key part of plant drought responses, and contribute to drought survival. Findings suggested distinct biome‐specific responses that partially depended on species climate‐of‐origin precipitation or aridity index, which allowed some species to recover from xylem embolism. These findings provide improved understanding required to predict the response of diverse plant communities to drought. Our results provide a framework for predicting future vegetation shifts in response to climate change.     

Will a 385 million year‐struggle for light become a struggle for water and for carbon? – How trees may cope with more frequent climate change‐type drought events

Trees are exceptional organisms that have evolved over some 385 million years and have overtaken other plants in order to harvest light first. However, this advantage comes with a cost: trees must transport water all the way up to their crowns and inherent physical limitations make them vulnerable to water deficits. Because climate change scenarios predict more frequent extreme drought events, trees will increasingly need to cope with water stress. Recent occurrences of climate change‐type droughts have had severe impacts on several forest ecosystems. Initial experimental studies have been undertaken and show that stomatal control of water loss hinders carbon assimilation and could lead to starvation during droughts. Other mechanisms of drought‐induced mortality are catastrophic xylem dysfunction, impeded long‐distance transport of carbohydrates (translocation) and also symplastic failure (cellular breakdown). However, direct empirical support is absent for either hypothesis. More experimental studies are necessary to increase our understanding of these processes and to resolve the mystery of drought‐related tree mortality. Instead of testing the validity of particular hypothesis as mechanisms of drought‐induced tree mortality, future research should aim at revealing the temporal dynamics of these mechanisms in different species and over a gradient of environmental conditions. Only such studies will reveal whether the struggle for light will become a struggle for water and/or for carbon in drought‐affected areas.     

Forest responses to simulated elevated CO2 under alternate hypotheses of size‐ and age‐dependent mortality

Elevated atmospheric carbon dioxide (eCO₂) is predicted to increase growth rates of forest trees. The extent to which increased growth translates to changes in biomass is dependent on the turnover time of the carbon, and thus tree mortality rates. Size‐ or age‐dependent mortality combined with increased growth rates could result in either decreased carbon turnover from a speeding up of tree life cycles, or increased biomass from trees reaching larger sizes, respectively. However, most vegetation models currently lack any representation of size‐ or age‐dependent mortality and the effect of eCO₂ on changes in biomass and carbon turnover times is thus a major source of uncertainty in predictions of future vegetation dynamics. Using a reduced‐complexity form of the vegetation demographic model the Functionally Assembled Terrestrial Ecosystem Simulator to simulate an idealised tropical forest, we find increases in biomass despite reductions in carbon turnover time in both size‐ and age‐dependent mortality scenarios in response to a hypothetical eCO₂‐driven 25% increase in woody net primary productivity (wNPP). Carbon turnover times decreased by 9.6% in size‐dependent mortality scenarios due to a speeding up of tree life cycles, but also by 2.0% when mortality was age‐dependent, as larger crowns led to increased light competition. Increases in aboveground biomass (AGB) were much larger when mortality was age‐dependent (24.3%) compared with size‐dependent (13.4%) as trees reached larger sizes before death. In simulations with a constant background mortality rate, carbon turnover time decreased by 2.1% and AGB increased by 24.0%, however, absolute values of AGB and carbon turnover were higher than in either size‐ or age‐dependent mortality scenario. The extent to which AGB increases and carbon turnover decreases will thus depend on the mechanisms of large tree mortality: if increased size itself results in elevated mortality rates, then this could reduce by about half the increase in AGB relative to the increase in wNPP.     

Growth, carbon-isotope discrimination, and drought-associated mortality across a Pinus ponderosa elevational transect

Drought‐ and insect‐associated tree mortality at low‐elevation ecotones is a widespread phenomenon but the underlying mechanisms are uncertain. Enhanced growth sensitivity to climate is widely observed among trees that die, indicating that a predisposing physiological mechanism(s) underlies tree mortality. We tested three, linked hypotheses regarding mortality using a ponderosa pine (Pinus ponderosa) elevation transect that experienced low‐elevation mortality following prolonged drought. The hypotheses were: (1) mortality was associated with greater growth sensitivity to climate, (2) mortality was associated with greater sensitivity of gas exchange to climate, and (3) growth and gas exchange were correlated. Support for all three hypotheses would indicate that mortality results at least in part from gas exchange constraints. We assessed growth using basal area increment normalized by tree basal area [basal area increment (BAI)/basal area (BA)] to account for differences in tree size. Whole‐crown gas exchange was indexed via estimates of the CO₂ partial pressure difference between leaf and atmosphere (p_a?p_c) derived from tree ring carbon isotope ratios (δ¹³C), corrected for temporal trends in atmospheric CO₂ and δ¹³C and elevation trends in pressure. Trees that survived the drought exhibited strong correlations among and between BAI, BAI/BA, p_a?p_c, and climate. In contrast, trees that died exhibited greater growth sensitivity to climate than trees that survived, no sensitivity of p_a?p_c to climate, and a steep relationship between p_a?p_c and BAI/BA. The p_a?p_c results are consistent with predictions from a theoretical hydraulic model, suggesting trees that died had a limited buffer between mean water availability during their lifespan and water availability during drought – i.e., chronic water stress. It appears that chronic water stress predisposed low‐elevation trees to mortality during drought via constrained gas exchange. Continued intensification of drought in mid‐latitude regions may drive increased mortality and ecotone shifts in temperate forests and woodlands.     

The importance of low atmospheric CO2 and fire in promoting the spread of grasslands and savannas

The distribution and abundance of trees can be strongly affected by disturbance such as fire. In mixed tree/grass ecosystems, recurrent grass‐fuelled fires can strongly suppress tree saplings and therefore control tree dominance. We propose that changes in atmospheric [CO₂] could influence tree cover in such metastable ecosystems by altering their postburn recovery rates relative to flammable herbaceous growth forms such as grasses. Slow sapling recovery rates at low [CO₂] would favour the spread of grasses and a reduction of tree cover. To test the possible importance of [CO₂]/fire interactions, we first used a Dynamic Global Vegetation Model (DGVM) to simulate biomass in grassy ecosystems in South Africa with and without fire. The results indicate that fire has a major effect under higher rainfall conditions suggesting an important role for fire/[CO₂] interactions. We then used a demographic model of the effects of fire on mesic savanna trees to test the importance of grass/tree differences in postburn recovery rates. We adjusted grass and tree growth in the model according to the DGVM output of net primary production at different [CO₂] relative to current conditions. The simulations predicted elimination of trees at [CO₂] typical of the last glacial period (180 ppm) because tree growth rate is too slow (15 years) to grow to a fire‐proof size of ca. 3 m. Simulated grass growth would produce an adequate fuel load for a burn in only 2 years. Simulations of preindustrial [CO₂] (270 ppm) predict occurrence of trees but at low densities. The greatest increase in trees occurs from preindustrial to current [CO₂] (360 ppm). The simulations are consistent with palaeo‐records which indicate that trees disappeared from sites that are currently savannas in South Africa in the last glacial. Savanna trees reappeared in the Holocene. There has also been a large increase in trees over the last 50–100 years. We suggest that slow tree recovery after fire, rather than differential photosynthetic efficiencies in C₃ and C₄ plants, might have been the significant factor in the Late Tertiary spread of flammable grasslands under low [CO₂] because open, high light environments would have been a prerequisite for the spread of C₄ grasses. Our simulations suggest further that low [CO₂] could have been a significant factor in the reduction of trees during glacial times, because of their slower regrowth after disturbance, with fire favouring the spread of grasses.     

Differences in xylem and leaf hydraulic traits explain differences in drought tolerance among mature Amazon rainforest trees

Considerable uncertainty surrounds the impacts of anthropogenic climate change on the composition and structure of Amazon forests. Building upon results from two large‐scale ecosystem drought experiments in the eastern Brazilian Amazon that observed increases in mortality rates among some tree species but not others, in this study we investigate the physiological traits underpinning these differential demographic responses. Xylem pressure at 50% conductivity (xylem‐P₅₀), leaf turgor loss point (TLP), cellular osmotic potential (π_o), and cellular bulk modulus of elasticity (ε), all traits mechanistically linked to drought tolerance, were measured on upper canopy branches and leaves of mature trees from selected species growing at the two drought experiment sites. Each species was placed a priori into one of four plant functional type (PFT) categories: drought‐tolerant versus drought‐intolerant based on observed mortality rates, and subdivided into early‐ versus late‐successional based on wood density. We tested the hypotheses that the measured traits would be significantly different between the four PFTs and that they would be spatially conserved across the two experimental sites. Xylem‐P₅₀, TLP, and π_o, but not ε, occurred at significantly higher water potentials for the drought‐intolerant PFT compared to the drought‐tolerant PFT; however, there were no significant differences between the early‐ and late‐successional PFTs. These results suggest that these three traits are important for determining drought tolerance, and are largely independent of wood density—a trait commonly associated with successional status. Differences in these physiological traits that occurred between the drought‐tolerant and drought‐intolerant PFTs were conserved between the two research sites, even though they had different soil types and dry‐season lengths. This more detailed understanding of how xylem and leaf hydraulic traits vary between co‐occuring drought‐tolerant and drought‐intolerant tropical tree species promises to facilitate a much‐needed improvement in the representation of plant hydraulics within terrestrial ecosystem and biosphere models, which will enhance our ability to make robust predictions of how future changes in climate will affect tropical forests.     

Carbon reserves and canopy defoliation determine the recovery of Scots pine 4 yr after a drought episode

? Severe drought may increase physiological stress on long-lived woody vegetation, occasionally leading to mortality of overstory trees. Little is known about the factors determining tree survival and subsequent recovery after drought. ? We used structural equation modeling to analyse the recovery of Scots pine (Pinus sylvestris) trees 4 yr after an extreme drought episode occurred in 2004-2005 in north-east Spain. Measured variables included the amount of green foliage, carbon reserves in the stem, mistletoe (Viscum album) infection, needle physiological performance and stem radial growth before, during and after the drought event. ? The amount of green leaves and the levels of carbon reserves were related to the impact of drought on radial growth, and mutually correlated. However, our most likely path model indicated that current depletion of carbon reserves was a result of reduced photosynthetic tissue. This relationship potentially constitutes a feedback limiting tree recovery. In addition, mistletoe infection reduced leaf nitrogen content, negatively affecting growth. Finally, successive surveys in 2009-2010 showed a direct association between carbon reserves depletion and drought-induced mortality. ? Severe drought events may induce long-term physiological disorders associated with canopy defoliation and depletion of carbon reserves, leading to prolonged recovery of surviving individuals and, eventually, to delayed tree death.     

Dominant tree species are at risk from exaggerated drought under climate change

Predicting the consequences of climate change on forest systems is difficult because trees may display species‐specific responses to exaggerated droughts that may not be reflected by the climatic envelope of their geographic range. Furthermore, few studies have examined the postdrought recovery potential of drought‐susceptible tree species. This study develops a robust ranking of the drought susceptibility of 21 tree species based on their mortality after two droughts (1990s and 2000s) in the savanna of north‐eastern Australia. Drought‐induced mortality was positively related to species dominance, negatively related to the ratio of postdrought seedlings to adults and had no relationship to the magnitude of extreme drought within the species current geographic ranges. These results suggest that predicting the consequences of exaggerated drought on species’ geographic ranges is difficult, but that dominant species like Eucalyptus with relatively slow rates of population recovery and dispersal are the most susceptible. The implications for savanna ecosystems are lower tree densities and basal area.     

Xylem hydraulic safety and construction costs determine tropical tree growth

Faster growth in tropical trees is usually associated with higher mortality rates, but the mechanisms underlying this relationship are poorly understood. In this study, we investigate how tree growth patterns are linked with environmental conditions and hydraulic traits, by monitoring the cambial growth of 9 tropical cloud forest tree species coupled with numerical simulations using an optimization model. We find that fast‐growing trees have lower xylem safety margins than slow‐growing trees and this pattern is not necessarily linked to differences in stomatal behaviour or environmental conditions when growth occurs. Instead, fast‐growing trees have xylem vessels that are more vulnerable to cavitation and lower density wood. We propose the growth ‐ xylem vulnerability trade‐off represents a wood hydraulic economics spectrum similar to the classic leaf economic spectrum, and show through numerical simulations that this trade‐off can emerge from the coordination between growth rates, wood density, and xylem vulnerability to cavitation. Our results suggest that vulnerability to hydraulic failure might be related with the growth‐mortality trade‐off in tropical trees, determining important life history differences. These findings are important in furthering our understanding of xylem hydraulic functioning and its implications on plant carbon economy.     

The Effect of Habitat Association and Edaphic Conditions on Tree Mortality during El Niño‐induced Drought in a Bornean Dipterocarp Forest

The effects of El Niño‐induced droughts on dipterocarp forests must be quantified to evaluate the implications of future global climatic changes for the tropical forests of Southeast Asia. We studied the mortality of trees ≥ 1 cm in diameter in a lowland dipterocarp forest in Borneo before, during, and after the 1997/1998 El Niño drought. The annual mortality rates were 1.30, 1.75, and 1.66 percent/yr for the pre‐drought, drought, and post‐drought periods, respectively. The effect of drought was tree size‐dependent being greater for larger trees. Modified logistic regression analysis revealed a significant interaction effect between species' habitat association and edaphic condition on mortality rates in all periods. For species associated with wet habitat, drought effect was greater in dry conditions than in wet conditions, in both the drought and post‐drought periods. The mortality rates of dry‐habitat species were less affected by the drought both in dry and wet conditions. A similar pattern was also found in common Dipterocarpaceae species; mortality rates increased more in species associated with wet‐habitat in the drought and post‐drought periods. Species and families with higher mortality in the pre‐drought period tended to experience greater mortality increases during the drought and post‐drought periods. These results suggest that changes in drought regimes alter the species composition and spatial distribution of dipterocarp forests.     

Drought timing and local climate determine the sensitivity of eastern temperate forests to drought

Projected changes in temperature and drought regime are likely to reduce carbon (C) storage in forests, thereby amplifying rates of climate change. While such reductions are often presumed to be greatest in semi‐arid forests that experience widespread tree mortality, the consequences of drought may also be important in temperate mesic forests of Eastern North America (ENA) if tree growth is significantly curtailed by drought. Investigations of the environmental conditions that determine drought sensitivity are critically needed to accurately predict ecosystem feedbacks to climate change. We matched site factors with the growth responses to drought of 10,753 trees across mesic forests of ENA, representing 24 species and 346 stands, to determine the broad‐scale drivers of drought sensitivity for the dominant trees in ENA. Here we show that two factors—the timing of drought, and the atmospheric demand for water (i.e., local potential evapotranspiration; PET)—are stronger drivers of drought sensitivity than soil and stand characteristics. Drought‐induced reductions in tree growth were greatest when the droughts occurred during early‐season peaks in radial growth, especially for trees growing in the warmest, driest regions (i.e., highest PET). Further, mean species trait values (rooting depth and ψ₅₀) were poor predictors of drought sensitivity, as intraspecific variation in sensitivity was equal to or greater than interspecific variation in 17 of 24 species. From a general circulation model ensemble, we find that future increases in early‐season PET may exacerbate these effects, and potentially offset gains in C uptake and storage in ENA owing to other global change factors.     

Prolonged tropical forest degradation due to compounding disturbances: Implications for CO2 and H2O fluxes

Drought, fire, and windstorms can interact to degrade tropical forests and the ecosystem services they provide, but how these forests recover after catastrophic disturbance events remains relatively unknown. Here, we analyze multi‐year measurements of vegetation dynamics and function (fluxes of CO₂ and H₂O) in forests recovering from 7 years of controlled burns, followed by wind disturbance. Located in southeast Amazonia, the experimental forest consists of three 50‐ha plots burned annually, triennially, or not at all from 2004 to 2010. During the subsequent 6‐year recovery period, postfire tree survivorship and biomass sharply declined, with aboveground C stocks decreasing by 70%–94% along forest edges (0–200 m into the forest) and 36%–40% in the forest interior. Vegetation regrowth in the forest understory triggered partial canopy closure (70%–80%) from 2010 to 2015. The composition and spatial distribution of grasses invading degraded forest evolved rapidly, likely because of the delayed mortality. Four years after the experimental fires ended (2014), the burned plots assimilated 36% less carbon than the Control, but net CO₂ exchange and evapotranspiration (ET) had fully recovered 7 years after the experimental fires ended (2017). Carbon uptake recovery occurred largely in response to increased light‐use efficiency and reduced postfire respiration, whereas increased water use associated with postfire growth of new recruits and remaining trees explained the recovery in ET. Although the effects of interacting disturbances (e.g., fires, forest fragmentation, and blowdown events) on mortality and biomass persist over many years, the rapid recovery of carbon and water fluxes can help stabilize local climate.     

Drought stress limits the geographic ranges of two tree species via different physiological mechanisms

Range shifts are among the most ubiquitous ecological responses to anthropogenic climate change and have large consequences for ecosystems. Unfortunately, the ecophysiological forces that constrain range boundaries are poorly understood, making it difficult to mechanistically project range shifts. To explore the physiological mechanisms by which drought stress controls dry range boundaries in trees, we quantified elevational variation in drought tolerance and in drought avoidance‐related functional traits of a widespread gymnosperm (ponderosa pine – Pinus ponderosa) and angiosperm (trembling aspen – Populus tremuloides) tree species in the southwestern USA. Specifically, we quantified tree‐to‐tree variation in growth, water stress (predawn and midday xylem tension), drought avoidance traits (branch conductivity, leaf/needle size, tree height, leaf area‐to‐sapwood area ratio), and drought tolerance traits (xylem resistance to embolism, hydraulic safety margin, wood density) at the range margins and range center of each species. Although water stress increased and growth declined strongly at lower range margins of both species, ponderosa pine and aspen showed contrasting patterns of clinal trait variation. Trembling aspen increased its drought tolerance at its dry range edge by growing stronger but more carbon dense branch and leaf tissues, implying an increased cost of growth at its range boundary. By contrast, ponderosa pine showed little elevational variation in drought‐related traits but avoided drought stress at low elevations by limiting transpiration through stomatal closure, such that its dry range boundary is associated with limited carbon assimilation even in average climatic conditions. Thus, the same climatic factor (drought) may drive range boundaries through different physiological mechanisms – a result that has important implications for process‐based modeling approaches to tree biogeography. Further, we show that comparing intraspecific patterns of trait variation across ranges, something rarely done in a range‐limit context, helps elucidate a mechanistic understanding of range constraints.     

Long‐term response of temperate canopy trees to removal of browsing from an invasive arboreal herbivore in New Zealand

Defoliation of forest tree canopies by herbivores and other agents, leading to tree mortality and reduced productivity, threatens the ecological stability of forests globally. This study shows that long‐term control of a mammalian arboreal folivore (brushtail possums; Trichosurus vulpecula Phalangeridae) reduces crown dieback and increases foliage cover in browsing‐damaged canopy trees. We monitored indices of possum density, possum browsing, tree foliage cover and crown dieback for 20 years following initiation of possum control in 1994 that repeatedly reduced possum densities to near zero every 5–6 years and kept the population below 35% of pre‐control levels over the entire period. Observable possum browsing was recorded on 20–49% of individuals of three palatable tree species at the time of first control. Those percentages fell to zero after control and never exceeded 2–10% for individual species over the next 19 years. We recorded significant increases in foliage cover attributable to recovery from defoliation by possums for all three species during the first 10 years. Large increases in foliage cover occurred on individuals that were heavily browsed in 1994 (mean increases: 36–89%), but mean population increases were modest (3–19%) because only 10–19% of trees were initially heavily browsed. Twenty‐year mortality rates were similar for plants with, or without, initial possum browsing, indicating no residual impact of pre‐control browsing on tree mortality. Times for full recovery of crown foliage cover varied from 10 years for the youngest trees and faster growing species to more than 20 years for mature individuals of the slowest growing species.     

The role of remnant trees in carbon sequestration,vegetation structure and tree diversity of early succession regrowing fallows in eastern Sierra Leone

Remnant tree presence affects forest recovery after slash‐and‐burn agriculture. However, little is known about its effect on above‐ground carbon stocks, especially in Africa. We focused our study on Sierra Leone, part of the Upper Guinean forests, an important centre of endemism threatened by encroachment and forest degradation. We studied 99 (20‐m‐radius) plots aged 2–10 years with and without remnant trees and compared their above‐ground carbon stocks, vegetation structure (stem density, basal area) and tree diversity. Above‐ground carbon stocks, stem density, basal area, species richness and tree diversity increased significantly with fallow age. Remnant tree presence affected significantly tree diversity, species dominance and above‐ground carbon stocks, but not vegetation structure (stem density, basal area). Number of remnant trees and number of species of remnant trees were also important explanatory variables. Although other factors should be considered in future studies, such as the size and dispersal modes of remnant trees, our results highlight that more strategic inclusion of remnant trees is likely to favour carbon stock and forest recovery in old fallows. To our knowledge, this is the first study on early succession regrowing fallows in West Africa.     

Amazonia trees have limited capacity to acclimate plant hydraulic properties in response to long‐term drought

The fate of tropical forests under future climate change is dependent on the capacity of their trees to adjust to drier conditions. The capacity of trees to withstand drought is likely to be determined by traits associated with their hydraulic systems. However, data on whether tropical trees can adjust hydraulic traits when experiencing drought remain rare. We measured plant hydraulic traits (e.g. hydraulic conductivity and embolism resistance) and plant hydraulic system status (e.g. leaf water potential, native embolism and safety margin) on >150 trees from 12 genera (36 species) and spanning a stem size range from 14 to 68 cm diameter at breast height at the world's only long‐running tropical forest drought experiment. Hydraulic traits showed no adjustment following 15 years of experimentally imposed moisture deficit. This failure to adjust resulted in these drought‐stressed trees experiencing significantly lower leaf water potentials, and higher, but variable, levels of native embolism in the branches. This result suggests that hydraulic damage caused by elevated levels of embolism is likely to be one of the key drivers of drought‐induced mortality following long‐term soil moisture deficit. We demonstrate that some hydraulic traits changed with tree size, however, the direction and magnitude of the change was controlled by taxonomic identity. Our results suggest that Amazonian trees, both small and large, have limited capacity to acclimate their hydraulic systems to future droughts, potentially making them more at risk of drought‐induced mortality.     

No evidence that elevated CO2 gives tropical lianas an advantage over tropical trees

Recent studies indicate that lianas are increasing in size and abundance relative to trees in neotropical forests. As a result, forest dynamics and carbon balance may be altered through liana‐induced suppression of tree growth and increases in tree mortality. Increasing atmospheric CO₂ is hypothesized to be responsible for the increase in neotropical lianas, yet no study has directly compared the relative response of tropical lianas and trees to elevated CO₂. We explicitly tested whether tropical lianas had a larger response to elevated CO₂ than co‐occurring tropical trees and whether seasonal drought alters the response of either growth form. In two experiments conducted in central Panama, one spanning both wet and dry seasons and one restricted to the dry season, we grew liana (n = 12) and tree (n = 10) species in open‐top growth chambers maintained at ambient or twice‐ambient CO₂ levels. Seedlings of eight individuals (four lianas, four trees) were grown in the ground in each chamber for at least 3 months during each season. We found that both liana and tree seedlings had a significant and positive response to elevated CO₂ (in biomass, leaf area, leaf mass per area, and photosynthesis), but that the relative response to elevated CO₂ for all variables was not significantly greater for lianas than trees regardless of the season. The lack of differences in the relative response between growth forms does not support the hypothesis that elevated CO₂ is responsible for increasing liana size and abundance across the neotropics.     

Importance of resin ducts in reducing ponderosa pine mortality from bark beetle attack

The relative importance of growth and defense to tree mortality during drought and bark beetle attacks is poorly understood. We addressed this issue by comparing growth and defense characteristics between 25 pairs of ponderosa pine (Pinus ponderosa) trees that survived and trees that died from drought-associated bark beetle attacks in forests of northern Arizona, USA. The three major findings of our research were: (1) xylem resin ducts in live trees were >10% larger (diameter), >25% denser (no. of resin ducts mm⁻²), and composed >50% more area per unit ring growth than dead trees; (2) measures of defense, such as resin duct production (no. of resin ducts year⁻¹) and the proportion of xylem ring area to resin ducts, not growth, were the best model parameters of ponderosa pine mortality; and (3) most correlations between annual variation in growth and resin duct characteristics were positive suggesting that conditions conducive to growth also increase resin duct production. Our results suggest that trees that survive drought and subsequent bark beetle attacks invest more carbon in resin defense than trees that die, and that carbon allocation to resin ducts is a more important determinant of tree mortality than allocation to radial growth.     

A retrospective,dual‐isotope approach reveals individual predispositions to winter‐drought induced tree dieback in the southernmost distribution limit of Scots pine

Winter‐drought induced forest diebacks in the low‐latitude margins of species' distribution ranges can provide new insights into the mechanisms (carbon starvation, hydraulic failure) underlying contrasting tree reactions. We analysed a winter‐drought induced dieback at the Scots pine's southern edge through a dual‐isotope approach (Δ¹³C and δ¹⁸O in tree‐ring cellulose). We hypothesized that a differential long‐term performance, mediated by the interaction between CO₂ and climate, determined the fates of individuals during dieback. Declining trees showed a stronger coupling between climate, growth and intrinsic water‐use efficiency (WUEi) than non‐declining individuals that was noticeable for 25 years prior to dieback. The rising stomatal control of water losses with time in declining trees, indicated by negative Δ¹³C‐δ¹⁸O relationships, was likely associated with their native aptitude to grow more and take up more water (suggested by larger tracheid lumen widths) than non‐declining trees and, therefore, to exhibit a greater cavitation risk. Freeze‐thaw episodes occurring in winter 2001 unveiled such physiological differences by triggering dieback in those trees more vulnerable to hydraulic failure. Thus, WUEi tightly modulated growth responses to long‐term warming in declining trees, indicating that co‐occurring individuals were differentially predisposed to winter‐drought mortality. These different performances were unconnected to the depletion of stored carbohydrates.