Global change‐type drought‐induced tree mortality: vapor pressure deficit is more important than temperature per se in causing decline in tree health

Drought‐induced tree mortality is occurring across all forested continents and is expected to increase worldwide during the coming century. Regional‐scale forest die‐off influences terrestrial albedo, carbon and water budgets, and land‐surface energy partitioning. Although increased temperatures during drought are widely identified as a critical contributor to exacerbated tree mortality associated with “global‐change‐type drought”, corresponding changes in vapor pressure deficit (D) have rarely been considered explicitly and have not been disaggregated from that of temperature per se. Here, we apply a detailed mechanistic soil–plant–atmosphere model to examine the impacts of drought, increased air temperature (+2°C or +5°C), and increased vapor pressure deficit (D; +1 kPa or +2.5 kPa), singly and in combination, on net primary productivity (NPP) and transpiration and forest responses, especially soil moisture content, leaf water potential, and stomatal conductance. We show that increased D exerts a larger detrimental effect on transpiration and NPP, than increased temperature alone, with or without the imposition of a 3‐month drought. Combined with drought, the effect of increased D on NPP was substantially larger than that of drought plus increased temperature. Thus, the number of days when NPP was zero across the 2‐year simulation was 13 or 14 days in the control and increased temperature scenarios, but increased to approximately 200 days when D was increased. Drought alone increased the number of days of zero NPP to 88, but drought plus increased temperature did not increase the number of days. In contrast, drought and increased D resulted in the number of days when NPP = 0 increasing to 235 (+1 kPa) or 304 days (+2.5 kPa). We conclude that correct identification of the causes of global change‐type mortality events requires explicit consideration of the influence of D as well as its interaction with drought and temperature.     

Shrub facilitation drives tree establishment in a semiarid fog‐dependent ecosystem

Questions

The exceptional occurrence of tall rain forest patches on foggy coastal mountaintops, surrounded by extensive xerophytic shrublands, suggests an important role of plant–plant interactions in the origin and persistence of these patches in semi‐arid Chile. We asked whether facilitation by shrubs can explain the growth and survival of rain forest tree species, and whether shrub effects depend on the identity of the shrub species itself, the drought tolerance of the tree species and the position of shrubs in regard to wind direction.

Location

Open area–shrubland–forest matrix, Fray Jorge Forest National Park, Chile.

Methods

We recorded survival after 12 years of a ~3600 tree saplings plantation (originally ~30‐cm tall individuals) of Aextoxicon punctatum, Myrceugenia correifolia and Drimys winteri placed outside forests, beneath the shrub Baccharis vernalis, and in open (shrub‐free) areas. We assessed the effects of neighbouring shrubs and soil humidity on survival and growth along a gradient related to the direction of fog movement.

Results

B. vernalis had a clear facilitative effect on tree establishment and survival since, after ~12 years, saplings only survived beneath the shrub canopy. Long‐term survival strongly depended on tree species identity, drought tolerance and position along the soil moisture gradient, with higher survival of A. punctatum (>35%) and M. correifolia (>14%) at sites on wind‐ and fog‐exposed shrubland areas. Sites occupied by the shrub Aristeguietia salvia were unsuitable for trees, presumably due to drier conditions than under B. vernalis.

Conclusions

Interactions between shrubs and fog‐dependent tree species in dry areas revealed a strong, long‐lasting facilitation effect on planted tree's survival and growth. Shrubs acted as benefactors, providing sites suitable for tree growth. Sapling mortality in the shrubland interior was caused by lower soil moisture, the consequence of lower fog loads in the air and thus insufficient facilitation. While B. vernalis was a key ecosystem engineer (nurse) and intercepted fog water that dripped to trees planted underneath, drier sites with A. salvia were unsuitable for trees. Consequently, nurse effects related to water input are strongly site and species specific, with facilitation by shrubs providing a plausible explanation for the initiation of forest patches in this semi‐arid landscape.     

Contrasting Demographic Structure of Short‐ and Long‐lived Pioneer Tree Species on Amazonian Forest Edges

Although tropical forests have been rapidly converted into human‐modified landscapes, tree species response to forest edges remains poorly examined. In this study, we addressed four pioneer tree species to document demographic shifts experienced by this key ecological group and make inferences about pioneer response to forest edges. All individuals with dbh ≥ 1 cm of two short‐lived (Bellucia grossularioides and Cecropia sciadophylla) and two long‐lived species (Goupia glabra and Laetia procera) were sampled in 20 1‐ha forest edge plots and 20 1‐ha forest interior plots in Oiapoque and Manaus, Northeast and Central Amazon, respectively. As expected, pioneer stem density with dbh ≥ 1 cm increased by around 10–17‐fold along forest edges regardless of species, lifespan, and study site. Edge populations of long‐lived pioneers presented 84–94 percent of their individuals in sapling/subadult size classes, whereas edge populations of short‐lived pioneers showed 56–97 percent of their individuals in adult size classes. These demographic biases were associated with negative and positive net adult recruitment of long‐ and short‐lived pioneers, respectively. Our population‐level analyses support three general statements: (1) native pioneer tree species proliferate along forest edges (i.e., increased density), at least in terms of non‐reproductive individuals; (2) pioneer response to edge establishment is not homogeneous as species differ in terms of demographic structure and net adult recruitment; and (3) some pioneer species, particularly long‐lived ones, may experience population decline due to adult sensitivity to edge‐affected habitats.     

Testing the Home‐Site Advantage in Forest Trees on Disturbed and Undisturbed Sites

Restoration of plant populations is often undertaken using seed or plants from local sources because it is assumed they will be best adapted to the prevailing conditions. However, the effect of site disturbance on local adaptation has rarely been examined. We assessed local adaptation in three southwestern Australian forest tree species (Eucalyptus marginata, Corymbia calophylla, and Allocasuarina fraseriana) using reciprocal transplant trials at disturbed and undisturbed sites. Performance of plants within the trials was assessed over 2 years. Planting location accounted for the majority of the variation in most measures of performance, although significant variation of percent emergence among source populations was also detected. In all species, percent emergence and survival of plants sourced from Darling Range populations was significantly higher than that of plants from the Swan Coastal Plain, regions of contrasting edaphic and climatic environment. Survival of E. marginata over the first 18 months and emergence of C. calophylla were both higher in local plants, providing at least weak evidence for local adaptation. Where a local advantage was observed, the relative performance of local and nonlocal seed did not vary among disturbed and undisturbed sites. Evidence for enhanced establishment from local seed in at least one species leads us to recommend that where sufficient high‐quality seed supplies exist locally, these should be used in restoration. We also recommend longer‐term studies to include the possibility of local adaptation becoming evident at later life history stages.     

Quantifying the effects of drought on abrupt growth decreases of major tree species in Switzerland

Drought entails important effects on tree physiology, which may result in short‐ to long‐term radial growth decreases. While the majority of studies have focused on annual drought‐related variability of growth, relatively little is known about sustained growth decreases following drought years. We apply a statistical framework to identify climatic factors that induce abrupt growth decreases and may eventually result in tree mortality. We used tree‐ring data from almost 500 standing dead trees and 200 living trees in eight sites of the Swiss network of strict forest reserves, including four of the most important Central European tree species (Abies alba, Picea abies, Fagus sylvatica and Quercus spp.). First, to assess short‐term growth responses to drought under various climate and site conditions, we calculated correlations and linear mixed‐effects models between ring‐width indices (RWIs) and drought based on the Standardized Precipitation Evapotranspiration Index (SPEI). Second, to quantify drought effects on abrupt growth decreases, we applied distributed lag nonlinear models (DLNMs), which account for both delayed effects and the nonlinear relationship between the SPEI and the occurrence of abrupt growth decreases. Positive correlations between RWIs and the SPEI indicated short‐term growth responses of all species, particularly at arid sites. Results of the DLNMs revealed species‐specific growth responses to drought. For Quercus spp., abrupt growth decreases were more likely to occur several years following severe drought, whereas for P. abies, A. alba, and F. sylvatica abrupt growth decreases started frequently immediately in the drought year. We conclude that the statistical framework allows for quantifying the effects of drought intensity on the probability of abrupt growth decreases, which ultimately contributes to an improved understanding of climate impacts on forest community dynamics.     

An ecological genetic delineation of local seed‐source provenance for ecological restoration

An increasingly important practical application of the analysis of spatial genetic structure within plant species is to help define the extent of local provenance seed collection zones that minimize negative impacts in ecological restoration programs. Here, we derive seed sourcing guidelines from a novel range‐wide assessment of spatial genetic structure of 24 populations of Banksia menziesii (Proteaceae), a widely distributed Western Australian tree of significance in local ecological restoration programs. An analysis of molecular variance (AMOVA) of 100 amplified fragment length polymorphism (AFLP) markers revealed significant genetic differentiation among populations (Φ_PT = 0.18). Pairwise population genetic dissimilarity was correlated with geographic distance, but not environmental distance derived from 15 climate variables, suggesting overall neutrality of these markers with regard to these climate variables. Nevertheless, Bayesian outlier analysis identified four markers potentially under selection, although these were not correlated with the climate variables. We calculated a global R‐statistic using analysis of similarities (ANOSIM) to test the statistical significance of population differentiation and to infer a threshold seed collection zone distance of ~60 km (all markers) and 100 km (outlier markers) when genetic distance was regressed against geographic distance. Population pairs separated by >60 km were, on average, twice as likely to be significantly genetically differentiated than population pairs separated by <60 km, suggesting that habitat‐matched sites within a 30‐km radius around a restoration site genetically defines a local provenance seed collection zone for B. menziesii. Our approach is a novel probability‐based practical solution for the delineation of a local seed collection zone to minimize negative genetic impacts in ecological restoration.     

The combined effects of a long‐term experimental drought and an extreme drought on the use of plant‐water sources in a Mediterranean forest

Vegetation in water‐limited ecosystems relies strongly on access to deep water reserves to withstand dry periods. Most of these ecosystems have shallow soils over deep groundwater reserves. Understanding the functioning and functional plasticity of species‐specific root systems and the patterns of or differences in the use of water sources under more frequent or intense droughts is therefore necessary to properly predict the responses of seasonally dry ecosystems to future climate. We used stable isotopes to investigate the seasonal patterns of water uptake by a sclerophyll forest on sloped terrain with shallow soils. We assessed the effect of a long‐term experimental drought (12 years) and the added impact of an extreme natural drought that produced widespread tree mortality and crown defoliation. The dominant species, Quercus ilex, Arbutus unedo and Phillyrea latifolia, all have dimorphic root systems enabling them to access different water sources in space and time. The plants extracted water mainly from the soil in the cold and wet seasons but increased their use of groundwater during the summer drought. Interestingly, the plants subjected to the long‐term experimental drought shifted water uptake toward deeper (10–35 cm) soil layers during the wet season and reduced groundwater uptake in summer, indicating plasticity in the functional distribution of fine roots that dampened the effect of our experimental drought over the long term. An extreme drought in 2011, however, further reduced the contribution of deep soil layers and groundwater to transpiration, which resulted in greater crown defoliation in the drought‐affected plants. This study suggests that extreme droughts aggravate moderate but persistent drier conditions (simulated by our manipulation) and may lead to the depletion of water from groundwater reservoirs and weathered bedrock, threatening the preservation of these Mediterranean ecosystems in their current structures and compositions.     

Diversity patterns of eucalypt canopy arthropods in eastern and western Australia

1. As part of a larger study on canopy arthropods and birds, a 1‐year chemical knockdown study was carried out in one Western Australian forest, where jarrah Eucalyptus marginata and marri E. (Corymbia) calophylla were sampled, and one eastern Australian (New South Wales) forest, where narrow‐leaved ironbark E. crebra and grey box E. moluccana were sampled. 2. Ten trees of each species were sampled during each of the four seasons and the arthropods were sorted to morphospecies level. This paper documents the foliage‐associated component of arboreal arthropod communities and compares arthropod species richness within orders and families, between tree species, and between the two forest types. 3. Hymenoptera, Coleoptera, Diptera, and Araneae were the richest in species. Nine hundred and seventy‐six species in 173 families were found in the eastern Australian forest, while 687 species in 176 families were found in the western Australian forest. Only 53% of families were common to both forests, but almost half the families recorded were represented by fewer than five species. Species overlap between tree species in each region was 40–53%. 4. Analysis using nonparametric bootstrapping procedures showed that sampling of foliage was comprehensive and that only 4–9% more species would be expected with more intensive sampling of the canopy. Absolute richness, as well as differences between tree species and regions, therefore appear to be real and not the result of sampling errors. As a consequence, arthropod species richness in Australian eucalypt forests is shown to be substantially greater than previous estimates.     

New evidence of unexpectedly high animal density and diet diversity will benefit the conservation of the Critically Endangered western ringtail possum

A comprehensive and contemporary understanding of habitat and resource requirements has been critical to the conservation of multiple taxa and ecosystems globally. Until recently, much of the ecological knowledge that contributes to conservation priorities and strategies for the Critically Endangered western ringtail possum (Pseudocheirus occidentalis) was largely derived from decades‐old observations in peppermint (Agonis flexuosa) and marri‐jarrah (Corymbia calophylla and Eucalyptus marginanta) woodlands in the northern parts of the species range. These observations do not account for more recent evidence of their flexible use of habitat resources in other regions of its range. This may represent a significant conservation opportunity for the species through the identification of additional habitats that warrant protection. In a region where knowledge of their ecology is scarce, we used scat analysis and quantitative spotlighting to determine the diet and density of western ringtail possums in three vegetation types: peppermint, sheoak (Allocasuarina fraseriana) and marri‐eucalypt (C. calophylla, E. marginanta and Eucalyptus staerii) woodlands. Given the species’ reported dependence on peppermint woodlands and dominant canopy species for food sources, we hypothesised that western ringtail possums would be most abundant in this habitat type and that their diet would comprise the foliage of few (≤2 species) canopy species. We found western ringtail possums consumed a higher diversity of plant species than expected (8–14), exhibited dietary preference for non‐dominant canopy species and were present in all sampled vegetation types at substantially higher densities than previously recorded for the region (as high as 17 possums ha^?1). Our results confirm (i) the western ringtail possum is flexible in its use of habitat resources and (ii) the significant conservation value of sheoak and marri‐eucalypt woodlands in the southernmost portion of its distribution.     

Response of foundation macrophytes to near‐natural simulated marine heatwaves

Marine heatwaves have been observed worldwide and are expected to increase in both frequency and intensity due to climate change. Such events may cause ecosystem reconfigurations arising from species range contraction or redistribution, with ecological, economic and social implications. Macrophytes such as the brown seaweed Fucus vesiculosus and the seagrass Zostera marina are foundation species in many coastal ecosystems of the temperate northern hemisphere. Hence, their response to extreme events can potentially determine the fate of associated ecosystems. Macrophyte functioning is intimately linked to the maintenance of photosynthesis, growth and reproduction, and resistance against pathogens, epibionts and grazers. We investigated morphological, physiological, pathological and chemical defence responses of western Baltic Sea F. vesiculosus and Z. marina populations to simulated near‐natural marine heatwaves. Along with (a) the control, which constituted no heatwave but natural stochastic temperature variability (0HW), two treatments were applied: (b) two late‐spring heatwaves (June, July) followed by a summer heatwave (August; 3HW) and (c) a summer heatwave only (1HW). The 3HW treatment was applied to test whether preconditioning events can modulate the potential sensitivity to the summer heatwave. Despite the variety of responses measured in both species, only Z. marina growth was impaired by the accumulative heat stress imposed by the 3HW treatment. Photosynthetic rate, however, remained high after the last heatwave indicating potential for recovery. Only epibacterial abundance was significantly affected in F. vesiculosus. Hence both macrophytes, and in particular F. vesiculosus, seem to be fairly tolerant to short‐term marine heatwaves at least at the intensities applied in this experiment (up to 5°C above mean temperature over a period of 9 days). This may partly be due to the fact that F. vesiculosus grows in a highly variable environment, and may have a high phenotypic plasticity.     

Dampening effects of long‐term experimental drought on growth and mortality rates of a Holm oak forest

Forests respond to increasing intensities and frequencies of drought by reducing growth and with higher tree mortality rates. Little is known, however, about the long‐term consequences of generally drier conditions and more frequent extreme droughts. A Holm oak forest was exposed to experimental rainfall manipulation for 13 years to study the effect of increasing drought on growth and mortality of the dominant species Quercus ilex, Phillyrea latifolia, and Arbutus unedo. The drought treatment reduced stem growth of A. unedo (?66.5%) and Q. ilex (?17.5%), whereas P. latifolia remained unaffected. Higher stem mortality rates were noticeable in Q. ilex (+42.3%), but not in the other two species. Stem growth was a function of the drought index of early spring in the three species. Stem mortality rates depended on the drought index of winter and spring for Q. ilex and in spring and summer for P. latifolia, but showed no relation to climate in A. unedo. Following a long and intense drought (2005–2006), stem growth of Q. ilex and P. latifolia increased, whereas it decreased in A. unedo. Q. ilex also enhanced its survival after this period. Furthermore, the effect of drought treatment on stem growth in Q. ilex and A. unedo was attenuated as the study progressed. These results highlight the different vulnerabilities of Mediterranean species to more frequent and intense droughts, which may lead to partial species substitution and changes in forest structure and thus in carbon uptake. The response to drought, however, changed over time. Decreased intra‐ and interspecific competition after extreme events with high mortality, together with probable morphological and physiological acclimation to drought during the study period, may, at least in the short term, buffer forests against drier conditions. The long‐term effects of drought consequently deserve more attention, because the ecosystemic responses are unlikely to be stable over time.Nontechnical summaryIn this study, we evaluate the effect of long‐term (13 years) experimental drought on growth and mortality rates of three forest Mediterranean species, and their response to the different intensities and durations of natural drought. We provide evidence for species‐specific responses to drought, what may eventually lead to a partial community shift favoring the more drought‐resistant species. However, we also report a dampening of the treatment effect on the two drought‐sensitive species, which may indicate a potential adaptation to drier conditions at the ecosystem or population level. These results are thus relevant to account for the stabilizing processes that would alter the initial response of ecosystem to drought through changes in plant physiology, morphology, and demography compensation.     

Identification and Pathogenicity of Lasiodiplodia Species from Eucalyptus urophylla × grandis,Polyscias balfouriana and Bougainvillea spectabilis in Southern China

Species of Lasiodiplodia are important pathogens of a wide variety of plants covering a wide geographical distribution. These fungi can be associated with different symptoms such as stem cankers, shoot blights, fruit rots, dieback and gummosis. Diseases caused by Lasiodiplodia were surveyed on Eucalyptus urophylla × grandis, Polyscias balfouriana and Bougainvillea spectabilis in a nursery in southern China. Based on morphology characteristics and phylogenetic analyses of ITS rDNA sequences and translation elongation factor 1‐alpha (TEF‐1α) gene regions, four species of Lasiodiplodia were identified. Lasiodiplodia theobromae was identified from E. urophylla × grandis, P. balfouriana and B. spectabilis. L. hormozganensis, L. iraniensis and L. pseudotheobromae were identified from B. spectabilis. To our knowledge, with the exception of L. theobromae on E. urophylla × grandis, this study represents the first report of these fungi on the host plants. Pathogenicity tests showed that all Lasiodiplodia spp. obtained in this study are virulent to E. urophylla × grandis and B. spectabilis, and L. theobromae was virulent to P. balfouriana.     

Species‐specific differences in water uptake depth of mature temperate trees vary with water availability in the soil

• Temperate tree species differ in their physiological sensitivity to declining soil moisture and drought. Although species‐specific responses to drought have often been suggested to be the result of different water uptake depths, empirical evidence for such a mechanism is scarce.
• Here we test if differences in water uptake depths can explain previously observed species‐specific physiological responses of temperate trees to drought and if the water uptake depth of different species varies in response to declining soil moisture. For this purpose, we employed stable oxygen and hydrogen isotopes of soil and xylem water that we collected over the course of three growing seasons in a mature temperate forest in Switzerland.
• Our data show that all investigated species utilise water from shallow soil layers during times of sufficient soil water supply. However, Fraxinus excelsior, Fagus sylvatica and Acer pseudoplatanus were able to shift their water uptake to deeper soil layers when soil water availability decreased in the topsoil. In contrast, Picea abies, was not able to shift its water uptake to deeper soil layers.
• We conclude from our data that more drought‐resistant tree species are able to shift their water uptake to deeper soil layers when water availability in the topsoil is becoming scarce. In addition, we were able to show that water uptake depth of temperate tree species is a trait with high plasticity that needs to be characterised across a range of environmental conditions.

     

Silver fir and Douglas fir are more tolerant to extreme droughts than Norway spruce in south‐western Germany

Improving our understanding of the potential of forest adaptation is an urgent task in the light of predicted climate change. Long‐term alternatives for susceptible yet economically important tree species such as Norway spruce (Picea abies) are required, if the frequency and intensity of summer droughts will continue to increase. Although Silver fir (Abies alba) and Douglas fir (Pseudotsuga menziesii) have both been described as drought‐tolerant species, our understanding of their growth responses to drought extremes is still limited. Here, we use a dendroecological approach to assess the resistance, resilience, and recovery of these important central Europe to conifer species the exceptional droughts in 1976 and 2003. A total of 270 trees per species were sampled in 18 managed mixed‐species stands along an altitudinal gradient (400–1200 m a.s.l.) at the western slopes of the southern and central Black Forest in southwest Germany. While radial growth in all species responded similarly to the 1976 drought, Norway spruce was least resistant and resilient to the 2003 summer drought. Silver fir showed the overall highest resistance to drought, similarly to Douglas fir, which exhibited the widest growth rings. Silver fir trees from lower elevations were more drought prone than trees at higher elevations. Douglas fir and Norway spruce, however, revealed lower drought resilience at higher altitudes. Although the 1976 and 2003 drought extremes were quite different, Douglas fir maintained consistently the highest radial growth. Although our study did not examine population‐level responses, it clearly indicates that Silver fir and Douglas fir are generally more resistant and resilient to previous drought extremes and are therefore suitable alternatives to Norway spruce; Silver fir more so at higher altitudes. Cultivating these species instead of Norway spruce will contribute to maintaining a high level of productivity across many Central European mountain forests under future climate change.     

Sap‐feeding insects on forest trees along latitudinal gradients in northern Europe: a climate‐driven patterns

Knowledge of the latitudinal patterns in biotic interactions, and especially in herbivory, is crucial for understanding the mechanisms that govern ecosystem functioning and for predicting their responses to climate change. We used sap‐feeding insects as a model group to test the hypotheses that the strength of plant–herbivore interactions in boreal forests decreases with latitude and that this latitudinal pattern is driven primarily by midsummer temperatures. We used a replicated sampling design and quantitatively collected and identified all sap‐feeding insects from four species of forest trees along five latitudinal gradients (750–1300 km in length, ten sites in each gradient) in northern Europe (59 to 70°N and 10 to 60°E) during 2008–2011. Similar decreases in diversity of sap‐feeding insects with latitude were observed in all gradients during all study years. The sap‐feeder load (i.e. insect biomass per unit of foliar biomass) decreased with latitude in typical summers, but increased in an exceptionally hot summer and was independent of latitude during a warm summer. Analysis of combined data from all sites and years revealed dome‐shaped relationships between the loads of sap‐feeders and midsummer temperatures, peaking at 17 °C in Picea abies, at 19.5 °C in Pinus sylvestris and Betula pubescens and at 22 °C in B. pendula. From these relationships, we predict that the losses of forest trees to sap‐feeders will increase by 0–45% of the current level in southern boreal forests and by 65–210% in subarctic forests with a 1 °C increase in summer temperatures. The observed relationships between temperatures and the loads of sap‐feeders differ between the coniferous and deciduous tree species. We conclude that climate warming will not only increase plant losses to sap‐feeding insects, especially in subarctic forests, but can also alter plant‐plant interactions, thereby affecting both the productivity and the structure of future forest ecosystems.     

Factors affecting fall down rates of dead aspen (Populus tremuloides) biomass following severe drought in west‐central Canada

Increases in mortality of trembling aspen (Populus tremuloides Michx.) have been recorded across large areas of western North America following recent periods of exceptionally severe drought. The resultant increase in standing, dead tree biomass represents a significant potential source of carbon emissions to the atmosphere, but the timing of emissions is partially driven by dead‐wood dynamics which include the fall down and breakage of dead aspen stems. The rate at which dead trees fall to the ground also strongly influences the period over which forest dieback episodes can be detected by aerial surveys or satellite remote sensing observations. Over a 12‐year period (2000–2012), we monitored the annual status of 1010 aspen trees that died during and following a severe regional drought within 25 study areas across west‐central Canada. Observations of stem fall down and breakage (snapping) were used to estimate woody biomass transfer from standing to downed dead wood as a function of years since tree death. For the region as a whole, we estimated that >80% of standing dead aspen biomass had fallen after 10 years. Overall, the rate of fall down was minimal during the year following stem death, but thereafter fall rates followed a negative exponential equation with k = 0.20 per year. However, there was high between‐site variation in the rate of fall down (k = 0.08–0.37 per year). The analysis showed that fall down rates were positively correlated with stand age, site windiness, and the incidence of decay fungi (Phellinus tremulae (Bond.) Bond. and Boris.) and wood‐boring insects. These factors are thus likely to influence the rate of carbon emissions from dead trees following periods of climate‐related forest die‐off episodes.     

Carbon consequences of drought differ in forests that resprout

Prolonged drought and intense heat‐related events trigger sudden forest die‐off events and have now been reported from all forested continents. Such die‐offs are concerning given that drought and heatwave events are forecast to increase in severity and duration as climate change progresses. Quantifying consequences to carbon dynamics and storage from die‐off events are critical for determining the current and future mitigation potential of forests. We took stand measurements five times over 2+ years from affected and unaffected plots across the Northern Jarrah Forest, southwestern Australia, following an acute drought/heatwave in 2011. We found a significant loss of live standing carbon (49.3 t ha^?1), and subsequently a significant increase in the dead standing carbon pool by 6 months post‐die‐off. Of the persisting live trees, 38% experienced partial mortality contributing to the rapid regrowth and replenishment (82%–88%) of labile carbon pools (foliage, twigs, and branches) within 26 months. Such regrowth was not substantial in terms of net carbon changes within the timeframe of the study but does reflect the resprouting resilience of this forest type. Dead carbon generated by the die‐off may persist for centuries given low fragmentation and decay rates resulting in low biogenic emission rates relative to other forest types. However, future fire may threaten persistence of both dead and live pools via combustion and mortality of live tissue and impaired regrowth capacity. Resprouting forests are commonly regarded as resilient systems, however, a changing climate could see vulnerable portions of forests become carbon sources rather than carbon sinks.     

Aggregated recruitment patterns under adult crowns in Photinia glabra,a bird‐dispersed tree species

To clarify recruitment patterns of Photinia glabra, which is an evergreen, broad‐leaved, bird‐dispersed tree species, we analyzed spatial distribution in P. glabra recruits at each growth stage and demography of current‐year seedlings with respect to distributions of adults in a warm‐temperate secondary forest, western Japan. Although individuals ≥ 5 cm diameter at breast height (DBH) that had nearly produced fruits showed a random distribution, seedlings (≥ 1 year old, < 10‐cm stem length [SL]), small saplings (10 ≤ SL < 30 cm) and large saplings (≥ 30‐cm SL, < 5‐cm DBH) were clumped and associated with reproductive adults at approximately 2–3‐m scales, nearly equal to their average crown radius. Based on monitoring the demography of current‐year seedlings, emerged seedling density profoundly decreased, and no seedlings survived at longer than an adult's crown scales, with distance‐dependent mortality as a result of disease and herbivory not greatly affecting the current‐year seedling mortality. Thus, aggregated seed dispersal under the crown of adult P. glabra would directly influence the distribution of recruits for P. glabra in this forest. Of the bird‐dispersed tree species in this forest, P. glabra produced the highest amount of fruits during large crop years, and their fruits ripened during the late seasonal period (early January), suggesting that birds might be strongly attracted to these species, in turn leading to seeds being deposited mostly under the tree crowns. We propose that dispersal limitation would occur, even in a bird‐dispersed tree species such as P. glabra, owing to plant–bird interactions in the forest.     

Responses of plant biomass,photosynthesis and lipid peroxidation to warming and precipitation change in two dominant species (Stipa grandis and Leymus chinensis) from North China Grasslands

Influential factors of global change affect plant carbon uptake and biomass simultaneously. Although the effects from warming and precipitation change have been extensive studied separately, the responses of plant biomass, photosynthesis, and lipid peroxidation to the interaction of these factors are still not fully understood. In this study, we examined the physiological responses of two dominant plant species from grasslands of northern China with different functional traits to combinations of five simulated warming patterns and five simulated precipitation patterns in environment‐controlled chambers. Our results showed that the biomass, net CO₂ assimilation rate (P_n), maximal efficiency of photosystem II photochemistry (F_v/F_m), and chlorophyll content (Chl) of Stipa grandis and Leymus chinensis were enhanced by moderate warming and plus precipitation, but they declined drastically with high temperature and drought. High temperature and drought also led to significant malondialdehyde (MDA) accumulation, which had a negative correlation with leaf biomass. The lower level of lipid peroxidation in leaves of S. grandis suggests that this species is better protected from oxidative damage under heat stress, drought stress and their interactive conditions than L. chinensis. Using the subordinate function values method, we found S. grandis to be more sensitive to climate change than L. chinensis and the gross biomass and root biomass of S. grandis and the leaf biomass of L. chinensis were most sensitive to climate change. Furthermore, the P_n of both S. grandis and L. chinensis had a significant linear relationship with F_v/F_m and Chl, indicating that carbon assimilation may be caused by nonstomatal limitations.     

Assessing forest vulnerability to climate warming using a process‐based model of tree growth: bad prospects for rear‐edges

Growth models can be used to assess forest vulnerability to climate warming. If global warming amplifies water deficit in drought‐prone areas, tree populations located at the driest and southernmost distribution limits (rear‐edges) should be particularly threatened. Here, we address these statements by analyzing and projecting growth responses to climate of three major tree species (silver fir, Abies alba; Scots pine, Pinus sylvestris; and mountain pine, Pinus uncinata) in mountainous areas of NE Spain. This region is subjected to Mediterranean continental conditions, it encompasses wide climatic, topographic and environmental gradients, and, more importantly, it includes rear‐edges of the continuous distributions of these tree species. We used tree‐ring width data from a network of 110 forests in combination with the process‐based Vaganov–Shashkin‐Lite growth model and climate–growth analyses to forecast changes in tree growth during the 21st century. Climatic projections were based on four ensembles CO₂ emission scenarios. Warm and dry conditions during the growing season constrain silver fir and Scots pine growth, particularly at the species rear‐edge. By contrast, growth of high‐elevation mountain pine forests is enhanced by climate warming. The emission scenario (RCP 8.5) corresponding to the most pronounced warming (+1.4 to 4.8 °C) forecasted mean growth reductions of ?10.7% and ?16.4% in silver fir and Scots pine, respectively, after 2050. This indicates that rising temperatures could amplify drought stress and thus constrain the growth of silver fir and Scots pine rear‐edge populations growing at xeric sites. Contrastingly, mountain pine growth is expected to increase by +12.5% due to a longer and warmer growing season. The projections of growth reduction in silver fir and Scots pine portend dieback and a contraction of their species distribution areas through potential local extinctions of the most vulnerable driest rear‐edge stands. Our modeling approach provides accessible tools to evaluate forest vulnerability to warmer conditions.