The functional diversity-productivity relationship of woody plants is climatically sensitive

Plot-scale experiments indicate that functional diversity (FD) plays a pivotal role in sustaining ecosystem functions such as net primary productivity (NPP). However, the relationships between functional diversity and NPP across larger scale under varying climatic conditions are sparsely studied, despite its significance for understanding forest-atmosphere interactions and informing policy development. Hence, we examine the relationships of community-weighted mean (CWM) and functional dispersion (FDis) of woody plant traits on NPP across China and if such relationships are modulated by climatic conditions at the national scale. Using comprehensive datasets of distribution, functional traits, and productivity for 9120 Chinese woody plant species, we evaluated the distribution pattern of community-weighted mean and functional dispersion (including three orthogonal trait indicators: plant size, leaf morphology, and flower duration) and its relationships with NPP. Finally, we tested the effects of climatic conditions on community-weighted mean/functional dispersion-NPP relationships. We first found overall functional diversity-NPP relationships, but also that the magnitude of these relationships was sensitive to climate, with plant size community-weighted mean promoting NPP in warm regions and plant size functional dispersion promoting NPP in wet regions. Second, warm and wet conditions indirectly increased NPP by its positive effects on community-weighted mean or functional dispersion, particularly through mean plant size and leaf morphology. Our study provides comprehensive evidence for the relationships between functional diversity and NPP under varying climates at a large scale. Importantly, our results indicate a broadening significance of multidimensional plant functional traits for woody vegetation NPP in response to rising temperatures and wetter climates. Restoration, reforestation actions and natural capital accounting need to carefully consider not only community-weighted mean and functional dispersion but also their interactions with climate, to predict how functional diversity may promote ecosystem functioning under future climatic conditions.

Functional diversity enhances dryland forest productivity under long-term climate change

Short-term experimental studies provided evidence that plant diversity increases ecosystem resilience and resistance to drought events, suggesting diversity to serve as a nature-based solution to address climate change. However, it remains unclear whether the effects of diversity are momentary or still hold over the long term in natural forests to ensure that the sustainability of carbon sinks. By analyzing 57 years of inventory data from dryland forests in Canada, we show that productivity of dryland forests decreased at an average rate of 1.3% per decade, in concert with the temporally increasing temperature and decreasing water availability. Increasing functional trait diversity from its minimum (monocultures) to maximum value increased productivity by 13%. Our results demonstrate the potential role of tree functional trait diversity in alleviating climate change impacts on dryland forests. While recognizing that nature-based climate mitigation (e.g., planting trees) can only be partial solutions, their long-term (decadal) efficacy can be improved by enhancing functional trait diversity across the forest community.

Decline in stability of forest productivity in the tropics as determined by canopy water content

The impacts of low soil moisture (SM) and high vapour pressure deficit (VPD) on tree's photosynthesis and productivity are ultimately realized by changing water content in the canopy leaves. In this study, variations in canopy water content (CWC) that can be detected from microwave remotely sensed vegetation optical depth (VOD) have been proposed as a promising measure of vegetation water status, and we first reported that the regulation of CWC on productivity stability is universally applicable for global forests. Results of structural equation model (SEM) also confirmed the significant negative effect of CWC on coefficient of variation (CV) of productivity, indicating that the decrease in CWC could inevitably induce the instability of forest productivity under climate change. The most significant decrease (p < 0.01) of CWC is observed primarily in evergreen broadleaf forest in the tropics, implying an increasing instability of the most important carbon sink in terrestrial ecosystem.

Drought survival and recovery in grasses: Stress intensity and plant-plant interactions impact plant dehydration tolerance

Plant dehydration tolerance confers drought survival in grasses, but the mortality thresholds according to soil water content (SWC), vapour pressure deficit (VPD) and plant-plant interactions are little explored. We compared the dehydration dynamics of leaf meristems, which are the key surviving organs, plant mortality, and recovery of Mediterranean and temperate populations of two perennial grass species, Dactylis glomerata and Festuca arundinacea, grown in monocultures and mixtures under a low-VPD (1.5 kPa) versus a high-VPD drought (2.2 kPa). The lethal drought index (LD₅₀ ), that is, SWC associated with 50% plant mortality, ranged from 2.87% (ψs = -1.68 MPa) to 2.19% (ψs = -4.47 MPa) and reached the lowest values under the low-VPD drought. Populations of D. glomerata were more dehydration-tolerant (lower LD₅₀ ), survived and recovered better than F. arundinacea populations. Plant-plant interactions modified dehydration tolerance and improved post-drought recovery in mixtures compared with monocultures. Water content as low as 20.7%-36.1% in leaf meristems allowed 50% of plants to survive. We conclude that meristem dehydration causes plant mortality and that drought acclimation can increase dehydration tolerance. Genetic diversity, acclimation and plant-plant interactions are essential sources of dehydration tolerance variability to consider when predicting drought-induced mortality.

Analysis of coniferous tree growth gradients in relation to regional pollution and climate change in the Miyun Reservoir Basin, China

Forests play a crucial role in regulating regional climate and mitigating local air pollution, but little is known about their responding to such changes. This study aimed to examine the potential responses of Pinus tabuliformis, the major coniferous tree species in the Miyun Reservoir Basin (MRB), along an air pollution gradient in Beijing. Tree rings were collected along a transect, and ring width (basal area increment, BAI) and chemical characteristics were determined and related to long-term climatic and environmental records. The results showed that Pinus tabuliformis showed an overall increase in intrinsic water-use efficiency (iWUE) at all sites, but the relationships between iWUE and BAI differed among the sites. The contribution of atmospheric CO₂ concentration (c_a) to tree growth was significant at the remote sites (> 90%). The study found that air pollution at these sites might have caused further stomatal closure, as evidenced by the higher δ¹³C levels (0.5 to 1‰ higher) during heavy pollution periods. The analysis of tree ring δ¹⁵N also revealed the potential of using δ¹⁵N to fingerprint major nitrogen (N) deposition, as shown in the increasing tree ring δ¹⁵N, and major nitrogen losses due to denitrification and leaching, as shown in the higher δ¹⁵N in tree rings during heavy rainfall events. Overall, the gradient analysis indicated the contributions of increasing c_a, increasing water deficit and elevated air pollution to tree growth and forest development. The different BAI trajectories suggested that Pinus tabuliformis has the ability to adapt to the harsh environment in the MRB.

Meta-analysis shows that plant mixtures increase soil phosphorus availability and plant productivity in diverse ecosystems

Soil phosphorus (P) availability is critical to plant productivity in many terrestrial ecosystems. How soil P availability responds to changes in plant diversity remains uncertain, despite the global crisis of rapid biodiversity loss. Our meta-analysis based on 180 studies across various ecosystems (croplands, grasslands, forests and pot experiments) shows that, on average, soil total P, phosphatase activity and available P are 6.8%, 8.5% and 4.6%, respectively, higher in species mixtures than in monocultures. The mixture effect on phosphatase activity becomes more positive with increasing species and functional group richness, with more pronounced increases in the rhizosphere than in the bulk soil. The mixture effects on soil-available P in the bulk soil do not change, but with increasing species or functional group richness these effects in the rhizosphere soil shift from positive to negative. Nonetheless, enhanced soil phosphatase activity stimulated available P in diverse species mixtures, offsetting increased plant uptake effects that decrease soil-available P. Moreover, the enhancement effects of species richness on soil phosphatase activity are positively associated with increased plant productivity. Our findings highlight that preserving plant diversity could increase soil phosphatase activity and P availability, which sustain the current and future productivity of terrestrial ecosystems.

Higher tree diversity is linked to higher tree mortality

Examining the relationship between tree diversity and ecosystem functioning has been a recent focus of forest ecology. Particular emphasis has been given to the impact of tree diversity on productivity and to its potential to mitigate negative global change effects; however, little attention has been paid to tree mortality. This is critical because both tree mortality and productivity underpin forest ecosystem dynamics and therefore forest carbon sequestration. Neglecting tree mortality leaves a large part of the picture undocumented. Here we show that increasingly diverse forest stands have increasingly high mortality probabilities. We found that the most species-rich stands in temperate biomes had mortality probabilities more than sevenfold higher than monospecific stands (∼0.6% year−1 in monospecific stands to 4.0% year−1 in the most species-rich stands) while in boreal stands increases were less pronounced but still significant (∼1.1% year−1 in monospecific stands to 1.8% year−1 in the most species-rich stands). Tree species richness was the third-most-important predictor of mortality in our models in temperate forests and the fifth-most-important predictor in boreal forests. Our results highlight that while the promotion of tree diversity undoubtedly has many positive effects on ecosystem functioning and the services that trees provide to humanity, it remains important to consider all aspects of forest dynamics in order to properly predict the implications of maintaining and promoting tree diversity.

Development of a Climate-Sensitive Matrix Growth Model for Larix gmelinii Mixed-Species Natural Forests and Its Application for Predicting Forest Dynamics under Different Climate Scenarios

Larix gmelinii natural forests, which are of great ecological and economic importance, are mainly distributed in northeast China. Sustainable management of these forests play a vital role in ecological security in northeast China, especially in the context of climate change. Forest growth models, which support forest management decision-making, are lacking for Larix gmelinii natural forests, hampering the prescription of forest management strategies. In this study, we produced a climate-sensitive, transition-matrix model (CM) for Larix gmelinii natural forests. For comparison, a variable transition model without including climate change effects (NCM) and a fixed-parameter model (FM) were also built. We examined the performance of the CM, NCM, and FM by conducting short- (5 years) and long-term (100 years) simulations. The results showed that for short-term prediction, no significant difference was observed among the three predictive models. However, the long-term prediction ability of the CM under the three different RCPs was superior to that of the FM and NCM. The number of trees and basal area were predicted to increase under climate change, which might result in natural disasters, such as snow break, windthrow, and forest fire. Silvicultural practices, such as reducing the intermediate thinning interval and the enrichment planting of slow-growing trees, should be implemented to mitigate the deleterious effects of climate change.

Rapid functional shifts across high latitude forests over the last 65 years

Global environmental changes have strongly affected forest demographic rates, particularly amplified tree mortality in high latitude forests (e.g., two to five times greater mortality probability over the half-century). Although forest functional composition is critical for multitrophic biodiversity and ecosystem functioning, it remains unclear how functional composition has changed over time across large high latitude regions, which have been warming twice the rate of the globe as a whole. Using extensive spatial and long-term forest inventory data (17,107 plots monitored 1951-2016) across Canada, we found that after accounting for stand age-dependent functional shifts, functional composition shifted toward fast-growing deciduous broadleaved trees and higher drought tolerance over time. The temporal shift toward deciduous broadleaved trees was consistent across the baseline climate. However, over the study period, drought tolerance increased (or shade tolerance decreased) by 300% in colder boreal regions, while drought tolerance did not shift significantly in warmer temperate climates. A further analysis accounting for temporal changes in atmospheric CO₂ , temperature, and water availability indicated that the functional composition of colder regions shifted toward drought tolerance more rapidly with rising CO₂ than warmer regions, suggesting the greater vulnerability of boreal forests than temperate forests under ongoing global environmental changes. Future ecosystem management practices should consider spatial differences in functional responses to global environmental change, focusing on high latitude forests experiencing higher rates of warming and compositional changes.

Role of Mixed-Species Stands in Attenuating the Vulnerability of Boreal Forests to Climate Change and Insect Epidemics

We investigated whether stand species mixture can attenuate the vulnerability of eastern Canada's boreal forests to climate change and insect epidemics. For this, we focused on two dominant boreal species, black spruce [Picea mariana (Mill.) BSP] and trembling aspen (Populus tremuloides Michx.), in stands dominated by black spruce or trembling aspen ("pure stands"), and mixed stands (M) composed of both species within a 36 km² study area in the Nord-du-Québec region. For each species in each stand composition type, we tested climate-growth relations and assessed the impacts on growth by recorded insect epidemics of a black spruce defoliator, the spruce budworm (SBW) [Choristoneura fumiferana (Clem.)], and a trembling aspen defoliator, the forest tent caterpillar (FTC; Malacosoma disstria Hübn.). We implemented linear models in a Bayesian framework to explain baseline and long-term trends in tree growth for each species according to stand composition type and to differentiate the influences of climate and insect epidemics on tree growth. Overall, we found climate vulnerability was lower for black spruce in mixed stands than in pure stands, while trembling aspen was less sensitive to climate than spruce, and aspen did not present differences in responses based on stand mixture. We did not find any reduction of vulnerability for mixed stands to insect epidemics in the host species, but the non-host species in mixed stands could respond positively to epidemics affecting the host species, thus contributing to stabilize ecosystem-scale growth over time. Our findings partially support boreal forest management strategies including stand species mixture to foster forests that are resilient to climate change and insect epidemics.

A fingerprint of climate change across pine forests of Sweden

Climate change has likely altered high-latitude forests globally, but direct evidence remains rare. Here we show that throughout a ≈1000-km transect in Scots pine (Pinus sylvestris L.) forests in Sweden, mature trees in ≈2015 had longer needles with shorter lifetimes than did trees in ≈1915. These century-scale shifts in needle traits were detected by sampling needles at 74 sites from 2012 to 2017 along the same transect where needle traits had been assessed at 57 sites in 1914-1915. Climate warming of ≈1 °C all along the transect in the past century has driven this temporal shift in foliage traits known to be physiologically critical to growth and carbon cycling processes. These century-scale changes in Scandinavian Scots pine forests represent a fingerprint of climate change on a fundamental biological element, the leaf, with repercussions for productivity and sensitivity to future climate, which are likely to be mirrored by similar changes for evergreen conifers across the boreal biome.

Interactive effects of elevated CO2 , warming, reduced rainfall, and nitrogen on leaf gas exchange in five perennial grassland species

Global changes can interact to affect photosynthesis and thus ecosystem carbon capture, yet few multi-factor field studies exist to examine such interactions. Here, we evaluate leaf gas exchange responses of five perennial grassland species from four functional groups to individual and interactive global changes in an open-air experiment in Minnesota, USA, including elevated CO₂ (eCO₂ ), warming, reduced rainfall and increased soil nitrogen supply. All four factors influenced leaf net photosynthesis and/or stomatal conductance, but almost all effects were context-dependent, i.e. they differed among species, varied with levels of other treatments and/or depended on environmental conditions. Firstly, the response of photosynthesis to eCO₂ depended on species and nitrogen, became more positive as vapour pressure deficit increased and, for a C₄ grass and a legume, was more positive under reduced rainfall. Secondly, reduced rainfall increased photosynthesis in three functionally distinct species, potentially via acclimation to low soil moisture. Thirdly, warming had positive, neutral or negative effects on photosynthesis depending on species and rainfall. Overall, our results show that interactions among global changes and environmental conditions may complicate predictions based on simple theoretical expectations of main effects, and that the factors and interactions influencing photosynthesis vary among herbaceous species.

A high biodiversity mitigates the impact of ocean acidification on hard-bottom ecosystems

Biodiversity loss and climate change simultaneously threaten marine ecosystems, yet their interactions remain largely unknown. Ocean acidification severely affects a wide variety of marine organisms and recent studies have predicted major impacts at the pH conditions expected for 2100. However, despite the renowned interdependence between biodiversity and ecosystem functioning, the hypothesis that the species’ response to ocean acidification could differ based on the biodiversity of the natural multispecies assemblages in which they live remains untested. Here, using experimentally controlled conditions, we investigated the impact of acidification on key habitat-forming organisms (including corals, sponges and macroalgae) and associated microbes in hard-bottom assemblages characterised by different biodiversity levels. Our results indicate that, at higher biodiversity, the impact of acidification on otherwise highly vulnerable key organisms can be reduced by 50 to >90%, depending on the species. Here we show that such a positive effect of a higher biodiversity can be associated with higher availability of food resources and healthy microbe-host associations, overall increasing host resistance to acidification, while contrasting harmful outbreaks of opportunistic microbes. Given the climate change scenarios predicted for the future, we conclude that biodiversity conservation of hard-bottom ecosystems is fundamental also for mitigating the impacts of ocean acidification.

Rootstock determines the drought resistance of poplar grafting combinations

To increase yield and/or enhance resistance to diseases, grafting is often applied in agriculture and horticulture. Interspecific grafting could possibly be used in forestry as well to improve drought resistance, but our understanding of how the rootstock of a more drought-resistant species can affect the grafted plant is very limited. Reciprocal grafts of two poplar species, Populus cathayana Rehder (less drought-resistant, C) and Populus deltoides Bart. ex Marsh (more drought-resistant, D) were generated. Four grafting combinations (scion/rootstock: C/C, C/D, D/D and D/C) were subjected to well-watered and drought stress treatments. C/D and D/C had a higher diameter growth rate, leaf biomass, intrinsic water-use efficiency (WUEi) and total non-structural carbohydrate (NSC) content than C/C and D/D in well-watered condition. However, drought caused greater differences between P. deltoides-rooted and P. cathayana-rooted grafting combinations, especially between C/D and D/C. The C/D grafting combination showed higher resistance to drought, as indicated by a higher stem growth rate, net photosynthetic rate, WUEi, leaf water potential, proline concentration and NSC concentration and maintenance of integrity of the leaf cellular ultrastructure under drought when compared with D/C. D/C exhibited severely damaged cell membranes, mitochondria and chloroplasts under drought. The scion genotype caused a strong effect on the root proline concentration: the P. cathayana scion increased the root proline concentration more than the P. deltoides scion (C/C vs D/C and C/D vs D/D) under water deficit. Our results demonstrated that mainly the rootstock was responsible for the drought resistance of grafting combinations. Grafting of the P. cathayana scion onto P. deltoides rootstock resulted in superior growth and biomass when compared with the other three combinations both in well-watered and drought stress conditions.

Complementarity effects are strengthened by competition intensity and global environmental change in the central boreal forests of Canada

Increases in niche complementarity have been hypothesised to reduce the intensity of interspecific competition within natural forests. In regions currently experiencing potentially enhanced growth under global environmental change, niche complementarity may become even more beneficial. However, few studies have provided direct evidence of this mechanism. Here, we use data from 180 permanent sample plots in Manitoba, Canada, with a full spatial mapping of all stems, to show that complementarity effects on average increased with neighbourhood competition intensity and temporally rising CO₂ , warming and water availability. Importantly, complementarity effects increased with both shade tolerance and phylogenetic dissimilarity between the focal tree and its neighbours. Our results provide further evidence that increasing stand functional and phylogenetic diversity can improve individual tree productivity, especially for individuals experiencing intense competition and may offer an avenue to maintain productivity under global environmental change.