Global importance of large-diameter trees Lutz, James A.; Furniss, Tucker J.; Johnson, Daniel J. ...
Global ecology and biogeography,
July 2018, Volume:
27, Issue:
7/8
Journal Article
Peer reviewed
Open access
Aim: To examine the contribution of large-diameter trees to biomass, stand structure, and species richness across forest biomes. Location: Global. Time period: Early 21st century. Major taxa studied: ...Woody plants. Methods: We examined the contribution of large trees to forest density, richness and biomass using a global network of 48 large (from 2 to 60 ha) forest plots representing 5,601,473 stems across 9,298 species and 210 plant families. This contribution was assessed using three metrics: the largest 1% of trees ≥ 1 cm diameter at breast height (DBH), all trees ≥ 60 cm DBH, and those rank-ordered largest trees that cumulatively comprise 50% of forest biomass. Results: Averaged across these 48 forest plots, the largest 1% of trees ≥ 1 cm DBH comprised 50% of aboveground live biomass, with hectare-scale standard deviation of 26%. Trees ≥ 60 cm DBH comprised 41% of aboveground live tree biomass. The size of the largest trees correlated with total forest biomass (r2 = .62, p < .001). Large-diameter trees in high biomass forests represented far fewer species relative to overall forest richness (r2 = .45, p < .001). Forests with more diverse large-diameter tree communities were comprised of smaller trees (r2 = .33, p < .001). Lower large-diameter richness was associated with large-diameter trees being individuals of more common species (r2 = .17, p = .002). The concentration of biomass in the largest 1% of trees declined with increasing absolute latitude (r2 = .46, p < .001), as did forest density (r2 = .31, p < .001). Forest structural complexity increased with increasing absolute latitude (r2 = .26, p < .001). Main conclusions: Because large-diameter trees constitute roughly half of the mature forest biomass worldwide, their dynamics and sensitivities to environmental change represent potentially large controls on global forest carbon cycling. We recommend managing forests for conservation of existing large-diameter trees or those that can soon reach large diameters as a simple way to conserve and potentially enhance ecosystem services.
1. Drought-induced tree mortality is expected to increase globally due to climate change, with profound implications for forest composition, function and global climate feedbacks. How drought is ...experienced by different species is thought to depend fundamentally on where they access water vertically below-ground, but this remains untracked so far due to the difficulty of measuring water availability at depths at which plants access water (few to several tens of metres), the broad temporal scales at which droughts at those depths unfold (seasonal to decadal), and the difficulty in linking these patterns to forest-wide species-specific demographic responses. 2. We address this problem through a new eco-hydrological framework: we used a hydrological model to estimate below-ground water availability by depth over a period of two decades that included a multi-year drought. Given this water availability scenario and 20 year long-records of species-specific growth patterns, we inversely estimated the relative depths at which 12 common species in the forest accessed water via a model of water stress. Finally, we tested whether our estimates of species relative uptake depths predicted mortality in the multi-year drought. 3. The hydrological model revealed clear below-ground niches as precipitation was decoupled from water availability by depth at multi-annual scale. Species partitioned the hydrological niche by diverging in their uptake depths and so in the same forest stand, different species experienced very different drought patterns, resulting in clear differences in species-specific growth. Finally, species relative water uptake depths predicted species mortality patterns after the multi-year drought Species that our method ranked as relying on deeper water were the ones that had suffered from greater mortality, as the zone from which they access water took longer to recharge after depletion. 4. Synthesis. This research changes our understanding of how hydrological niches operate for trees, with a trade-off between realized growth potential and survival under drought with decadal scale return time. The eco-hydrological framework highlights the importance of species-specific below-ground strategies in predicting forest response to drought. Applying this framework more broadly may help us better understand species coexistence in diverse forest communities and improve mechanistic predictions of forests productivity and compositional change under future climate.
In tropical forests, understory herbaceous angiosperms (herbs), which can comprise up to 40% of plant species richness, have received relatively little attention compared with trees, and their ...diversity patterns and drivers remain poorly understood. While tropical tree diversity has been shown to be driven primarily by water availability, we hypothesized that herb diversity may be equally or more limited by light availability.
To test the importance of water and light in shaping herb diversity, we surveyed herb communities in 13 one‐ha plots along a rainfall gradient in a seasonally dry tropical forest landscape in India. In each plot, herbs were censused thrice during the year in 47–50 one‐m2 subplots. We examined drivers of herb diversity and ground cover at the landscape scale (across plots) and local scale (among subplots) using simple linear regression and linear mixed models, respectively.
At the landscape scale, herb diversity and ground cover were negatively related to rainfall and soil moisture, in contrast with previous studies of tropical forest trees. This indicates that water was not limiting for herbs. Instead, light availability, which was negatively correlated with rainfall, appears to drive patterns of herb diversity and ground cover across the gradient. Herb diversity and ground cover were uniformly low across the gradient during the dry season, indicating that water limitation does have a seasonal impact. When water was abundant (rainy season), herbs took advantage of higher light availability at more open (lower‐rainfall) plots.
Consistent with landscape‐scale patterns, herb richness at the subplot level was positively related to light availability, with its effect stronger at higher‐rainfall (more closed‐canopy) sites. Herb species richness declined with increasing subplot soil moisture in all sites and seasons, suggesting negative impacts of water logging and high moisture.
Synthesis. Diversity of understory herbs is limited by light availability at both local and landscape scales in this region, with the impact of water limitation restricted to the dry season. Our study shows that tree diversity patterns cannot be assumed to hold for other plant lifeforms, and conservation and restoration efforts must consider unique strategies for different lifeforms.
Diversity of understory herbs is limited by light availability at both local and landscape scales in this region, with the impact of water limitation restricted to the dry season. Our study shows that tree diversity patterns cannot be assumed to hold for other plant lifeforms, and conservation and restoration efforts must consider unique strategies for different lifeforms.
The extent to which interspecific niche differences structure plant communities is highly debated, with extreme viewpoints ranging from fine-scaled niche partitioning, where every species in the ...community is specialized to a distinct niche, to neutrality, where species have no niche or fitness differences. However, there exists a default position wherein niches of species in a community are determined by their evolutionary and biogeographic histories, irrespective of other species within the community. According to this viewpoint, a broad range of pair-wise niche overlaps—from completely overlapping to completely distinct—are expected in any community without the need to invoke interspecific interactions. We develop a method that can test for both habitat associations and niche differences along an arbitrary number of spatial and temporal niche dimensions and apply it to a 24-yr data set of the eight dominant woody-plant species (representing 84% and 76% of total community abundance and basal area, respectively) from a 50-ha permanent plot in a southern Indian tropical dry forest, using edaphic, topographic, and precipitation variables as niche axes. Species separated into two broad groups in niche space—one consisting of three canopy species and the other of a canopy species and four understory species—along axes that corresponded mainly to variation in soil P, Al and a topographic index of wetness. Species within groups tended to have significantly greater niche overlap than expected by chance. Community-wide niche overlap in spatial and temporal niche axes was never smaller than expected by chance. Species-habitat associations were neither necessary nor sufficient preconditions for niche differences to be present. Our results suggest that this tropical dry-forest community consists of several tree species with broadly overlapping niches, and where significant niche differences do exist, they are not readily interpretable as evidence for niche differentiation. We argue, based on a survey of the literature, that many of the observed niche differences in tropical forests are more parsimoniously viewed as autecological differences between species that exist independently of interspecific interactions.
Questions: Water availability is known to be a first-order driver of plant diversity; yet water also affects fire regimes and soil fertility, which, in turn, affect plant diversity. We examined how ...precipitation, fire and soil properties jointly determine woody plant diversity. Specifically, we asked how woody plant diversity varies along a sharp precipitation gradient (about 600–1,800 mm mean annual precipitation MAPwithin a ∼45-km distance) exhibiting considerable variation in long-term fire burn frequency and soil fertility, in a southern Indian seasonally dry tropical forest (SDTF) landscape. Location: Mudumalai, Western Ghats, India. Methods: Woody plants ≥1-cm DBH were enumerated in 19 1-ha permanent plots spanning a range of tropical vegetation types from dry thorn forest, through dry and moist deciduous forest to semi-evergreen forest. Burn frequencies were derived from annual fire maps. Six measures of surface soil properties – total exchangeable bases (Ca + Mg + K), organic carbon (OC), total N, pH, plant available P and micronutrients (Fe + Cu + Zn + Mn) were used in the analyses. Five measures of diversity – species richness, Shannon diversity, the rarefied/extrapolated versions of these two measures, and Fisher's α – were modelled as functions of MAP, annual fire burn frequency and the principal components of soil properties. Results: Most soil nutrients and OC increased with MAP, except in the wettest sites. Woody productivity increased with MAP, while fire frequency was highest at intermediate values of MAP. Woody plant diversity increased with MAP but decreased with increasing fire frequency, resulting in two local diversity maxima along the MAP gradient – in the semi-evergreen and dry thorn forest – separated by a low-diversity central region in dry deciduous forest where fire frequency was highest. Soil variables were, on the whole, less strongly correlated with diversity than MAP. Conclusions: Although woody plant diversity in this landscape, representative of regional SDTFs, is primarily limited by water availability, our study emphasizes the role of fire as a potentially important second-order driver that acts to reduce diversity in this landscape.
Global change is impacting forests worldwide, threatening biodiversity and ecosystem services including climate regulation. Understanding how forests respond is critical to forest conservation and ...climate protection. This review describes an international network of 59 long‐term forest dynamics research sites (CTFS‐ForestGEO) useful for characterizing forest responses to global change. Within very large plots (median size 25 ha), all stems ≥1 cm diameter are identified to species, mapped, and regularly recensused according to standardized protocols. CTFS‐ForestGEO spans 25°S–61°N latitude, is generally representative of the range of bioclimatic, edaphic, and topographic conditions experienced by forests worldwide, and is the only forest monitoring network that applies a standardized protocol to each of the world's major forest biomes. Supplementary standardized measurements at subsets of the sites provide additional information on plants, animals, and ecosystem and environmental variables. CTFS‐ForestGEO sites are experiencing multifaceted anthropogenic global change pressures including warming (average 0.61 °C), changes in precipitation (up to ±30% change), atmospheric deposition of nitrogen and sulfur compounds (up to 3.8 g N m⁻² yr⁻¹and 3.1 g S m⁻² yr⁻¹), and forest fragmentation in the surrounding landscape (up to 88% reduced tree cover within 5 km). The broad suite of measurements made at CTFS‐ForestGEO sites makes it possible to investigate the complex ways in which global change is impacting forest dynamics. Ongoing research across the CTFS‐ForestGEO network is yielding insights into how and why the forests are changing, and continued monitoring will provide vital contributions to understanding worldwide forest diversity and dynamics in an era of global change.
In Amazonian tropical forests, recent studies have reported increases in aboveground biomass and in primary productivity, as well as shifts in plant species composition favouring fast-growing species ...over slow-growing ones. This pervasive alteration of mature tropical forests was attributed to global environmental change, such as an increase in atmospheric CO2 concentration, nutrient deposition, temperature, drought frequency, and/or irradiance. We used standardized, repeated measurements of over 2 million trees in ten large (16-52 ha each) forest plots on three continents to evaluate the generality of these findings across tropical forests. Aboveground biomass increased at seven of our ten plots, significantly so at four plots, and showed a large decrease at a single plot. Carbon accumulation pooled across sites was significant (+0.24 MgC ha(-1) y(-1), 95% confidence intervals 0.07, 0.39 MgC ha(-1) y(-1)), but lower than reported previously for Amazonia. At three sites for which we had data for multiple census intervals, we found no concerted increase in biomass gain, in conflict with the increased productivity hypothesis. Over all ten plots, the fastest-growing quartile of species gained biomass (+0.33 0.09, 0.55 % y(-1)) compared with the tree community as a whole (+0.15 % y(-1)); however, this significant trend was due to a single plot. Biomass of slow-growing species increased significantly when calculated over all plots (+0.21 0.02, 0.37 % y(-1)), and in half of our plots when calculated individually. Our results do not support the hypothesis that fast-growing species are consistently increasing in dominance in tropical tree communities. Instead, they suggest that our plots may be simultaneously recovering from past disturbances and affected by changes in resource availability. More long-term studies are necessary to clarify the contribution of global change to the functioning of tropical forests.
The growth and survival of individual trees determine the physical structure of a forest with important consequences for forest function. However, given the diversity of tree species and forest ...biomes, quantifying the multitude of demographic strategies within and across forests and the way that they translate into forest structure and function remains a significant challenge. Here, we quantify the demographic rates of 1961 tree species from temperate and tropical forests and evaluate how demographic diversity (DD) and demographic composition (DC) differ across forests, and how these differences in demography relate to species richness, aboveground biomass (AGB), and carbon residence time. We find wide variation in DD and DC across forest plots, patterns that are not explained by species richness or climate variables alone. There is no evidence that DD has an effect on either AGB or carbon residence time. Rather, the DC of forests, specifically the relative abundance of large statured species, predicted both biomass and carbon residence time. Our results demonstrate the distinct DCs of globally distributed forests, reflecting biogeography, recent history, and current plot conditions. Linking the DC of forests to resilience or vulnerability to climate change, will improve the precision and accuracy of predictions of future forest composition, structure, and function.
(a) We fit growth and survival models to 1,961 species across 20 tropical and temperate forest plots. (b) Species were clustered into Growth‐Survival‐Stature Modes (GSSMs) based on demographic rates. At each plot we calculated demographic diversity (DD) as the area occupied by species in demographic space, and demographic composition (DC) as the relative abundance of each GSSM. (c) DD peaks at intermediate levels of species richness. (d) Aboveground biomass (AGB) and carbon residence time are not related to DD across plots, but are related to DC, specifically the relative abundance of high‐survival, large‐statured GSSMs 5 and 6.
Pseudogymnoascus destructans is the causative agent of white-nose syndrome, a disease that has caused the deaths of millions of bats in North America. This psychrophilic fungus proliferates at low ...temperatures and targets hibernating bats, resulting in their premature arousal from stupor with catastrophic consequences. Despite the impact of white-nose syndrome, little is known about the fungus itself or how it infects its mammalian host. P. destructans is not amenable to genetic manipulation, and therefore understanding the proteins involved in infection requires alternative approaches. Here, we identify hydrolytic enzymes secreted by P. destructans, and use a novel and unbiased substrate profiling technique to define active peptidases. These experiments revealed that endopeptidases are the major proteolytic activities secreted by P. destructans, and that collagen, the major structural protein in mammals, is actively degraded by the secretome. A serine endopeptidase, hereby-named Destructin-1, was subsequently identified, and a recombinant form overexpressed and purified. Biochemical analysis of Destructin-1 showed that it mediated collagen degradation, and a potent inhibitor of peptidase activity was identified. Treatment of P. destructans-conditioned media with this antagonist blocked collagen degradation and facilitated the detection of additional secreted proteolytic activities, including aminopeptidases and carboxypeptidases. These results provide molecular insights into the secretome of P. destructans, and identify serine endopeptidases that have the clear potential to facilitate tissue invasion and pathogenesis in the mammalian host.
Among the local processes that determine species diversity in ecological communities, fluctuation‐dependent mechanisms that are mediated by temporal variability in the abundances of species ...populations have received significant attention. Higher temporal variability in the abundances of species populations can increase the strength of temporal niche partitioning but can also increase the risk of species extinctions, such that the net effect on species coexistence is not clear. We quantified this temporal population variability for tree species in 21 large forest plots and found much greater variability for higher latitude plots with fewer tree species. A fitted mechanistic model showed that among the forest plots, the net effect of temporal population variability on tree species coexistence was usually negative, but sometimes positive or negligible. Therefore, our results suggest that temporal variability in the abundances of species populations has no clear negative or positive contribution to the latitudinal gradient in tree species richness.
We quantified temporal population variability for tree species in 21 large forest plots and found much greater variability for higher latitude plots with fewer tree species. In addition, we used a fitted mechanistic model to show that among the forest plots, the net effect of temporal population variability on tree species coexistence was usually negative, but sometimes positive or negligible. Therefore, our results suggest that temporal population variability has no clear negative or positive contribution to the latitudinal gradient in tree species richness.
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