Wood decomposition is faster in open habitats than closed-canopy forests, with the mechanisms unclear. When allowing access, termites outcompete fungi during wood decomposition. If increasing canopy ...density shifts the dominance from termites to fungi, decomposition rates will fall. We conducted wood decomposition experiments in 148 plots of two regenerating forest sites (Site A vs. B) in subtropical China with tree species diversity and composition (for example, specific leaf area) manipulated. Branches of seven species with varying quality were deployed and measured for wood mass-loss rates and decomposer abundance at two time steps (that is, after 1 year and 2 years). We also monitored understory microclimate and predatory ant diversity in 40 plots of Site A. We found that higher canopy density drove the shift of dominance from termites to fungi via facilitating predatory ants and wetter–cooler microclimate. Furthermore, termites promoted decomposition after 1 year especially for the species with highest wood quality, but fungi slowed decomposition after 2 years especially for two species with lower quality. Notably, canopy density negatively correlated with wood mass-loss rates in closed-canopy Site A but not so in more open Site B. Tree diversity and community-weighted mean specific leaf area increased canopy density but did not affect abundance nor decomposition. Our study highlights the importance of canopy density for retarding fine wood decomposition in closed-canopy regenerating forests, probably via shifting the dominance from more (that is, termites) to less efficient decomposer (that is, fungi). Future cross-regional studies are necessary to test the generality of canopy density effect on wood decomposition and the related mechanisms.
ABSTRACT
Land‐use change is fragmenting natural ecosystems, with major consequences for biodiversity. This paper reviews fragmentation trends – historical and current – in China, the fourth largest ...country on Earth, and explores its consequences. Remote sensing makes it possible to track land‐use change at a global scale and monitor fragmentation of dwindling natural landscapes. Yet few studies have linked fragmentation mapped remotely with impacts on biodiversity within human‐modified landscapes. Recent reforestation programs have caused substantial increases in forest cover but have not stopped fragmentation, because the new forests are mostly monocultures that further fragment China's remnant old‐growth lowland forests that harbour the highest levels of biodiversity. Fragmentation – and associated biodiversity declines – is unevenly distributed in China's forests, being most problematic where agricultural expansion is occurring in the southwest and northeast, serious in the densely populated eastern regions where urbanisation and transport infrastructure are modifying landscapes, but less of a problem in other regions. Analyses of temporal trends show that the drivers of forest fragmentation are shifting from mainly agricultural expansion to urbanisation and infrastructure development. Most of China's old‐growth forests persist in small, isolated fragments from which many native species have disappeared, on land unsuitable for human utilisation. Fragmentation throughout China is likely to have major consequences on biodiversity conservation, but few studies have considered these large‐scale processes at the national level. Our review fills this research gap and puts forward a systematic perspective relevant to China and beyond.
1. Evergreen broad-leaved forests are widely distributed in eastern Asia with evergreen broad-leaved (EBL) and deciduous broad-leaved (DBL) tree species coexisting under the same climatic regime, ...raising questions as to the underlying mechanisms. Since EBL and DBL species differ in leaf life span, a key component of resource economic strategies, their coexistence might be attributed to regeneration niche partitioning across habitats varying in resource supply. 2. We investigated the effects of variation in insolation and topography on regeneration of EBL and DBL species in a subtropical EBL forest of eastern China after an ice storm that caused severe canopy disturbance. 3. Using a mixed-effects modelling framework and census data from 2011 to 2014 on 8,548 wild seedlings of 123 species, we quantified habitat preferences of EBL and DBL species during post-disturbance regeneration and how their survival and height relative growth rates varied among habitats. 4. The relative density of DBL seedlings (proportional to all seedlings) was greater in habitats with greater (canopy gaps) compared to habitats with lesser (understorey) insolation and increased with canopy gap size. However, DBL seedlings were not more frequent in higher (valleys) compared to lower (ridges) fertility habitats. Although DBL seedlings exhibited larger differences in growth between higher and lower resource habitats than EBL seedlings, their growth rates did not increase with canopy gap size. Seedlings of EBL species had high survival in all habitats, but larger DBL seedlings survived equally well on ridges. Consequently, the relative density of DBL seedlings declined in valleys, so that by 2014 it became more similar in valley and ridge habitats, whereas it remained higher in gaps than in the understorey, and especially in larger gaps. 5. Synthesis. Specialization on contrasting topographic habitats is considered the primary mechanism mediating coexistence between deciduous broad-leaved and evergreen broad-leaved species. Our results, however, suggest this may not always be true, since seedlings of deciduous broad-leaved and evergreen broad-leaved species partitioned regeneration niches based on light more so than topography. We propose that coexistence of deciduous broad-leaved and evergreen broadleaved species can strongly depend upon canopy disturbance to create a mosaic of habitat patches, including high light gaps favouring regeneration of deciduous broad-leaved species.
Biodiversity experiments have shown that species loss reduces ecosystem functioning in grassland. To test whether this result can be extrapolated to forests, the main contributors to terrestrial ...primary productivity, requires large-scale experiments. We manipulated tree species richness by planting more than 150,000 trees in plots with 1 to 16 species. Simulating multiple extinction scenarios, we found that richness strongly increased stand-level productivity. After 8 years, 16-species mixtures had accumulated over twice the amount of carbon found in average monocultures and similar amounts as those of two commercial monocultures. Species richness effects were strongly associated with functional and phylogenetic diversity. A shrub addition treatment reduced tree productivity, but this reduction was smaller at high shrub species richness. Our results encourage multispecies afforestation strategies to restore biodiversity and mitigate climate change.
While the number of studies on the role of biodiversity on ecosystem functioning is steadily increasing, a key component of biogeochemical cycling in forests, dead wood decay, has been largely ...neglected. It remains widely unknown whether and how dead wood decay is affected by diversity loss in forests. We studied the hierarchical effects of tree species diversity on wood decay rates in a subtropical forest landscape in southeast China via its influence on fungal OTU richness and invertebrate diversity using piecewise structural equation models. The experiment was conducted in natural forest plots that span a wide gradient of tree species diversity embedded in a heterogeneous topography. To account for interactions between macro‐invertebrates and fungi, that potentially modify the influence of tree biodiversity and climate on dead wood decay, we compared a macro‐invertebrate exclusion treatment with a control treatment that allowed access to all types of decomposers.
Diversity effects of trees on wood decay rates were mostly negative and mediated by the diversity of macro‐invertebrates. However, the effects of tree species diversity or fungal OTU richness and macro‐invertebrate diversity on wood decay rates were comparatively weak. Temperature affected decay rates positively and had the strongest influence in all treatments. While the exclusion of macro‐invertebrates did not lead to a reduction of wood decay rates, our results suggest that they may however have a mediating role in the process. In the presence of invertebrates the predictability of wood decay rates was higher and we observed a tendency of a stronger temperature control.
Our results suggest that there is evidence for diversity effects on wood decomposition, but the temperature control is still more important. Thus, an increase in mean annual temperature will increase carbon and nutrient turnover through wood decomposition in subtropical forest irrespective of biotic composition.
Altered microclimatic conditions and higher disturbance at forest edges create environmental stress and modify resource gradients from edge to interior, changing the selection pressures acting on ...individuals. Although community‐weighted trait‐mean (CWM) shifts along edge gradients have been widely documented at the species level, it is unclear how edge effects act at the individual level, and whether the direction of intraspecific trait shifts mirrors that of CWM shifts in response to edge effects. On 20 islands in the Thousand Island Lake, China, we established 484 plots (2 × 2 m) in a stratified random design across distances of 0–128 m from the forest edge. Within each plot, we sampled leaves (n = 34 768) from within and among all 2993 individuals of 68 species and measured five leaf traits (leaf area, LA; specific leaf area, SLA; leaf dry matter content, LDMC; thickness, LT; chlorophyll content, LCC). Using generalized linear mixed models, we found that different leaf traits exhibited contrasting shifts in inter‐ versus intraspecific trait variation in response to edge effects. For SLA, LT and LCC, negative covariance between inter‐ and intraspecific trait shifts resulted in dampening of community‐wide trends compared to CWM response to edge effects. In contrast, the community‐wide trend for LDMC was reinforced due to positive covariance between inter‐ and intraspecific trait shifts, while for LA the direction of covariance shifted from negative to positive on small versus large islands. Together, edge effects alter selection regimes in reassembling plant communities. Predicting the community‐wide consequences depends on the degree to which there is negative versus positive covariance between species sorting and within‐species adaptation. The widely‐used CWM approach can mask contrasting trait selection pressures acting on individuals within local populations. Individual‐level trait variation can improve understanding of community re‐assembly trajectories in response to global environmental change.
Habitat loss and fragmentation has long been considered the primary cause for biodiversity loss and ecosystem degradation worldwide, and is a key research topic in landscape ecology.
The classical environmental control model assumes that species distribution is determined by the spatial variation of underlying habitat conditions. This niche-based model has recently been ...challenged by the neutral theory of biodiversity which assumes that ecological drift is a key process regulating species coexistence. Understanding the mechanisms that maintain biodiversity in communities critically depends on our ability to decompose the variation of diversity into the contributions of different processes affecting it. Here we investigated the effects of pure habitat, pure spatial, and spatially structured habitat processes on the distributions of species richness and species composition in a recently established 24-ha stem-mapping plot in the subtropical evergreen broad-leaved forest of Gutianshan National Nature Reserve in East China. We used the new spatial analysis method of principal coordinates of neighbor matrices (PCNM) to disentangle the contributions of these processes. The results showed that (1) habitat and space jointly explained ~53% of the variation in richness and ~65% of the variation in species composition, depending on the scale (sampling unit size); (2) tree diversity (richness and composition) in the Gutianshan forest was dominantly controlled by spatially structured habitat (24%) and habitat-independent spatial component (29%); the spatially independent habitat contributed a negligible effect (6%); (3) distributions of richness and species composition were strongly affected by altitude and terrain convexity, while the effects of slope and aspect were weak; (4) the spatial distribution of diversity in the forest was dominated by broad-scaled spatial variation; (5) environmental control on the one hand and unexplained spatial variation on the other (unmeasured environmental variables and neutral processes) corresponded to spatial structures with different scales in the Gutianshan forest plot; and (6) five habitat types were recognized; a few species were statistically significant indicators of three of these habitats, whereas two habitats had no significant indicator species. The results suggest that the diversity of the forest is equally governed by environmental control (30%) and neutral processes (29%). In the fine-scale analysis (10 × 10 m cells), neutral processes dominated (43%) over environmental control (20%).
Island biogeography theory describes the relationship between island size, isolation and biodiversity, but it does not address the effects on ecosystem processes such as wood decomposition. Wood ...decomposition rates can be expected to increase with decomposer diversity, which increases with island size and decreases with increasing island isolation. However, island size and isolation effects on wood decomposition may also act by impacting substrate composition, forest composition and biotic interactions among decomposers. In this study, branch‐wood decomposition rates of three locally dominant tree species deployed at 154 plots on 15 islands in Thousand Island Lake, China, were measured over one year. The results showed that decomposition rates in coarse‐mesh bags where both termites and fungi were allowed access increased with island size for the two tree species preferred by the termites. By contrast, decomposition rates in fine‐mesh bags which only fungi could access decreased with island size for the two tree species preferred by fungi. The positive effects of island size on termite feeding activity were mediated by a higher deadwood diversity, a more diverse forest composition and a higher soil moisture content, with forest composition shaping soil moisture. For fungal diversity, the negative effects of island size were attributable to a higher soil moisture content and termite feeding activity. Island isolation decreased fungal diversity but increased wood decomposition rates. Our study indicates that island size affects wood decomposition by altering the distribution of decomposers and their relative contributions to wood decomposition. Size–decomposition relationship further depends on the affinity between substrates and decomposer taxa. In general, our results show that ecosystem processes may be affected by island size and isolation, but the patterns are more complex than expected from the island biogeography theory. This discrepancy is largely due to the different responses and trade‐offs among different functional groups.
Context
The study of habitat fragmentation is fraught with definitional and conceptual challenges. Specifically, a multi-scale perspective is needed to address apparent disagreements between ...landscape- and patch-based studies that have caused significant uncertainty concerning fragmentation’s effects.
Objectives
Here we test the hypothesis that habitat fragmentation alters biological communities by creating hierarchically nested selective pressures across plot-, patch-, and landscape-scales in the Thousand Island Lake, China. We assess the variation in fragmentation-diversity relationships contextualize the interplay of scale-dependent relationships.
Methods
This work utilized two datasets. At the island scale, islands were sampled to adequately capture the vast majority of species on each island using 5 × 5 m sampling cells. To capture intra-island variation, we sampled 400 m
2
plots set up along an edge gradient on each island. The variation in species richness at each scale was then compared using Wilcoxon tests, Spearman rank correlations, generalized linear models, and single-large-or-several-small (SLOSS) simulations.
Results
We find that edges have little impact on plot α-diversity and between-plot β-diversity, but increase the amount of β-diversity that can be attributed to nestedness at the plot-scale. We also find that the percent habitat in the surrounding landscape has a positive effect on species richness at the patch-scale and that small islands accumulate species faster than large islands of equal total size at the landscape-scale.
Conclusions
By observing interdependent effects at each scale, we find support for the hypothesis that habitat fragmentation’s effects are hierarchically structured. Therefore, multi-scale approaches are needed to understand the patterns, processes, and consequences of fragmentation.
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