Scale-dependent changes in ecosystem temporal stability over six decades of succession

Temporal and Spatial Dynamics of Soil Carbon Cycling and Its Response to Environmental Change in a Northern Hardwood Forest

The timescales over which soil carbon responds to global change are a major uncertainty in the terrestrial carbon cycle. Radiocarbon measurements on archived soil samples are an important tool for addressing this uncertainty. We present time series (1969-2023) of radiocarbon measurements for litter (Oi/Oe and Oa/A) and mineral (0-10 cm) soils from the Hubbard Brook Experimental Forest, a predominantly hardwood forest in the northeastern USA. To estimate soil carbon cycling rates, we built different autonomous linear compartmental models. We found that soil litter carbon cycles on decadal timescales (Oi/Oe: ~7 years), whereas carbon at the organic-mineral interface (Oa/A), and mineral soil (0-10 cm) carbon cycles on centennial timescales (~104 and 302 years, respectively). At the watershed-level, the soil system appears to be at steady-state, with no observed changes in carbon stocks or cycling rates over the study period, despite increases in precipitation, temperature, and soil pH. However, at the site-level, the Oi/Oe is losing carbon (-15 g C m-2 year-1 since 1998). The observed decline in carbon stocks can be detected when the Oi and Oe layers are modeled separately. This pattern suggests that the rapidly cycling litter layer at the smaller scale is responding to recent environmental changes. Our results highlight the importance of litter carbon as an "early-warning system" for soil responses to environmental change, as well as the challenges of detecting gradual environmental change across spatial scales in natural forest ecosystems.

Climate Variability Modulates the Temporal Stability of Carbon Sequestration by Changing Multiple Facets of Biodiversity in Temperate Forests Across Scales

Climate variability poses a significant threat to ecosystem function and stability. Previous studies suggest that multiple facets of biodiversity enhance the temporal stability of forest ecosystem functioning through compensatory effects. However, as climate change intensifies, two key questions remain unresolved: (1) the mechanisms by which different biodiversity facets sustain the temporal stability of carbon sequestration across spatial scales and (2) how climate variability influences biodiversity and stability at different scales. In this study, based on data from 262 natural communities in the temperate forests of northeastern China, we aggregated metacommunities at varying spatial extents. Using ordinary-least squares regression, we examined the relationships between different facets of biodiversity and the temporal stability of carbon sequestration (hereafter, "stability") across scales. We then employed mixed-effects models to assess how multiple facets of biodiversity influence biotic stability mechanisms at different scales. Additionally, we applied piecewise structural equation modeling to disentangle the relationships among climate variability, multiple facets of biodiversity, and stability across scales. Our findings indicate that biodiversity facets (taxonomic, functional, and phylogenetic diversity) enhance ecosystem stability at multiple scales primarily through insurance effects. Temperature variability was negatively correlated with all biodiversity facets, and declines in biodiversity were associated with reduced ecosystem stability at different scales. Precipitation variability, in contrast, was negatively correlated with α diversity facets but positively correlated with β diversity facets. Unexpectedly, precipitation variability exhibited an overall positive correlation with stability across scales. These results suggest that increasing temperature variability may pose a greater threat to temperate forest ecosystems in the future. Thus, preserving multiple facets of biodiversity across spatial scales will be critical for mitigating the adverse effects of climate warming. Furthermore, the impact of precipitation variability cannot be overlooked in arid and semi-arid regions. Our study provides novel insights into biodiversity conservation under global climate change.

Temporal stability of forest productivity declines over stand age at multiple spatial scales

There is compelling experimental evidence and theoretical predictions that temporal stability of productivity, i.e., the summation of aboveground biomass growth of surviving and recruitment trees, increases with succession. However, the temporal change in productivity stability in natural forests, which may undergo functional diversity loss during canopy transition, remains unclear. Here, we use the forest inventory dataset across the eastern United States to explore how the temporal stability of forest productivity at multi-spatial scales changes with stand age during canopy transition. We find that productivity stability decreases with stand age at the local and metacommunity scales. Specifically, consistent declines in local diversity result in less asynchronous productivity dynamics among species over succession, consequently weakening local stability. Meanwhile, increasing mortality and the transition from conservative to acquisitive species with succession weaken species and local stability. Successional increases in species composition dissimilarity among local communities cause more asynchronous productivity dynamics among local communities. However, the decline in local stability surpasses the rise in asynchronous productivity dynamics among local communities, resulting in lower metacommunity stability in old forests. Our results suggest lower productivity stability in old-growth forests and highlight the urgency of protecting diversity at multiple spatial scales to maintain productivity stability.

Ectomycorrhizal Dominance Increases Temporal Stability of Productivity at Multiple Spatial Scales Across US Forests

Mycorrhizas are fundamental to plant productivity and plant diversity maintenance, yet their influence on the temporal stability of forest productivity across scales remains uncertain. The multiscale stability theory clarifies that the temporal stability (γ stability) of metacommunity-several local communities connected through species dispersal-can be decomposed into the temporal stability of local communities (α stability) and asynchrony among them. Here, based on the forest inventory dataset from the United States and the multiscale stability theory, we explored how mycorrhizal strategy influences forest stability across scales and their underlying mechanisms. At the local scale, we found that α stability increased with ectomycorrhizal dominance due to the higher temporal stability of ectomycorrhizal trees. Additionally, higher α diversity associated with mixed mycorrhizal strategies promoted species asynchrony. At the metacommunity scale, the stabilizing effect of ectomycorrhizal dominance surpassed that of mixed mycorrhizal strategies on the asynchrony among local communities (i.e., spatial asynchrony), resulting in higher γ stability with increasing ectomycorrhizal dominance. Our research suggests the stabilizing effects of ectomycorrhizal dominance on the temporal stability of forest productivity, highlighting the importance of protecting ectomycorrhizal forests to maintain productivity under climate change, especially in the boreal-temperate ecotone where ectomycorrhizal trees are threatened by global change.

Accelerated succession in Himalayan alpine treelines under climatic warming

Understanding how climate change influences succession is fundamental for predicting future forest composition. Warming is expected to accelerate species succession at their cold thermal ranges, such as alpine treelines. Here we examined how interactions and successional strategies of the early-successional birch (Betula utilis) and the late-successional fir (Abies spectabilis) affected treeline dynamics by combining plot data with an individual-based treeline model at treelines in the central Himalayas. Fir showed increasing recruitment and a higher upslope shift rate (0.11 ± 0.02 m yr-1) compared with birch (0.06 ± 0.03 m yr-1) over the past 200 years. Spatial analyses indicate strong interspecies competition when trees were young. Model outputs from various climatic scenarios indicate that fir will probably accelerate its upslope movement with warming, while birch recruitment will decline drastically, forming stable or even retreating treelines. Our findings point to accelerating successional dynamics with late-successional species rapidly outcompeting pioneer species, offering insight into future forest succession and its influences on ecosystem services.

Aridity-dependent shifts in biodiversity-stability relationships but not in underlying mechanisms

Climate change will affect the way biodiversity influences the stability of plant communities. Although biodiversity, associated species asynchrony, and species stability could enhance community stability, the understanding of potential nonlinear shifts in the biodiversity-stability relationship across a wide range of aridity (measured as the aridity index, the precipitation/potential evapotranspiration ratio) gradients and the underlying mechanisms remain limited. Using an 8-year dataset from 687 sites in Mongolia, which included 5496 records of vegetation and productivity, we found that the temporal stability of plant communities decreased more rapidly in more arid areas than in less arid areas. The result suggests that future aridification across terrestrial ecosystems may adversely affect community stability. Additionally, we identified nonlinear shifts in the effects of species richness and species synchrony on temporal community stability along the aridity gradient. Species synchrony was a primary driver of community stability, which was consistently negatively affected by species richness while being positively affected by the synchrony between C3 and C4 species across the aridity gradient. These results highlight the crucial role of C4 species in stabilizing communities through differential responses to interannual climate variations between C3 and C4 species. Notably, species richness and the synchrony between C3 and C4 species independently regulated species synchrony, ultimately affecting community stability. We propose that maintaining plant communities with a high diversity of C3 and C4 species will be key to enhancing community stability across Mongolian grasslands. Moreover, species synchrony, species stability, species richness and the synchrony between C3 and C4 species across the aridity gradient consistently mediated the impacts of aridity on community stability. Hence, strategies aimed at promoting the maintenance of biological diversity and composition will help ecosystems adapt to climate change or mitigate its adverse effects on ecosystem stability.

Temporal dynamics of Grime's CSR strategies in plant communities during 60 years of succession

Grime's competitive, stress-tolerant, ruderal (CSR) theory predicts a shift in plant communities from ruderal to stress-tolerant strategies during secondary succession. However, this fundamental tenet lacks empirical validation using long-term continuous successional data. Utilizing a 60-year longitudinal data of old-field succession, we investigated the community-level dynamics of plant strategies over time. Our findings reveal that while plant communities generally transitioned from ruderal to stress-tolerant strategies during succession, initial abandonment conditions crucially shaped early successional strategies, leading to varied strategy trajectories across different fields. Furthermore, we found a notable divergence in the CSR strategies of alien and native species over succession. Initially, alien and native species exhibited similar ruderal strategies, but in later stages, alien species exhibited higher ruderal and lower stress tolerance compared to native species. Overall, our findings underscore the applicability of Grime's predictions regarding temporal shifts in CSR strategies depending on both initial community conditions and species origin.

A successional shift enhances stability in ant symbiont communities

Throughout succession, communities undergo structural shifts, which can alter the relative abundances of species and how they interact. It is frequently asserted that these alterations beget stability, i.e. that succession selects for communities better able to resist perturbations. Yet, whether and how alterations of network structure affect stability during succession in complex communities is rarely studied in natural ecosystems. Here, we explore how network attributes influence stability of different successional stages of a natural network: symbiotic arthropod communities forming food webs inside red wood ant nests. We determined the abundance of 16 functional groups within the symbiont community across 51 host nests in the beginning and end stages of succession. Nest age was the main driver of the compositional shifts: symbiont communities in old nests contained more even species abundance distributions and a greater proportion of specialists. Based on the abundance data, we reconstructed interaction matrices and food webs of the symbiont community for each nest. We showed that the enhanced community evenness in old nests leads to an augmented food web stability in all but the largest symbiont communities. Overall, this study demonstrates that succession begets stability in a natural ecological network by making the community more even.

Soil organic carbon loss decreases biodiversity but stimulates multitrophic interactions that promote belowground metabolism

Soil organic carbon (SOC) plays an essential role in mediating community structure and metabolic activities of belowground biota. Unraveling the evolution of belowground communities and their feedback mechanisms on SOC dynamics helps embed the ecology of soil microbiome into carbon cycling, which serves to improve biodiversity conservation and carbon management strategy under global change. Here, croplands with a SOC gradient were used to understand how belowground metabolisms and SOC decomposition were linked to the diversity, composition, and co-occurrence networks of belowground communities encompassing archaea, bacteria, fungi, protists, and invertebrates. As SOC decreased, the diversity of prokaryotes and eukaryotes also decreased, but their network complexity showed contrasting patterns: prokaryotes increased due to intensified niche overlap, while that of eukaryotes decreased possibly because of greater dispersal limitation owing to the breakdown of macroaggregates. Despite the decrease in biodiversity and SOC stocks, the belowground metabolic capacity was enhanced as indicated by increased enzyme activity and decreased enzymatic stoichiometric imbalance. This could, in turn, expedite carbon loss through respiration, particularly in the slow-cycling pool. The enhanced belowground metabolic capacity was dominantly driven by greater multitrophic network complexity and particularly negative (competitive and predator-prey) associations, which fostered the stability of the belowground metacommunity. Interestingly, soil abiotic conditions including pH, aeration, and nutrient stocks, exhibited a less significant role. Overall, this study reveals a greater need for soil C resources across multitrophic levels to maintain metabolic functionality as declining SOC results in biodiversity loss. Our researchers highlight the importance of integrating belowground biological processes into models of SOC turnover, to improve agroecosystem functioning and carbon management in face of intensifying anthropogenic land-use and climate change.