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What are typical values and dynamic status of live‐tree biomass pools in old‐growth, mesic, cool temperate forests? A handful of biomass density estimates in eastern North American temperate forests show large biomass/carbon reserves on a per‐area basis. However, it is less clear whether these ecosystems are, over multi‐decade scales, typically steady‐state or non‐equilibrial carbon pools. Previous studies have suggested both possibilities, but claims are based on inferences from short‐term studies or proxy data sets. An unusually long‐term and extensive data set from repeatedly sampled permanent plots (84 yr, ca. 10 ha sample area, 6–8 measurements), from old‐growth conifer‐hardwood forest in northern Michigan, USA, allows direct estimation of multi‐decade trends in aboveground live‐tree biomass. Results confirm prior suggestions of high‐biomass density for old‐growth temperate forests (averaging >300 Mg/ha), but, despite significant decade‐scale variation, show no overall, long‐term directional change. Study plots typically show multi‐decade trends of gradually increasing biomass density, interrupted by sharp declines attributed to intermediate‐severity disturbances, with recovery of pre‐disturbance biomass density requiring upwards of a half‐century. At the stand scale, biomass dynamics are strongly historically contingent, and short‐term studies may yield biased or misleading results. Disturbance legacies, through demographic and structural effects, can have multi‐decade effects on vulnerability to further disturbance. While this study shows no general trend in aboveground biomass pools, it suggests that changes in disturbance regime may drive important feedbacks in biomass pool dynamics.

 

Global change drivers (GCDs) are expected to alter community structure and consequently, the services that ecosystems provide. Yet, few experimental investigations have examined effects of GCDs on plant community structure across multiple ecosystem types, and those that do exist present conflicting patterns. In an unprecedented global synthesis of over 100 experiments that manipulated factors linked to GCDs, we show that herbaceous plant community responses depend on experimental manipulation length and number of factors manipulated. We found that plant communities are fairly resistant to experimentally manipulated GCDs in the short term (<10 y). In contrast, long-term (≥10 y) experiments show increasing community divergence of treatments from control conditions. Surprisingly, these community responses occurred with similar frequency across the GCD types manipulated in our database. However, community responses were more common when 3 or more GCDs were simultaneously manipulated, suggesting the emergence of additive or synergistic effects of multiple drivers, particularly over long time periods. In half of the cases, GCD manipulations caused a difference in community composition without a corresponding species richness difference, indicating that species reordering or replacement is an important mechanism of community responses to GCDs and should be given greater consideration when examining consequences of GCDs for the biodiversity–ecosystem function relationship. Human activities are currently driving unparalleled global changes worldwide. Our analyses provide the most comprehensive evidence to date that these human activities may have widespread impacts on plant community composition globally, which will increase in frequency over time and be greater in areas where communities face multiple GCDs simultaneously.