Intermediate disturbances drive long‐term fluctuation in old‐growth forest biomass: an 84‐yr temperate forest record

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.