Landform and soils control successional trajectories of forest composition and biomass accumulation in a north-temperate forest landscape

Long-term changes in the composition and biomass of temperate forests are fundamental topics in ecosystem ecology. A longstanding paradigm that forest biomass accumulation reaches steady-state within roughly a century is increasingly at odds with empirical results from throughout the temperate forest biome, although mechanisms for this theory-data mismatch remain elusive. One key limitation is the difficulty of observing long-term changes within a spatial design that integrates all ecosystem components (e.g., landforms, soils, vegetation). In the present study, we synthesized >70 years of repeated-measure forest inventory data from census plots distributed across a range of landforms, ecosystem, soil, and vegetation cover types in northern Michigan, in order to: 1) quantify temporal patterns of forest compositional change and aboveground biomass accumulation; 2) identify divergent successional pathways on the landscape; 3) test potential mechanisms for different pathways. Biomass accumulation did not reach steady state, but rather continued to aggrade through three centuries of stand development, reaching totals approaching 450 Mg ha-1 on some sites. Successional changes in composition and biomass accumulation differed by landform, with moraines supporting increasingly hardwood-dominated forests that were twice as productive as conifer-dominated lake-marginal wetlands. Forests on outwash plains had intermediate biomass production. The upland landforms (moraine and outwash) spanned 5 parent materials, each differing in soil silt+clay content, which explained 95% of the spatial variation in biomass accumulation. Results suggest that forests across glaciated north temperate regions can accumulate biomass for centuries, at rates mediated by soil and parent material properties.