Forest demography and biomass accumulation rates are associated with transient mean tree size vs density scaling relations

Linking individual and stand-level dynamics during forest development reveals a scaling relationship between mean tree size and tree density in forest stands, which integrates forest structure and function. However, the nature of this so-called scaling law and its variation across broad spatial scales remains unquantified and its linkage with forest demographic processes and carbon dynamics remains elusive. Here we develop a theoretical framework and compile a broad-scale dataset of long-term sample forest stands (n = 1433) from largely undisturbed forests to examine the association of temporal mean tree size vs density scaling trajectories (slopes) with biomass accumulation rates and the sensitivity of scaling slopes to environmental and demographic drivers. The results empirically demonstrate a large variation of scaling slopes, ranging from -4 to -0.2, across forest stands in tropical, temperate and boreal forest biomes. Steeper scaling slopes are associated with higher rates of biomass accumulation, resulting from a lower offset of forest growth by biomass loss from mortality. In North America, scaling slopes are positively correlated with forest stand age and rainfall seasonality, thus suggesting a higher rate of biomass accumulation in younger forests with lower rainfall seasonality. These results demonstrate the strong association of the transient mean tree size vs density scaling trajectories with forest demography and biomass accumulation rates, thus highlighting the promise of leveraging forest structure properties to predict forest demography, carbon fluxes and dynamics at broad spatial scales. Significance Statement Mean tree size vs density scaling relationships are thought to predict forest function at broad spatial scales. Here we develop a theoretical framework based upon demographic processes and empirical evidence from forest inventory data to demonstrate a strong association of the transient mean tree size and density scaling trajectories (slopes) with forest demography and biomass accumulation rates. This strong association is pervasive across forest biomes and suggests a negative relationship between scaling slope and biomass accumulation rate (resource availability). Our results highlight the promise of leveraging forest structure (i.e., inferred from high resolution remote sensing data or fused into size-structured demographic models) to evaluate forest demography, carbon fluxes and dynamics at broad spatial scales.