Inter-annual variability and spatial coherence of net primary productivity across a western Oregon Cascades landscape

Inter-annual variability (IAV) of forest Net Primary Productivity (NPP) is a function of both extrinsic (e.g., climate) and intrinsic (e.g., stand dynamics) drivers. As estimates of NPP in forests are scaled from trees to stands to the landscape, an understanding of the relative effects of these factors on spatial and temporal behavior of NPP is important. Although a high degree of spatial coherence (i.e., the degree of spatial synchrony over time) is often assumed, this inherent behavior is rarely examined. Quantifying this term may improve future predictions as site-level estimates are scaled up spatially. We quantified the spatial coherence of bole biomass production (BBP) within and between trees, and bole-related NPP (NPPB) between sites of varying age, elevation, moisture, and species composition across a forested landscape in the western Cascade Range of Oregon. Within sites, individual trees with lower than average BBP were the most coherent. IAV of BBP increased as average tree BBP increased and spatial coherence was reduced. Among sites, NPPB was the most spatially coherent (r = 0.92) between young sites, while older sites and comparisons between age classes revealed a much larger range in spatial coherence (r = −0.18 to 0.85). Our findings indicate climate variability may be of greater importance for spatial coherence between young sites, and that intrinsic factors could be decreasing spatial coherence between older sites or sites not in close proximity. The wide range in spatial coherence between sites found in this study, coupled with the complex land use history patterns across forested landscapes, has significant implications for modeling and scaling of NPP.