Forest tree diversity and species composition can exert strong effects on carbon (C) sequestration. Recent studies in the carbon-rich forests of the Pacific Northwest have demonstrated significant associations between forest tree diversity and productivity. Such relationships are important to quantify in the region since these forests represent some of the most productive and carbon-dense forests in North America. Here we use a permanent plot network in a Pacific Northwest forest to examine native tree species’ contributions to productivity over a decade. We examined plot-level and individual tree carbon accumulation over two- and four- year measurement intervals (2006, 2008, 2012, 2016.) In each survey, allometric equations were used to estimate total plot biomass and biomass C. Tree richness, biomass-based indices of species diversity (Shannon’s H’), evenness, and species dominance were also generated. Based on previous work, we hypothesized that overstory diversity would be a significant predictor of plot productivity, and that different species would exhibit distinct contributions to total productivity of the forest over time. To add temporal resolution to trends in species-specific productivity, we used a tree core method to determine size-based annual biomass increment in individual species throughout the past 20 years of growth.
Results/Conclusions
Despite earlier findings in the system, we did not find evidence to support the hypothesis that overstory richness, diversity, or evenness is a significant factor in the productivity of this forest. Nevertheless, our data suggest that higher carbon sequestration rates were often found in high diversity stands. Our cumulative data suggest forest-wide net C sequestration increment of 6.2 Mg ha-1 yr-1 between 2006 and 2008, 6.05 Mg ha-1 yr-1 between 2008 and 2012, and 4.3 Mg ha-1 yr-1 between 2012 and 2016. These values are consistent with over-all high sequestration values for the region. Individual species dominance can also have a significant effect on patterns in C uptake through time. Preliminary analysis shows that tree species’ contributions to total forest C have changed over the course of the study, where Tsuga heterophylla demonstrated increased C uptake through time, Pseudotsuga menziesii productivity was less than what might be expected based on average plot biomass, and Alnus rubra stands declined in C through time. In combination, these data provide a unique lens on changing species dominance and forest C patterns through secondary succession in one of the most carbon-dense forests in North America.