Abstract
While aboveground biomass and forest productivity can vary over abiotic gradients (e.g., temperature and moisture gradients), biotic factors such as biodiversity and tree species stand dominance can also strongly influence biomass accumulation. In this study we use a permanent plot network to assess variability in aboveground carbon (C) flux in forest tree annual aboveground biomass increment (ABI), tree aboveground net primary productivity (ANPPtree), and net soil CO2 efflux in relation to diversity of coniferous, deciduous, and a nitrogen (N)-fixing tree species (Alnus rubra). Four major findings arose: (1) overstory species richness and indices of diversity explained between one third and half of all variation in measured aboveground C flux, and diversity indices were the most robust models predicting measured aboveground C flux; (2) trends suggested decreases in annual tree biomass increment C with increasing stand dominance for four of the five most abundant tree species; (3) the presence of an N-fixing tree species (A. rubra) was not related to changes in aboveground C flux, was negatively related to soil CO2 efflux, and showed only a weak negative relationship with aboveground C pools; and (4) stands with higher overstory richness and diversity typically had higher soil CO2 efflux. Interestingly, presence of the N-fixing species was not correlated with soil inorganic N pools, and inorganic N pools were not correlated with any C flux or pool measure. We also did not detect any strong patterns between forest tree diversity and C pools, suggesting potential balancing of increased C flux both into and out of diverse forest stands. These data highlight variability in second-growth forests that may have implications for overstory community drivers of C dynamics.
Highlights
Forests of the Pacific Northwest (PNW) can be highly productive [1,2,3,4], and as temperate forests they may store more biomass carbon (C) per unit area than most other ecosystems [1,5,6,7,8].Second-growth forest productivity can be highly variable, and understanding reasons for this variability is an important research priority with implications for ecosystem C dynamics and global C cycles
We focused on three key measurable C flux measures in a natural second-growth PNW forest: net changes in aboveground tree C pools (hereafter; aboveground biomass increment (ABI), aboveground net primary productivity of trees (ANPPtree)), and net soil CO2 efflux
We found significant positive relationships between aboveground biomass increment C flux (ΑΒΙ) and overstory richness classified by the five most dominant overstory species (Figure 2) and all 12 tree species (Table 2)
Summary
Second-growth forest productivity can be highly variable, and understanding reasons for this variability is an important research priority with implications for ecosystem C dynamics and global C cycles. While a large portion of variability in forest C flux (both above- and belowground) occurs in response to abiotic variation [9,10,11,12,13,14,15,16,17,18,19], biotic gradients in plant community composition and diversity may be especially important as drivers of patterns in C cycling and productivity even at fine scales [4,7,20,21,22,23]. Studies across productivity gradients have suggested the reverse relationship where productivity predicts forest tree diversity in large-scale regional datasets [16,29]
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