Pacific Northwest second-growth forest carbon dynamics in relation to plant community drivers

Background/Question/Methods Examinations of relationships between diversity and productivity in second-growth forests are important because these forest types are increasingly common at the landscape level (as less old-growth remains), and tree diversity can vary between monoculture plantations to functionally-diverse species mixtures. In order to determine patterns in carbon (C) storage and flux rates in a Pacific Northwest second-growth forest, we assess variability in above-ground net primary productivity (ANPP), Net soil CO₂ efflux (NCER), and C storage in relation to presence of coniferous, deciduous, and a nitrogen (N) fixing tree species.

Results/Conclusions Four major findings arose from our study: 1) Mean ANPP was only mildly different among stands dominated by evergreen, deciduous, and an N-fixing tree species; 2) conifer dominated plots had an estimated 3.12 Mg ha^-1 y^-1 ± 0.95 SE higher ANPP than hardwood dominated plots (P<0.04), while conifer and hardwood dominated plots were not significantly different in estimated net changes in total above-ground C stocks (P>0.05, average 4.72 ± 0.74 SE Mg ha^-1 y^-1), or NCER (P>0.05; Average 4.0 ± 0.42 SE mmol C m^-2 s^-1). 3) We found a positive linear relationship between ANPP and tree species richness, where overstory richness explained 46% of NPP (r²=0.46, P<0.05).  We also find a positive relationship between average growing-season (April-October) NCER and productivity (r²=0.75, P<0.05), additionally plots with more tree species generally had higher NCER (r²=0.30, P<0.05); and 4) variation in overstory stand dominance was strongly predictive of understory plant community composition (MRPP P<0.05). We found relationships between understory plant diversity and NCER which were weak for species richness (r²=0.004, P<0.05) and stronger for Shannon’s (r²=0.22, P<0.05) and Simpson’s (r²=0.28, P<0.05) diversity indices. However, ANPP, net changes in C stocks, and understory plant communities were unrelated (P>0.05). These data highlight variability in second-growth forests that may have strong implications for net ecosystem productivity (NEP). We estimate NPP at 7.75 Mg C ha^-1 yr^-1, and suggest that this and other plot level measurements imply an upper bounds on NEP of ~3.7 Mg C ha^-1 yr^-1, and a lower bounds of  ~2.7 Mg C ha^-1 yr^-1.