PS 52-117 - Pacific Northwest second-growth forest carbon dynamics in relation to plant community drivers

Wednesday, August 5, 2009
Exhibit Hall NE & SE, Albuquerque Convention Center
Justin L. Kirsch1, Alexandra N. Kazakova2, Emily B. Anderson3, Margaret Pryor3 and Dylan G. Fischer4, (1)Ecosystem Ecology Laboratory, The Evergreen State College, Olympia, WA, (2)School of Environmental and Forest Sciences, University of Washington, Seattle, WA, (3)Field Ecology Lab, The Evergreen State College, Olympia, WA, (4)Environmental Studies Program, The Evergreen State College, Olympia, WA
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.

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