Examining the effects of functional diversity on tree growth and vigor across California and Oregon conifer-hardwood forest landscapes

Christopher Looney, Ecosystem Function and Health Program, USDA Forest Service Pacific Southwest Research Station, Davis, CA and Jonathan W. Long, USDA Forest Service Pacific Southwest Research Station, Davis, CA

Background/Question/Methods
Functional diversity among tree species has been found to enhance aboveground productivity in a variety of forest types worldwide. Enhanced productivity in tree-species mixtures may reflect facilitation or complementarity, i.e., contrasting species traits that allow for more efficient resource sharing. In addition to growth, functional diversity may enhance tree survival and drought resistance under climate change. In the mixed-conifer/hardwood forests of California and Oregon, USA, where oaks are the major source of hardwood biodiversity, increasingly frequent wildfires and drought, combined with historic fire exclusion, have altered forest succession, population structure, and site quality in ways that vary across complex terrain. Understanding whether functional differences in conifer and hardwood resource use substantially alter tree performance is critical to restoring and managing these disturbed systems for climate-change adaptation. We examined individual-tree growth and vigor, as indicted by volume and live crown ratio (LCR), respectively, using Forest Inventory and Analysis (FIA) data for conifer, evergreen and deciduous hardwood functional groups. We investigated whether stand age, topography, neighborhood tree competition, tree size, and drought modified functional diversity effects. We examined data on FIA plots across broad environmental gradients. Comparisons of generalized linear mixed models were used to infer neighborhood functional diversity effects on tree (stems ≥ 12.7cm DBH) growth and LCR.
Results/Conclusions
Individual-tree conifer growth and LCR responses to neighborhood functional diversity were contingent on competition. Under low competition, conifer growth and LCR were higher in functionally similar neighborhoods under low competition and relatively higher in functionally diverse stands under high competition. In contrast to conifers, growth of deciduous and evergreen hardwood species did not vary with functional diversity. Deciduous and evergreen hardwood LCR improved with functional diversity in dense stands. For both conifers and deciduous hardwoods, LCR increased with functional diversity in smaller trees. We found weaker evidence that greater functional diversity sustained higher LCR under drought in conifers, while drought was not a plausible modifier of functional diversity effects for hardwoods. These findings suggest that functional-diversity effects present tradeoffs for managing California and Oregon conifer-hardwood forests. Ecosystem management that prioritizes high functional diversity appears to sustain tree growth and vigor at the high stand densities resulting from historical fire suppression but may adversely impact conifer growth and vigor under the low-competition conditions typical of recently restored stands.