Species richness, functional diversity, and ecosystem function in drying boreal Alaskan wetlands

Background/Question/Methods

Boreal lakes and wetlands represent a globally significant reservoir of organic carbon. In Alaska, climate warming has been linked to an estimated 18% loss in net lake surface water over the last 30 years. The consequences of this trend for lake-margin wetlands are unknown. Climate-driven lake drying could affect carbon storage in boreal wetlands directly, or by controlling the composition of local plant communities. Variation in plant diversity could regulate carbon stocks if carbon fluxes are sensitive to the presence of specific functional traits in the plant community (the functional composition hypothesis) or if functional trait diversity promotes the efficient usage of available resources by wetland plants (the functional diversity hypothesis).

We investigated relationships between historical drying trends, aboveground biomass and soil carbon, and the diversity and composition of plant species and functional traits in boreal wetlands using structural equation modeling. Our goal was to determine whether drying trends are associated with significant changes in above and below-ground carbon stocks, and whether these changes are mediated by the functional composition or functional diversity of wetland plant communities. Models were fit using data from field surveys and remote-sensing analyses of wetlands surrounding 94 lakes  in the Yukon Flats National Wildlife Refuge in interior Alaska. Lakes were characterized as drying or stable by linear regression, using estimates of lake area derived from satellite imagery collected between 1980 and the present.

Results/Conclusions

We detected significant decreasing trends in lake area at 22 of our samples sites. Vascular species richness and aboveground biomass were both significantly higher at drying sites. Preliminary results show that variation in aboveground biomass could be best explained by a combination of lake drying trends, species diversity, functional diversity, and functional composition as predictor variables (R2 = 0.69). Functional diversity and functional composition were both significant predictors of aboveground biomass. However, functional diversity and biomass were negatively correlated, which suggests that diversity did not increase the efficiency of resource usage by the plant community.

Climate-driven drying trends over the last 30 years are significantly altering carbon stocks in boreal Alaskan wetlands. These relationships appear to be a function of both direct mechanisms (such as changes in soil conditions), and indirect effects mediated by changes in the functional composition of wetland communities. Management and land-use decisions regarding Alaskan wetlands should account for the effects of plant species composition on ecosystem services relating to carbon storage and climate regulation.