Climate induced changes in biome distribution, NPP, and hydrology for potential vegetation of the Upper Midwest US

Background/Question/Methods

Climate change has the potential to severely impact ecosystems throughout the world, causing changes in structure and function as well as services provided. With climate change underway, recent historical climate records offer the opportunity to study the impacts of recent climate change on ecosystems and explore the range and magnitude of possible impacts sustained thus far. We investigated the impacts of recent climate change on potential vegetation distributions (i.e. trees, grasses, and shrubs) and carbon and water cycling across the Upper Midwest US from 1948-2007 using the Agro-IBIS dynamic global vegetation model. We drove the model using a historical, gridded daily climate data set at a spatial resolution of 5’ x 5’.

Results/Conclusions

Model results showed an average change in total net primary productivity of 41 g C m^-2 yr^-1 over the study period based on linear regression analysis. Increased summer relative humidity, increased annual precipitation and decreased mean maximum summer temperature were key variables contributing to these positive trends. Mechanisms for increased productivity included a reduction in soil moisture stress (e.g., increased available water), as well as increased assimilation due to the positive effect of humidity on stomatal conductance.  Model results also showed an average total increase in annual groundwater recharge throughout the region of 39 mm yr^-1 driven by increases in annual precipitation. Evapotranspiration had a variable spatial trend over the 60-year period, with total change over the study period averaging 4.6 mm yr^-1. We also analyzed potential changes in plant functional type (PFT) distributions at the biome level, but hypothesize that the model may be unable to adequately capture competitive interactions among PFTs at smaller spatial scales. An analysis of the bioclimatic envelopes for PFTs common to the region revealed no significant change to the boreal conifer tree climatic domain over the study period yet did reveal a slightly expanded domain for the temperate deciduous tree.

In conclusion, potential biome and PFT distributions did not change significantly between 1948 and 2007, yet climate change has contributed to substantial changes in coupled carbon and water cycling in natural ecosystems of the Upper Midwest US. We suggest that incorporating recent, high-resolution climate records into ecological studies can offer valuable insight into the heterogeneous nature of climate change and its impacts on ecosystems.