Net primary productivity in the U.S. northern Lake States region: The influence of climate, weather and forest type

Background/Question/Methods: Climate change and increasing weather variability are forcing land managers to explore strategies that create resistance and resilience in ecosystem services.  One strategy is to maximize functional diversity; niche differentiation theory suggests that greater functional diversity should ensure that some components of the ecosystem can continue to function across a wide range of environmental conditions.  Although positive relationships have been observed between functional diversity and stability, most studies have focused on relatively small spatial and/or temporal scales.  Thus, the practical benefits of management actions that impact functional diversity remain unclear.   This study examined how 20 years of remote sensing-derived net primary productivity (NPP) in forests of the northern Lake States responded to long-term climate and short-term weather fluctuations, and how those relationships were influenced by forest type and forest type evenness.    In addition, this study addressed two integrative questions: 1) Is there evidence for complementarity among forest types in how NPP is influenced by weather?  2) Is evenness in forest type abundance (a general measure of functional diversity) related to long-term NPP mean or variability?

Results/Conclusions: Results indicate that 53% of long-term mean NPP can be accounted for by climatic conditions; important climate variables (all positive relationships) included spring precipitation (r²=0.17), annual temperature (r²=0.48) and temperature in winter, spring, summer and fall (r²=0.32, 0.46, 0.37 and 0.44, respectively.)  NPP variability was only weakly related to climate.  Forest types displayed differences in NPP mean and variability, but those differences were not statistically significant after controlling for climate.  54% of annual NPP deviations could be accounted for by weather fluctuations; important weather precipitation included annual, winter and fall precipitation in the prior year and winter precipitation two years prior while important weather temperature (all positive) included annual, spring and fall temperatures in the current year, and spring temperature two years prior.  Relationships between NPP deviations and weather fluctuations differed among forest types in magnitude, although not in direction.  These differences in NPP-weather relationships among forest types remained statistically significant even after controlling for climate, suggesting subtle evidence for forest type complimentarity.  In addition, even after controlling for climate, forest type evenness was positively related to long-term mean NPP and negatively related to NPP variability, suggesting that greater functional diversity may enhance NPP sustainability.