Long-term silviculture experiments impact stand-level weather sensitivity, resistance, and resilience
Extreme weather events are expected to increase in frequency and severity as a result of climate change, and numerous studies have demonstrated the often intimate association between climate variability and tree growth. Evidence suggests that manipulation of forest structure can influence how trees respond to extreme weather events. We used detailed dendrochronological data from four long-term experimental forests in the upper Lakes States (Minnesota and Wisconsin) and New England (New Hampshire and Maine) to examine how the relationships between inter-annual tree biomass increment and climate variability are affected by long-term management regime. The silvicultural experiments resulted in a diversity of stand structural and compositional conditions. Given the effects of stand structure on growing conditions and tree response to severe weather; we examined the following questions (1) which monthly and seasonal climate variables most affect stand-level biomass increment? (2) How are stand resistance and resilience to extreme weather events affected by stand structure? We used dendrochronological techniques, historic climate data, and response function analysis to identify the importance of climate variables and timing of those variables to annual biomass increment. To calculate resistance and resilience of stands during and after extreme weather events, we used historic inventory measurements and tree ring analysis to calculate historic stand structure and biomass increment.
In red pine-dominated communities in the Lake States, growth was positively associated with higher spring temperatures and summer precipitation, but negatively associated with higher fall temperatures in all treated plots. Manipulation of stand density appeared to reduce climate sensitivity in northern hardwood systems in the Lake States, as only untreated, control plots showed a response to climate variability with a negative association with summer temperature. An opposite trend was detected in the more humid northern hardwood sites in New Hampshire, where growth in control plots had no significant correlations with climate, and growth in some treated plots was positively correlated with spring and summer temperature and precipitation. In Maine, growth in some treatments was positively correlated with higher spring and summer temperatures and fall moisture. Initial results suggest that stand-level resistance and recovery are not affected by stand structure, but resilience is affected. Overall, these results indicate that alterations to stand structure via management treatments may impart differential responses to climate variability, with impacts varying according to prevailing climatic regime and composition. An understanding of these relationships will help inform future management decisions aimed at reducing climate change impacts.