Climate change and forest decline: A review of the red spruce case

Climate change as a contributor to the decline of red spruce is investigated. Previous climatic response model results are reviewed and more detailed time-dependent modeling of tree growth-climate interactions are performed using the Kalman filter. These new results show that there is a clear temporal and elevational dependence in the response of red spruce to climate. Influence of abnormally warm prior-August temperatures become increasingly time-dependent with decreasing elevation, which is contrary to the elevational gradient in the severity of decline. Thus, this variable, which had been implicated in red spruce declines from previous studies, is unlikely to be a primary cause of the current decline. However, it may be implicated in earlier declines at low elevations. Prior-December temperatures are influential at all elevations, but time-dependent only at the highest elevational zone. The emergence of a strongly time-dependent prior-November temperature response is clearly associated with a time-trend in the temperature record. Thus, it is likely that red spruce is responding, in a transient sense, to changing climate. An additional transient response to current-July temperatures is not associated with any unusual behavior in the data and is, as yet, unexplained. These results show that red spruce is not in equilibrium with its climatic environment, which may have made it more susceptible to damage caused by natural and anthropogenic factors.