Variable synchrony in insect outbreak cycling across a forest landscape gradient: multi-scale evidence from trembling aspen in Alberta

Using multi-scale trembling aspen tree-ring width data from Alberta, we show that scaling has a profound influence on dendroecological inferencing. At all scales of sampling there is a significant climatological signal whose strength is nevertheless superseded by the pervasive effect of insect herbivory. At the smallest spatial scale, 20km x 20km, we demonstrate a quasi-periodic pattern of sharp growth reductions, and the existence of negative spatial correlations amongst successive outbreaks. At the intermediate spatial scale of 20km x 80 km, we show that the period 1930-1963 was marked by extremely low correlations in aspen ring widths, with much higher correlations occurring both before and after, despite high variance in precipitation that might have caused ring widths to correlate under moisture limitation. At the largest spatial scale of the entire province, aspen ring widths in the boreal forest became de-correlated abruptly in 1916, after cycling synchronously for the previous 80 years. This occurred despite a warming climate that heightened moisture-limitation. Something in the environment prevents some outbreak cycles from rising to maximum intensity and from spreading to maximum extent. This appears to be the key to better predictability and management of forest insects and forests at short and long time scales.