Uprooting in boreal spruce forests: long-term variation in disturbance rate

Uprooted trees provide many kinds of exposed and colonizable substrates and may mediate coexistence of plant species. Here we present, for the first time, the temporal forest floor disturbance pattern caused by uprooted trees over a long period of time (120 years). There was a significant correlation between the frequency of high winds and number of uprooted trees, and the fall direction was closely related to the main direction of high winds. The temporal distribution was strongly aggregated, with many uprootings in the 1890s and the 1970s, resulting in large variations in disturbance rate between different decades. This implies periods with low availability of exposed soil. To interpret traits among species dependent on the disturbance as adaptations to some mean rate may thus be strongly misleading. However, the occurrence and importance of such bottleneck periods is hard to evaluate, as studies of the process of uprooting have only documented numbers and rates of uprooted trees and not the availability of exposed soil. We recommend more retrospective studies to evaluate long-term variation in disturbance regime parameters and studies on the temporal availability of exposed and colonizable soil.