Abstract
Wind is an important disturbance in many forested regions, but research has largely focused on immediate to short-term (<10 years) effects on species composition and structure of stands and landscapes. We used a set of ten, 0.4-ha plots established in 1935 and measured every 5–13 years to examine the cumulative effects of multiple wind disturbance events in the coastal forests of Oregon. Since 1935 there have been ten documented wind events in coastal Oregon with hurricane-force winds. Most of the eight windstorms since 1962 noticeably influenced at least one plot; however, no individual storm clearly influenced all plots. Net rate of biomass accumulation of plots was negative when biomass mortality exceeded 29–41 Mg/ha per 5-year period (or 3.4%–5.8% per 5 years). In contrast, wind disturbance did not clearly reduce NPP (Net Primary Production) in any plot until biomass mortality exceeded 50%. Major wind-related mortality episodes in individual plots did not necessarily strongly influence the average loss of biomass across all plots. For example, a biomass loss of 88% in one plot resulted in an average loss of only 3.4% across all plots. In contrast, the cumulative effect of multiple plots with repeated wind-related mortality did decrease biomass at the multi-plot level. Depending on the plot, wind caused 16%–59% of the total mortality over time, and the proportion of wind-related mortality has increased 5- to 8-fold since 1940. The majority (80%) of wind-related mortality was “immediate”, but some trees survived up to 35 years after being significantly wind damaged. Despite western hemlock having a wind-related mortality rate that was at least twice that of Sitka spruce, the number of stems of the former species increased over time. This trend may be related to the predominant recovery mechanism which was the release of existing small trees and the fact that the majority of ingrowth was western hemlock. This study suggests that wind disturbance is a heterogeneous, gradual process and that while individual events have a detectable impact on structure and function, the largest impacts, at least in systems where it reoccurs each decade, are cumulative.
Highlights
IntroductionMany processes lead to periodic pulses of mortality in forest stands and landscapes which can result in changes of forest structure and function that are evident for decades to centuries [1]
Many processes lead to periodic pulses of mortality in forest stands and landscapes which can result in changes of forest structure and function that are evident for decades to centuries [1].These disturbance effects are dependent on the spatial scale examined, as well as the spatial heterogeneity of the disturbance: the death of a few trees can lead to significant change at the scale of 100 m2, but may be viewed as part of “normal” mortality at the scale of more than 1 ha
Wind is an important disturbance agent in many forested regions of the globe, though its effects on forest structure vary with the frequency, intensity and duration of each event, as well as the interaction with topography and antecedent conditions such as soil moisture [14,20]
Summary
Many processes lead to periodic pulses of mortality in forest stands and landscapes which can result in changes of forest structure and function that are evident for decades to centuries [1]. These disturbance effects are dependent on the spatial scale examined, as well as the spatial heterogeneity of the disturbance: the death of a few trees can lead to significant change at the scale of 100 m2 , but may be viewed as part of “normal” mortality at the scale of more than 1 ha. The coastal forests of Oregon provide an example of the heterogeneous, hierarchical mosaic of disturbance In this region, fire and wind are the primary processes causing major episodes of mortality that lead to long-term structural and functional change. Despite influencing over 20 times more area than the 1933 Tillamook burn, the 1962 Columbus Day Storm killed a similar volume of timber: 11 billion board feet versus 10 billion board feet, respectively [6,7,8]
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