Change in Tree Spatial Pattern After Severe Wind Disturbance in Four North American Northern Hardwood and Sub-Boreal Forests

Chris J Peterson

doi:10.3389/ffgc.2020.00057

Abstract

Growing appreciation for the importance of forest structural features to forest functioning has led to increased focus on tree spatial patterns. While the influence of disturbances on composition and diversity has been well-studied, understanding of how disturbances influence spatial pattern is still far from complete. In particular, very few studies have examined the change in spatial pattern as a result of wind disturbance. Here results are presented for change in spatial pattern after severe wind disturbance in four eastern North American forests. It is hypothesized that, as in other types of disturbances, severe wind will cause an increase in aggregation among survivors, and aggregated distribution of mortality. These trends were indeed observed; at very small distances (e.g. 5-10 m), all four sites showed significantly greater aggregation after wind disturbance. Mortality was also significantly aggregated at three of the four sites, but at different distances. Unexpectedly, at slightly larger distances (e.g. 30-50m), all four sites showed significant decreases in aggregation; it is suggested that this second trend may generalize across disturbance types. A hypothesis proposed by Davis et al. (2005) is that intermediate disturbance severity produces the greatest aggregation; this hypothesis is tested in the four sites reported here, and the hypothesis is rejected. Two refinements to this hypothesis are proposed for further testing in future research.

Highlights

Ecologists, conservationists, and forest managers have increasingly recognized in recent years that forest structural characteristics may be as important as species composition and diversity for forest health, recovery from disturbance, and sustainable management (Franklin et al, 2000; Franklin and Van Pelt, 2004; McElhinny et al, 2005; Mitchell, 2013; Fraver et al, 2017)
Spatial pattern of trees is among the important structural characteristics, with the potential to influence a wide variety of forest processes (Pacala and Deutschmann, 1995; Turnbull et al, 2007; Hart and Marshall, 2009)
The spatial patterns of most of the major species were highly aggregated at the smallest distances (e.g., 5–30 m, Figure 2), with the aggregation typically decreasing at greater distances; this trend is consistent across the four sites

Summary

INTRODUCTION

Ecologists, conservationists, and forest managers have increasingly recognized in recent years that forest structural characteristics may be as important as species composition and diversity for forest health, recovery from disturbance, and sustainable management (Franklin et al, 2000; Franklin and Van Pelt, 2004; McElhinny et al, 2005; Mitchell, 2013; Fraver et al, 2017). A variety of methods for spatial analysis are available, but for analysis of point patterns of trees, Ripley’s K(t) second-order statistic is widely used (Dale, 1999) This method totals all neighbors with a set of increasingly large circles centered on a focal plant, and sums these totals across all focal plants; the result is compared for each size circle (i.e., each sample distance) to a Poisson expectation to test for departure from. For case #2 above, an analogous process was followed, except that the 20 random data sets were constructed based on the observed number of fallen trees (‘mortality’) In both of these cases, the result is a test for significant non-randomness among the mortality or the survivors, constrained by the pre-disturbance tree distribution. Two additional measures were plotted against severity as alternative ways to test the hypothesis: (a) the mean of the three highest NDF values within a given site; and (b) the percent increase in NDF values for the 5 and 10 m distances (these were inevitably the distances with the greatest aggregation among the distances tested)

RESULTS

DISCUSSION

DATA AVAILABILITY STATEMENT