Development of spatial pattern in large woody debris and debris dams in streams

Abstract

The spatial distribution of large woody debris (LWD) in streams was evaluated using Neighbor K statistics, following extensive wood deposition from an ice storm in the eastern Adirondack Mountains (New York). Two years after wood deposition, we surveyed individual pieces of LWD in one stream and surveyed debris dam locations in eight streams within the ice storm area. To examine the linear pattern of debris dams within a stream, we used a one-dimensional version of Ripley's K, a second-order statistic that evaluates the spatial pattern of points within a landscape. Both aggregated and segregated (regularly spaced) distributions of wood were identified. Individual pieces of LWD were aggregated at spatial extents ranging from 0 to 40 m and were segregated at spatial extents ranging from 80 to 100 m. In two streams, we found that debris dams were segregated at distances ranging from 100 to 300 m relative to randomly chosen locations, but debris dams showed no significant spatial pattern in six other study streams. Previous studies of wood distribution in streams have not observed segregated distribution patterns. Spatial segregation of debris dams in the study area likely occurred in response to regularly spaced stream features or processes that allow movement of individual pieces of LWD toward more stable accumulation points. Neighbor K statistics can be used to identify and describe spatial pattern in large woody debris, and such patterns can be used to help evaluate and identify processes responsible for their generation.