Long‐term large wood load fluctuations in two low‐order streams in Southern Chile

Abstract

AbstractThe eco‐hydrogeomorphic significance of large wood (LW) and its potential for increasing downstream hazards during extreme floods have been widely recognized. We used LW data collected for a 10‐year period from the two low‐order streams of Pichún (Pi) and Vuelta de Zorra (VZ) in Southern Chile to (a) determine if the abundance and dimensions of individual LW pieces change with time, (b) quantify wood load fluctuations during the 10‐year period, and (c) assess the role of LW recruitment from the riparian forests to explain wood load fluctuations during the study period. Nine years after the first survey, the number of LW pieces in Pi and VZ diminished by 60 and 40%, respectively. Despite the reduction in these numbers, in Pi, the LW dimensions did not change significantly during the study. In VZ, the dimensions exhibited statistically significant differences, despite being within the same class. In both catchments, the LW load fluctuated during a 10‐year period, but the drivers of change differed. Although tree toppling was the recruitment mechanism responsible for LW in both stream cases, the high wood load measured in Pi at the beginning of the study suggested massive tree recruitments before the first survey, followed by wood exports which were higher than inputs in the subsequent 10‐year period. In VZ, LW load decreased during the first 9 years (mean annual rate of ~9.2 m3 year−1) and then increased by ~12.1 m3 year−1 in year 10. At VZ, the inputs consisted of single trees that were recruited from the riparian area and by upstream flotation, while exports occurred by downstream fluvial transport. Wood inputs and exports occurred asynchronously and led to LW load fluctuations at decadal and annual intervals. Land management and tree species thus exert a major influence on wood inventory and budget in streams. © 2020 John Wiley & Sons, Ltd.