Breakdown of instream wood in low order forested streams of the Southern Chilean mountain ranges

Abstract

We analyzed instream wood depletion or breakdown in terms of wood breakage (defined as the reduction in length) and decay (reduction in diameter) on channel segments of three low-order mountain streams located in southern Chile. We used a unique database, composed of 1049 individually tagged logs, which were measured and their position was georeferenced between 2005 and 2014, and re-measured and re-georeferenced after the rainy winter period of 2015. Results showed that median breakdown at the end of the survey ranged between 5 and 27% and between 10 and 25% for median decay, with highest values of 83 and 72% respectively. While median annual breakage ranged between less than 1 and 4 %·year−1 and median annual decay between 1 and 4 %·year−1. According to our results, breakdown of instream wood is influenced by many factors, the most significant being initial log size (both length and diameter), their orientation, residence time and fluvial transport (i.e. displacement length). Wood density and initial tree height at the time of recruitment were also analyzed, aiming to better understand breakage and decay processes. Results showed that in-channel logs were, as expected, less dense than living trees, and very decayed logs significant loose density compared to fresh logs. The differences between initial log size and potential minimum tree height at the time of recruitment ranged between 29 and 64%. We believe that our findings illuminates on wood depletion processes on low order streams in the Chilean mountain ranges, and can be easily extrapolated to other streams with similar conditions; however, the natural heterogeneity among river environments and hydrological conditions during the survey period may influence breakage and decay processes. Therefore, we encourage other researchers to provide data on breakdown of instream wood from different regions.