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

Abstract

The 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.

Dynamics of large wood added to Midwestern USA streams

“…Our results also suggest that high discharge events (e.g., >1.5 times mean discharge) are more important than mean discharge when active channel dimensions are less than log dimensions. A similar but shorter study in a shallow Chilean stream, but with a width and discharge higher than our study streams, also found that wood was not mobilized until a major discharge event (Iroumé et al, 2020). In larger systems, however, log mobilization during even more frequent floods may be a significant consideration for wood addition or other restoration activities (Wohl et al, 2016).…”

Fluvial transport of coarse particulate organic matter in a coastal mountain stream of a rainy‐temperate evergreen broadleaf forest in southern Chile

“…Main forest types in the Erlenbach torrent (coniferous) and VZ (broadleaved) are perennial, and litterfall rates, which significantly contribute to CPOM yields (Findlay et al ., 1997), are almost identical (0.36–0.37 kg CPOM per m 2 and year). LW load was not very different in both streams and accounted for 7.9 m 3 /100 m in the Swiss Erlenbach (Jochner et al ., 2015) and between 6.2 and 11.6 m 3 /100 m after three surveys spanning 10 years in the VZ (Iroumé et al ., 2020). We can assume that the contribution of comparable LW loads to CPOM in terms of OM retention and smaller logs floating downstream, or the deteriorating rates of wood pieces into CPOM‐size particles (Vera et al ., 2014; Bunte et al ., 2016), might not be too different between these two streams, with similar wet and temperate climatic conditions.…”

“…Annual and decadal CPOM loads were normalized by forested catchment area (in this case, the entire catchment) and by the area of a seam along the stream banks together with the stream surface. For the seam along the stream banks we arbitrarily set a 10.6 m buffer for the width of the active channel (which is the mean width of the 1557 m segment used in this stream to study LW dynamics, see Iroumé et al ., 2020) plus 25 m along both banks for third‐order stream segments, and 3 m per channel width plus the same 25 m each side for first‐ and second‐order stream segments. With these assumptions, the area of a seam along the stream banks plus the stream surface was calculated to be 21 ha.…”

“…and the appearance of Tepualia stipularis (tepú). Thorough descriptions of the characteristics of these forests and specially of the vegetation of the riparian area are available in Iroumé et al (2010Iroumé et al ( , 2020. The native Chusquea quila bamboo species is an important constituent of the understory of the riparian forest (González and A. IROUMÉ ET AL.…”

Spatial Distribution of Drifted-wood Hazard following the July 2017 Sediment-hazards in the Akatani river, Fukuoka Prefecture, Japan

“…The DW is recruited (a) from forested and wooded hillslopes notably from debris flows and landsides (e.g., Benda and Sias, 2003), (b) from the river-banks and (c) from previously deposited DW across the floodplain, and the recruitment is often related to planform changes due to high-energy event erosion such as flashfloods (e.g., Lucia et al, 2018). Often, it is not a linear process, and the drifted wood will have variable residence and delivery time in the catchment depending on the hydro-geomorphologic and landcover characteristics of a catchment (e.g., Iroume et al, 2020).…”