Post-harvest windthrow and recruitment of large woody debris in riparian buffers on Vancouver Island

Abstract

Large woody debris (LWD) provides structural complexity to small streams. Riparian buffers are intended to provide long-term vegetation cover and supplies of LWD, but post-harvest windthrow often occurs. To evaluate the impacts of windthrow in riparian buffers and identify the components for a small stream LWD recruitment model, we sampled 26 streams in immature and older stands in wind-exposed areas of southwestern and northern Vancouver Island. These treed buffer strips had been exposed following clearcut harvest of adjacent timber on both sides 1-20 years previously. For stream sections 100 m long in each buffer, all logs greater than 7.5 cm diameter that spanned at least part of stream channel were measured. A total of 658 logs were recorded. Windthrown trees were comparable in characteristics to the trees that made up the buffer. The majority of logs derived from windthrown trees were oriented perpendicular to the stream channel and were suspended above the stream channel. Even 20 years after harvesting, two-thirds of the logs were still suspended above the stream. Logs in older buffers were more decayed, and the decay rate depended on tree species and initial diameter. Log height above stream was negatively correlated with log decay class and time since logging. Log length declined with time since harvest exposure and decay class. Sediment was exposed on upturned roots and within mineral soil pits. The volume of soil retained on upturned rootwads declined over time, but some soil remained even after 20 years. Very little of this exposed sediment was close enough to the creek to result in sediment delivery.

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

“…As reported by Bahuguna et al . (2010, 2012), narrower riparian buffers, such as those at Pi (the width of this stream is smaller than the ‘one site‐potential tree height’ defined in Burton et al ., 2016), may increase short‐term LW inputs to the streams as the remaining riparian trees are more exposed to post‐harvest windthrow. P. radiata logs were part of the 53% of in‐stream LW found in Pi and recorded in the 2008 survey, while this tree species made up only 21% of the basal area of the Pi riparian forest.…”

Decadal‐scale changes in suspended wood after riparian recruitment in managed stands in headwater streams of coastal British Columbia, Canada

“…Therefore, our approach may have added bias in the analysis if any of the situations mentioned earlier prevailed in our sampling. In this study, our objective was to assess general patterns of wood changes in the long term starting from one sampling scheme that was not initially designed for this objective (Bahuguna et al, 2012). Therefore, it is hard to know if any of these conditions is strongly affecting our results.…”

“…However, given the geometry of small stream valleys in the Pacific Northwest, it is more likely that trees longer than channel width will not enter the channel immediately after tree fall. Instead, they tend to stay suspended above the channel in ‘bridge’ positions ( sensu Wohl et al, 2010) for years or decades (Hassan et al, 2005; Bahuguna et al, 2010; Jones et al, 2011; Bahuguna et al, 2012), where they have less influence on stream ecological and hydrogeomorphic processes compared to in‐stream logs (Richmond and Fausch, 1995; Jones et al, 2011).…”

“…However, these management techniques may be insufficient to protect all ecological processes and habitat features of riparian and stream ecosystems. For example, narrower riparian buffers may increase short term LW inputs to the streams as the remaining riparian trees are more exposed to post‐harvest windthrow (Bahuguna et al, 2010; Bahuguna et al, 2012).…”

“…Recently, Bahuguna et al . (2012) studied the LW conversion from suspended to non‐suspended condition using a chronosequence approach, but our study advances our understanding by returning to the same streams to track changes on individual LW pieces over time.…”