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Postharvest windthrow and recruitment of large woody debris in riparian buffers Bahuguna, Devesh

Abstract

Large woody debris (LWD) is an important component of forest ecosystems and provides structural complexity to small streams. Riparian buffers are intended to provide long term supplies of LWD, but post harvest windthrow often occurs. To document the impacts of windthrow in riparian buffers and identify the components needed for small stream LWD recruitment modeling, I sampled 39 small streams at the Malcolm Knapp Research Forest (MKRF) and on Vancouver Island. I took two basic approaches. In the small stream experiment at MKRF a series of small clearcuts were harvested in 1998 in a 70 year old second growth stand. I measured LWD in 10m and 30m buffer treatments, and in the unharvested control. I added samples in mature and old-growth stands for comparison. In the second approach, I retrospectively sampled buffers that were exposed by harvesting from 0-20yrs ago on southwestern and northeastern Vancouver Island. In both studies, all logs greater than 7.5 cm diameter at mid-creek, in decay class 1 to 4 that spanned at least part of stream channel width were measured. There was no significant difference in the number of spanning and in-creek logs in 10m and 30m buffer given the short term monitoring of woody debris in the buffers. The majority of windthrown trees were still suspended above the stream channel years after a windthrow event. The height above stream was negatively correlated with log decay class and the buffer age class. The number of logs was higher in immature stands than mature stands. As the stems per hectare in riparian stands increases, so does the frequency of spanning LWD. The frequency of logs in decay classes 3 and 4 was higher in older buffers, and deciduous LWD decayed more quickly than conifers. Interestingly, the log length was found to be shorter in advance stage of decay. Key elements in a conceptual model of LWD recruitment via windthrow are the geometry of initial log position, log size, species and decay rate.

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