Effects of thinning buffers and fluvial geomorphology on down wood dynamics in headwater streams in western Oregon, USA
Quantifying the effects of silvicultural practices and geomorphology on the distribution of down wood in headwater streams may be useful for designing strategies that maintain critical stream-riparian habitat features in managed forests. We examined instream wood in 34 reaches at 6 sites in western Oregon from pretreatment through two thinning harvest entries. All sites regenerated following clearcuts 4-6 decades previously. Experimental treatments included three no-harvest streamside buffer treatments (~6 m, 15-m minimum, and 70-m widths) embedded within thinned uplands (1st thin: 200 trees/ha [tph]; 2nd thin: 85 tph). The control consisted of unthinned reference units (~400 tph). Instream wood was measured 1) pretreatment, 2) year 5 post 1st thinning, 3) year 9-13 post 1st thinning, and 4) year-1 post 2nd thinning (year 12-14 post 1st thinning). We developed hierarchical models to examine relations of wood volume to thinning buffer width, geomorphology, zone within stream prism zone (1-summer wet channel, 2-bankfull channel, 3-suspended above bankfull channel) and decay stage (early vs. late) over this 14-year timeframe. Additionally, to characterize the primary zone of origin for instream wood, we examined relationships between in-stream wood volume and the distance to the source of the wood.
There was no evidence for effects of thinning or buffer width on the volume of instream wood. However, volume increased with drainage basin area. Within reaches, volume was related to a three-way interaction among zone, sampling year and decay stage. Wood in late stages of decay was more abundant than early-stage decayed wood within all zones and years. Volume in zone 2 increased substantially from pre-treatment levels after the first thinning (year 5). The lack of treatment effects suggests increases may be caused by downslope movement of wood during storms. The volume of wood for which sources could be identified accounted for over 90% of the total for wood in early stages of decay, with 82% coming from within 15 m of the stream. In contrast, sources could be identified for <45% of the total volume of wood in late stages of decay. However, 85% of this wood came from distances within 15 m of the stream. Our results point to a future deficit of wood in late-stages of decay. The strong relationship between wood inputs and distance to source suggests the greatest potential for managing future instream wood recruitment is within 15 m of headwater streams.