Abstract
ABSTRACTIn‐channel large woody debris (LWD) promotes quality aquatic habitat through sediment sorting, pool scouring and in‐stream nutrient retention and transport. LWD recruitment occurs by numerous ecological and geomorphic mechanisms including channel migration, mass wasting and natural tree fall, yet LWD sourcing on the watershed scale remains poorly constrained. We developed a rapid and spatially extensive method for using light detection and ranging data to do the following: (i) estimate tree height and recruitable tree abundance throughout a watershed; (ii) determine the likelihood for the stream to recruit channel‐spanning trees at reach scales and assess whether mass wasting or channel migration is a dominant recruitment mechanism; and (iii) understand the contemporary and future distribution of LWD at a watershed scale. We utilized this method on the 78‐km‐long Narraguagus River in coastal Maine and found that potential channel‐spanning LWD composes approximately 6% of the valley area over the course of the river and is concentrated in spatially discrete reaches along the stream, with 5 km of the river valley accounting for 50% of the total potential LWD found in the system. We also determined that 83% of all potential LWD is located on valley sides, as opposed to 17% on floodplain and terrace surfaces. Approximately 3% of channel‐spanning vegetation along the river is located within one channel width of the stream. By examining topographic and morphologic variables (valley width, channel sinuosity, valley‐side slope) over the length of the stream, we evaluated the dominant recruitment processes along the river and often found a spatial disconnect between the location of potential channel‐spanning LWD and recruitment mechanisms, which likely explains the low levels of LWD currently found in the system. This rapid method for identification of LWD sources is extendable to other basins and may prove valuable in locating future restoration projects aimed at increasing habitat quality through wood additions. Copyright © 2011 John Wiley & Sons, Ltd.
Highlights
The presence of large woody debris (LWD) in streams is associated with an array of positive ecological impacts (Gurnell et al, 2002)
We acknowledge that LWD of many size ranges influences channel morphology and habitat heterogeneity, we focused on channel‐spanning wood as the metric
We successfully developed a method to use lidar data to evaluate potential wood recruitment at the watershed scale and applied this method to assess spatial and temporal variation in potential wood sources in the Narraguagus River basin of coastal Maine, USA
Summary
The presence of large woody debris (LWD) in streams is associated with an array of positive ecological impacts (Gurnell et al, 2002). Studies have shown wood to be an important mechanism in promoting stream habitat heterogeneity through sediment storage and sorting (Beschta, 1979; Assani and Petit, 1995; Thompson, 1995), pool formation (Montgomery et al, 1995; Abbe and Montgomery, 1996) and in‐stream nutrient transport (Bilby, 2003). Owing to the importance of LWD as a habitat‐forming agent, a large literature has developed concerning the ecological role of in‐ stream wood (Gregory et al, 2003) and the physical. The delivery of wood to streams has been studied extensively, with particular regard to the Pacific Northwest, and has been shown to occur by several processes (Benda et al, 2003). The distance of trees from the channel has been shown to be an important factor in LWD delivery
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