Characterizing the variability of wood in streams: simulation modelling compared with multiple-reach surveys

摘要

Wood abundance in streams is an indicator of its likely geomorphological and ecological importance. However, wood volume estimates can be highly variable, due in part to natural variability and the methodology used to characterize it. We measured wood volume in streams of similar sizes and riparian forest conditions using extended field surveys and simulation modelling. We surveyed a total of 3.1 km along four tributary streams of New Zealand's Waihaha River to obtain an estimate of wood volume in streams with similar basin positions (second order), forest types (podocarp/hardwood forest), disturbance histories (post volcanic eruption of Taupo ca. 180 AD) and stream sizes (2-3 m bankfull width). A `sliding window' analysis was conducted whereby the wood volume was calculated for a 'window' (i.e., reach survey of fixed length) that was progressively moved upstream in 10 m increments. The resulting frequency distributions of wood volume were bimodal and represented the range and relative proportion of reach volumes possible from the wood surveys. The wood volume based on all streams surveyed (23 m(3)/100 m) was equivalent to the 64th percentile of the sliding window distribution, suggesting that a randomly placed study reach would be likely to underestimate wood volume. The bimodal distribution was attributed to the inclusion/exclusion of relatively large (>= 10 m(3)), but rare (0.3 logs/100 m) logs. We also examined the variability of reach-level wood volume estimates (200 m and 400 m) for the Waihaha tributaries using the model OSU Stream Wood. The volume frequency distributions from the simulations were similar to those from the empirical approach, except that they were unimodal. We attribute the unimodal distribution to the greater number of reach-scale estimates used in the simulations (n=2000). The two independent approaches characterized the variation of wood volumes possible for this forest type and stream width. Copyright (c) 2007 John Wiley & Sons, Ltd.