Forest Investment Account

Abstract of FIA Project Y051062

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Cumulative watershed effects of forestry practices on stream ecosystems

Author(s): Zhang, Yixin; Richardson, John S.
Imprint: Vancouver, B.C. : University of British Columbia, 2005
Subject: Forest Investment Account (FIA), Riparian Ecology, British Columbia, Forests and Forestry, Watersheds
Series: Forest Investment Account (FIA) - Forest Science Program


Watershed effects of past forestry practices accumulate in forested landscapes where operations and management have occurred for long periods of time, and impact on stream biodiversity and ecosystem processes. Theory predicts that cumulative watershed effects of timber harvest relating to forest cover change alter stream habitat structures and influence forest fluvial species richness and abundance. By comparing watersheds with different forest practice history through old growth forests to recently disturbed forests, this study investigated impacts of past forest practices associating with cumulative watershed effects on stream habitats and the present-day diversity of benthic macroinvertebrates in Chilliwack River basin area. We found characteristic difference of stream habitats, which stream reaches in well-matures forests had coarser substrates than did stream reaches in young-growth forests impacted by recent forest practices. A diagram of two principal components separated six reference sites in well-matures forests from five test sites in young-growth forests. Using a modelling technique - partial least squares projection to latent structures (PLS), we investigated the relationship between multiple environmental variables and ecological responses of benthic communities. Predictive PLS models were developed based on environmental variables at reference stream reaches. Results indicated that reference reaches with high species richness and high relative abundance of benthic invertebrates were characterized by low volume of LWD and low amounts of LWD in fresh or heavily decayed conditions, low accumulation level of FPOM and CPOM on stream substrate. These PLS models were modified by excluding the variable of mean forest age, and were used to predict species richness and relative abundance of benthic communities, as well as different functional feeding groups at the test sites, which were impacted by recently past forestry practices. The residuals, i.e. differences between observed and predicted values, were used to evaluate impact strength of mean forest age at a watershed scale. A significant impact of past forestry practices was detected on both species richness and relative abundance of benthic invertebrates at the test sites. Species richness and the relative abundance at the test sites were significantly lower than expected. Thus, the funding of this study provides evidence for the importance of past forest management practices associating with cumulative watershed effects in influencing habitat alteration and determining present-day stream biodiversity. This study also indicates that the predictive model approach combining with intensive field observation has potential strengths on predictive cumulative watershed effect assessment for forest biodiversity conservation and sustainable managements.
Yixin Zhang and John S. Richardson

For further information, please contact Yixin Zhang, University of British Columbia (

Updated September 08, 2005 

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