Cumulative watershed effects of forest operations on stream habitat and biodiversity

Theory predicts that cumulative watershed effects of timber harvest alter stream habitat structure and influence fluvial species richness and abundance. By comparing watersheds with different forest practice histories, we investigated the impacts of past forest practices associating with cumulative watershed effects on streams ecosystems in Chilliwack River basin area. We found characteristic differences of stream habitats being related to past forestry operation. Stream reaches in well-mature forests had coarser substrate than stream reaches in young-growth forests impacted by recent forest practices.  A diagram of two principal components separated reference sites in well-mature forests from recently impacted sites. 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. Through PLS modelling and field-survey data analyzing, we found that reference reaches with high benthic species richness and high relative abundance were characterized by low volume of large woody debris, less FPOM and CPOM accumulated on substrate. A significant impact of past forestry practices was detected on both species richness and relative abundance of benthic invertebrates at the test stream sites that were disturbed by recent forest operations. Species richness and the relative abundance of benthos at the test sites were significantly lower than the PLS model expected. The overall biomass of invertebrate communities at recently disturbed sites was significantly lower than that at the reference sites.  The results provide evidence for the importance of past forest practices associating with cumulative watershed effects in influencing habitat alteration and determining present-day stream biodiversity. This study suggests that the predictive model approach combining with intensive field observation has potential strength on assessment of cumulative watershed effect for forest biodiversity conservation and ecosystem management.