Plant diversity and effects of harvesting on riparian reserves in forested landscapes

Background/Question/Methods

Managed forests are semi-natural landscapes that comprise a matrix of vegetation over large areas in the Pacific Northwest (PNW). Embedded within this landscape are numerous sites with physical and biological characteristics distinct from the surrounding matrix, including non-forested (rocky balds, meadows, wetlands) and forested (riparian buffers) habitats. In many cases, these areas are designated as reserves and little management activity occurs within them. These reserves can make disproportionately large contributions to local and regional biodiversity. Previous research in managed forests found that reserves contained 186 species of plants, 42% of the managed forest flora in only 7% of its area. Riparian buffers are the most common type of reserve in PNW managed forests, but little is known about the impacts of upslope logging to the ecological characteristics of these buffers. We characterized and compared forest understory vegetation in riparian buffers and adjacent managed stands along small streams at 70 sites in western Washington using a chronosequence design. Each site consisted of a 50-70 year old riparian buffer and an upslope managed stand. Managed stands were categorized into 7 age classes (1-2, 3-4, 5-6, 7-10, 11-30, 31-50 and 51-70 years since harvest). Plant cover and species richness were recorded at each site. Community composition, species richness, and total plant cover were analyzed using ANOVA, PERMANOVA, and NMDS. We were particularly interested in the effects of stand type (buffer or upslope), age class, and the interaction between these factors.

Results/Conclusions

Species composition differed significantly for main effects (stand type and age class) and their interactions. Upslope harvesting did not affect species richness within riparian buffers. Plant cover in riparian buffers peaked at 82% 7-10 years after upslope areas were harvested; a pattern similar to, but more subtle than, what occurred in upslope stands. Our data indicates that riparian areas along small forested streams support a unique understory plant community and that this community is temporarily altered by upslope forest harvesting activities. Further research on other reserve types is needed to develop a more comprehensive understanding of role these locations play in maintaining plant diversity in managed forest landscapes.