Changes in ground flora of forested riparian areas following a disturbance gradient

Forested riparian areas provide unique ecological, economical, and recreational opportunities.  Geomorpholocial stability, wildlife habitat, and water quality all depend on appropriate vegetative species composition and abundance in the riparian zone and uplands.  Riparian areas are easily degraded by certain land use practices; therefore, it is important to understand the short and long term impacts of disturbance in these areas in order to balance watershed health and productivity with resource use.  We investigated the effects of timber harvesting intensity on species composition and biomass of woody and herbaceous plants in managed riparian forests of northern Minnesota three years after harvest.  One of two different harvesting intensities was randomly assigned to the riparian management zone (RMZ) of each site, with an unharvested riparian control located upstream.  The upland of each riparian zone was clearcut.  Differences in light availability among treatments at the understory level were significantly different (P < 0.0001), reflecting a microclimate gradient of increasing light availability with increasing harvest intensity.  Significant pre-to post-harvest differences in the amounts of woody (P = 0.005) and herbaceous (P = 0.0008) biomass were found between the two RMZ treatments, and between treatments and the controls, likely as a response to increased light levels.  Shrub biomass accumulation also increased with harvest intensity.  Preliminary results indicate less regeneration of shade tolerant and long-lived tree species in the “low” RMZ compared to the “medium” RMZ, possibly due to competition with other understory species.  If the trends in the “low” RMZ continue over time, lack of recruitment of shade tolerant and long-lived tree species into the overstory and changes in herbaceous understory composition may decrease the functional width of the riparian area and compromise ecosystem processes.