Bottom-up and top-down factors control ecosystem processes in headwater streams, and changes in riparian vegetation due to management practices and pathogen outbreaks can shift the importance of these controls. In the southern Appalachians, significant mortality of riparian Tsuga canadensis has led to increased growth of the evergreen shrub, Rhododendron maximum. Rhododendron inhibits tree recruitment and growth, and its removal is viewed as a potential management strategy to restore hardwood forests. Initial studies suggest that static measures of stream basal resources respond minimally to rhododendron removal despite changes to bottom-up factors. Here, we examine how top-down control by stream macroconsumers affects ecosystem response to rhododendron removal. We predict that increased light and nutrients created through rhododendron removal will lead to increased rates of algal accrual and leaf decomposition, but that omnivorous crayfish will diminish algal and amplify leaf litter response variables. We conducted 32-day experiments (summer) using paired crayfish exclusion and access plots (n=5), which were nested within 300 m stream reaches of both a reference and treatment (rhododendron cut and burned) stream. Experiments were conducted at three points in time: pre-treatment (Pre), post-cut (Post-YR1) and post-burn (Post-YR2).
Results/Conclusions
As rhododendron canopy cover was removed, benthic sediment increased. Variation in flow-conditions were driven by large inter-annual differences in rainfall, with Post-YR2 having noticeably reduced flow. Both stream reaches showed minimal algal accrual in pre-treatment and Post-YR1, but in Post-YR2 there was a significant interaction effect: crayfish access plots in the cut/burn treatment reach had >21x lower algal accrual than crayfish exclusion plots. Leaf decomposition rates responded more to crayfish exclusion than rhododendron removal. Pre-treatment and Post-YR2, decomposition rates were 2x greater in the presence of crayfish. In Post-YR1, top down effects of crayfish were somewhat reduced in the treatment stream (c.a. 1.25x higher). Our study suggests that crayfish consumers influence how stream basal resources respond to rhododendron removal. Crayfish controlled the increased algal standing crop following rhododendron removal and during low-flow. Leaf decomposition responded minimally to rhododendron removal, with rates declining after one year and returning to pre-treatment rates after two years. Crayfish significantly increased leaf decomposition rates regardless of treatment. Our initial findings may not persist over the long-term if crayfish abundance ultimately declines in response to rhododendron loss, given that rhododendron leaves are an important summer food resource and refugia for crayfish.