Assessing the importance of a dominant understory shrub, rhododendron maximum, to riparian forest and headwater stream ecosystems of the southern Appalachian mountains
Dudley, Maura Patricia
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As species composition changes rapidly worldwide in response to anthropogenic stressors, there is a growing need to understand how species loss will affect ecosystem function. Human activities (e.g., introduction of species-specific pests and pathogens) can lead to loss of individual species from ecosystems, and the consequences of this loss can be significant and irreversible if the extirpated species is dominant, structurally unique, and without a functional substitute. Large-scale experimental manipulations of species presence are needed to proactively identify dominant species, and quantitatively assess how their loss might alter ecosystem function. Study of dominant riparian plant species, located at the interface between terrestrial and headwater stream ecosystems, may be especially informative because of their potential role in structuring both systems. In the southern Appalachians, managers propose to selectively remove riparian thickets of a dominant evergreen shrub, Rhododendron maximum, with the goal of increasing hardwood tree recruitment. The objective of this dissertation is to examine how loss of riparian rhododendron could alter ecosystem function in riparian forest and headwater stream ecosystems of the southern Appalachians. First, I use a detailed review of previous studies from forest ecosystems to document rhododendron’s unique role in structuring, stabilizing, and modulating riparian forest ecosystems. Then, using an experimental reach-scale removal of riparian rhododendron (300 m), I determine how the loss of rhododendron affects stream ecosystems by comparing pre-removal (1 yr) with post-removal (1, 2, 3 yr) measurements in response to two different rhododendron removal methods: (1) Cut+Remove; and (2) Cut+Burn. My findings indicate that loss of rhododendron leaf litter inputs decreases the quantity and increases the quality of summer detrital food resources. Finally, macroconsumer exclusion experiments, nested within a rhododendron removal (Cut+Burn) and control reach, enabled me to quantify the importance of crayfish in controlling algal growth through top-down processes that mediated algal response to loss of rhododendron, thus explaining why algal standing stocks remained unchanged despite large declines in canopy cover. Together, these studies provide strong evidence that rhododendron plays an essential role in structuring both riparian forest and headwater stream ecosystems.