The transition from riparian to upland forest plant communities on headwater streams in the southern Sierra Nevada, California, United States1,2

Abstract

Fixed-width buffer zones on rivers and streams are designed to protect the diverse riparian community and its important function in the ecosystem. However, recent data suggest that riparian areas of some western forests have become more fire prone because of restrictions on fuel reduction treatments within buffer zones. Surprisingly little is known about where and how the plant community transitions from riparian to upland vegetation, but understanding that transition would inform the restoration of riparian forest structure and function and its associated management applications (e.g., prescribed fire or mechanical thinning) that may be necessary to achieve restoration. Using data collected from the Kings River Experimental Watersheds, we assessed the transition in plant structure and composition from riparian near-stream areas to upland locations, in mixed-conifer and red fir forests on headwater streams of the southern Sierra Nevada, CA. Our data strongly support the conclusion that the riparian zone, as evidenced by the riparian plant community, extends beyond 10 m from the stream on these narrow, first- and second-order streams. The herbaceous community at 10 m from the stream was distinct from the upland community and had greater similarity to locations closer to the stream in mixed-conifer forest. Species richness was three to four times greater in riparian areas compared with upland areas, and there was little overlap of the more-abundant herbaceous species. In addition, riparian forests were generally denser, with smaller trees in mixed-conifer forest, but had similar stand structure to upland forests in red fir forest. Differences between mixed-conifer and red fir forests may reflect different departures from the historical fire regime in these two community types. Compared with riparian areas of wetter climates, riparian areas of dry climates, such as the Sierra Nevada, may harbor even greater species diversity relative to nearby upland areas, indicating that buffer zones of restricted management may be justified in these forests if the goal is the preservation of biodiversity. However, perhaps more important, these findings, along with recent literature, highlight the need to identify site-specific goals when undertaking restoration of riparian forests in the western USA: herbaceous biodiversity, fuels reduction, historic tree composition and structure, and/or water quality.