Riparian vegetation guilds: applications to small streams of the interior Pacific Northwest
Riparian flow guilds have been proposed as a method of measuring riparian ecosystem integrity in large alluvial rivers, especially rivers with regulated or overallocated instream flows, or strong potential for flows to be reduced by climate change. The riparian flow guild concept identifies groups of species with similar life-history strategies that result from similar physiological requirements and morphological attributes. These trait-based riparian guilds respond to common environmental stressors within the riparian environment such as flooding, drying, and soil moisture availability. In smaller streams however, where hillslopes often directly connect to stream corridors, fluvial and hydrologic processes may work in tandem with riparian and watershed management to influence guild abundance. Here, we build on the concept of riparian flow guilds by identifying riparian disturbance guilds- riparian guilds whose functional and morphological attributes correspond to multiple disturbance or resource axes. We used 26 environmental tolerance and morphological attributes in 30 species to identify five riparian disturbance guilds: a tall, deeply-rooted coniferous tree guild, a rapidly-growing, drought-plastic shrub guild, a low-stature hydrophytic shrub guild, vegetative reproduction guild, and a short-statured, shade-tolerant, understory shrub guild. We modeled these guilds’ presence and abundance, finding that each guild responded to a variety of climatic, disturbance, and watershed management attributes. Each guild corresponded to climatic and watershed disturbance attributes that were related to the traits most characteristic of that guild. Most notably, we found that complimentary coexisting guilds or mutually exclusive guilds were strong predictors of guild presence and abundance. From these observations, we conclude that riparian disturbance guilds respond not only to environmental variability tied to each guilds attributes, but also niche partitioning in which different life history strategies can coexist under comparable disturbance regimes.