Relationships among hydrogeomorphic processes and the distribution, age and stand characteristics of woody species in Great Basin upland riparian areas

Riparian ecosystems often constitute less than one percent of the central Great Basin landscape but provide critical ecosystem services. Shrubs and trees are fundamental components of these riparian ecosystems that can provide stabilization of sediment and resistance to stream down-cutting. This can promotes ground-water recharge and maintenance of elevated water tables. Fluvial processes shape landforms and riparian woody species distribution across those fluvial landforms. In the arid to semi-arid west, riparian woody species are distributed along vertical elevation gradients within stream reaches (i.e., height above and distance from the channel) and along longitudinal elevation gradients within watersheds (i.e., contributing area and local bedrock) according to their life history and ecophysiological traits. Thus, knowledge of the hydrogeomorphic context at both watershed and stream reach scales is essential for understanding woody species establishment and persistence in riparian ecosystems. This study was located in the central Great Basin across four study watersheds characterized by small watershed size, low relief, and narrow valley floors. Streams were relatively high stream gradient, channel bed material was coarse to fine-grained and bedrock geology varied among watersheds. The study examined the influences of hydrogeomorphic setting and flood disturbance on the distribution, age and stand characteristics of four “key” woody riparian species (Betula occidentalis, Salix exigua, Salix lutea, and Populus tremuloides) with different ecological amplitudes and life history characteristics. Three questions were addressed: (1) How do hydrogeomorphic factors affect the spatial patterning of riparian woody species with different ecological amplitudes and life history traits ? (2) How do the hydrogeomorphic setting and flood events affect establishment of woody species with different ecological amplitudes and life history traits? (3) How does the hydrogeomorphic setting affect the stand structure of woody species with different ecological amplitudes and life history traits? Geomorphic data were sampled along stream cross-sections and included all inset terraces and surfaces from the channel thalweg to the valley floor (upland) surface. Woody vegetation data were sampled along the same cross-sections for size and abundance. Watershed data pertaining to each cross-section were obtained following field data collection (e.g. contributing area, local bedrock). The strongest influences on spatial patterning of the woody species were longitudinal gradients in the watershed and vertical gradients perpendicular to the stream within stream reaches. Ecological amplitudes and life history traits often aligned directly with those gradients. P. tremuloides dominated the upper watersheds where cooler temperatures were coincident with species requirements. Salix spp. dominated moderate elevations and active channel zones in all of the study watersheds demonstrating its tolerance of flood disturbance and inundation. B. occidentalis was most abundant at intermediate to low elevations with constricted valleys and high stream power. B. occidentalis often occurred in reaches exhibiting channel incision and lowering water tables, as indicated by steep bank angles and high entrenchment ratios, and had low recruitment. Tree age structures showed a pulse of seedling establishment following regional flooding in 1983/1985. Seedling establishment following the 1983 and 1985 floods was watershed-specific and highly predictable (72.6 %) according to environmental context. Seedling establishment was positively associated with the incised alluvial process zone, small clasts in the channel bed, low stream gradients, and a high number of inset stream terraces. As in other riparian areas of the semi-arid west woody species seedling establishment depended on bare, moist alluvial substrates with constant soil moisture provided by fine-grained bed material. Woody species stand characteristics were sensitive to geomorphic process zone type and specific watershed characteristics. Young, low density stands and new establishment due to flooding were strongly related to the incised alluvial process zone and specific watershed characteristics. Riparian succession and stand structure were only slightly related to geomorphic variables in these watersheds. The study watersheds differ in relative sensitivity to disturbance, but all are in an incisional phase. The 1983/1985 flood resulted in widespread stream incision and reinitiated successional processes. Flood effects were most pronounced in the alluvial process zone which is characterized by active deposition and erosion and in San Juan Canyon which has volcanic lithology, flashy flows and compounding perturbations (roads in valley bottom, beaver dams). Abundance of newly established stands was low for B. occidentalis. Because of the location of B.occidentalis in areas prone to flood disturbance and incision, this species has generally low recruitment and is of management concern