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

MOLLY JEAN FERRY, University of Nevada - Reno, Department of Natural Resources and Environmental Science

JEANNE CHAMBERS, Research Plant Ecologist, USFS Rocky Mountain Research Station

DR. PETER WEISBERG, Assistant Professor of Landscape Ecology, University of Nevada - Reno, Department of Natural Resources and Environmental Science

Abstract:

This 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. Four watersheds across the central Great Basin characterized by small watershed size, low relief, and narrow valley floors were sampled. Streams were relatively high stream gradient, channel bed material was coarse to fine-grained and bedrock geology varied among watersheds. 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?

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.

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.

Funding provided by: Great Basin Ecosystem Management Project of the USFS Rocky Mountain Research Station

Project duration: 2008 - 2010