RELATIONSHIPS BETWEEN RIPARIAN COVER AND THE COMMUNITY STRUCTURE OF HIGH DESERT STREAMS

Many studies on cool, forested streams have shown that removal of riparian canopy leads to higher incident radiation, blooms in algal and macroinvertebrate populations, and concomitant increases in salmonid abundance. In warm, high-elevation desert streams, however, an open canopy may not increase salmonid density. Our seven study reaches on 3rd-order tributaries of the John Day River in eastern Oregon included riparian areas ranging from denuded, heavily grazed streambanks to intact conifer forest. Average summer solar inputs to these sites varied from 165 to 2230 megajoules/m2, and stream temperatures were influenced by the density and extent of canopy. Densities of steelhead trout (Oncorhynchus mykiss) and sculpin (Cottus spp.) decreased significantly with greater incident radiation and higher stream temperatures, although many warmwater cyprinids increased in abundance in unshaded sites. Periphyton standing crops (g ash-free dry mass/m2) closely tracked solar inputs and was, in turn, strongly positively correlated with biomasses of total invertebrates and of grazers. Collector, shredder, and predator biomasses, and numerical abundances of all invertebrate groups, did not change with canopy density. The abundances of chironomids and baetids were unrelated to increases in light or algal resources, in contrast to studies in Cascades and Coast Range streams where irruptions of these taxa occurred in canopy openings. In our streams the large-bodied caddisfly Dicosmoecus accounted for the higher total invertebrate biomass observed in exposed sites. These insects composed 55-96% of the total biomass in open reaches but only 0-1.4% in the three most shaded sites. Increases in total invertebrate biomass with increasing light levels or periphyton were not observed when Dicosmoecus were removed from the analysis. Dicosmoecus are consumed infrequently by juvenile trout or other small fish species common in John Day tributaries; consequently, extensive openings in the canopy appear to produce few advantages to upper trophic levels in these streams.