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

Many studies on cool, forested streams have shown that removal of ripa rian canopy leads to higher incident radiation, blooms in algal and ma croinvertebrate populations, and concomitant increases in salmonid abu ndance. In warm, high-elevation desert streams, however, an open canop y may not increase salmonid density. Our seven study reaches on 3rd-or der tributaries of the John Day River in eastern Oregon included ripar ian areas ranging from denuded, heavily grazed streambanks to intact c onifer 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 (Oncorh ynchus mykiss) and sculpin (Cottus spp.) decreased significantly with greater incident radiation and higher stream temperatures, although ma ny warmwater cyprinids increased in abundance in unshaded sites. Perip hyton standing crops (g ash-free dry mass/m2) closely tracked solar in puts and was, in turn, strongly positively correlated with biomasses o f total invertebrates and of grazers. Collector, shredder, and predato r biomasses, and numerical abundances of all invertebrate groups, did not change with canopy density. The abundances of chironomids and baet ids were unrelated to increases in light or algal resources, in contra st to studies in Cascades and Coast Range streams where irruptions of these taxa occurred in canopy openings. In our streams the large-bodie d caddisfly Dicosmoecus accounted for the higher total invertebrate bi omass 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 f rom the analysis. Dicosmoecus are consumed infrequently by juvenile tr out or other small fish species common in John Day tributaries; conseq uently, extensive openings in the canopy appear to produce few advanta ges to upper trophic levels in these streams.