Canopy closure after four decades of postlogging riparian forest regeneration reduces cutthroat trout biomass in headwater streams through bottom-up pathways

Abstract

Recovery from timber harvest is widespread across North America, but few studies have evaluated long-term stream responses to riparian harvest. We revisited five stream reach pairs where in 1976, periphyton chlorophyll a, predatory invertebrate biomass, and cutthroat trout (Oncorhynchus clarkii clarkii) biomass were elevated in reaches where canopies were more open following timber harvest. After four decades of riparian regeneration, mean canopy openness, chlorophyll a, predatory invertebrate biomass, and cutthroat trout biomass declined in harvested reaches relative to paired old-growth reference reaches. In one reach pair, the harvested reach remained more open than the control reach. In accordance with the hypothesis that light exerts strong controls on predator biomass via bottom-up processes in these forested headwaters, trout biomass was also greater in the harvested reach in this pair in 2014. Changes in large wood and pool area over this time interval do not account for chlorophyll a, predatory invertebrate biomass, or cutthroat trout biomass responses. These results provide empirical support for conceptual models relating changes in riparian canopy cover to primary production and bottom-up controls on consumer populations.