Substrates mediate responses of forest bryophytes to a gradient in overstory retention

Abstract

Forest bryophytes are sensitive to the disturbances and environmental changes associated with forest management. We asked whether the substrates on which bryophytes grow mediate responses to exposure following canopy removal. We measured bryophyte cover and richness in 0.1 m2 quadrats on the forest floor, decayed logs, and tree bases along a gradient of dispersed overstory retention (100%, 40%, and 15% of initial basal area) 7 to 8 years after harvest of mature Pseudotsuga forests. Cover, local richness, and, to a lesser degree, species evenness declined steeply across the retention gradient on decayed logs and tree bases but not on the forest floor. Liverworts were more sensitive than mosses, particularly on decayed logs and on the southwestern aspects of trees (>97% declines in cover under 15% retention). Richness and evenness at the treatment scale also declined sharply on decayed logs and on the southwestern aspects of trees but changed little or increased under 40% retention on the forest floor. Our results indicate that even moderate levels of dispersed retention cannot sustain the abundance and overall diversity of wood-associated bryophytes in these forests. During regeneration harvests, conservation of these species may require retention of intact forest aggregates in which substrate quality and microclimatic stability can be maintained.