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 m 2 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. Résumé :Les bryophytes forestières sont sensibles aux perturbations et aux changements environnementaux reliés à l'aménagement forestier. Nous nous demandons si les substrats sur lesquels se développent les bryophytes modifient leurs réactions à l'exposition provoquée par l'enlèvement du couvert forestier. Nous avons mesuré le couvert et la richesse des bryophytes dans des quadrats de 0,1 m 2 établis sur la couverture morte, sur des billes de bois décomposées et sur la base des arbres le long d'un gradient de rétention dispersée du couvert forestier (100, 40 et 15 % de la surface terrière initiale), de 7 à 8 ans après la coupe de forêts matures de Pseudotsuga. Le couvert, la richesse locale et, dans une moindre mesure, la régularité des espèces ont fortement diminué sur les billes décomposées et la base des arbres le long du gradient de rétention, mais pas sur la couverture morte. Les hépatiques étaient plus sensibles que les mousses, particulièrement sur les billes décomposées et du côté sud-ouest des arbres (diminution du couvert supérieure à 97 % sous une rétention de 15 %). La richesse et la régularité à l'échelle du traitement ont aussi fortement diminué sur les billes décomposées et le côté sud-ouest des arbres, mais ont peu changé ou ont augmenté sous 40 % de rétention sur la couverture morte. Nos résultats indiquent que même un niveau modéré de rétention dispersée ne peut soutenir l'abondance et la diversité totale des bryophytes associées au bois dans ces forêts. Lors des coupes de régénération, la conservation de ces espèces peut nécessiter la rétention de bouquets intacts de forêt dans lesquels la qualité du substrat et la stabilité microclimatique peuvent êt...

Bryophyte abundance, diversity and composition after retention harvest in boreal mixedwood forest

“…A key result of our study, consistent with our initial hypothesis, was that increased levels of retention moderated the impact on bryophyte abundance, richness, diversity and composition, as compared to clear‐cuts. This is in agreement with previous studies (Caners et al., ; Dovčiak et al., ; Halpern, Dovciak, Urgenson, & Evans, ). In addition, however, our work reveals that effects of retention harvesting differ across the four dominant merchantable stand types in the western boreal forest.…”

Slow recovery of bryophyte assemblages in middle-aged boreal forests regrown after clear-cutting

“…The most negatively affected groups in this, as in many other studies (e.g. Halpern et al, 2014), were liverworts and species growing mainly on tree-related substrates (mostly decomposing wood, but also bark of tree bases and of trunks of broadleaved trees). In upland forests also bryophytes living on the ground and on rocks and boulders had lower numbers of species in the treatment stands (Dynesius et al, 2009).…”

“…Microclimate has repeatedly been shown to be important for bryophytes (in particular liverworts) growing on deadwood or tree bases (e.g. Hylander et al, 2005;Halpern et al, 2014). In this study, a less prominent role of climate is indicated by the equally large treatment effect in upland and stream-side stands ( Fig.…”

Human-sensitive bryophytes retreat into the depth of forest fragments in central European landscape

“…This suggests that fragments of open oak thermophilic stands have best preserved forest habitat and the species that require it, regardless of long-term human management which has been moderate over the last few decades. In any case, some forest bryophyte species require shaded conditions with a stable moisture microclimate, which is found only in densely forested habitats (Löbel et al 2006;Halpern et al 2014). These species are rare or absent in forest fragments in our study area with long history of management, as are other bryophytes associated with remnants of unmanaged forests in Europe (Ó dor et al 2006).…”