Bryophytes in fir waves: Forest canopy indicator species and functional diversity decline in canopy gaps

Abstract

Co-ordinating Editor: Tohru Nakashizuka Abstract Aims: Bryophytes can cover three quarters of the ground surface, play key ecological functions, and increase biodiversity in mesic highelevation conifer forests of the temperate zone. Forest gaps affect species coexistence (and ecosystem functions) as suggested by the gap and gapsize partitioning hypotheses (GPH, GSPH). Here we test these hypotheses in the context of highelevation forest bryophyte communities and their functional attributes. Study Site: Spruce–fir forests on Whiteface Mountain, NY, USA. Methods: We characterized canopy openness, microclimate, forest floor substrates, vascular vegetation cover, and moss layer (cover, common species, and functional attributes) in three canopy openness environments (gap, gap edge, forest canopy) across 20 gaps (fir waves) (n = 60); the functional attributes were based on 16 morphologic, reproductive, and ecological bryophyte plant functional traits (PFTs). We tested GPH and GSPH relative to bryophyte community metrics (cover, composition), traits, and trait functional sensitivity (functional dispersion; FDis) using indicator species analysis, ordination, and regression. Results: Canopy openness drove gradients in groundlevel temperature, substrate abundance and heterogeneity (beta diversity), and understory vascular vegetation cover. The GPH was consistent with (a) the abundance patterns of forest canopy indicator species (Dicranum fuscescens, Hypnum imponens, and Tetraphis pellucida), and (b) FDis based on three PFTs (growth form, fertility, and acidity), both increasing with canopy cover. We did not find support for GPH in the remaining species or traits, or for GSPH in general; gap width (12–44 m) was not related to environmental or bryophyte community gradients. Conclusions: The observed lack of variation in most bryophyte metrics across canopy environments suggests high resistance of the bryophyte layer to natural canopy gaps in highelevation forests. However, responses of forest canopy indicator species suggest that canopy mortality, potentially increased by changing climate or insect pests, may cause declines in some forest canopy species and consequently in the functional diversity of bryophyte communities.