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

摘要

Aims Bryophytes can cover three quarters of the ground surface, play key ecological functions, and increase biodiversity in mesic high-elevation conifer forests of the temperate zone. Forest gaps affect species coexistence (and ecosystem functions) as suggested by the gap and gap-size partitioning hypotheses (GPH, G(S)PH). Here we test these hypotheses in the context of high-elevation 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 G(S)PH 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 ground-level 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 G(S)PH 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 high-elevation 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.