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

Abstract

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. K E Y W O R D S Abies balsamea, bryophyte community, Ellenberg indicator values, environmental gradient, fir wave, forest gaps, functional dispersion, functional trait, gap partitioning hypothesis, indicator species, moss layer 236 | Additional supporting information may be found online in the Supporting Information section at the end of the article. Appendix S1 Whiteface Mountain characteristics: location and appearance of fir wave zone Appendix S2 Supplementary methods. Appendix S3 Supplementary results. How to cite this article: Berdugo MB, Dovciak M. Bryophytes in fir waves: Forest canopy indicator species and functional diversity decline in canopy gaps. J Veg Sci. 2019;30:235-246.

The Roles of the Moss Layer in Mediating Tree Seedling Environmental Stress, Mercury Exposure, and Regeneration in High-Elevation Conifer Forests

“…22°N and 73.54°W) in northeastern United States. Details of the study area were previously described in Berdugo and Dovciak (2019) and elsewhere (for example, Battles and others 2003;others 2017b, 2021). Briefly, Whiteface Mountain is an isolated massif with the summit at 1,485 m above sea level (asl) and well-developed elevational climatic and vegetation zones whose ecology has been extensively studied over the past half century (for example, Sprugel 1976;Battles and others 2003;Aleksic and others 2009;Wason and others 2017b).…”

“…Thus, fine scale microsite conditions affect seedling growth and survival, and thus ultimately tree seedling distributions on the forest floor (Dovc ˇiak and others 2003; Mori and others 2004;Bac ˇe and others 2012). However, various microsite conditions such as substrate type and canopy openness often covary in complex ways (Jonsson and others 2015;Berdugo and Dovciak 2019), underscoring the importance of better understanding how fine-scale processes may affect responses of tree seedling banks to global environmental change (Fisichelli and others 2014).…”

“…To elucidate the role of the moss layer in the dynamics of high-elevation balsam fir forests, we tested four hypotheses across the two contrasting canopy environments (gaps, closed canopies) defining this balsam fir-dominated forest: Seedling density (H1) and seedling recruitment rates into taller seedling size class (H2) are higher on moss than on other substrates; and seedlings established on the moss layer experience less instant physiological stress (H3) and less exposure to mercury (H4) than seedlings on other substrates. We anticipate that the moss layer, with similar thickness and species composition in both canopy environments (Berdugo and Dovciak 2019), favors seedling establishment by creating microsites that mitigate seedling stress and reduce mercury exposure compared to other dominant substrates (leaf litter). We also expect differences in the size of the moss-effect between closed canopies and forest gaps as a warmer microclimate in forest gaps (Berdugo and Dovciak 2019) suggests a more stressful environment for seedlings.…”

“…We anticipate that the moss layer, with similar thickness and species composition in both canopy environments (Berdugo and Dovciak 2019), favors seedling establishment by creating microsites that mitigate seedling stress and reduce mercury exposure compared to other dominant substrates (leaf litter). We also expect differences in the size of the moss-effect between closed canopies and forest gaps as a warmer microclimate in forest gaps (Berdugo and Dovciak 2019) suggests a more stressful environment for seedlings.…”

Island biogeography: an avenue for research in bryology

“…In bryophytes, a humpshape relationship between alpha diversity and elevation has been consistently reported (Henriques et al 2017a, Boch et al 2019, Hernández-Hernández et al 2019, Marline et al 2020 and interpreted in terms of the optimal climatic conditions that prevail at mid-elevation for bryophytes. among other components of diversity, functional diversity has increasingly appeared as a complementary metric to taxonomic diversity (Spitale 2016, Øien et al 2018, Berdugo & dovciak 2019. In oceanic islands, functional diversity correlates with elevation (Henriques et al 2017a).…”