Past studies have demonstrated successions of epiphyte communities on trees of progressively larger diameters, suggesting the existence of temporal resource-gradients associated with aging/growing phorophyte hosts. The objective of this study was to identify possible resource gradients associated with the diameter/age of Acer saccharum (Sugar Maple) in northern New York. We determined epiphytic bryophyte cover by species on 102 Sugar Maples (min–max = 11–84 cm diameter at breast height) from 12 Adirondack northern hardwood forest stands. We extracted a 12.6-cm2 bark sample from each tree to analyze in the laboratory for moisture-holding capacity, surface-moisture availability, drying rates, and leachate cation and nitrogen concentrations. We collected throughfall and stemflow from 15 trees (19–79 cm dbh) at a separate site over the course of a growing season and analyzed samples for cation and nitrogen concentrations. Bark mass per unit surface area (g cm-2) was positively correlated with tree diameter, reflecting increasing bark thickness with age. Bark moisture-holding capacity (H2O as % dry mass) was independent of tree diameter, but bark surface-moisture availability (g H2O cm-2 bark) increased with diameter as a result of thickening bark. Bark drying rates were negatively correlated with bark mass (thickness). Cation (Ca2+, Mg2+, K+) concentrations in bark leachate were all positively correlated with tree diameter, but NH4+ and DON concentrations varied independently of tree diameter, and NO3- concentrations were typically below detection limits. Stemflow became enriched 10- to 20-fold with dissolved cations but not with dissolved nitrogen. Percent cover of several mesophytic and calciphilic epiphytes (e.g., Anomodon rugelii, Brachythecium laetum, Neckera pennata, and Porella platyphylla) were positively correlated with cation concentration in bark leachate, bark thickness, and moisture availability, and negatively correlated with bark drying rate. The results of this study are consistent with hypotheses that increased moisture and nutrient availability and slower drying rates of bark on large-diameter trees may account for increasing total cover and species richness of bryophtyes and increasing dominance of mesophytic and calciphilic bryophytes on larger trees. We extend McCune's similar gradient hypothesis with an analogous set of nutrient-based gradients, and offer an alternative mechanism for McCune's original time-based moisture gradient.
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Vol. 26 • No. 1