Elevational Distributions of Forest Floor Mosses and a Test of Competition at Two Elevations in Western Oregon

Factors controlling mosses on the forest floor in western North America are poorly understood. We examined elevational distributions for six of the most abundant large forest floor mosses; based on those distributions, a transplant experiment of two species evaluated if interspecific interactions can be mediated by climatic context. Mosses had species-specific elevational profiles, with Rhytidiopsis robusta more prominent at higher elevations, while Hylocomium splendens, Kindbergia oregana, Rhytidiadelphus loreus, and Rhytidiadelphus triquetrus were more prominent at lower elevations. Homalothecium megaptilum was bimodal, peaking at middle and low elevations. We selected Rhytidiadelphus triquetrus and Rhytidiopsis robusta for a transplant experiment because each is prominent at different elevations and they are similar in stature. Moss mat squares cut from the forest floor at middle elevations were transplanted in a single- or mixed-species pattern at two sites, one high elevation and one low elevation. We recorded changes in percent cover within the squares over one year as well as outgrowth onto bare soil and litter. Hypothesized relative species performances based on elevational distributions were mostly not supported. The low-elevation associated species (Rhytidiadelphus triquetrus) outperformed the high-elevation species (Rhytidiopsis robusta) at the high-elevation site, both in a mixture and as a monoculture. At the lower site, Rhytidiadelphus triquetrus grew well in a mixture, but the monoculture declined. Furthermore, Rhytidiopsis robusta grew faster at low elevation than at high, both in a mixture and monoculture, despite being more abundant at high elevations. Poor performance of both species at high elevations raises interesting questions about what factors limit moss mats in general at higher elevations in the Cascade Range. Key Points Rhytidiopsis robusta frequents higher elevations; Hylocomium splendens, Kindbergia oregana, Rhytidiadelphus loreus, and R. triquetrus frequent lower elevations. Elevational distributions of dominant forest floor moss species do not predict responses to transplantation to sites at different elevations. The generally poor performance of transplants to higher elevations and lower abundance of forest floor mosses at higher elevations raise the question of what factors limit these moss mats at higher elevations in the Cascade Range.