Competitive ability of an epilithic moss, Thuidium tomentosum Schimp., under different light treatments in a subtropical lower montane forest in Puerto Rico

Bryophyte competition in tropical vegetation has been considered a weak shaping drive in the assembly of the communities. Studies on tropical bryophytes considered that changes in species composition are originated mainly by direct interactions with abiotic factors rather than by inter-specific interactions. Here I investigated the competitive ability of Thuidium tomentosum, as well as the importance of its flagelliform branches and the competitive hierarchy with two epilithic bryophyte species: Ceratolejeunea cornuta and Leucoloma cruegerianum. Relative growth rate (RGR) of T. tomentosum was compared across three species combinations, three levels of light (full light, intermediate light and full shade), and two clipping levels (clipped and non-clipped flagelliform branches) during a nine months in a secondary forests of the Ciales municipality. Bryophyte hierarchies were estimated using standardized growth rates during three months in the Toro Negro Commonwealth Forest. Changes in RGR and relative competitive performance (RCP) were analyzed with a mixed model ANOVA for repeated measures. The combination of T. tomentosum with only one of the species showed an effect coherent with the resource competition hypothesis, but the combination of the three species together showed a positive interaction coherent with facilitation hypothesis (F = 8.62, p < 0.001). Light intensity modified the competitive ability of T. tomentosum with higher growth rates under full light and intermediate light than in complete shade (F = 2.76, p = 0.0076). Flagelliform branches seemed to have a role in the lateral expansion of T. tomentosum under different light conditions (F = 3.73, p = 0.025) but did not enhance its competitive ability (F = 0.08, p = 0.98). The competitive ability of T. tomentosum was favored by the presence of C. cornuta and L. cruegerianum and at the same time was reduced in the presence of a sole species. Thuidium tomentosum showed the best competitive performance and was at the top of the hierarchy. Ceratolejeunea cornuta and L. cruegerianum interchanged their role under different light conditions (F = 11.7, p = 0.001) indicating that competition is an important element structuring the epilithic bryophyte communities of mountain forests in Puerto Rico. Changes in the competitive hierarchies across environmental gradients are a new approach to explain the coexistence of numerous bryophyte species in tropical mountains.