Signatures of autogenic epiphyte succession for an aspen chronosequence

Abstract

Question Is the succession of cryptogamic epiphyte communities (lichens) consistent with autogenic processes and life‐history trade‐offs? Location Northern Britain (Scotland). Methods We subsampled epiphytes from the lower bole of aspen (Populus tremula L.) and tested for two signatures in community succession. First, we used resampling among microhabitats to estimate species richness (sR) and species density (ρ) for trees along a chronosequence. We tested sR and ρ against patterns consistent with autogenic and life history‐mediated succession. Second, we constructed null model communities from a regional species pool to test for significant dispersion in multivariate trait diversity. Results We show that species density (ρ) declined along the tree age chronosequence. This decline was consistent with the observed unimodal pattern in species richness (sR) for a habitat that has a non‐stationary area (increasing lower bole habitat area with tree age). We found under‐dispersed trait diversity for younger trees, tending towards equilibrium between the regional species pool and local community for mid‐ and older‐aged trees. Over‐dispersion was evident for communities with the highest species richness. Conclusions We conclude that lichen epiphyte communities undergo succession structured by autogenic processes (interference competition) and may also be structured by life‐history trends, e.g. the competition–colonization trade‐off. However, we suggest that the stochastic extirpation of competitive and dominant species may allow pioneer species to persist in the long term, explaining equilibrium in trait diversity observed for older‐aged trees.

Trees as habitat islands: Temporal variation in alpha and beta diversity in epiphytic laurel forest bryophyte communities

“…hump‐shaped models reported in previous studies for epiphytes (contrast e.g. Johansson et al., ; Nascimbene et al., ; Ellis & Ellis, ). As tree size, and hence, habitat availability, increases with tree age, a clearer signal might emerge from analyses of species densities at the scale of tens of cm 2 rather than total species richness at the scale of entire trees (Ellis & Ellis, ).…”

“…The quadratic product of time ( T 2 ), which was expected to significantly contribute to the observed variation in species richness if community assembly is progressively constrained as trees age, was included in some, but not all of the best models (ΔAICc < 2), thus contributing less to explain the observed patterns in epiphyte species richness as compared to T . While a hump‐shaped relationship has recurrently been reported in community‐level succession studies (for review see Ellis & Ellis, ), the lack of a clear signal of community saturation reflects the contrasting support for monotonic vs . hump‐shaped models reported in previous studies for epiphytes (contrast e.g.…”

“…Johansson et al., ; Nascimbene et al., ; Ellis & Ellis, ). As tree size, and hence, habitat availability, increases with tree age, a clearer signal might emerge from analyses of species densities at the scale of tens of cm 2 rather than total species richness at the scale of entire trees (Ellis & Ellis, ). Host‐tree height, which can be considered as a surrogate for the extent of the colonizable area, was, however, not included in the best‐fit models for LSTs, and in only one of the three competing models for ESTs, with a low contribution to this model.…”

“…Second, the role of competition in shaping bryophyte communities during colonization has been questioned (Wilson, Steel, Newman, & Tangney, ). In this context, the unbounded relationship between epiphytic species richness and tree age (Boudreault, Gauthier, & Bergeron, ; Fritz, Brunet, & Caldiz, ; Johansson, Rydin, & Thor, ; Jönsson, Thor, & Johansson, ; Kantvilas & Jarman, ; Lie, Arup, Grytnes, & Ohlson, ; Thor, Johansson, & Jönsson, ; but see Ellis & Ellis, ; Johansson et al., ; Nascimbene, Marini, & Nimis, ) has been interpreted in terms of the unrestricted increase in species richness in the absence of competition in unsaturated communities.…”

Testing the competition–colonization trade‐off with a 32‐year study of a saxicolous lichen community

“…However, this trade-off has never been explicitly tested, perhaps because documenting competitive outcomes in lichen communities requires long-term data. Indirect evidence for such a trade-off exists in the form of observations of young forests being dominated by species with good dispersal abilities (e.g., Gjerde et al 2012) and a unimodal diversity patterns over a chronosequence of epiphytic lichens on aspens (Ellis and Ellis 2012). Interestingly, other studies of lichen colonization found a relationship between mode of reproduction and colonization ability (Lo¨bel et al 2009, Johansson et al 2012).…”