Life on a leaf: The development of spatial structure in epiphyll communities

Abstract

1. The spatial structure of biotic communities can be shaped by niche-based or stochastic processes, and the importance of both can change through time. Nichebased processes include neighbour interactions, which can change in intensity and quality as communities develop in dependence of environmental conditions. Epiphylls, miniature communities of liverworts, lichens, algae and fungi on leaves, develop only in relatively moist forests, but their leaf-surface habitat is still characterized by moisture stress, especially in more exposed parts of the forest. As neighbours may alleviate moisture stress, we expected that in forest gaps (dryer, brighter), epiphyll communities would show more positive neighbour interactions, resulting in more clustered spatial patterns, than in closed forest, in accordance with the stress-gradient hypothesis.2. To understand how the processes shaping epiphyll communities change through time and differ between gaps and closed forest, we examined the fine-scale spatial structure within and between epiphyll functional groups (algae, fungi, lichens and liverworts) during 12 months on 60 leaves across 12 sites in a tropical rainforest in Panama. We analysed changes in spatial positions and spatial associations within and between functional groups using spatial point pattern analysis based on repeated photography. Epiphyll densities increased through time for most epiphyll functional groups.However, although epiphyll groups changed their positions through time, the general types of spatial patterns remained similar for most groups: only lichen patterns shifted from mostly random to mostly aggregated through time. We found only random and aggregated spatial patterns within groups, while among groups we additionally found segregated spatial associations, but no consistent temporal trend. Patterns did not differ between gaps and closed forest, thus not supporting the stress-gradient hypothesis. 4. Synthesis. Our results only weakly support the assumption that communityshaping processes change from neutral to niche-based as communities develop. We do provide empirical evidence that epiphyll communities are dynamic

Disentangling the roles of chance, abiotic factors and biotic interactions among epiphytic bryophyte communities in a tropical rainforest (Yunnan, China)

“…In agreement with our hypothesis H4a, co-occurrences involved species pairs from the same community (moss-moss pairs or liverwort-liverwort pairs), whereas non-coincidences involved pairs from different communities (moss-liverwort pairs). Negative interactions leading to spatial segregation were similarly reported in epiphyll communities composed of various communities, but play a minor role among species from the same community (Me zaka et al 2022). This suggests that specific traits of moss and liverwort, or potentially, lineagespecific allelopathic substances, shape their biotic interactions, promoting facilitation among species from the same community and competition among species from different life forms.…”

“…As Mežaka et al . (2022) hypothesized in epiphyllous communities, the contribution of random factors to community composition can be expected to decrease on ageing trees. Following an optimal stage of ontogenetic development of the host tree, during which both primary and late successional species can be found, competition is expected to play an increasing role, as suggested by the decrease in species richness following an optimum reported in epiphytic lichens (Ellis & Ellis 2013).…”

“…The importance of biotic interactions in epiphytic community composition is also expected to vary vertically on the same tree because of variations in light intensity, relative humidity, wind exposure, temperature, and nutrient availability. The stress gradient hypothesis, primarily developed in terrestrial ecosystems (Bertness & Callaway 1994;Qi et al 2018), has most recently been applied to epiphytic communities (Me zaka et al 2022;Spicer & Woods 2022) and provides a framework for predicting when and where interactions among epiphytes may be important. For epiphytes, the outer canopy is a high stress environment characterized by high exposure to UV radiation and large daily temperature and relative humidity fluctuations, wherein facilitation is expected to prevail (Soliveres et al 2015).…”

“…The relevance of facilitation by bryophytes for vascular epiphytes is such that bryophyte cover contributed more to models of vascular epiphyte richness than host tree size, which has been identified as the major predictor of epiphyte richness (Zotz & Vollrath 2003;Zotz & Schultz 2008;Francisco et al 2021). As light exposure and daily variations in temperature and relative humidity decrease towards the tree base, interactions are expected to become more competitive (see Spicer & Woods 2022 for review), especially among epiphytes from different communities, as reported in moss, liverwort, and epiphyllous lichen communities (Me zaka et al 2022).…”

“…2018), has most recently been applied to epiphytic communities (Mežaka et al . 2022; Spicer & Woods 2022) and provides a framework for predicting when and where interactions among epiphytes may be important. For epiphytes, the outer canopy is a high stress environment characterized by high exposure to UV radiation and large daily temperature and relative humidity fluctuations, wherein facilitation is expected to prevail (Soliveres et al .…”