Interplay of above‐ and belowground resource limitations: a competition–facilitation shift maintains species coexistence

Abstract

Forest ecosystems are commonly characterised by a hierarchy of resources. During a disturbance of a forest community, increased light availability in the understorey can support competitive interactions at the expense of facilitation. This may overwhelm the role of belowground resource heterogeneity in maintaining species coexistence and so result in biotic homogenisation of a site. We re-surveyed species composition while estimating interspecific interactions along a microtopographic (moisture) gradient of a temperate swamp forest after increasing the availability of light into the undergrowth. We measured temporal changes in species diversity, compositional dissimilarity measures and functional traits, while belowground resource heterogeneity remained unchanged over time. To explain observed changes in terms of prevailing interspecific interactions, we evaluated temporal changes in the competition-facilitation balance. The productivity of understorey vegetation increased strongly over the five years of the study. This was accompanied by an increase in species diversity and the maintenance of compositional dissimilarity. The relative importance of competitive and facilitative interactions along the microtopographic gradient did not change over time. Rather than an overall biotic homogenisation of a site, we observed a spread of resource acquisitive species within the resource-rich sites and of stress-tolerant species within the resource-limited sites. We have shown that the temporary increase in productivity did not mitigate the effects of belowground resource heterogeneity on plant species turnover, which is likely maintained by contrasting interspecific interactions prevalent in different parts of the resource gradient. This suggests contrasting community assembly processes maintain diversity and productivity even in a local community.

Soil toxicity and species dominance rather than nutrient availability drive plant species richness in swamp forests of Central Europe

“…Our results suggest that the importance of environmental harshness may depend on habitat type. Contrary to the original expectation that short moisture gradients in swamp vegetation do not provide enough variability in soil toxicity (Lavers & Field, 2006), a characteristic feature of temperate swamp forests is the intra‐ and inter‐annual dynamics of water levels (Douda et al., 2021; Hulík & Douda, 2017). These temporal changes enhance the ranges and combinations of moisture conditions and nutrient availability in the local plant community (i.e.…”

“…Pielech et al., 2015). This situation commonly leads to the release of competitive pressure exerted by canopy trees and increase in light transmission to understory vegetation (Douda et al., 2021; Reich et al., 2012). In our case, overstory canopy shading and waterlogged‐induced toxicity hampered the decomposition rate of organic matter, as indicated by the higher C/N ratio, however without a significant impact on the richness pattern.…”

“…In forest ecosystems, this pattern is linked to variation in light and soil conditions (nutrients and water), which strongly shape interspecific interactions and species diversity (Douda et al, 2021;Kumar et al, 2018;Reich et al, 2012). The RBCM also integrates the potential role of environmental toxicity (originally referred to as environmental harshness) on community richness (Lavers & Field, 2006).…”

“…The number of potentially coexisting species increases with each new combination of limiting resources because each species in the plant community is limited by a different resource (Tilman & Pacala, 1993). In forest ecosystems, this pattern is linked to variation in light and soil conditions (nutrients and water), which strongly shape interspecific interactions and species diversity (Douda et al., 2021; Kumar et al., 2018; Reich et al., 2012). The RBCM also integrates the potential role of environmental toxicity (originally referred to as environmental harshness) on community richness (Lavers & Field, 2006).…”

Contrasting responses of forest phenological guilds to complex floodplain change

“…We found a significant decrease in the number of species with summer phenology, with an average decrease of almost 50% species per vegetation plot. Reduced light perception below the canopy due to successional changes or infrequent management interventions is considered one of the most important causes of forest biodiversity decline (Chudomelová et al., 2017; Douda et al., 2021; Doudová et al., 2022; Hédl et al., 2010). In this study, however, we did not find support for the negative effect of canopy shading on the species richness of the understorey.…”