The assembly of a plant network in alpine vegetation

Abstract

Questions Positive and negative associations among species influence the structure of plant communities. Yet how these plant associations are assembled at the community level is poorly understood. We propose a new approach that combines spatial ecology, network theory and trait‐based ecology to examine the assembly of plant–plant associations at the community level. Location Gemmipass, Swiss Alps. Methods We fully mapped an alpine plant community at the individual plant level, recording both plant coordinates and functional traits for each individual. We identified non‐random species associations using spatial point‐pattern analysis and partialled out the effect of abiotic heterogeneity. We then analysed the plant network structure and used plant traits to predict species associations. Results We identified 36 significant spatial associations between plant species, 34 positive and two negatives. Dominant, stress‐tolerant species such as Dryas octopetala, Linaria alpina and Leontodon montanus were highly connected in the network, whereas rare, water‐ and nutrient‐demanding species such as Saxifraga aizoides, Galium anisophyllon and Thymus praecox were less connected compared to random expectation. The plant network was clustered, meaning that species were overall more connected among each other than expected by chance. Conclusions Positive associations among species characterized the studied plant community. Besides the primary effect of associations of the “foundation” species D. octopetala with other species, these “subordinate” plants were also associated with each other. Our study reveals the assembly of plant communities as driven by positive associations among stress‐tolerant pioneer species, highlighting their role in supporting the cohesiveness of alpine plant communities.

Perspectives for ecological networks in plant ecology

“…When looking at spatial patterns of species distribution within communities, positive or negative species associations are based on spatial co-occurrence data that are used as a proxy for inferring positive or negative interactions among species, respectively (Burns and Zotz 2010; Saiz and Alados 2011a; Montesinos-Navarro et al 2018;Losapio et al 2018a). Contrary to plant networks based on fitness measurements, networks based on co-occurrence are representative of the spatial organisation of communities and can be expressed with different spatial resolution.…”

“…Bayesian Networks) at community scale, including covariates that bear additional ecological information (such as abiotic conditions) and focusing on fine spatial resolution (i.e. centimetres) can help building informative plant interaction networks (Saiz et al 2017;Staniczenko et al 2017;Montesinos-Navarro et al 2018;Losapio et al 2018a).…”

“…For this reason, pairwise interactions do not necessarily scale in multispecific context as interactions between two species change in the presence of other species (Losapio et al 2019). On the other hand, limitations associated with co-occurrence networks can be partially overcome by accounting for environmental heterogeneity, plant dispersal patterns and survey scale (Saiz et al 2018;Losapio et al 2018a). This can be done by modelling plant distribution first as a function of environmental factors and second in response to neighbours.…”

“…Using this approach, several authors have explored different factors that affect the spatial structure of plant communities. For instance, Fuller et al (2008) have shown that tree size was related to the spatial organization of tropical-forest networks, and Losapio et al (2018a) have found that a small number of stress-tolerant species were important for the structure an alpine-tundra network. However, the consideration of both positive and negative links simultaneously remains an overlooked aspect of plant networks which can be important for species coexistence given that a balance between positive and negative associations can have stabilising effects for the network structure (Saiz et al 2017).…”

Unifying facilitation and recruitment networks

“…() found that the best model explaining an observed facilitation network in subtropical Brazilian forest–grasslands ecotones included species abundance but not phylogeny. Despite the relevance of plant traits determining plant–plant interactions (Losapio, de la Cruz, Escudero, Schmid, & Schöb, ; Soliveres et al., ), so far, no study has assessed the phenotypic structure of RNs to test to which extent the relevant plant traits structuring the networks are evolutionarily conserved. There has been some attempt to explain the frequency of recruitment in RNs based on the phenotype of the plants involved, showing that facilitation is stronger between species with contrasting communities of associated arbuscular mycorrhizal fungi (AMF) (Montesinos‐Navarro et al., ).…”