Abstract
Facilitation is a positive interaction demonstrated to be one of the important factors shaping the regeneration niche of trees, mostly under stressful conditions which is currently studied in the frame of complex ecological networks. The protection provided by benefactor plants for tree seedlings playing the role of beneficiaries is documented mainly in arid and semi-arid habitats or in situations where herbivores’ pressure constitutes the main stressful factor for tree regeneration. One of the iconic Transylvanian landscapes is the wood-pasture, also one of the oldest agro-forestry systems to which recent forest expansion in abandoned agricultural fields or pastures is added. The proposed work represents a preliminary investigation on the association between benefactor plants, mostly spiny shrubs (Rosa canina, Crataegus monogyna, Prunus spinosa as the most frequently encountered benefactors) and tree seedlings (Quercus spp., Tilia spp., Carpinus betulus, Fraxinus angustifolia, Pyrus pyraster as most frequently encountered beneficiaries), in four different locations from North-Western and Western Romania, wood-pastures, abandoned pastures and abandoned agricultural fields under the consideration that the main stressful factor is represented by livestock grazing. Bipartite, qualitative merged network was generated depicting the interaction between beneficiaries and benefactors. Commonly used metrics were calculated: connectivity, nestedness, modularity, betweenness centrality and centralization compared to similar facilitation networks presented in the literature. Facilitation network is characterized by high nestedness (N=0.896), lack of modularity, relatively high connectance (C=0.233), features encountered in mutualistic networks also. Betweenness centrality scores highlighted the keystone benefactor and beneficiary species, while betweenness centralization score (0.192) indicates the fact that there are several species sharing the dominant position in terms of interactions. The analysis of measurement data (seedlings’ and benefactor plants’ heights, distance from focal seedlings to nearest benefactor species and orientation) showed that there is common pattern in orientation (most of the benefactor species oriented toward South or South West) also in dimensional variability (MANOVA results).
Highlights
Nurse plants; Bipartite facilitation network; Nestedness; Modularity; Betweennes centrality (Gómez-Aparicio et al 2004)
The present study aims the establishment of facilitative interactions between benefactor woody plants and tree seedlings in wood pastures which are characteristic for North-Western and Western Romanian landscapes and in abandoned meadows, pastures or agricultural lands
Site locations: Sampling areas were selected in secondary wood pastures in North Western and Western Romania, in four sites (1 - wooded pasture adjacent to Hoia recreational forest near the city of Cluj-Napoca, 2 - wood pasture near forest stands in the area of Stana commune, degraded pasture near Miersig commune, 3 adjacent to mixed broadleaved forest dominated by Quercus cerris and 4 - a strip of wood pasture adjacent to forest stands near Traian Vuia commune) (Table 1)
Summary
Nurse plants; Bipartite facilitation network; Nestedness; Modularity; Betweennes centrality (Gómez-Aparicio et al 2004). Traditional wood pastures which are semi-natural ecosystems cover over 10,000 ha, largest areas being located in Transylvania, from silvo steppe to colline zone (Hartel et al 2013) and are considered EMERALD priority habitats Another largely represented habitat type, the abandoned or postagricultural lands (D’Orangeville et al 2008) in the proximity of the forest stands contribute to forest expansion with the participation of facilitative interactions. Networks are entities composed of nodes (for instance species, genes, proteins, habitats, individuals of a population) and the connection among them, characterized by specific topology and properties In this context, the study of facilitation networks’ properties sheds light on the assembly mechanisms of plant communities (considered not so long ago as being shaped mainly by competition) and biodiversity maintenance mechanisms (ValienteBanuet et al 2006). Both properties are considered to be drivers of network dynamics (Fortuna et al 2010)
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