Living at the edge: the functional niche occupation of woody plant communities in the submediterranean ecotone

Abstract

Background and aims Submediterranean areas are rich ecotones, where slight modifications in environmental conditions can lead to substantial changes in the composition of plant communities. They thus offer an ideal scenario to examine plant community assembly. In this study, we followed a trait-based approach including intraspecific variability to elucidate (1) the relationship between niche occupancy components and species richness, (2) the processes governing the assembly of these communities, and (3) the contribution of intraspecific trait variability in shaping the functional trait space. Methods We measured eight morphological and chemical traits in 405 individuals across 60 plots located in different forest communities (Mediterranean, Eurosiberian and Mixed) coexisting within a submediterranean ecosystem in central Spain. We calculated three niche occupancy components related to Hutchinson’s n-dimensional hypervolumes: the total functional volume of the community, the functional overlap between species within the community and the average functional volume per species, and then used null models to explore the relative importance of habitat filtering, limiting similarity and intraspecific variability as assembly patterns. Key Results Both habitat filtering and niche differentiation drive the community assembly of Mediterranean communities, whereas limiting similarity and hierarchical competition shape Eurosiberian communities. Intraspecific responses were mostly explained by shifts in species’ niches across functional space (changes in the position of the centroids of hypervolumes). Conclusions Different assembly mechanisms govern the structure of Mediterranean, Eurosiberian and Mixed plant communities. Combining niche occupancy components with a null model approach at different spatial scales offers new insights into the mechanisms driving plant community assembly. Considering intraspecific variability is indispensable for understanding the mechanisms governing species coexistence in species-rich ecotones.

Linking functional composition moments of the sub-Mediterranean ecotone with environmental drivers

“…At the other end of the environmental gradient (see also de la Riva et al, 2023), we found the F. sylvatica community with predominantly acquisitive strategies at the whole plant level. In fact, species with a profligate water-use strategy, such as F. sylvatica (de Tomaś , dominated the lowest areas of Montejo where deeper soils are found (de la Riva et al, 2023), which could explain the shift in the hydraulic dimension along the soil depth gradient better than soil nutrients. This profligate use of water and nutrients of F. sylvatica makes it a strong fast-growing species that promotes strong competitive exclusion (Meyer et al, 2003;Rodrıǵuez-Calcerrada et al, 2011;Castaño-Santamaria et al, 2021).…”

“…In each FI, 125 circular plots of 30-m diameter were systematically established in a 100 x 100 m square grid covering the entire forest. Based on the last FI, we divided the forest into six different areas (communities hereafter) according to the distribution of the three most abundant tree species, i.e., beech, Pyrenean oak and sessile oak (see also de la Riva et al, 2023). An additional community dominated by shrubs where trees are scarce and dispersed was included.…”

“…Overall, the CWMs showed a trend that was in line with the acquisitive-conservative axis of resource uptake (Reich, 2014;Dıáz et al, 2016;de la Riva et al, 2016b) at aboveground level. Plant communities dominated by Mediterranean species (Q. pyrenaica, Shrubland and Mixed 1 and 2) are dominated by species with low SLA and SRA, and high LDMC, thick root diameters (Rdi), and high leaf and root C:N. These communities are found in harsh areas which are characterized by low soil depths with stony soils, steep slopes or high insolation (de la Riva et al, 2023;, conditions that probably limit water availability. In fact, this water shortage promoted the prevalence of communities with a conservative hydraulic architecture (i.e.…”

“…When belowground strategies were considered, however, Shrubland community was the most acquisitive (high CWM) but also showed the highest positive skewness, indicating the presence of subordinate species with conservative strategies. This community occupies flat areas with shallow soils (and thus lower water availability) and high soil nutrient contents in Montejo (de la Riva et al, 2023). In these areas, greater aridity would promote the convergence of traits towards conservative strategies (Cornwell and Ackerly, 2009;de la Riva et al, 2018b;Carrascosa et al, 2023).…”

“…The Iberian Peninsula, in the south of Europe, represents one of the largest ecotones of this kind (Sańchez de Dios et al, 2009), being the low-latitudinal edge for the distribution of many central European species (Gil et al, 2010;Dorado-Liñań et al, 2017;Hernańdez-Lambraño et al, 2021). Sub-Mediterranean regions host Mediterranean and Eurosiberian species as well as many endemic species (Vila-Vicosa et al, 2020a;Box, 2021;Cantoral et al, 2023), creating a mosaic of unique plant communities with contrasting assembly processes (de la Riva et al, 2023). Sub-Mediterranean regions in the Iberian Peninsula are very sensitive to slight environmental alterations (Sańchez de Dios et al, 2009;Vila-Vicosa et al, 2020a), which makes them highly vulnerable to ongoing climatic change and thus critical areas for conservation (Benito Garzoń et al, 2008;Cantoral et al, 2023).…”