Spatial and temporal patterns of plant community assembly in teo biogeographic regions: a taxonomic and functional approach.

Abstract

One of the main aims of plant community ecology throughout its history has been to understand the processes governing the assembly of plant communities. In the last decades, and in the face of the threat in biodiversity posed by global change, this interest has renewed. Despite the great number of studies carried out to understand plant community assembly, still many questions remain unclear, such as the relative influence of different processes on community assembly patterns across space and time, or the relationship between different characteristics of plant communities and environmental factors. In this regard, the aim of this thesis has been to increase our understanding of how plant communities work, by describing the patterns found in plant communities and trying to infer the processes driving them.

To achieve this goal, we studied the spatial and temporal variation of plant communities in two ecosystems located in different biogeographic regions in Navarra (north of Spain): a temperate forest located in the Señorío de Bertiz Natural park, in the Atlantic region, and the Special Conservation Area of Ugarra (locality of Napal), in the Mediterranean region. In the Bertiz forest the study design consists on 102 permanent plots of 400 m2 each that were established throughout a basin of 132 ha, and which were sampled in 2006 and 2016. In the Napal grassland there were 12 permanent plots of 1m2 that were sampled annually for 14 years (2004-2017). Half of the plots in Napal were fertilized in 2003. In addition, from 2019 to 2021, we carried out extensive inventories around Napal to obtain the regional species pool. In all studies we applied a taxonomic and a functional approach, using functional traits. In the Bertiz forest we studied the spatial variation of the forest understory throughout an environmental gradient of light and soil moisture. Further, we studied changes in the forest understory in response to canopy gap dynamics over a decade. In Napal grassland, we carried out two temporal studies to analyze the effect of fertilization on semi-natural Mediterranean grasslands. Firstly, we studied plant community stability over 14 years and tried to discover its main drivers in control and fertilized plots, distinguishing year-to-year variation from long-term trends to avoid spurious conclusions when interpreting temporal patterns. Secondly, we studied over 14 years the dynamics of annual communities in response to their interactions with perennial herbs and woody species, and in response to inter-annual weather fluctuations.

We found that in Bertiz forest light is the most limiting resource. Topography leads to spatial resource heterogeneity, while canopy gaps lead to spatial and temporal resource heterogeneity, which is key for maintaining the taxonomic and functional diversity of understory communities. Understory species composition also changes following gap dynamics, but it does it slowly, lasting more than a decade. In the Napal grassland community stability is driven by species richness, asynchrony and conservative traits. However, the relationship between stability and its drivers differs between year-to-year and long-term scale. Fertilization protects annuals from drought and cold and slows-down shrub encroachment in the long-term, but strengthens competition, decreases species richness and reduces stability in Mediterranean grasslands. In both study sites, abiotic filtering and competition drive community assembly, but the patterns they produce depend on the characteristic of the community analysed, the approach used, the functional trait studied and time span. Other processes such as dispersal also influence understory species composition following gap dynamics, seed bank longevity, species asynchrony and community stability. Studying spatio-temporal patterns in different regions and combining the taxonomic and functional approach can effectively advance our understanding of plant communities.