Introduction
Disentangling the drivers of β-diversity (the site-to-site variability in species composition) provides insights into the processes that govern community assembly (Chase 2010; Kraft et al. 2011). β-diversity can arise from community assembly processes involving deterministic selection, when environmental heterogeneity creates different niches that shape the occurrences of species in a community, and stochastic aspects related to dispersal limitation and ecological drift (Lalibertéet al. 2014; Mori et al. 2018). Interspecific variations in plant traits determine the capacity for individuals to grow, reproduce, and disperse within and among habitats, and therefore play important roles in determining the relative importance of deterministic selection (McGill et al. 2006; Blonder 2018). Traits that have been considered thus far are largely related to ecological strategy axes along which plant species vary in their abilities to acquire and allocate resources (Adler et al. 2014; Salguero-Gómez et al. 2016). As a fundamental trait that significantly varies across angiosperms (2400-fold) and precisely correlates with diverse phenotypic characters at cellular and organismal level, genome size (GS, i.e. nuclear DNA content), has received relatively little attention in the context of its role in community assembly. Genome size is generally constant within a species, which has functional consequences for species’ environmental tolerances, dispersal capacity, and interactions with other species (Knight & Ackerly 2002; Herben et al. 2012). The potential impact of plant genome size on community assembly processes is starting to be recognized (Greilhuber & Leitch 2013; Pellicer et al. 2018), but to date, its role in shaping β-diversity has not been tested empirically.
A key determinant of β-diversity is environmental filtering. The strength of the relationship between local environmental conditions and species environmental requirements affects the establishment and persistence of species (Laliberté et al. 2014; van Breugelet al. 2019). Environmental filtering is hypothesized to differ for large- vs. small- GS species (Faizullah et al. 2021). First, the small-GS species grow faster due to short cell cycle duration and are subject to fewer material costs for packing DNA, allowing them to achieve optimal growth across a wider range of environments (Knight & Beaulieu 2008; Hessen et al. 2010). Second, according to the ‘large genome constraint hypothesis’, the optimal growth for large-GS species is only achievable under conditions of stress-free or high resource availability (Knight et al. 2005). It has been hypothesized that there will be selection for species with small genomes in nutrient-depleted soils as a way to reduce the biochemical cost of synthesising DNA, which is rich in nitrogen (N) and phosphorus (P) (Leitch & Leitch 2008; Hessen et al. 2010). Indeed, a recent study showed that large-GS species became more dominant under conditions with higher nutrient availability (Guignard et al. 2016). Thus, we expect that environmental filtering would have stronger effects on large-GS species than that for small-GS species.
To assess the roles that genome size plays in plant community assembly, we used data from 520 plant communities in 52 sites (10 plant communities per site) along a 3200-km transect in the temperate grasslands of northern China (Fig. 1). We measured plant genome size [the amount of DNA in a gamete nucleus or 1C-value, representing the DNA content of the whole complement of chromosomes for the organism, irrespective of the degree of generative polyploidy; C-values have been used as a reference value for genome size, for details see Greilhuberet al. (2005)] for 161 herbaceous species occurring along the transect (Fig. 1; Table S1-2). Generalized dissimilarity models [GDMs (Ferrier et al. 2007; Fitzpatrick et al. 2013)] were used to quantify the effects of genome size, environmental variation, and geographical distance on β-diversity along the gradient. We also estimated the importance of genome size as a continuous trait in driving species distribution with a joint species distribution model (Hierarchical Modelling of Species Communities [HMSC (Ovaskainenet al. 2017)]. HMSC can simultaneously model the environmental responses of multiple species accounting for shared habitats and evolutionary histories (i.e., phylogenetic correlations) and calculated the community weighted mean genome size along the environmental gradient (Tikhonov et al. 2020). We hypothesized that environmental filtering would play a more important role in driving the β-diversity for large-GS than that for small-GS species. Finally, to confirm the findings of our observational transect study, we also analysed data from a 10-yr field experiment manipulating resource availability to confirm that large-GS species and their higher resource requirements would be favored over small-GS species after reduction of resource limitation.