Effect of dominant species on community phylogenetic structure in a grassland restoration
Knowledge of how intraspecific variation in dominant species influences the structure of plant communities is needed to understand the ecological consequences of different propagule sources used in restoration. Our aim was to determine the phylogenetic patterns of community structure during the first four years of grassland assembly in relation to population sources of the dominant grasses. We conducted a field experiment in which two population sources (cultivar and non-cultivar) of three dominant grasses (Sorghastrum nutans, Andropogon gerardii, and Schizachyrium scoparium) were seeded into whole-plots (n=12) with subplots (n=36) assigned to three unique subordinate species pools. Three gene sequences (ITS1, 5.8s, and ITS2) were used to build a phylogenetic tree of all taxa occurring in the plots over four years from initial seeding in 2006 to 2009. The genetic sequences were aligned using Bioedit. Mega 5.2 using maximum parsimony was used to build the tree. To quantify community phylogenetic structure, mean pairwise distance (MPD), mean nearest taxon distance (MNTD), net relatedness index (NRI), nearest taxon index (NTI), average phylogenetic distinctness (AvPD), and Faith's phylogenetic diversity (PD) were calculated separately for all taxa (n=110), monocots (n=27), all forbs (n=83), Asteraceae (n=30), Fabaceae (n=16), and other forb taxa (n=37). Statistical analyses were carried out using R and SAS.
NRI values for monocots, Fabaceae, and other forb taxa showed that these groups were phylogenetically clustered with high cluster rates in the first year of community assembly (2006) and low rates in (2009), while all taxa considered together showed a decrease in phylogenetic overdispersion through time. NTI values for all forbs indicated shifts in closely related species from overdispersion to a high clustering rate through time. There were differences in MPD and MNTD between monocots, and all taxa. In monocots, MPD and MNTD varied between population sources and species pools through time, while when all taxa were considered a similar pattern was seen for MNTD, but MPD only varied through time. AvPD in all forbs and monocots exhibited an interaction between dominant grass source and species pools. A similar pattern was seen for NRI in the Asteraceae. Monocots, Fabaceae, and all forbs showed decreasing values of PD through time. These changes in phylogenetic structure were associated with declining numbers of taxa through time. In this study, incorporation of species phylogeny into quantification of community structure revealed the importance of seeded species pools and dominant species population source.