Grassland restoration characteristics influence phylogenetic and taxonomic structure of plant communities and suggest assembly mechanisms

Abstract

Phylogenetic and species‐based taxonomic descriptions of community structure may provide complementary information about the mechanisms driving community assembly across different environments. Environmental filtering may have similar effects on taxonomic and phylogenetic diversity under the assumption of niche conservatism, whereas competitive exclusion could produce contrasting patterns in these diversity metrics. In grassland restorations, these diversity patterns might then reveal potential assembly mechanisms underlying the impacts of restoration and management conditions on community structure. We compared plant community structure (alpha diversity, composition, and within‐site beta diversity) from both phylogenetic and taxonomic perspectives. Using surveys from 120 tallgrass prairie restorations in four regions of the Midwestern United States, we examined the effects of four potential drivers or environmental gradients: precipitation in the first year of restoration, seed mix richness, time since last prescribed fire, and restoration age, and included soil conditions as a covariate. First‐year precipitation influenced taxonomic community structure, but had weak effects on phylogenetic diversity and composition. Similarly, greater seed mix richness increased taxonomic diversity but did not influence phylogenetic diversity. Taxonomic, but not phylogenetic, diversity generally was lower in older restorations and those with a longer time since the last prescribed fire. These drivers consistently explained more variation in taxonomic than phylogenetic diversity and composition, perhaps in part because species turnover was largely among related species, producing weak impacts on phylogenetic community measures. An impact of precipitation on taxonomic but not phylogenetic diversity suggests that there may not be large differences in drought tolerance among clades that would cause phylogenetic patterns to arise from this environmental filter. Declining taxonomic diversity but not phylogenetic diversity is consistent with competitive exclusion as an assembly mechanism when competition is strongest between related species. Synthesis. This research shows how studying taxonomic and phylogenetic diversity of ecosystem restorations can inform plant community ecology and help natural resource managers better predict the outcomes of restoration actions and management.

Is phylogenetic and functional trait diversity a driver or a consequence of grassland community assembly?

“…Khalil et al () was the only other study here suggesting that FD expressed through intraspecific trait variation can be a driver of community assembly, especially if dominant species impact FD of subordinate species. In contrast, Barber et al () found no impact of environmental gradients such as restoration age, time since fire, seed mix richness and precipitation in first year of planting on PD, suggesting PD is not a driver or passenger of community assembly.…”

“…Griffin‐Nolan et al () showed a similar pattern, where changes in FD and PD did not consistently match; as functional traits changed from drought‐tolerance to drought‐avoidance, PD did not differ accordingly. Barber et al () showed that while PD did not change with environmental context, SD did respond to all the environmental gradients they analysed. Lastly, in their systematic review, Cadotte et al () found that while FD and PD are often positively correlated, functional and phylogenetic dispersion sometimes show opposite patterns.…”

“…Studies included in this issue often used standard measures of species diversity, functional diversity and phylogenetic diversity. Species richness (Barber et al, ; Miller, Li, LaForgia, & Harrison, ), diversity indices (Barber et al, ; Brandt, Seabloom, & Cadotte, ) and dissimilarity indices (Barber et al, ; Yang et al, ) were used to describe SD. Standard functional richness, evenness, dissimilarity indices and community weighted means were used to describe functional trait composition (Galland et al, ; Griffin‐Nolan et al, ), and intraspecific trait variation was measured to capture impacts of competition (Galland et al, ; Khalil, Gibson, & Baer, ).…”

“…Brandt et al () used a unique experimental approach—removal of above‐ground biomass and N‐additions to investigate the dual impacts of disturbance and resource availability. Barber et al () used 120 tallgrass prairie restorations to explore the PD and SD consequences of a variety of management and climatic variables in an uncontrolled field setting. Griffin‐Nolan et al () manipulated water availability to look at drought over 4 years while Miller et al () used a 19‐year time series to investigate the impacts of real droughts.…”

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“…As a measure of biodiversity, phylogenetic diversity differs from taxonomic diversity as it takes into account the phylogenetic and evolutionary relationships among taxa based upon distance matrices of pairwise dissimilarities derived from molecular data [15]. Taxonomic and phylogenetic diversity patterns may differ over the course of a restoration reflecting key and heretofore unknown aspects of community assembly [13,16,17].…”