Disentangling the processes driving plant assemblages in mountain grasslands across spatial scales and environmental gradients

1. Habitat filtering and limiting similarity are well-documented ecological assembly processes that hierarchically filter species across spatial scales, from a regional pool to local assemblages. However, information on the effects of fine-scale spatial partitioning of species, working as an additional mechanism of coexistence, on community patterns, is much scarcer. 2. In this study, we quantified the importance of fine-scale spatial partitioning, relative to habitat filtering and limiting similarity, in structuring grassland communities in the western Swiss Alps. To do so, 298 vegetation plots (2 m × 2 m ) each with five nested subplots (20 cm × 20 cm) were used for trait based assembly tests (i.e. comparisons with several alternative null expectations), examining the observed plot and subplot level α-diversity (indicating habitat filtering and limiting similarity) and the between-subplot β-diversity of traits (indicating fine-scale spatial partitioning). We further assessed variations in the detected signatures of these assembly processes along a set of environmental gradients. 3. We found habitat filtering to be the dominating assembly process at the plot level with diminished effect at the subplot level, while limiting similarity prevailed at the subplot level with weaker average effect at the plot level. Plot-level limiting similarity was positively correlated with fine-scale partitioning suggesting that the trait divergence may result from a combination of competitive exclusion between functionally similar species and environmental micro-heterogeneities. Overall, signatures of assembly processes only marginally changed along environmental gradients but the observed trends were more prominent at the plot than at the subplot scale. Synthesis: Our study emphasises the importance of considering multiple assembly processes and traits simultaneously across spatial scales and environmental gradients to understand the complex drivers of plant community composition

Pollination and dispersal trait spectra recover faster than the growth form spectrum during spontaneous succession in sandy old‐fields

Question: Spontaneous succession is the most natural and cost‐effective solution for grassland restoration. However, little is known about the time required for the recovery of grassland functionality, i.e., for the recovery of reproductive and vegetative processes typical of pristine grasslands. Since these processes operate at different scales, we addressed the question: do reproductive and vegetative processes require different recovery times during spontaneous succession? Location: Kiskunság sand region (Central Hungary). Methods: As combinations of plant traits can be used to highlight general patterns in ecological processes, we compared reproductive (pollination‐ and dispersal‐related) and vegetative (growth form) traits between recovered grasslands of different age (<10 years old; 10–20 years old; 20–40 years old) and pristine grasslands. Results: During spontaneous succession, the reproductive trait spectra became similar to those of pristine grasslands earlier than the vegetative ones. In arable land abandoned for 10 years, pollination‐ and dispersal‐related trait spectra did not show significant difference to those of pristine grasslands; anemophily and anemochory were the prevailing strategies. Contrarily, significant differences in the growth form spectrum could be observed even after 40 years of abandonment; in recovered grasslands erect leafy species prevailed, while the fraction of dwarf shrubs and tussock‐forming species was significantly lower than in pristine grasslands. Conclusions: The recovery of the ecological processes of pristine grasslands might require different amounts of time, depending on the spatial scale at which they operate. The reproductive trait spectra recovered earlier than the vegetative one, since reproductive attributes first determine plant species sorting at the regional level towards their respective habitats. The recovery of the vegetative trait spectrum needs more time as vegetative‐based interactions operate on a smaller spatial scale. Thus, vegetative traits might be more effective in the long‐term assessment of restoration success than the reproductive ones

Plant functional and taxonomic diversity in European grasslands along climatic gradients

Aim: European grassland communities are highly diverse, but patterns and drivers of their continental-scale diversity remain elusive. This study analyses taxonomic and functional richness in European grasslands along continental-scale temperature and precipitation gradients. Location: Europe. Methods: We quantified functional and taxonomic richness of 55,748 vegetation plots. Six plant traits, related to resource acquisition and conservation, were analysed to describe plant community functional composition. Using a null-model approach we derived functional richness effect sizes that indicate higher or lower diversity than expected given the taxonomic richness. We assessed the variation in absolute functional and taxonomic richness and in functional richness effect sizes along gradients of minimum temperature, temperature range, annual precipitation, and precipitation seasonality using a multiple general additive modelling approach. Results: Functional and taxonomic richness was high at intermediate minimum temperatures and wide temperature ranges. Functional and taxonomic richness was low in correspondence with low minimum temperatures or narrow temperature ranges. Functional richness increased and taxonomic richness decreased at higher minimum temperatures and wide annual temperature ranges. Both functional and taxonomic richness decreased with increasing precipitation seasonality and showed a small increase at intermediate annual precipitation. Overall, effect sizes of functional richness were small. However, effect sizes indicated trait divergence at extremely low minimum temperatures and at low annual precipitation with extreme precipitation seasonality. Conclusions: Functional and taxonomic richness of European grassland communities vary considerably over temperature and precipitation gradients. Overall, they follow similar patterns over the climate gradients, except at high minimum temperatures and wide temperature ranges, where functional richness increases and taxonomic richness decreases. This contrasting pattern may trigger new ideas for studies that target specific hypotheses focused on community assembly processes. And though effect sizes were small, they indicate that it may be important to consider climate seasonality in plant diversity studies

Macroevolutionary patterns in European vegetation

Question Habitat‐specific species pools are shaped by ecological and evolutionary processes such as speciation, extinction, and migration. However, their role is poorly known because of the lack of robust data on species pools across a large number of plant community types and large areas. Here, we analyse a unique dataset of species pools of diagnostic species for all European vegetation types, asking: (a) what are the patterns of phylogenetic structure and phylogenetic beta‐diversity across European vegetation types and biomes; (b) what are the drivers of these patterns; and (c) is there a signal of niche conservatism at the level of biomes and broad categories of vegetation types? Location Europe, Canary Islands, Madeira, Azores, Cyprus, Caucasus, Iceland and Greenland. Methods We built a dataset comprising 10,804 vascular plant species (almost 85% of the European flora) assigned to 106 vegetation types representing all European vegetated habitats, grouped into 11 biomes. This dataset represented habitat‐specific species pools. We analysed the phylogenetic structure of the species pools and related it to distribution range sizes of individual vegetation types, their successional status, levels of disturbance and environmental stress. Results In European vegetation, phylogenetic overdispersion is associated with late‐successional habitats: several forest types, aquatic vegetation, and rock‐cliff vegetation serve as depositories of relict lineages. In contrast, phylogenetic clustering is typical of early successional and disturbed vegetation in anthropogenic, coastal and saline habitats, and in open‐canopy Mediterranean vegetation. The phylogenetic similarity of vegetation types is higher within than between broad categories of vegetation types and biomes. Conclusions The variable phylogenetic structure of European vegetation types is a heritage of evolutionary processes in the Tertiary and Quaternary. Habitat‐specific species pools of different vegetation types and biomes have been formed by different evolutionary processes as indicated by the observation that certain clades are significantly associated with certain vegetation types or biomes, hence indicating the phylogenetic niche conservatism

On the need for phylogenetic ‘corrections’ in functional trait-based approaches

There is considerable uncertainty about if, and when, phylogenetic information is needed to answer various ecological questions about trait-based ecological studies. It has been recommended that both functional and phylogenetic information should be combined, and some researchers have even suggested that functional information for species should be ‘corrected’ because species are not phylogenetically independent. Here, we address these issues by identifying key types of questions in functional trait-based ecology and discussing the utility of phylogenetic information for answering them, either as a correction or in combination with functional traits. Phylogenetic analyses are identified as essential to answer questions related to the evolution of adaptations to abiotic and biotic conditions. However, we argue that phylogenetic information is not always relevant for functional trait studies, and should not be incorporated into ecological analyses without clear justification. Phylogenetic relatedness between species should not be considered a bias to be corrected, but rather an evolutionary signal that allows results to be interpreted at different evolutionary scales. Furthermore, if traits are conserved, phylogeny can be used as a proxy for missing information on traits and functional trait diversity. We conclude by providing guidelines on when to apply, and how to interpret, results obtained using phylogenetic information for a variety of ecological questions linked to functional traits

Ecological assembly rules in plant communities--approaches, patterns and prospects.

Understanding how communities of living organisms assemble has been a central question in ecology since the early days of the discipline. Disentangling the different processes involved in community assembly is not only interesting in itself but also crucial for an understanding of how communities will behave under future environmental scenarios. The traditional concept of assembly rules reflects the notion that species do not co-occur randomly but are restricted in their co-occurrence by interspecific competition. This concept can be redefined in a more general framework where the co-occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive. Here we present a survey and meta-analyses of 59 papers that compare observed patterns in plant communities with null models simulating random patterns of species assembly. According to the type of data under study and the different methods that are applied to detect community assembly, we distinguish four main types of approach in the published literature: species co-occurrence, niche limitation, guild proportionality and limiting similarity. Results from our meta-analyses suggest that non-random co-occurrence of plant species is not a widespread phenomenon. However, whether this finding reflects the individualistic nature of plant communities or is caused by methodological shortcomings associated with the studies considered cannot be discerned from the available metadata. We advocate that more thorough surveys be conducted using a set of standardized methods to test for the existence of assembly rules in data sets spanning larger biological and geographical scales than have been considered until now. We underpin this general advice with guidelines that should be considered in future assembly rules research. This will enable us to draw more accurate and general conclusions about the non-random aspect of assembly in plant communities

Applying the dark diversity concept to nature conservation

Linking diversity to biological processes is central for developing informed and effective conservation decisions. Unfortunately, observable patterns provide only a proportion of the information necessary for fully understanding the mechanisms and processes acting upon a particular population or community. Here, we suggest conservation managers utilise the often overlooked information imbedded in species absences, paying particular attention to the dark diversity (i.e. a set of species that are absent from a site but can potentially disperse to and establish there, in other words, the absent portion of a habitat-specific species pool). Together with existing ecological metrics, concepts and conservation tools, dark diversity can be used to complement and further develop conservation prioritisation and management decisions through an understanding of biodiversity relativized by its potential (i.e. its species pool). Furthermore, through a detailed understanding of the population, community and functional dark diversity, the restoration potential of degraded habitats can be more rigorously assessed further, and so to, the likelihood of successful species invasions. We suggest the application of the dark diversity concept is currently an underappreciated source of information that is valuable for conservation applications ranging from macro-scale conservation prioritization to more locally-scaled restoration ecology and the management of invasive species. This article is protected by copyright. All rights reserved