Little is known about changes in the significance of environmental filtering and neutral processes during the succession of plant communities. Generally, a succession is expected to be trait-driven and a shift is predicted from high importance of seed dispersal ability in initial phases to importance of strong competitive ability in later stages. In this study, we consider a community assembly trait-driven in the case of a shift in functional traits during succession and randomly assembled in the case there is no shift in functional traits during succession. We therefore tested whether urban plant communities show trait convergence or trait divergence in relation of successional stage, water and nutrient availability. At two scales (100 × 100 cm and 10 × 10 cm), we analysed traits relevant to seed dispersal, stress tolerance and competitive ability and compared real data with null models. We subsequently used regression trees to associate convergence and divergence to plot age and stress due to low water and soil nutrient availability. Most traits were neither significantly convergent nor divergent and the variance explained by the regression trees was often lower than 20 %. We found divergence in seed number instead of the expected convergence towards high seed number. In accord with expectations we found low seed terminal velocity and annual life span in early succession. Convergence in seed bank longevity occurred at intermediate plot age, and convergence in the combination of life span and lateral spread showed the relevance of competition filtering in the most fertile plots. On the other hand, competition-induced convergence in canopy height or specific leaf area was not supported by our results. We concluded that, based on the traits considered in this study, the overwhelming evidence was that community assembly during the first 40 years of succession was driven by trait-neutral mechanisms.
To what extent does plant clonality contribute to the assemblage of species in communities? Two apparently contrasting, and largely untested, hypotheses envisage the potential role of plant clonal traits in community assembly: (i) environmental filters constrain coexisting species to have functionally similar traits (i.e. trait convergence); (ii) niche differentiation selects for functionally dissimilar species (i.e. trait divergence) allowing them to exploit different spatial and temporal niches. These hypotheses are assessed using a large dataset of 369 plots (100 m2) covering altitudes between 4100 and 5800 m a.s.l. and including the major vegetation types found in Ladakh, NW Himalaya. Patterns of clonal traits, coexistence and turnover were assessed using a functional diversity partitioning framework in the context of different null models. Functional diversity was expressed both for morphologically delimited clonal growth forms (17 categorical growth forms) and for functionally delimited clonal characters (combining 16 different traits differentiating the 17 growth forms). PERMANOVA revealed that both α (within-plots) and β (between-plots) functional diversity varied across environmental conditions and vegetation types highlighting a filtering effect on clonal traits. Alpha diversity, however, was more stable across habitats than β diversity. Despite the significant turnover of clonal traits across habitats, most of the diversity of clonal traits was found within plots, with a higher trait divergence than expected by chance, which suggests that niche differences determine species coexistence. While both trait convergence and trait divergence were detected, convergence was stronger when using null models that shuffled all species in the regional pool across plots and functional diversity expressed in terms of different clonal growth forms. Divergence, in contrast, was detected mostly when using null models that shuffled species cover across species co-occurring in given plots and considering functional diversity in terms of clonal traits. By detecting both trait convergence and trait divergence this study supports both initial hypotheses and brings new evidence on the relevance of clonal traits as a function of species that both inhabit different environments and coexist.
We investigated patterns of trait-based community assembly in grasslands sampled along a moisture gradient (216 species) using three continuous species-level traits (maximum species height, minimum species height, seed mass), as well as seven nominal traits (flowering phenology, fruit type, pollen vector, clonal growth organs – CGOs, branching type, leaf distribution, lateral spread), some of which are unusual in that a species may possess more than one state per trait simultaneously (e.g. CGO). Additionally, this study tests whether patterns of assembly vary with plot biomass and moisture using both presence-absence and abundance-weighted analyses (two plot sizes: 25 × 25 cm, 75 × 75 cm). Plant species in these grasslands were randomly organized with respect to both maximum and minimum species height; however, coexisting plant species had a significantly smaller mean seed mass than expected by chance, and were significantly convergent in seed mass, consistent with observations from one previous study, and with theory related to environmental or competitive filtering. These patterns were consistent across plot sizes, and were similar whether analyses were abundance-weighted or not, although partitioned analyses show that these patterns are most pronounced in wet grasslands. Grasslands were non-randomly assembled with respect to five of our ten traits including seed mass, fruit type, CGO, branching type and leaf distribution. Among these, only patterns of branching type clearly conformed to the predictions from Limiting Similarity Theory, suggesting that variation in this trait may contribute to species coexistence in these grasslands. In two cases (fruit type, branching type), results differed in significance depending on whether analysis used presence-absence or abundance data; incorporating abundance may be more relevant, however, cover-based abundance measures in small plots can bias trait representation in favour of size over ramet number. In general, patterns were consistent across the two plot sizes. For four traits (seed mass, flowering phenology, leaf distribution and lateral spread) plot-level effect sizes of our tests varied significantly with plot-level biomass production; the slope of these relationships was positive for seed mass, but negative for flowering phenology, leaf distribution and lateral spread. For seven of ten traits, plot-level effect sizes varied significantly with plot-level soil moisture, with positive regression slopes for some traits (flowering phenology, leaf distribution), and negative slopes for others (minimum height, seed mass, fruit type, pollen vector, CGO). These results indicate that community assembly rules related to different functional traits can be responsive to variation in either biomass or soil moisture, or both.