The Journal of Ecology editors are delighted to announce that Sandra Lavorel is our Eminent Ecologist for 2017. In recognition of her work, we have asked Sandra to select 10 of her most influential Journal of Ecology papers to make up a special virtual issue.
These next two papers relate to endeavours to identify how plant functional traits influence community assembly. The papers are representative of two complementary approaches prevalent in community ecology, and also of a general shift in the literature from experimental tests of the mechanisms structuring communities to numerical, inductive approaches based in plant traits.
The first paper, led by Nicolas Gross as part of his PhD research, is an example of a classic experimental approach using assembled communities under garden conditions (Gross et al. 2007). In this experiment we planted single or mixed functional group assemblages of grasses with exploitative and conservative strategies which coexist in the subalpine grasslands, and placed them under two fertilisation treatments (none/intermediate).
In fertilised treatments, increased biomass production in the mixed as compared to single functional group assemblages, associated with decreased competition intensity, demonstrated functional complementarity between the two groups. Species with similar traits competed principally for light, and conversely there was complementary light use when the two functional groups were mixed. Although this should be better investigated for greater numbers of species, this effect may reflect size hierarchies within each of the two groups.
As a result, light interception in the fertilised mixtures was equivalent or greater than that of either single functional group assemblages, which was particularly true for conservative species. Under fertilisation, leaf nutrient use efficiency of conservative grasses also increased when grown in mixtures as compared to single group assemblages, likely linked to their greater root allocation. Altogether these results illustrated the interplay among different traits that results in complementary resource use between functional groups. Importantly, insights from this study were strengthened by the focus on functional variations within grasses, thereby eliminating possible confounding by qualitative trait differences across life forms.
The second paper, led by Francesco de Bello and Marie Vandewalle as part of Marie’s PhD research, is an example of an inductive approach for identifying mechanisms of community assembly from field occurrence along environmental gradients, here a contrast between grazed and abandoned grasslands in the Swedish island of Öland (de Bello et al. 2013).
Using composition and trait data we calculated taxonomic and functional dissimilarity and analysed whether functional dissimilarity or convergence were associated with species co-occurrence. The originality of the study was that the analysis was repeated at three scales: 10×10 cm, 50×50 cm and management plots within the landscape. Dissimilarity was observed at the finest scale suggesting niche differentiation.
This result would be broadly consistent with the conclusion by Gross et al. (2007) about functional complementarity of closely coexisting species. At the 50×50 cm scale functional convergence was detected, interpreted as environmental filtering, but such convergence was not found across patches within the landscape. First, functional convergence in response to environment is indeed the central hypothesis of trait-environment relationships such as those investigated in the two early papers with Sue McIntyre. Second, this analysis suggested that such relationships may sometimes be concealed by within-plot environmental heterogeneity.
Indeed this type of pattern is the likely reason why several alternative response groups with different trait syndrome are found for a single set of plot-level environmental conditions, as discovered in the McIntyre et al. analysis (2001). This finding has important methodological implications, meaning that working on mean traits at plot scale may not always appropriately detect actual functional responses and that more sophisticated multivariate approaches may be required (Kleyer et al. 2012).
Third, the mechanism suggested for the lack of convergence or divergence at plot scale was dispersal limitation, which maintained within plot functional heterogeneity and segregation depending on environmental filtering. Interestingly, this segregation was also stronger in grazed as compared to abandoned plots, consistent with the creation and maintenance of spatial heterogeneity by grazing. Consequently, ecologists interested in plant community responses to environment would be well advised to measure environmental, for instance soil parameters at fine scales, though of course this is resource-intensive.
You can read Sandra’s virtual issue on the journal website, or view the individual papers below.