Plant assemblages do not respond homogenously to local variation in environmental conditions: functional responses differ with species identity and abundance

Abstract

Questions We investigated some commonly held assumptions of community assembly theory needed to provide accurate predictions of changes in plant species assemblages across environmental gradients or following environmental change. Do (1) dominant and subordinate species respond in the same way to changes in environmental variables; (2) plant species assemblages show higher interspecific than intraspecific trait responses; and (3) co‐existing dominant species differ in their responses to the same environmental variables? Location Islands in Lakes Uddjaure and Hornavan, northern Sweden. Methods We explored the responses of forest understorey vegetation assemblages to variation in environmental resources across a chronosequence of 30 lake islands that differ in fire history, above‐ground and below‐ground resource availability and species diversity. For one plot on each island, we measured specific leaf area, leaf dry matter content and foliar N and P of all dominant and subordinate understorey plant species to assess species‐specific and weighted and non‐weighted community‐level trait responses to variation across islands in all major local environmental drivers. Results Consistent with our expectations, we found that species responses to environmental conditions were not homogenous within assemblages, and that responses of dominant and subordinate species differed. Further, intraspecific variation was often an important component of local‐scale plant community‐level responses. Responses were often relatively consistent across species, but dominant species sometimes showed contrasting responses of the same trait to the same environmental factor. Finally, environmental factors that influenced community average trait values also affected functional diversity. Conclusions This study has shown that several common assumptions that underpin community assembly theory do not necessarily hold, and this can cause inaccuracies in predicting plant functional composition responses to changes in environmental variables. Because these assumptions are central to current models that predict vegetation responses to environmental change, it is crucial to further test in which particular environmental context and to what extent these assumptions are critical for model accuracy.

Linkage of plant trait space to successional age and species richness in boreal forest understorey vegetation

“…), represents 78% of the vascular plant species present, and plays an important role in the functioning of the boreal forest (Nilsson & Wardle ). Previous studies on the islands (Lagerström, Nilsson & Wardle ; Kumordzi, Wardle & Freschet ) have shown that in the current system, overstorey and understorey vegetation structure and composition are controlled by different environmental factors. For each island, we used a 10‐m‐radius plot area established directly adjacent to a set of pre‐existing experimental plots used for previous studies on the islands (Wardle et al .…”

“… The proportion (%) of the total number of plants hits per total intercept of 200 points, representing a relative measure of understorey shrub density (Kumordzi, Wardle & Freschet ). …”

“…). Consistent with Kumordzi, Wardle & Freschet (), we used the total number of intercepts of all understorey species as a measure of total vegetation density which is interpreted as a relative measure of the level of competition exerted by the whole understorey plant community (Al‐Mufti et al . ).…”

“…(), changes in within‐ and between‐species functional diversity of understorey vegetation were assessed across the 30 communities in response to key aspects of local environmental variables, that is ecosystem successional age, plant species richness and density of understorey vegetation, and spatial heterogeneity of soil N and P. We selected these five environmental variables because they usually change considerably across communities (including in our system) and their effects on plant community assembly are well established (Gundale et al . ; Kumordzi, Wardle & Freschet ). We focused on specific leaf area (SLA), a key plant functional trait that is strongly indicative of plant ecological strategy and can be easily measured for many individuals (Diaz, Cabido & Casanoves ; Evans & Poorter ).…”

Contrasting responses in leaf nutrient-use strategies of two dominant grass species along a 30-yr temperate steppe grazing exclusion chronosequence

“…The magnitude of within species plasticity in resorption efficiency, with for instance an average shift from 77 to 50 % in N resorption efficiency of L. chinensis along the chronosequence, is even larger than these commonly observed across species of contrasting functional types (Aerts 1996;Kobe et al 2005;Freschet et al 2010b) and can therefore have potentially important consequences for plant-soil feedbacks and ecosystem functioning (Hobbie 1992;Lü et al 2012a;Freschet et al 2013). The species-specific responses suggest potentially complex responses of plant communities to changing environmental conditions (Kichenin et al 2013;Kumordzi et al 2014) potentially leading to large uncertainty in projections of plant assemblages under changing climate and land-use (Thuiller et al 2008).…”