The Arcana of Plant Communities
The Arcana of Plant Communities
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Community theory drew in the last decades the main principles of the structure and dynamics of biotic communities. Nonetheless, the context- and/or species-dependence of biotic interactions and community assemblages remains the common rule. A first aspect of my research aims to address the structuring role of these processes in plant communities.
From Competition to Facilitation among plants
From Competition to Facilitation among plants
My PhD dealt with the role of the interactions between the tree canopy, herb layer and tree seedlings for the forest dynamics under global environmental changes. Transplanting seedlings from contrasted species in design crossing tree and herb removal treatments along environmental gradients, I described the changes in the role of positive and negative, direct and indirect effects of these interactions for the forest dynamics. In a mountain post-cultural forest where land-use legacy led to stable transient exploitative stands, I examined the effect of 2003 European heatwave on the survival of transplanted seedlings. As a result, the canopy from adult trees, depending on the environmental conditions and seedlings species, has buffered or reversely amplified the effect of such extreme climatic event on forest recruitment [13] (Fig. 3). In a scenario with a future climate including more frequent extreme events, such interplay between plant interactions, environmental conditions and species functional attributes could lead to divergent trajectories in the landscape where post-cultural deciduous forest could maintain it transient stage in dryer patches and shift toward new successional dynamics in wet conditions (see alternative trajectories hypothesis developed for mountain tundra [14]).
In the Rhone’s floodplain forests, Acer negundo, originating from North America, forms mono-dominant stands between the native Salix alba community on riverbank and Fraxinus angustifolia community on the upper terraces (Fig. 4). A similar transplant-removal design highlighted a rarely evidenced mechanism of indirect facilitation where the canopy of the adult invasive A. negundo favours the growth of its own seedlings by reducing the abundance of strong herbaceous competitors [15, 16].
The complete experimental design revealed a large panel of plant interactions, shifting from the containment to the facilitation of the invasive species, amplifying or buffering the effect of global changes on the vegetation. Beyond the Stress Gradient Hypothesis [17] and subsequent debates [18,19], the outcomes of plant interactions and their relative importance for the structure of communities result from multiple specific mechanisms depending on species and conditions (Fig. 5). This context- and species-dependence constitutes the mechanistic part to integrate for a real consideration of biotic interactions in the modelling of the vegetation response to global changes.
Of Plant Strategy
Of Plant Strategy
How plants interact and constitute communities depends, for a part, on their strategies in terms of resource acquisition and storage, reproduction, tolerance to stress and disturbance. The seminal concept of the r-K continuum [20] describing the trade-off between the exploitation and the conservation of resources was largely re-interpreted in the literature [21,22]. An aspect of my research questions the definition of these strategies.
The transplant-removal designs of my PhD shed unexpected light on the plant strategy conceptualisation. First, the invasive Acer negundo challenges the classical survival-growth trade-off proposed in forest science [23] (Fig. 6). But beyond an invasive advantage, we found that such particular suite of traits can be shared by other exploitative species, including natives [24]. Second, in the mountain design, the seedling performances during the first years of growth distinguished r-K patterns (so called hare and tortoise strategies respectively, Fig. 7a) allocating their resource either to growth or survival that shaped the seedling population fitness and also their response to plant interactions the next years (Fig. 7b).
Plant Functional Types are a powerful classification of the vegetation to stress global patterns and general principles. However, in the context of tundra shrubification, where field observations reveal large divergent patterns, we found that the traditional functional segregation into evergreen and deciduous dwarf shrub can hide species-specific behaviours of high interest in the context of long-term response of arctic communities to environmental changes [25] (Fig. 8).
The development of functional traits framework represents one of the strongest breakthroughs of plant ecology in the last decades. The mechanistic perspective invites to prioritise traits directly linked to environmental parameters such as frost-tolerance or water use efficiency to explain the plant distribution patterns. However, it appears that the integrative dimension of the soft traits prioritised in the literature as SLA or LDMC gives them higher explanative power than physiological measures [26].
The generalisation of specific functional traits in most of biomes allows an exploration of general patterns cascading from the community responses to their effect on the changing environment [27]. However, a focus at the local scale indicates that the intraspecific variability cannot be eluded regarding interspecific variability, especially when addressing interactions mechanisms or community response to environmental changes [28,29]. More specifically, along an environmental gradient in the French Alps, the change in trait values for a species or for a group of species appeared strongly depended upon the position of measured individuals on their ecological niche (Fig. 9). Moreover, the decoupling between the functional characterisation from above and belowground traits has been recently pointed out [30]. The TraitDivNet call, which I answered to, aims at collecting above and belowground traits on the same individuals at the global scale is one of the examples of the still necessary developments needed in this field of ecology to tend towards a more comprehensive representation of the biotic communities structuring.
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