DEMO-TRAITS · Tree demography, functional traits and climate change

Tree demography, functional traits and climate change

Forests play a crucial role all over the world. Forest houses a significant proportion of global biodiversity and world terrestrial carbon stocks, and they also contribute (directly or indirectly) to the livelihoods of local populations. Forests have always been dynamics and experiencing important changes. However with the on-going global change (such as climate change, N deposition, species invasion, biodiversity loss and habitat degradation) changes are occurring at an unprecedented speed and may lead to potential degradation of the provision of ecosystems services. Our ability to predict the future of this ecosystem remains however extremely limited. This is because the response of forest to the on-going global change is an emergent property of the dynamics of all species and their complex interactions. Given the very large number of tree species on earth, understanding each of their responses to all their pairwise interactions is a daunting task. In the last ten years there has been increasing interest in the use of phenotypic traits to describe the diversity of species in a handful of dimension that would capture key function and trade-off faced by plants and thus organising the species in term of their ecological strategies (in a similar way as the periodic table of element in chemistry). For instance, phenotypic traits and their associated trade-offs have been shown to have globally consistent effects on individual plant physiological functions. This presents a very promising approach to increase our predictive ability of forest dynamics, but so far progresses have been limited. This is because we have limited understanding of how trait-based trade-offs translate into competitive interactions between species, particularly for long-lived forest ecosystems. Competition is a key filter through which ecological and evolutionary success is determined. A long-standing hypothesis is that the intensity of competition decreases as two species diverge in trait values (trait dissimilarity). The few studies that have explored links between traits and competition have shown that linkages were more complex than this, as particular trait values may also confer competitive advantage independently from trait dissimilarity. Empirical investigations have been limited so far to a few particular locations, restricting our ability to find general mechanisms that link traits and competition in the main vegetation types of the world. In this Marie-Curie project we quantified the links between traits and competition, measured as the influence of neighbouring trees on growth of a focal tree. The framework of this analysis is novel in two important ways: (i) competition is analysed at an unprecedented scale covering all the major forest biomes on Earth, and (ii) the influence of traits on competition is partitioned among four fundamental mechanisms. A competitive advantage for trees with some trait values compared to others can arise through: (1) permitting faster maximum growth in absence of competition; (2) exerting a stronger competitive effect, meaning that competitor species possessing those traits suppress more strongly the growth of their neighbours; or (3) permitting a better tolerance of competition, meaning that growth of species possessing those traits is less affected by competition from neighbours. Finally, (4) competition can promote trait diversification, if increasing trait dissimilarity between species reduces interspecific competition compared to intraspecific competition. The analysis of this Marie-Curie project shows how these four mechanisms are connected to three key traits that describe plant strategies worldwide. These traits are wood density (an indicator of a trade-off between stem construction cost and strength), specific leaf area (SLA, an indicator of a trade-off between leaf construction cost and leaf longevity), and maximum height (an indicator of a trade-off between access to light and early reproduction). We analyse basal area growth of more than 3 million trees from more than 2500 species, in more than 140000 plots across all major forested biomes of the earth. This database represents an unprecedented attempt at describing tree species ecological strategies at global scale. The analyses performed within this Marie-Curie project show, that fast maximum growth of a focal tree was correlated negatively with its wood density in all biomes and positively with its specific leaf area in most biomes. Low wood density was also correlated with a low ability to tolerate competition and a low competitive impact on neighbours (competitive effect), and high specific leaf area with a low competitive effect. Thus, traits generate trade-offs between performances with vs. without competition. This trait-based trade-off is a key ingredient in the classical model of successional coexistence in forests, where fast-growing species are more abundant in early successional stages where competitors are absent or rare, and are later replaced by slow-growing species in late successional stages where competitors become more abundant. Coordination between traits value conferring high competitive impact and traits value conferring high competitive tolerance is generally expected. However, in agreement with previous studies, we found little evidence for such coordination (only for wood density, where high density conferred both strong competition impact and strong competition tolerance). Competition within species was stronger than between species, but the degree of trait dissimilarity between species had little influence in weakening competition. No benefit of dissimilarity was detected for specific leaf area and wood density and only a weak benefit for maximum height. Trait dissimilarity effects are widely considered to be a key mechanism by which traits affect competition, but this has rarely been supported with field data. Our analysis shows at global scale that trait dissimilarity effects are weak or absent. Analyses allowing for different effects among biomes did not show any particular biome behaving consistently differently from the others. This lack of context dependence in trait effects may seem surprising, but reinforces that competition for light is important in most forests, and this may explain why we find consistency across such diverse forest types. When competition is described as interactions between pairs of species (as it traditionally has been), the number of different interactions rises as the power of number of species, and becomes quickly intractable. Also this species-pair approach does not lead naturally to generalization across forests on different continents having different composition. The globally consistent links that this Marie-Curie project report between traits and competition have considerable promise for predicting species interactions governing forest communities across different forest biomes and continents of the globe. The analysis presented here also demonstrates that trait dissimilarity is not the major determinant of local-scale competitive impacts on tree growth, at least for these three traits. In contrast, the trait-based trade-off in performance with vs. without competition, reported here, could promote coexistence of species with diverse traits, provided disturbances create a mosaic of successional stages. A challenge for the future is moving beyond tree growth to analyse all key demographic rates and life history stages of trees, to analyse how traits influence competitive outcomes at the population level and control stable coexistence. Website: http://www.irstea.fr/en/kunstler Contact: Georges Kunstler. georges.kunstler@irstea.fr

Data: CORDIS, © European Union