At the interface of ecophysiology and functional ecology, our research involves different disciplines (chemistry, geography, mathematics), tools (simulation and statistical modelling) and playground (tundra, taiga, temperate grasslands and forests, wetlands and drylands), but can be sumed up through four main contributions, presented below:

Contribution 1 – From intraspecific to interspecific levels: Disentangling trait coordination in plants

Journal articles: A1, A7, A10, A12 (see publication page)

Figure 1: Simulating plant performance response to variation in specific leaf area and tiller density parameters in the Gemini model. Plant performance was estimated in terms of plant biomass and C:N ratio.

Plant species face strategic dilemmas: For instance, species achieving high rate of leaf carbon assimilation do so at the expense of leaf longevity and vice versa. Quantifying and questioning such fundamental trade-offs between functional traits usually observed across species is central to plant ecology. We disentangled such trade-offs by developing and using a mechanistic individual-based model for grassland functioning (Gemini, Grassland Ecosystem Model with Individual Interactions). We showed that trait coordination at the interspecific level is tightly constrained by physiological and allometric rules existing at the intraspecific level. In a n-dimensional space, each species got a unique combination of traits, for which the plant fitness was maximised. Maximizing each species fitness under new conditions was only achieved through the joint modification of coordinated traits. Our studies particularly revealed the importance to identify the main axes of trait coordination among species. The model is available online for scientific and student communities.

Contribution 2 – From interspecfic to community levels: The importance of considering multiple axes of species specialization

Journal Articles: A3, A6, A9, A15

Figure 2: From interspecific differences to community assembling and emerging ecosystem functioning (adapted from Maire et al 2012 NPH).

As plant functional traits determine how species respond to their environment and also how they affect local resources, it has been proposed that species’ positions within a multidimensional trait space can represent their functional niche. It is important to note that more than one or two dimensions can be required to explain the species success and impact within different communities. The number of dimensions should depend on the complexity of the environmental structuring factors. For instance, we demonstrate the existence of an NH4+ / NO3- trade-off for root influx capacity, that only occured on the third axis of a principal component analysis. Such trade-off seems to indicate that grasses invest preferentially in NH4+ or NO3- in transporters. Contrasted metabolic pathways for the uptake and the assimilation of these both N forms may favor the occurrence of such trade-off. We assume that the preference for either NH4+ or NO3- can have strong impact on plant ecophysiology, plant coexistence (e.g. Boudsocq et al. 2012), plant community (Maire et al. 2012 in NPH) and ecosystem functioning (Pontes et al 2015).

Two fundamental axes of species specialization...

Figure 3: Summary of path analysis for nitrogen yield (N Yield) and leaf N content (LNC) of component traits related to root N acquisition and to shoot N utilization in 13 co-occurring pasture grass species (see Maire et al. 2009).

Two assembling rules are usually depicted in order to understand how a structured community emerges locally from a regional pool of species: The habitat-filtering (HF) rule selects species from the regional pool because they possess a similar trait syndrome suitable for a given habitat, while the niche differentiation (ND) rule selects species based on their trait dissimilarity limiting the competition and favoring the coexistence between them. We showed that these two rules are not necessarily in opposition but can jointly be at play to explain the abundance of species within grassland communities. HF largely dominated ND under favorable environments, while the ND effect increased with environmental severity. The joint effect of HF and ND on the community structure was possible as each process operated on an independent functional trait. For instance, HF selected dominant species based on plant height under favorable environments, while ND selected coexisting species that displayed a difference in growth precocity with the dominant species. We highlighted the need to better describe the niche of species using a minimum of four independent traits, so that the structure of the community could be highly explained.

The theory developed in this series of articles is well cited in my field, especially to say that the HF process includes both abiotic adaptation as well as exclusive competition of species within/out of communities (e.g. Kraft et al. 2014, Funct Ecol), and beyond. In animal ecology, it is used to explain the assembling of stream-breeding anuran communities (VargasSalinas & Amézquita 2013, Evol Ecol) while in cellular biology, it is used to describe the coexistence of cancerous cells with healthy ones (Yang et al 2013, J Cell Biochem).

Contribution 3 – Interhabitat level: Biotic and abiotic mechanisms of the soil organic matter mineralisation [Journal Articles A1, A5, A11, A22, A25]

The soil stores more carbon into soil organic matters than is contained in the atmosphere or in the living biomass. We discovered two universal mechanisms, one biotic, the second abiotic, that are able to mineralise soil organic matters and release carbon back into the atmosphere in important quantities.

Regarding the abiotic mechanism, we have shown that endoenzymes released from dead organisms can be stabilized in soil particles and have access to suitable substrates and co-factors to permit a production of CO2 over several months. This result was remarkable, because until then, the CO2 production under ambient pressure and temperature had always been regarded as intrinsically linked to living cells. This study reveals a 'soil memory' from past microbial communities, which was dependent on between-habitat differences in clay content and soil pH. These results were confirmed by an independent study (Blankinship et al 2014, Soil Biol Biochem) and the ‘extracellular respiration’ is now measured routinely in soil ecology studies (McDaniel et al 2014, Oecologia; Sinsabaugh et al 2015, Biogeochemistry).

Regarding the biotic mechanism, we have identified a fungal functional strategy that derive the energy obtained from fresh litter to decompose recalcitrant soil organic matter in return for nutrients. The intensity of this mechanism depended on soil fertility. The supply of nutrients obtained from soil organic matter decomposition may help synchronizing plant nutrient requirements. This work was published in Soil Biology & Biogeochemistry (A5), confirmed on a large set of soils (A25), and is very well cited in the soil ecology community.

Contribution 4 – Photosynthetic strategies of plant species at the global scale [Journal Articles A8, A13, A14, A16,A21,A24]

Photosynthesis is the source of nearly all energy available to living things. Understanding its adaptation to the constraints of the environment is still one of the most discussed topics in ecophysiology. Collaborating with two international teams, I have extended the development of two theories of photosynthesis through the use of two datasets to better understand the spatial and temporal regulation of photosynthesis by several hundred plant species (TRY & Globamax).

Through the theory of the coordination of photosynthesis, we demonstrated that the photosynthetic machinery is actively co-regulated by its two main biochemical limitations (nitrogen and light), when one considers the average environmental conditions experienced by plants during the Rubisco lifetime (Rubisco is the main enzyme involved in CO2 fixation). This theory has been included into the Gemini model developed during my thesis (see above the contribution 1) and into an Earth System Model (Wang 2013 Biogeosciences), making progress towards a better prediction of the carbon dynamics at the global scale.

Through the least-cost theory of photosynthesis, we have demonstrated that in opposition to the coordination of the two biochemical limitations of the photosynthesis, its biochemical and biophysical (CO2 and H2O transfers between atmosphere and leaf) limitations are completely decoupled, making room for two photosynthetic strategies. The first strategy relies on the different ways of transporting and using water into leaves. The second strategy relies on the different ways of capturing and keeping nitrogen into leaves. The aridity, temperature and elevation of the habitat determine which N or H2O strategy is best suited to maximize carbon fixation for plants.