Metallicolous grasslands and plant interactions

What are metallicolous grasslands and plant interactions?

Metallicolous grasslands are plant communities developping on soils with high content of metal elements (copper, lead, arsenic, zinc, ect.). These habitats are either natural soils deriving from the weathering of rocks with high metal content or from old mine site tailings. Plant species occurring in these habitats are generally stress-tolerant species. These metallicolous grasslands are both exceptional in terms of biodiversity, as they are the result of tens or hundreds of generations of evolution in stressful environments, and in terms of rarity, as there is only a limited number of sites where they can be observed. They are also unique resources that could be exploited for the development of phyto-managment technologies including revegetation.

When plants are growing close to each other, they can have an effect either positive or negative on their neighbour. These plants are interacting. In mettalicolous species, present species are expected to be proned to positive interactions (i.e. facilitation), similar to other stressed environments (dry or cold for instance). Facilitation indicates that some metallicolous species, by their presence and development, can modify their surrounding environments allowing the mitigation of stress factors. Then development of neighboring beneficiary species is improved.

Objectives of this research axis:

The main aim of this research axis is to better understand several factors along metallic gradients in soils (from low to very high presence) including (i) functional strategies of plants (how these species grow, survive, reproduce in the studied environments), (ii) the importance and direction of plant interactions and their changes along metallic gradients, (iii) determine which species are associated as a result of facilitation, (iv) what the driving mechanisms of facilitative effects are. The anticipated results should help to propose solutions for ecological management of these habitats.

Methodological approaches are diverse. They combine: (i) measurements of plant functional traits to better characterize species strategies, (ii) observation of patterns of association among species and (iii) in situ plant interaction experiments. These different approaches are implemented in different sites of the Biros Valley, either at the mountain belt at the Bocard d’Eylie and the old Uretz’s ore-whashing plant (La Plagne site) or in the Chichoué area at the subalpine belt.

Main researchers:

Florian Delerue, GE lab,  Associate Professor at the Polytechnical National Institute (Bordeaux-INP) (fdelerue_at_ensegid.fr)

Richard Michalet, EPOC lab, Full Professor at University of Bordeaux.

David Nemer, EPOC lab, PhD student, assessing the influence of species provenances on their responses to plant interactions.

Hugo Randé, EPOC lab, PhD student, assessing the driving mechanisms of facilitation between species.

Main achievements:

See publications (data and publications) regarding functionnal strategies of plant species in metalliferous ecosystems and on plant-plant interactions

A brief synthesis is presented here:

First, the functional syndromes of metallophyte species proved to be more diverse than expected with regard to the conservation or rapid acquisition of soil resources (functional axis of the Leaf Economic Spectrum). Two major findings emerged: 1. The observed functional syndromes, together with environmental measurements (WP1), consistently show that in addition to metal toxicity, disturbance (via soil instability) is a key ecological factor shaping ecosystem functioning on mining residues. 2. A novel association was identified between the Leaf Economic Spectrum and the exclusion or accumulation of metals in aboveground plant parts. This discovery led to further sampling on additional sites to assess the generality of this relationship.

Regarding interaction dynamics, both observational and experimental approaches showed a gradual shift from competition to facilitation as pollution levels increased. This supports the Stress Gradient Hypothesis (SGH) and contradicts alternative models within the studied systems. Specifically: 1. Metallophyte species are effectively excluded from less contaminated zones due to competitive pressure. 2. Facilitation increases when interacting species or individuals are functionally distinct, contributing to the maintenance of functional diversity. Since metallophyte species themselves are functionally diverse, they can also facilitate one another, likely explaining why facilitation remains strong even under the most extreme conditions.

As for the facilitation processes, they are mainly linked to microclimatic improvements due to shading and transpiration by metallophyte species. Further ongoing analyses are investigating potential effects related to soil stabilization.

Additionally, negative effects of elemental allelopathy were demonstrated in the less contaminated areas of the gradients. In these zones, species that accumulate metals in their leaves (and consequently in their litter) interact with metal-sensitive species that have low tolerance. We also demonstrated the link between these effects and the functional syndromes of metallophyte plants. So-called acquisitive plants, which also tend to accumulate metals, have the most pronounced effects because: 1. they transpire more (enhancing microclimatic conditions), and 2. they produce leaf litter rich in metals that decomposes rapidly (elemental allelopathy).

These findings are summarized in the illustration below. However, these facilitative effects disappeared during the heatwave of summer 2022. Given the critical role of facilitation in contaminated zones, this raises important questions about the future dynamics of these communities under climate change.