1. Biogeochimistry in Forest Ecosystems

Juliette HEROUET, Thomas DROUET

The main focus of this work is to use the natural variation of stable isotopes to understand element cycling in forest ecosystems. Strontium (Sr) isotopes are used as a proxy for calcium (Ca) and offer a powerful tool to discriminate between atmospheric and mineral weathering sources of Ca in tree nutrition. The origin and fluxes of Ca is examined at several field sites with contrasting nutrient status. The coupling of this isotopic method with the archive properties of tree-rings (dendrochemistry) adds an important piece to the knowledge of the soil acidification story. We are currently testing the use of other isotopic tracers (multi-isotopic approach: Sr-Pb-Nd) to provide data on mineral weathering, atmospheric depositions and anthropogenic sources of nutrients. Such method appears to be a promising way to determine the impact of several tree species on the soil properties and the nutrient biogeochemistry. This work is associated with the development of simple computer model to examine the role of internal fluxes (recycling) and highlight the strategies of nutrient acquisition by trees. Financial support is provided by the Fonds National de la Recherche Scientifique (FNRS, Belgium) through the convention FRFC nr 2.4607.07.

2. Ecology of Plant Invasion


Alien invasive plants represent an important dimension of global change. In NW Europe, a number of
alien species are rapidly spreading (Fallopia japonica, Senecio inaequidens, Solidago gigantea, Impatiens glandulifera, Prunus serotina, Heracleum mantegazzianum, Rosa rugosa). It is suspected that these species have profound impacts on soil chemical properties and ecosystem functioning.
We examine the impacts of these species on organic matter cycling. This include litterbags experiments in situ, assessment of change in the composition of soil animal communitites, and alterations in nitrogen mineralisation due to changes in the soil microbe communties.
We also test if other components of global change, specifically eutrophication, may enhance the invasive capacity and impacts of alien invasive species. This is studied in cultivation in semi-controlled conditions.
An important study case are Japanese knotweeds (Fallopia div. taxa). Different taxa are spreading in Belgium. We compare functional traits of a representative sample of genotypes in order to test if differences in invasive capacity among them might be due to differences in some ecophysiological attributes, including specific leaf area, shoot architecture, nitrogen economy and phenotypic plasticity.
This research is supported by the Fonds de la recherche scientifique (FRS/FNRS) (FRFC project in collaboration with ULg-Gembloux AgroBioTech, Lab. of Ecology) and the Belgian Science Policy (Science for a Sustainable Development, Project Alien Impact, in collboration with UA, VUB, UCL and ULg-Gembloux AgroBioTech).

3. Ecology and Evolution of heavy metal Tolerance and Accumulation

3.1. Evolutionary ecology of a zinc hyperaccumulator

Nausicaa NORET and Pierre MEERTS

Thlaspi caerulescens, a Cd-Zn hyperaccumulator, is used as a model species to address fundamental questions on the evolutionary and adaptive significance of heavy metal hyperaccumulation in plants:
- Is hyperaccumulation a defence against herbivores and pathogens?
- How the variation in hyperaccumulation is structured within and among populations in relation with life-history traits?

3.2. Ecology and evolution of copper/cobalt tolerant plants in Katanga (DR Congo)


In Katanga (DR Congo), natural outcrops of bedrock contaminated with Cu and Co (“copper hills”) are colonized by unique vegetation, consisting of heavy metal tolerant species (cuprophytes). A number of species are endemic of those sites. Plant communities on those sites have an extremely variable species composition, in relation to complex environmental gradients. A number of species have evolved the extraordinary capacity to hyperaccumulate Cu and/or Co. Our research develops in three directions.
-Firstly, we examine if cuprophytes might be domesticated in order to restore soil which has been degraded by mining activities (phytostabilisation). This research is performed in the botanical garden at Lubumbashi and in situ in a polluted site.
-Secondly, we examine the factors that may explain variation in metal accumulation in plants in the field. This comprises cultivation experiments in various conditions.
-Thirdly, we analyse functional trait variation in plant communities along copper contamination gradients. The target traits include all morpho-pheno-physiological attributes that might be correlated to ecological niche (i.e. ecological optimum and amplitude along metal concentration gradients). In particular, we test if endemic species have evolved particular traits in response the selective pressure of high metal concentration in the soil.

This research is funded by the Coopération Universitaire au Développement (CUD, project REMEDLU, in collaboration with the University of Lubumbashi, and ULg-Gembloux AgroBioTech) and by the FRS/FNRS (FRFC project in collaboration with UCL (Mycothèque) and ULg-Gembloux AgroBioTech.

4. Taxonomy of the Central African flora


In collaboration with the Herbarium and Library of African Botany of ULB, our laboratory contributes to the elaboration of the Flore d'Afrique Centrale (DR Congo, Rwanda, Burundi), edited by the National Botanical Garden of Belgium. In particular, the complex genus Chlorophytum (Asparagaceae) is being revised, based on herbarium materials and original field observations, in collaboration with the Museum of Natural History of Oslo (molecular phylogeny).