Maître de recherche
Chargé de cours invité
Year Label Educational Organization 1996 Candidat ingénieur agronome Université catholique de Louvain 1999 Ingénieur agronome Université catholique de Louvain 2003 Diplômé d'études complémentaires en statistique Université catholique de Louvain 2006 Docteur en sciences agronomiques & ingénierie biologique Université catholique de Louvain
Research activities on forest ecosystem functioning
Given the various types of pressure affecting forest ecosystems (climate change, air pollution, increased harvesting intensity), one may wonder whether forests will be able to continue to ensure their essential roles for our society in the future (wood production, recreation, biodiversity conservation, watershed protection, carbon storage).
I chose to tackle three crucial issues that must be addressed to better understand the functioning of forest ecosystems under global change and to design sustainable silvicultural systems. For each of them, I present my research objectives and the approaches used to achieve them as well as the main results.
1.1 How tree species diversity affects forest ecosystem functioning?
As a legacy of past sylvicultural systems (conversion of oak coppices into high forests), we can find pure and mixed paches of oak and beech close to each other in the broadleaved forests of the Ardenne. This exciting opportunity was used to design a common garden experiment in order to compare the biogeochemical functioning of mixed stands of oak and beech with that of the corresponding pure stands.
Four plots were established on a same site (Baileux) located in the western part of the Ardenne: two plots in stands dominated either by oak or by beech and two plots in a mixture of both species. These plots are all situated on the same tableland (305-312 m elevation) and were selected in such a way that species composition was the main varying factor. On this experimental site, two PhD theses (including mine) and two additional field studies were completed to quantify the main carbon, water and nutrient fluxes.
During my PhD thesis, I focused on organic matter recycling and particularly on the processes that regulate leaf litterfall, decomposition and accumulation. The objective was to evaluate the relative importance of the different factors by which tree species composition can influence leaf litter dynamics in mixed stands of oak and beech.
The total litterfall amounts were similar in all stand types; however, in mixed stands, species composition of leaf litter varied from place to place according to the spatial distribution of the two tree species. A mechanistic and spatially-explicit leaf dispersal model accounting for leaf litter production (allometric equation), wind effects and release height (ballistic approach) was therefore developed to predict the spatial distribution of leaf litter in mixed stands (Jonard et al. 2006). Based on this expertise, I collaborated to the development of a seed dispersal model for Prunus serotina (Pairon et al. 2006).
In parallel, a transplantation litterbag experiment was conducted to evaluate separately the effects of leaf litter quality (including non-additive mixture effects) and of microenvironmental conditions on leaf decomposition. Leaf litterbags were incubated under oak, under beech and in mixed stands during three years. To better understand the environmental effects, ground climate was measured in the different stands and the effects of soil water content on decomposition were evaluated using roofs to simulate drought conditions. This study showed (i) that litter quality is the major factor regulating leaf litter decomposition and accumulation in stands with varying proportions of oak and beech, and (ii) that micro-environmental conditions are factors of secondary importance (Jonard et al. 2008). The decomposition of the forest floor was also studied from soil respiration measurements taken in situ and during laboratory incubation of forest floor samples (Jonard et al. 2007).
I was also involved in the other PhD thesis conducted on the same site by Frédéric André who investigated the influence of species composition and stand density on atmospheric deposition. In his work, he quantified the effects of canopy structure on the partitioning of rainfall into throughfall, stemflow and interception (André et al., 2008a,b; André et al., 2010) as well as on the associated ion fluxes, distinguishing external inputs (wet and dry deposition) from internal cycling (canopy exchanges) (André et al., 2007 and 2008c,d).
Finally, I participated in a study of tree transpiration whose objective was (i) to compare oak and beech tree sap flux density and mean stomatal conductance in the pure stands and within the mixed stand and (ii) to evaluate the mixture effect by comparing tree sap flux density and mean stomatal conductance of a same species in the pure and mixed stands (Jonard et al. 2011).
1.2 Impact of silvicultural practices (thinning, liming-fertilization, harvesting intensity) on tree nutrition
My first study on tree nutrition concerned the long-term effects of thinning on the foliar nutritional
status of Norway spruce. Based on thinning trials, I showed that the increase in the forest floor mass with stand density was accompanied by a parallel increase in the foliar N, P and K concentrations (Jonard et al., 2006). At that time, we hypothesized that the forest floor provides an alternative medium for root growth, more favorable to P uptake than the underlying mineral soil. During a postdoctoral stay in France (INRA Bordeaux, UMR TCEM), I combined a greenhouse experiment with a modeling approach to test this hypothesis. The experiment aimed at (i) comparing the P uptake of maritime pine seedlings growing either in mineral soil or in mineral soil covered with a forest floor layer and (ii) evaluating the contribution of the forest floor to P supply using an isotopic tracer (Jonard et al., 2009). The comprehensive description of the two soil-plant systems was used to develop and test a model integrating the processes that regulate P availability and P uptake (Jonard et al., 2010). These combined approaches allowed me to demonstrate the major role of the forest floor in P nutrition.
Following the widespread tree decline that occurred on acid soils in the 1970s and 1980s, liming was recommended as a treatment to prevent the negative effects of the so-called acid rain and to restore tree vitality. In 1995, various liming trials were installed throughout the Ardenne. I recently completed a synthesis of the results obtained in these trials. Liming improved significantly Mg and Ca nutrition, reduced the increase in defoliation that occurred during the three consecutive dry years (2003 to 2005) and increased stand productivity (Jonard et al. 2010). Afterwards, a dendrochronological study confirmed the positive effect of liming on tree radial growth; this effect was however reduced under drought conditions (Van der Perre et al. 2012).
Nutrient budget approaches were also developed to estimate the risks associated to increased harvesting intensity to meet the demand in biomass energy (André et al. 2010, Genet et al. 2011).
3. Is tree vitality threatened by global environmental change?
3.1 Lessons learned from the long-term monitoring of forest ecosystems
In the context of collaborations with the French forest services (ONF) and with the research unit BEF (INRA Nancy), I processed the foliar chemistry data of the French, Walloon and Luxembourg plots of forest monitoring. This statistical analysis revealed a general decrease in the foliar P concentration of all broadleaved species between 1993 and 2007 (Jonard et al. 2009). With the same partners, I put together different data sets collected in the Ardenne between 1978 and 2009 in order to characterize the long-term temporal trends in soil solution chemistry, foliar nutrition, and crown conditions. This study allowed us to propose hypotheses to explain the triggering of tree decline symptoms after the 2003 summer drought (Jonard et al. 2012).
3.2 Synthesis and integration of knowledge into process-based models
Within the modeling platform CAPSIS (CIRAD Montpellier, UMR AMAP), I am currently elaborating a model (HETEROFOR) describing tree growth and resource use (solar radiation, water and nutrients) in heterogeneous forests. The model is developed at the individual scale according to a process-based approach in order to account for interactions between individuals of different tree species and of different size and to take global change into account. In addition, HETEROFOR should allow us to better understand tree response to water and nutritional stresses. At first, it will be used to predict change in tree species composition in pure and mixed stands of oak and beech in the Ardenne and to analyze how competition between the two tree species will evolve according to different climate and silvicultural scenarios.
I am also involved in the Belspo project ECORISK for which I collaborate to the development of a nutrient module to be integrated in the stand-scale forest model ANAFORE.