Manuel Nicolas

Tree mineral nutrition is deteriorating in Europe

The response of forest ecosystems to increased atmospheric CO2 is constrained by nutrient availability. It is thus crucial to account for nutrient limitation when studying the forest response to climate change. The objectives of this study were to describe the nutritional status of the main European tree species, to identify growth-limiting nutrients and to assess changes in tree nutrition during the past two decades. We analysed the foliar nutrition data collected during 1992-2009 on the intensive forest monitoring plots of the ICP Forests programme. Of the 22 significant temporal trends that were observed in foliar nutrient concentrations, 20 were decreasing and two were increasing. Some of these trends were alarming, among which the foliar P concentration in F. sylvatica, Q. Petraea and P. sylvestris that significantly deteriorated during 1992-2009. In Q. Petraea and P. sylvestris, the decrease in foliar P concentration was more pronounced on plots with low foliar P status, meaning that trees with latent P deficiency could become deficient in the near future. Increased tree productivity, possibly resulting from high N deposition and from the global increase in atmospheric CO2, has led to higher nutrient demand by trees. As the soil nutrient supply was not always sufficient to meet the demands of faster growing trees, this could partly explain the deterioration of tree mineral nutrition. The results suggest that when evaluating forest carbon storage capacity and when planning to reduce CO2 emissions by increasing use of wood biomass for bioenergy, it is crucial that nutrient limitations for forest growth are considered.

Environmental drivers of ectomycorrhizal communities in Europe's temperate oak forests

Ectomycorrhizal fungi are major ecological players in temperate forests, but they are rarely used in measures of forest condition because large‐scale, high‐resolution, standardized and replicated belowground data are scarce. We carried out an analysis of ectomycorrhizas at 22 intensively monitored long‐term oak plots, across nine European countries, covering complex natural and anthropogenic environmental gradients. We found that at large scales, mycorrhizal richness and evenness declined with decreasing soil pH and root density, and with increasing atmospheric nitrogen deposition. Shifts in mycorrhizas with different functional traits were detected; mycorrhizas with structures specialized for long‐distance transport related differently to most environmental variables than those without. The dominant oak‐specialist Lactarius quietus, with limited soil exploration abilities, responds positively to increasing nitrogen inputs and decreasing pH. In contrast, Tricholoma, Cortinarius and Piloderma species, with medium‐distance soil exploration abilities, show a consistently negative response. We also determined nitrogen critical loads for moderate (9.5–13.5 kg N/ha/year) and drastic (17 kg N/ha/year) changes in belowground mycorrhizal root communities in temperate oak forests. Overall, we generated the first baseline data for ectomycorrhizal fungi in the oak forests sampled, identified nitrogen pollution as one of their major drivers at large scales and revealed fungi that individually and/or in combination with others can be used as belowground indicators of environmental characteristics.

The importance of biomass net uptake for a trace metal budget in a forest stand in north-eastern France.

The trace metal (TM: Cd, Cu, Ni, Pb and Zn) budget (stocks and annual fluxes) was evaluated in a forest stand (silver fir, Abies alba Miller) in north-eastern France. Trace metal concentrations were measured in different tree compartments in order to assess TM partitioning and dynamics in the trees. Inputs included bulk deposition, estimated dry deposition and weathering. Outputs were leaching and biomass exportation. Atmospheric deposition was the main input flux. The estimated dry deposition accounted for about 40% of the total trace metal deposition. The relative importance of leaching (estimated by a lumped parameter water balance model, BILJOU) and net biomass uptake (harvesting) for ecosystem exportation depended on the element. Trace metal distribution between tree compartments (stem wood and bark, branches and needles) indicated that Pb was mainly stored in the stem, whereas Zn and Ni, and to a lesser extent Cd and Cu, were translocated to aerial parts of the trees and cycled in the ecosystem. For Zn and Ni, leaching was the main output flux (>95% of the total output) and the plot budget (input-output) was negative, whereas for Pb the biomass net exportation represented 60% of the outputs and the budget was balanced. Cadmium and Cu had intermediate behaviours, with 18% and 30% of the total output relative to biomass exportation, respectively, and the budgets were negative. The net uptake by biomass was particularly important for Pb budgets, less so for Cd and Cu and not very important for Zn and Ni in such forest stands.

Forest soils in France are sequestering substantial amounts of carbon

The aim of this study was to assess whether French forest soils are sources or sinks of carbon and to quantify changes in soil organic carbon (SOC) stocks over time by resampling soil in long-term forest monitoring plots. Within each plot, and for each survey, soils were sampled at five points selected in five subplots and divided into layers. Composite samples were produced for each layer and subplot, then analysed for mass, bulk density and SOC. Linear mixed models were used to estimate SOC changes over 15years between two soil surveys carried out in 102 plots in France. A factor analysis and a budget approach were also used to identify which factors and processes were primarily responsible for SOC dynamics. Forest soils throughout France substantially accumulated SOC (+0.35MgCha-1yr-1) between 1993 and 2012. The SOC sequestration rate declined with stand age and was affected by stand structure. Uneven-aged stands sequestered more SOC than did even-aged stands (p<0.001). For the forest floor, the SOC sequestration rate estimated by the budget approach was in agreement with that based on stock comparison. This increasing SOC stock in the forest floor can be explained by recent changes in certain factors affecting litter decomposition (climate and litter quality). For the mineral soil, the budget approach was unable to replicate the observed SOC sequestration rate, probably because SOC stocks were not yet at equilibrium with litter inputs at the beginning of the monitoring period (contrary to our steady-state assumption). This explanation is also supported by the fact that the SOC sequestration rate decreased with stand age. As the SOC sequestration rate declines with stand age and is higher in uneven-aged stands, forest management has the potential to influence this carbon sink.

Modelling the impact of climate change and atmospheric N deposition on French forests biodiversity.

A dynamic coupled biogeochemical-ecological model was used to simulate the effects of nitrogen deposition and climate change on plant communities at three forest sites in France. The three sites had different forest covers (sessile oak, Norway spruce and silver fir), three nitrogen loads ranging from relatively low to high, different climatic regions and different soil types. Both the availability of vegetation time series and the environmental niches of the understory species allowed to evaluate the model for predicting the composition of the three plant communities. The calibration of the environmental niches was successful, with a model performance consistently reasonably high throughout the three sites. The model simulations of two climatic and two deposition scenarios showed that climate change may entirely compromise the eventual recovery from eutrophication of the simulated plant communities in response to the reductions in nitrogen deposition. The interplay between climate and deposition was strongly governed by site characteristics and histories in the long term, while forest management remained the main driver of change in the short term.

Environment and host as large-scale controls of ectomycorrhizal fungi

Explaining the large-scale diversity of soil organisms that drive biogeochemical processes—and their responses to environmental change—is critical. However, identifying consistent drivers of belowground diversity and abundance for some soil organisms at large spatial scales remains problematic. Here we investigate a major guild, the ectomycorrhizal fungi, across European forests at a spatial scale and resolution that is—to our knowledge—unprecedented, to explore key biotic and abiotic predictors of ectomycorrhizal diversity and to identify dominant responses and thresholds for change across complex environmental gradients. We show the effect of 38 host, environment, climate and geographical variables on ectomycorrhizal diversity, and define thresholds of community change for key variables. We quantify host specificity and reveal plasticity in functional traits involved in soil foraging across gradients. We conclude that environmental and host factors explain most of the variation in ectomycorrhizal diversity, that the environmental thresholds used as major ecosystem assessment tools need adjustment and that the importance of belowground specificity and plasticity has previously been underappreciated.Analyses of data from 137 forest plots across 20 European countries show that ectomycorrhizal fungal diversity is strongly influenced by environmental and host species factors and provide thresholds to inform ecosystem assessment tools

Sylviculture et protection des sols.

Ce travail tente d’evaluer les risques que font peser les pratiques forestieres actuelles sur la durabilite des fonctions des sols et des ecosystemes. La relation entre fonction de production et fonction de conservation des sols traduit une certaine antinomie entre ces deux options. Les contraintes pour la fertilite chimique du sol sont analysees pour les traitements des moins aux plus intensifs, c’est-a-dire des reserves forestieres aux cultures lignocellulosiques traitees a tres courte revolution. Dans les systemes extensifs, le recyclage biologique performant est durable si les exploitations se cantonnent aux seuls troncs. Les amendements permettent de corriger l’acidification lente et ineluctable des sols. Dans les traitements intensifs, la fertilite naturelle des sols forestiers ne peut soutenir la production ; seuls les sols de qualite physique convenable et avec apports d’intrants peuvent conduire a des productions elevees et soutenues, en particulier dans les systemes a courte revolution.

Author Correction: Environment and host as large-scale controls of ectomycorrhizal fungi

Change history: In the HTML version of this Article, author ‘Filipa Cox’ had no affiliation in the author list, although she was correctly associated with affiliation 3 in the PDF. In addition, the blue circles for ‘oak’ were missing from Extended Data Fig. 1. These errors have been corrected online.

L'apport des dispositifs de suivi vis-à-vis des enjeux de fonctionnement et de gestion des écosystèmes en relation avec les sols

Les changements environnementaux, particulièrement les pollutions atmosphériques et le changement climatique, pèsent fortement sur le fonctionnement futur des écosystèmes et donc sur la manière la plus adaptée de les gérer. Malgré les nombreux acquis de la recherche, une bonne part de la complexité de notre environnement reste mal comprise ; les modèles de prévision d’impact peinent souvent à reproduire les phénomènes observés, plus encore à les anticiper. Qui plus est, les changements environnementaux peuvent générer des conditions inédites remettant en question la validité d’une partie des connaissances acquises jusqu’ici et sur lesquelles les prédictions ont besoin de s’appuyer, à l’image par exemple des connaissances sur la sensibilité des arbres forestiers au climat (Lebourgeois et Mérian, 2011).

How monitoring networks contribute to the understanding and to the management of soil and forest ecosystems

Faced with the uncertainties linked to environmental change, the monitoring networks are an essential source of data in order to anticipate changes in ecosystems. In France, three national networks are concerned with the change in the soil and/or forest in relation to the soil. Initially designed to monitor the effect of atmospheric pollution on the health of forests, the systematic network for the monitoring of forest damage (RSSDF) and the RENECOFOR network correspond to the two extensive and intensive components of pan-European forest monitoring (ICP Forests). Created more recently, the RMQS specifically monitors soil quality by extending the grid for forest monitoring to other land uses (crops, meadows, natural or urban environments). Pioneering and recent initiatives regarding the speed of change of soils and forests, these networks have nonetheless already generated a large number of original results and have shown their potential to meet expectations that are becoming increasingly diversified.