Thierry Gauquelin

Secondary metabolites of Pinus halepensis alter decomposer organisms and litter decomposition during afforestation of abandoned agricultural zones

1. Over a century of agricultural abandonment across the Mediterranean region has favoured the installation of the pioneer expansionist species Aleppo pine (Pinus halepensis Miller). This species synthesizes a wide range of secondary metabolites that are partially released during needle decomposition , and which can thus affect the brown food chain. Litter decomposition is a key process connecting ecosystem structure and function, and involving microbial and faunal components. n2. The goal of this study was to determine the effect of chemical compounds from Aleppo pine needles on the litter decomposition process along a gradient of Mediterranean forest secondary succession. Using in situ litterbags, we compared the dynamics of decomposers, particularly the relative contributions of fungal and mesofauna biomass to litter mass loss (calculations based on the measured decomposer biomass, published fungal growth efficiency and mesofauna feeding rate), against the dynamics of secondary metabolites associated with decomposed needles in three successional stages (early, middle and late, i.e. pinewoods that were aged 10, 30 and over 60 years old). n3. Our first key finding was that fungi accounted for the largest portion of overall litter mass loss (60– 79%) and detritivorous mesofauna contributed to 8–12%. In the early stage of succession, fungal biomass after 6 months of decomposition was lower than in middle and late stages, and may be responsible for the delay in litter colonization by mesofauna. We linked this result to a clearly longer residence time for phenolic compounds in young pine forest, leading to an overall slowdown in the decomposition process. n4. Synthesis. Litter phenolic content emerged as a key functional trait for predicting litter decomposition , delaying the colonization of litter by decomposers in Mediterranean forest ecosystems. Another key finding is that the relative contributions of fungi and detritivores to needle mass loss were different between the successional stages. From the food-web perspective, the organic matter available for higher trophic levels thus remains unchanged beyond 30 years after pine colonization.

Climate change effects on litter decomposition: intensive drought leads to a strong decrease of litter mixture interactions

BackgroundLitter decomposition is a fundamental process of biogeochemical cycles, and there is axa0strong consensus that litter mixture interactions are one of the factors driving the decomposition process. A better understanding of how climate change can alter interactions between species and the litter decomposition process could facilitate projections of ecosystem functioning into the future.MethodsA 24-month litterbag decomposition experiment was carried out in a Mediterranean forest to analyze the effects of climate and species diversity changes on litter mixture interactions and the decomposition process.ResultsIn the control plot, synergistic interactions increased with time and species diversity in litter mixtures, leading to more efficient litter decomposition. Drier conditions obtained in the field with a rain exclusion device decreased decomposition rates, resulting in three-fold less synergistic interactions and five-fold more antagonistic interactions during the decomposition process. Furthermore, synergistic interactions were better preserved in the drought conditions with increasing number of species.ConclusionsOur findings underline how a longer drought season could strongly affect the relationship between biodiversity and ecosystem functioning. Drier climate led to slower mass loss rates and a strong shift in the litter mixture interactions, with fewer synergistic interactions and more antagonistic interactions.

2 Atmospheric Deposition in France and Possible Relation with Forest Decline

With the exception of a few pioneering studies such as the work of Albert Levy (1877-1907, in Ulrich and Williot 1993) at the turn of the last century, precipitation chemistry monitoring in rural areas really began in France at the end of the 1970s (BAPMON and EMEP networks), as a result of international studies showing the negative impact of precipitation acidity on surface waters and the possibilities of long-range pollution transport (Gorham and Gordon 1960; Zephoris et al. 1984). No dense deposition network covering the whole country was organized until the end of the 1980s, but a large number of local studies were initiated, the results of which were often never published.

Potential Shift in Plant Communities with Climate Change: Outcome on Litter Decomposition and Nutrient Release in a Mediterranean Oak Forest

The ongoing decline in biodiversity has fuelled concerns about its impact on ecosystem functioning. Mediterranean oak forests may prove very sensitive to global change, which could strongly influence the species composition of plant communities and thereby affect ecosystem processes. To determine the potential outcome of shifts in species composition on litter decomposition dynamics, we conducted a full-factorial decomposition experiment over a gradient of litter species diversity in a Mediterranean Downy oak (Quercus pubescens Willd.) forest. We used litter from the three dominant tree species naturally present in the Downy oak forest and litter from Aleppo pine (Pinus halepensis Mill.) in anticipation of its possible spread in the future with global change. Litter water holding capacity and N/P ratio were the most important and positive drivers of decomposition process. In contrast to other ecosystems where synergistic non-additive effects are prevalent, we observed 54% of additive and 46% of non-additive effects on litter mass loss in our Mediterranean ecosystem. These results could indicate less complementarity among decomposers for decomposition of diverse plant material in such a stressful climate. Moreover, dominant tree species are of key importance for nutrient availability, and the arrival of Aleppo pine would strongly reduce the N release during the decomposition process. Based on calculations of an annual partial nutrient budget at the ecosystem level, we showed that a shift in plant communities could affect nutrient release, ranging from 5 to 36% for N and 63 to 83% for P depending on the co-occurring tree species in the Downy oak forest.

Plant litter mixture partly mitigates the negative effects of extended drought on soil biota and litter decomposition in a Mediterranean oak forest

A major challenge of current ecological research is to determine the responses of plant and animal communities and ecosystem processes to future environmental conditions. Ecosystems respond to climate change in complex ways, and the outcome may significantly depend on biodiversity. We studied the relative effects of enhanced drought and of plant species mixture on soil biota and on litter decomposition in a Mediterranean oak forest. We experimentally reduced precipitation, accounting for seasonal precipitation variability, and created a single-species litter (Quercus pubescens), a two-species litter mixture (Q. pubescens + Acer monspessulanum) and a three-species litter mixture (Q. pubescens + A. monspessulanum + Cotinus coggygria). nIn general, drier conditions affected decomposers negatively, directly by reducing fungal biomass and detritivorous mesofauna, and also indirectly by increasing the predation pressure on detritivorous mesofauna by predatory mesofauna. This is reflected under drier conditions in that Collembola abundance decreased more strongly than Acari abundance. One Collembola group (i.e. Neelipleona) even disappeared completely. nIncreased drought strongly decreased litter decomposition rates. Mixed litter with two and three plant species positively affected soil biota communities and led to a more efficient litter decomposition process, probably through a greater litter quality. Faster decomposition in mixed litter can thus compensate slower decomposition rates under drier condition. nSynthesis. Our results highlight that, within our study system, drier climate strongly impacts on soil biodiversity and hence litter decomposition. Species-rich litter may mitigate such a decline in decomposition rates. Diverse plant communities should hence be maintained to reduce shifts in ecosystem functioning under climate change.

Effect of organic amendment on soil fertility and plant nutrients in a post-fire Mediterranean ecosystem

Backgrounds and aimsIn Mediterranean frequently burnt areas, the decrease of soil fertility leads to regressive vegetation dynamics. Organic amendments could help to accelerate post-fire ecosystem resilience, by improving soil properties and plant nutrition. This study was conducted to assess the potential of a composted biosolid to restore an early post-fire shrubland.MethodsAbout 50xa0Mg.ha−1 of fresh co-composted sewage sludge and green wastes were surface applied 7xa0months after fire on a silty-clayey soil. We monitored over a 2-year period organic matter and nutrient transfers to soil, nutrient responses of dominant plant species, and ecosystem contamination by potentially toxic trace elements.ResultsOver the experimental survey, compost rapidly and durably improved soil P2O5, MgO and K2O content, and temporarily increased N-(NO3− + NO2−) content. Plant nutrition was improved more or less durably depending species. The most positive compost effect was on plant and soil phosphorus content. Plant nutrient storage was not improved 2xa0years after amendment, suggesting luxury consumption. No contamination by trace elements was detected in soil and plant.ConclusionsThe use of compost after fire could help for rapidly restoring soil fertility and improving plant nutrition. The increase of soil nutrient pools after amendment emphazised the diversity of plant nutritional traits. Eutrophication risk could occur from high compost and soil P2O5 content.

Mediterranean forests, land use and climate change: a social-ecological perspective

Mediterranean forests are found in the Mediterranean basin, California, the South African Cape Province, South and southwestern Australia and parts of Central Chile. They represent 1.8xa0% of the world forest areas of which the vast majority is found in the Mediterranean basin, where historical and paleogeographic episodes, long-term human influence and geographical and climatic contrasts have created ecosystemic diversity and heterogeneity. Even if evergreen is dominant, deciduous trees are also represented, with different forest types including dense stands with a closed canopy (forests sensu stricto) and pre-forestal or pre-steppic structures with lower trees density and height. The Mediterranean basin is also a hot spot of forest species and genetic diversity, with 290 woody species versus only 135 for non-Mediterranean Europe. However, the characteristics of the Mediterranean area (long-standing anthropogenic pressure, significant current human activity and broad biodiversity) make it one of the world’s regions most threatened by current changes. Four examples of Mediterranean forest types, present in south and north of the Mediterranean basin and more or less threatened, are developed in order to show that linking “hard sciences” and humanities and social sciences is necessary to understand these complex ecosystems. We show also that these forests, in spite of specific climatic constraints, can also be healthy and productive and play a major ecological and social role. Furthermore, even if the current human activity and global change constitute a risk for these exceptional ecosystems, Mediterranean forests represent a great asset and opportunities for the future of the Mediterranean basin.

Use of urban composts for the regeneration of a burnt Mediterranean soil: A laboratory approach

In Mediterranean region, forest fires are a major problem leading to the desertification of the environment. Use of composts is considered as a solution for soil and vegetation rehabilitation. In this study, we determined under laboratory conditions the effects of three urban composts and their mode of application (laid on the soil surface or mixed into the soil) on soil restoration after fire: a municipal waste compost (MWC), a compost of sewage sludge mixed with green waste (SSC) and a green waste compost (GWC). Carbon (C) and nitrogen (N) mineralisation, total microbial biomass, fungal biomass and soil characteristics were measured during 77-day incubations in microcosms. The impact of composts input on hydrological behaviour related to erodibility was estimated by measuring runoff, retention and percolation (i.e. infiltration) of water using a rainfall simulator under laboratory conditions. Input of composts increased organic matter and soil nutrient content, and enhanced C and N mineralisation and total microbial biomass throughout the incubations, whereas it increased sporadically fungal biomass. For all these parameters, the MWC induced the highest improvement while GWC input had no significant effect compared to the control. Composts mixed with soil weakly limited runoff and infiltration whereas composts laid at the soil surface significantly reduced runoff and increased percolation and retention, particularly with the MWC.

Gender, mediterranean drought, and seasonality: photosystem II photochemistry in Pistacia lentiscus L.

In this work, photosystem II (PSII) photochemistry, leaf water potential, and pigment contents of male and female Pistacia lentiscus L. were investigated during a seasonal cycle at three different, arid locations: superior semiarid, inferior semiarid, and arid. The results showed that the gender, season, and the site conditions interacted to influence the quantum yield and pigment contents in P. lentiscus. Predawn leaf water status was determined only by the site and season. The annual patterns of PSII maximum quantum efficiency (Fv/Fm) were characterized by a suboptimal activity during the winter, especially, populations with the more negative water potential exhibited a lower chlorophyll (Chl) a content and chronic photoinhibition irrespective of a gender. We also demonstrated that both photochemical or nonphotochemical mechanisms were involved to avoid the photoinhibition and both of them depended on the season. This plasticity of photosynthetic machinery was accompanied by changes in carotenoids and Chl balance. In the spring, the female Fv/Fm ratio was significantly higher than in male individuals, when the sexual dimorphism occurred during the fruiting stage, regardless of site conditions. P. lentiscus sex-ratio in Mediterranean areas, where precipitations exceeded 500 mm, was potentially female-biased. Among the fluorescence parameters investigated, nonphotochemical quenching coefficient appeared as the most useful one and a correlation was found between Chl a content and Fv/Fm. These results suggest that functional ecology studies would be possible on a large scale through light reflectance analysis.

Evaluation of Plant Responses to Atmospheric Nitrogen Deposition in France Using Integrated Soil-Vegetation Models

The aim of this chapter is to give an overview of plant responses to nitrogen (N) deposition by using two dynamic biogeochemical models coupled with a vegetation module: VSD+-VEG and ForSAFE-VEG. The biogeochemical models were first validated on some French forest sites from the ICP-Forests network. A French vegetation table (which is now part of a European database) containing 230 species with their appropriate ecological environmental parameters, was set up. The outputs of each model in terms of plant response to atmospheric nitrogen deposition were compared to measured values for one forest stand. The two models underestimated the occurrence of certain herbs and grasses and overestimated (ForSAFE-VEG) or underestimated (VSD+-VEG) the presence of certain mosses. This allowed us to improve the validation and thus the calibration of some parameters. For the simulated period ForSAFE-VEG indicated some variations in the occurrence of the plant groups, the mosses group showing the highest increase and indicating a high sensitivity to atmospheric N deposition. No significant changes in the occurrence percentage of plant groups were observed by running the VSD+-VEG model, this model being less sensitive than ForSAFE-VEG to simulate tenuous vegetation changes. The observed changes over time in the dominant ground plant groups using ForSAFE-VEG could be related to changes in site environmental conditions, but only the influence of the maximum N deposition was obvious. Further investigations are needed to compare the performance of the two models on other sites, but these tests of the ForSAFE-VEG and VSD+-VEG models showed promise for simulating the link between N deposition and vegetation diversity.