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Combined effects of atmospheric pollution and climate change

Anne Probst (CNRS) takes the floor

Simulating the combined effects of atmospheric pollution and climate change on forest ecosystems

Summary

Human activity has been contributing to increasing sulphur and nitrogen levels in the atmosphere, most notably since the end of the 19th century. These compounds, emitted into the atmosphere where they can travel long distances, then fall as atmospheric deposits, often affecting forest ecosystems.

Sulphur emissions have recently been restricted at the European scale, but nitrogen compounds are still emitted in important quantities and are difficult to control due to their variety of sources and forms and to the complexity of the nitrogen cycle and its chemical transformations. However, within the framework of the Geneva Convention on Long-range Trans-boundary Air Pollution (1979), European countries are making an effort to reduce emissions and atmospheric deposits, notably for nitrogen compounds.

Nitrogen is, of course, an important nutrient for forests, but nitrogen deposition has a recognised impact on soil biogeochemistry, the nutrient balance, tree growth, and more generally on forest health and under-story plant biodiversity. These effects depend on the environmental characteristics specific to each forest, and, when combined with the effects of the global climate change now under way, impact and upset forest ecosystem functions, though we do not as yet understand the processes involved.

It is primordial in such a context to be able to predict the effects of atmospheric deposition on the forest ecosystems in France. To do this, models such as ForSAFE-VEG, have been developed, which couple biogeochemical and ecological data. They make it possible to simulate the long-term impacts of atmospheric deposition and climate change combined on the biogeochemical responses of the soil and to predict cascade effects on forest biodiversity, while also integrating local environmental characteristics in order to define critical loads (a tool designed to assess the sensitivity of an ecosystem to nitrogen deposition). Developing and applying such models depends on having robust entry and validation data (quality and quantity of precipitation, soil solution composition, soil characteristics, biomass estimates, vegetation surveys...). These data are indispensable and must be available for each site or ecosystem the models are applied to.

The first research work on this coupled modelling approach has shown that the models must be adapted to the territory under evaluation to be pertinent. In France, the data obtained from the 25 years of RENECOFOR observations is of exceptional value when calibrating and testing site-scale models, which will then be extrapolated to the forest-ecosystem scale, and spatialised at the national scale. This spacialisation will be a robust basis on which to agree on concrete Europe-wide emission-control measures. Around ten CATAENAT plots were selected to validate and calibrate the models, then the 102 RENECOFOR plots, combined with other well-informed networks in France (the EcoPlant data base with 6000 sites, in particular), were used in the extrapolation phase. Parametrising vegetation data makes it possible to elaborate logistic regression models capable of predicting changes in several hundred forest plant species in mainland France as they respond to major alternations in environmental conditions.

Climate change and changes in nitrogen deposition have an important influence on soil solution composition on one hand, and on plant community composition on the other hand. Results underline the combined impact of deposition and climate change scenarios on certain key biogeochemical forest soil parameters; they highlight a predominant effect of climate on base cation saturation percentages, and of deposition on the nitrogen cycle. Furthermore, impacts have been found on long-term species abundance and diversity, depending on the species and its ecological preferences, as illustrated for plot CHS 41 in Fig. a for nitrogen deposition, and in Fig. b for the combined deposition-climate change scenario. Responses for ferns, grasses, forbs, mosses and bushes have been simulated (Fig. a) and show the ambiguous effect of nitrogen: either inhibiting development (grasses) or favouring it (mosses).

RENECOFOR site CHS41 located in a sessile oak stand. a) Top: Changes in under-story vegetation cover (in %) over time, simulated by the ForSAFE-Veg model. Diamonds and dotted blue lines: recorded deposits and simulated deposits, respectively (Probst et al., 2015). B) Bottom: Changes in the Czekanowski criterion: comparison of plant community composition under several of climate scenarios (T°+3.4C (A2)) and with no change in climate (No_CC)) combined with nitrogen deposition (MFR: Maximum Feasible Reductions in N emissions, and CLE: Current LEgislation), with a baseline scenario (BKG) (Rizzetto et al., 2016).
RENECOFOR site CHS41 located in a sessile oak stand. a) Top: Changes in under-story vegetation cover (in %) over time, simulated by the ForSAFE-Veg model. Diamonds and dotted blue lines: recorded deposits and simulated deposits, respectively (Probst et al., 2015). B) Bottom: Changes in the Czekanowski criterion: comparison of plant community composition under several of climate scenarios (T°+3.4C (A2)) and with no change in climate (No_CC)) combined with nitrogen deposition (MFR: Maximum Feasible Reductions in N emissions, and CLE: Current LEgislation), with a baseline scenario (BKG) (Rizzetto et al., 2016). © Anne Probst / CNRS

Even though climate seems to be the main driver, as shown by its obvious impact on base cations from 2080 onwards and on plant species responses, reducing nitrogen emissions as much as possible (MFR) has a positive effect on biodiversity. However, peaks and significant variations in plant cover coupled with clearly similar trends in the short term (25 years) can be linked to one or more factors, in addition to the role played by climate change and atmospheric nitrogen deposits.

Silvicultural harvesting, which opens up the canopy, or climatic events (storm events) can influence cation concentrations in the soil. In addition, more light available in the understory can also increase the ground cover percentage of light-demanding and semi-light-demanding species, and cause a decrease in the abundance of shade-tolerant species. 

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