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In this review, the unique features and facts of long-term experiments are presented. Long-term experimental plots provide information of forest stand dynamics which cannot be derived from forest inventories or small temporary plots. Most comprise unthinned plots which represent the site specific maximum stand density as an unambiguous reference. By measuring the remaining as well as the removed stand, the survey of long-term experiments provides the total production at a given site, which is most relevant for examining the relationship between site conditions and stand productivity on the one hand and between stand density and productivity on the other. Thus, long-term experiments can reveal the site-specific effect of thinning and species mixing on stand structure, production and carbon sequestration. If they cover an entire rotation or even the previous and following generation on a given site, they reveal a species' long-term behaviour and any growth trends caused by environmental changes. Second, we exploit the unique data of European long-term experiments, some of which have been surveyed since 1848. We show the long-term effect of different density regimes on stand dynamics and an essential trade-off between total stand volume production and mean tree size. Long-term experiments reveal that tree species mixing can significantly increase stand density and productivity compared with monospecific stands. Thanks to surveys spanning decades or even a century, we can show the changing long-term-performance of different provenances and acceleration of stand production caused by environmental change, as well as better understand the growth dynamics of natural forests. Without long-term experiments forest science and practice would be not in a position to obtain such findings which are of the utmost relevance for science and practice. Third, we draw conclusions and show perspectives regarding the maintenance and further development of long-term experiments. It would require another 150years to build up a comparable wealth of scientific information, practical knowledge, and teaching and training model examples. Although tempting, long-term experiments should not be sacrificed for cost-cutting measures. Given the global environmental change and the resulting challenges for sustainable management, the network of long-term experiments should rather be extended regarding experimental factors, recorded variables and inter- and transdisciplinary use for science and practice.

Apport de l’approche par traits fonctionnels pour comprendre les liens plantes-faune du sol dans les écosystèmes forestiers. Vèmes Journées de travail TEBIS

International audience

Key message According to biomechanical processes, tree morphology (trunk inclination, height-to-diameter ratio and crown area) explains statistically silvicultural effects on growth stress variation.Abstract Growth stresses constitute the main mechanism allowing the tree to control its posture against its mechanical environment, but are also among the most important factors contributing to the depreciation of timber value. This study aimed at assessing the link between stand planting density and growth stress level in European beech (Fagus sylvatica) stands. Beech seedlings were planted in four plots corresponding to four planting densities: 2,500, 5,000, 10,000 and 40,000 stems/ha. They were left to grow for 26 years without any intervention, resulting in trees with highly different morphologies but of the same age and provenance. After 26 years of growth, both the tree morphology and growth stress indicators were measured on the standing trees in each plot and an attempt was made to establish a link between them. Our results showed that initial stand density influences growth stresses of the first order as a result of its impact on tree morphology. The best predictors of high growth stresses were high trunk inclination, high height-to-diameter ratio (slenderness factor) and low crown area. According to mechanosensing theories, these morphological criteria emphasised that growth stresses are due to a global mechanical stimulation rather than to local stem inclination alone. Research now has to be undertaken on new methods that combine the integrative assessment of tree morphology as well as its monitoring over time.

International audience; Global warming induces new constraints on forest ecosystems and requires forest management adaptation. The reduction of stand density is currently debated in France as a potential tool to face increasing climate change-induced tree mortality risk due to summer drought by improving forest resistance to water stress. Our study aimed to assess the potential impact of this forest management adaptation on soil biodiversity, i.e. the detrital food web, and ecosystem functioning, i.e. litter and soil organic matter decomposition processes. We took advantage of a large-scale, multi-site experimental network of long-term forestry trials experimentally manipulating oak stand density through thinning operations in contrasted stand age and local abiotic context. Our results provide evidence that reducing stand density will have substantial impacts on the detrital food web structure, with cascading effects on soil functioning. While microbial biomass had little response, the effect of stand density on microbial-feeding nematodes was much more contrasted. Microarthropods such as collembolan and oribatid mites as well as diplopoda and anecic earthworms were depleted by stand density reduction. In contrast, endogeic earthworms were beneficially affected. Litter quality and decomposability decreased with stand density reduction while standard litter decomposition increased. Therefore, in situ litter decomposition was lower and carbon sequestration higher at intermediate stand density. Exploratory analysis using causal diagrams, i.e. path analyses, highlighted that those changes were mainly related to understory vegetation, microclimate and soil pH conditions alterations Overall, our study emphasizes that managing stand density of oak forests at intermediate level appears as the best way to optimize the trade-off between improving forest resistance to drought and preserving soil biodiversity and ecosystem functioning.

National audience

The mechanical environment is a significant factor that controls tree growth. Growth response to mechanical stress inhibits primary growth, stimulates radial growth and produces acclimated wood. That results in trees designed at their proper limit of mechanical stability with a low margin. Several regionalized models under IPCC scenarios predict that winter storm frequency in Europ may increase, while chronic winds speed, that is responsible of growth acclimation, may decrease. As this biomechanical response is scarcely invoked when discussing forest vulnerability and acclimation to global change, assuming implicitly that other environmental effects are more significant, we will discuss its ecological relevancy, from observations of forest tree growth in natural both dried and windy conditions (Pinus halepensis in Spain) and models of tree mechanical stability related to tree size and allometry of growth. Then, wood responses to mechanical constraints include reaction and flexure woods, that differ from “normal” wood by their cellular and cell wall structure, with great consequences on wood properties. Wood acclimation to mechanical constraints should have feedback effects on carbon and water balance at the organ or whole tree level: for instance, reaction wood anatomy could induce a lower hydraulic conductivity that could result in lower stomatal conductance. We present some results from greenhouse experiments about how controlled mechanical stimuli modify local secondary growth rate and wood mechanical and hydraulical properties. We discuss how this acclimation of wood properties could change the margin of mechanical safety of trees, stating the whole complexity of the problem involving many feedback loops.

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International audience

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National audience

National audience

National audience