Bontemps, Jean-Daniel (2006) Productivity changes in pure and regular stands of beech (Fagus sylvatica L.) and sessile oak (Quercus petraea Liebl.) in the Northern half of France during the XXth century. PhD thesis Sciences Forestières, LERFoB, ENGREF.

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Abstract

Historical evolution of forest productivity is investigated for the French main two social broadleaved species - common beech (Fagus sylvatica L.) and sessile oak (Quercus petraea Liebl.) - in pure even-aged stands from State forests, with an aim of description and quantification at stand scale. The analysis therefore focuses on dominant height growth, a classical proxy for forest productivity. Using a retrospective approach (stem analysis), increments were sampled from two generations of stands associated in couples to ensure control of site conditions. The evolution of growth rate is estimated by statistical modelling of increments, accounting for the effect of stand developmental stage and site differences between couples. Growth trends were assessed at a regional scale, leading to the constitution of 2 samples for beech (Normandie/Picardie, Lorraine/Alsace) and 4 samples for oak (Normandie, val de Loire, plaine d'Allier, Lorraine/Alsace). Complementary analysis of dominant radial growth (ring measurements) and wood density (X-ray microdensitometry) at breast height was performed for beech on the Lorraine/Alsace sample. Climatic and trophic conditions in sampled stands were also assessed.
The increase in dominant height growth rate is widespread on the territory, although its intensity varies along a longitudinal gradient. Whereas it appears to be more pronounced in the Northeastern area for both species, it remains weak or recent in the Northwestern area. Most of the evolution formed in the second half of the century and is affected by crises of climatic origin, specific or common to both species, of which the most marked took place in the 1940's. Contrary to beech, the 1990's were beneficial to oak and it is a reason why trends detected on oak stands culminate at a higher level compared to those on beech. The secular increase in dominant height growth rate varies between +20% and +100% in extreme situations and reaches an approximate +50-60% common to both species in Northeastern France when eluding the effect of recent years. In this area, radial growth increase is similar to that of height growth. Wood density remained stable, so that biomass productivity could follow the trend estimated on dominant height growth. Analysis of site - growth relationships allows to tackle the issue of the origin of productivity changes and suggests multi-causal determinism. So far, climate would have had essentially short-term consequences. Moreover, opposite seasonal effects on tree growth could be awaited. Faster growth rate evolution in recent decades, together with its organisation in space and the sensitivity of species to trophic conditions, emphasize the possible key role of atmospheric nitrogen deposition in the response to environmental changes.

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