Jean-Christophe Hervé

Nitrogen footprint in a long-term observation of forest growth over the twentieth century

Environmental drivers of forest productivity increases have been much debated. Evidence for the suggested role of increasing nitrogen supply is lacking over long-term time scales. Tracking the footprint of environmental factors by using long-term growth records may thus prove decisive. We analysed growth chronologies of common beech in two areas of contrasting nutritional status in France. Dominant height growth was used as a proxy for productivity. Growth was compared between old and young paired stands sampled at the same sites to factor out effects of ageing and site. Growth chronologies were estimated with a statistical modelling procedure. The environmental causality of growth changes was addressed by combining (1) a comparison of growth changes between regions, (2) a regional comparison of growth chronologies with chronologies of environmental factors and (3) growth–environment relationships established from climate/soil data. Historical growth increases followed very similar courses in the two areas. Remarkably, the magnitude of change was 50% lower in the area that had reduced nutritional status and nitrogen deposition. Historical variations in environmental factors and growth were congruent with the roles of nitrogen availability and deposition, and of atmospheric CO2 increase. Low-frequency variations in climate and growth were not coincident. However, our analysis demonstrated the role of climatic anomalies in short-term growth variations. Growth–environment relationships further indicated a nitrogen constraint. These observations corroborate the enhancing role of increased nitrogen availability on forest biomass accumulation previously reported in ecosystem experiments and process-based modelling explorations.

L’expansion séculaire des forêts françaises est dominée par l’accroissement du stock sur pied et ne sature pas dans le temps

Après avoir connu un phénomène de transition forestière (Mather, 1992) dont l’ancienneté est d’ordre séculaire à pluridécennal selon les pays (Meyfroidt et Lambin, 2011), la forêt européenne présente actuellement, au plan mondial, la singularité de connaître à la fois une expansion de sa surface et de son stock sur pied (+ 0,08 %/an pour les surfaces et + 0,40 %/an pour le stock entre 1990 et 2015 ; FAO, 2015). La forêt française présente par ailleurs une des expansions les plus marquées d’Europe (Forest Europe, 2015), avec un accroissement du stock deux fois plus rapide qu’en surface (+ 0,65 %/an en France entre 1990 et 2015 pour les surfaces contre + 1,3 %/an pour le stock, qui atteint actuellement 2 600 millions de m3 ; IGN, 2016). La forêt française pourrait à ce titre devenir assez rapidement le second stock forestier d’Europe, derrière l’Allemagne (> 3 600 millions de m3 ; Forest Europe, 2015) et devant la Suède (actuellement > 2 900 millions de m3 ; Forest Europe, 2015). Parmi les autres pays présentant des singularités de leur expansion, on citera la Turquie (+ 2,3 millions d’ha sur la même période contre + 2,5 millions pour la France), dont la transition forestière est plus récente.

L’inventaire des ressources forestières en France : un nouveau regard sur de nouvelles forêts

The authors examine the reasons that led, in 2004, the French national Forest Inventory to abandon its regular method, which was applied at departemental level. They describe the features of the new, continuous, inventory method applied at the national level: sampling, calculation of results. Standard results obtained with this new method applied at the national scale using a 5-year “sliding window” are presented. The authors also show that this method now enables information to be gathered across a broad spatial and temporal range, for instance following a disturbance. They detail several important developments in this new inventory method that have since been introduced: a new biogeographic breakdown, a direct, on-site method for measuring wood volumes logged, adoption of a new and more satisfactory method for computing biological production, collection of more varied ecological data. The article underscores the strong upward trend in standing timber volumes, essentially deciduous species, in French forests. The conclusion provides some indications as to what future developments in the French Forest Inventory might be in coming years.

Comment apprécier le niveau d’exploitation des ressources forestières françaises ?

The rate of removals is the main factor to which reference is made to assess logging intensity of forest resources. However, other factors, commented on in this article, are also involved. In particular, an imbalance between the various stages of maturity may be grounds for more intensive logging (an aged forest on one in the process of being rejuvenated) or less intensive logging (young forest or one whose asset value is on the increase) as compared to net biological production. An adequate assessment of logging intensity is therefore based not only on the average national rate of removals but also on a breakdown of that rate between geographic areas or categories of species, the density and mortality rate of trees, and on the rate of removals for improvement and their rate for regeneration purposes.