Dominique Gelhaye

The dynamics of biomass and nutrient accumulation in a Douglas-fir (Pseudotsuga menziesii Franco) stand studied using a chronosequence approach

Abstract Douglas-fir is presently one of the dominant species used in France for afforestation. It was widely introduced without knowing exactly its effects on soils and superficial waters. The aim of this research was to obtain quantitative data on the biogeochemical functioning of a Douglas-fir ecosystem, and to provide relevant information to forest managers. The results presented here concern nutrient accumulation on a stand according to its stage of development. A chronosequence of stands was used to rapidly obtain information on ecosystem dynamics in particular because it is possible to assimilate stands of different ages to the different stage of development of a single stand. The present results make it possible to evaluate with sufficient accuracy nutrient losses at crop harvest. Several scenarios could be drawn using various rotation lengths and harvesting intensities. These results could be introduced in management models. Nevertheless, a direct relationship between nutrient losses and the immediate or delayed decrease of forest productivity is not easy to predict. It depends more on soil nutrient dynamics than on the present available nutrient pool. Comparison between the two main species used in forestry showed that the nutrient efficiency of Douglas-fir for biomass production was at least the same as for Norway spruce. This important conclusion needs to be confirmed by other measurements.

Nutrient budgets for a rotation of a Douglas-fir plantation in the Beaujolais (France) based on a chronosequence study

Abstract Nutrient fluxes were quantified in a chronosequence of three Douglas-fir stands in the Monts du Beaujolais (France). Measurements were taken for 6 years. Current annual and seasonal budgets were calculated and budgets for three theoretical rotation lengths were simulated. Results showed that current budgets were negative for most elements in the young stand, but that the deficit decreased with stand age. The budgets were balanced or positive in the older stand. Element deficits in the young stand were related to biomass accumulation and to considerable losses in seepage waters. High nitrification levels related to past land uses were responsible for leaching losses of nitrates and cations. Atmospheric deposition of nitrogen was an additional constraint in this N-saturated ecosystem. Simulated element budgets showed that whole tree harvesting should not be prescribed in these stands. Even when stem-only harvesting was practised, the balancing of the nutrient budgets would require a rotation length between 100 and 120 years, which is both technically and economically unrealistic. Sustainable management of such high production Douglas-fir forests would require nutrient inputs from fertilisation to maintain soil quality and their capacity to produce biomass at high rates.

The effects of limestone and of limestone plus NPK fertilization on the soil and mass balance of a spruce stand (Picea abies (L.) Karst.) in the Vosges mountains

Abstract Calcareous amendment and/or fertilization trials were carried out in a declining 66-year-old Norway spruce (Picea abies (L.) Karst.) stand in the Vosges mountains (northeast France, altitude 1100 m) in 1985. The aim was to test tree response to nutritional deficiencies (Ca + Mg) and to alleviate soil acidity. In 1988, experimental equipment was set up to collect atmospheric input (bulk precipitation, throughfall) and soil seepage water. The soils are podzolic (‘ocre-podzolique’), derived from an acid-poor granitic bedrock (‘granite du Valtin’). The soils are coarsely textured, hence very porous. They are very acid (pH 3.4 in the A1 horizon, and pH 4.1 in the Bs horizon). The cation exchange capacity (CEC) is highly saturated in exchangeable acidity: 86% in the A1 horizon. Ca2+ + Mg2+ saturation is very low ( Bulk precipitation is dilute and acidic (pH 4.5); ion concentration of the precipitation greatly increases after passing through the tree canopy. Dry deposition is moderate and has values in the lower range reported for Europe. Nitrification produced large amounts of NO3−-N and H+ ions in the O layer and organo-mineral horizons. Nitrate and aluminium dominate the chemical composition of the soil see-page water. Liming induces a considerable rise in the soil solution pH and the base cations (Ca2+ + Mg2+) and decreases nitrate and aluminium substantially. Liming alone seems to lower the rate of nitrification. Addition of fertilizers increases the deep drainage of base cations which could reduce the long term efficiency of liming. Improvement of tree health-status accords with the positive effects of liming and/or fertilization on the soils solid phase and solution.

Effects of the clear-cutting of a Douglas-fir plantation (Pseudotsuga menziesii F.) on the chemical composition of soil solutions and on the leaching of DOC and ions in drainage waters

The effects of the clear-cutting of a 70-year-old Douglas-fir plantation on the chemical composition of soil solutions and on leaching of nutrients in drainage waters were observed by a continuous monitoring, six years before and three years after the cutting. Forest harvesting was made with very limited soil disturbances. Results showed that the concentration of weakly fixed solutions did not change but that the concentration of gravitational solutions of the upper soil layers drastically fell down after the cutting. The limited increase in nutrients leached with drainage waters was only due to the increase in the water flux, which is difficult to quantify because of the presence of ground vegetation. The monitoring of numerous fluxes before and after the clear-cutting could explain the specific behaviour of the soil solutions. The limited losses of nutrients the after clear-cutting in a potentially responsive ecosystem were unexpected. The initial hypothesis was that the decrease in the mineralization and nitrification rates observed after the cutting was related to a stimulating effect of Douglas-fir on the activity of soil nitrifyers.RésuméLes effets de la coupe à blanc d’une plantation de Douglas de 70 ans ont été observés sur la composition chimique des solutions du sol et les pertes d’éléments par drainage, par un suivi mensuel pendant 6 ans avant, et 3 ans après la coupe. L’exploitation du peuplement a été réalisée avec une perturbation minimum du sol. Les résultats montrent que les solutions liées ont peu évolué après la coupe, alors que le changement des solutions libres a été drastique dans les horizons de surface du sol. Malgré des incertitudes sur le rôle de la végétation spontanée, le drainage d’éléments n’a pas fortement augmenté après la coupe. La prise en compte de l’ensemble des flux mesurés dans cette étude semble pouvoir expliquer les observations. Les pertes limitées après la coupe d’une plantation où l’activité nitrifiante était élevée avant la coupe étaient inattendues. L’hypothèse avancée est l’arrêt du contrôle stimulateur des populations nitrifiantes du sol après la coupe du Douglas.

Effects of the clear-cutting of a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation on chemical soil fertility

Abstract• Stand harvesting and regeneration were usually considered to be a critical phase for the sustainability of forest soils. The present study concerned the effects on soil chemical fertility of the clear-cutting of a highly productive Douglas-fir stand aged 67 years that was clear-cut with no disturbance.• Results showed that soil changes were rapid in the three-year period following the cutting. The forest floor mass considerably decreased and the mineral soil showed a limited but real acidification. Soil losses represented 4% of the available nutrients over a depth of 60 cm for N, 22% for K, 25% for Ca and 32% for Mg. Only P increased by 11%. Due to the spatial variability of forest soils, and despite regular re-sampling, confidence intervals were large and difficult to reduce.• The reversibility of the effects of the clear-cutting and its consequences on soil functions depend on the element: it should not be a problem for C, N and K, which would recover when the biological cycle is re-established once again in the young stand. Phosphorus is not at issue since it changes form in the soil. The depletion of Ca, and to a lesser extent of Mg, is of some concern as a result of limited soil reserves, the limited flux of cations from the mineral changes in the soil, the relatively large part of Ca and Mg in the forest floor, and the negative input-output budgets for those elements.• The duration of the impact of the clear-cutting on soil requires medium-term observations because it cannot be deduced from the current knowledge of this ecosystem.Résumé• La récolte des peuplements est une phase critique pour la durabilité des sols forestiers. Cette étude concerne les effets de la coupe à blanc d’un peuplement de Douglas de 67 ans, réalisée sans perturbation, sur la fertilité organo-minérale du sol.• Les résultats montrent que l’évolution du sol est rapide dans la période de trois ans après la coupe. La couche organique décroît fortement et le sol minéral se désature et s’acidifie. Les pertes représentent 4 % des éléments disponibles du sol sur 60 cm pour N, 22 % pour K, 25 % pour Ca et 32 % pour Mg. Seul P augmente de 11 %. Cependant, la variabilité spatiale des sols forestiers et la technique de l’observatoire, par échantillonnages successifs, conduisent à des intervalles de confiance larges autour de ces valeurs, intervalles qui s’avèrent difficile à réduire.• La réversibilité des modifications du sol dépend des éléments : elle ne devrait pas poser de problème pour C, N et K. P n’est pas en cause puisqu’il change de forme dans le sol mais aucune perte n’est notée. Les pertes de Ca voire de Mg sont les plus préoccupantes en raison des faibles réserves du sol, du flux limité de cations issu de l’altération des minéraux du sol, de la part relativement élevée des ces éléments dans les couches organiques, et de leur bilan entrées-sorties négatif.• La durée de l’effet de la coupe à blanc nécessite des observations sur le moyen terme car elle ne peut pas être déduite des connaissances actuelles sur cet écosystème.