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Soil respiration (SR) was monitored periodically throughout 2001 in a Scots pine (Pinus sylvestris L.) stand located in the Belgian Campine region. As expected for a temperate maritime forest, temperature was the dominant control over SR during most of the year. However, during late spring and summer, when soil water content (SWC) was limiting, SR was insensitive to temperature (Q(10) = 1.24). We observed that during prolonged rain-free periods, when SWC was less than 15% (v/v), SR decreased dramatically (up to 50%) and SWC took over control of SR. During such drought periods, however, rain events sometimes stimulated SR and restored temperature control over SR, even though SWC in the mineral soil was low. We hypothesize that restoration of temperature control occurred only when rain events adequately rewetted the uppermost soil layers, where most of the respiratory activity occurred. To quantify the rewetting capacity of rain events, an index (I(w)) was designed that incorporated rainfall intensity, time elapsed since the last rain event, and atmospheric vapor pressure deficit (a proxy for evaporative water losses). To simulate SR fluxes, a model was developed that included the effects of soil temperature and, under drought and non-rewetting conditions (I(w) and SWC < threshold), an SWC response function. The model explained 95% of the temporal variability in SR observed during summer, whereas the temperature function alone explained only 73% of this variability. Our results revealed that, in addition to temperature and SWC, rain plays a role in determining the total amount of carbon released from soils, even in a maritime climate.

We investigated the storage of carbon (C) in the soil, litter and various phytomass compartments in a 69-year-old Scots pine (Pinus sylvestris L.) stand in the Belgian Campine region, Brasschaat, Belgium. The total amount of C stored in the stand was 248.9 t•ha-1 , 47 % of which was in soil organic matter, 11 % in surface litter and 42 % in phytomass. More than 60 % of total C was stored belowground. Total phytomass C in the stand was 104 t•ha-1 ; most phytomass C was found in the stems (70 %). The root system was very shallow and contained only 14 % of the phytomass C, most of it in the coarse roots. Although total live fine root (< 1 mm) length was high (3.3 km•m-2), fine roots contributed only 2 % to total phytomass. (© Inra/Elsevier, Paris.) carbon storage / phytomass / Pinus sylvestris / roots / Scots pine Résumé-Phytomasse aérienne et souterraine et stock de carbone dans un peuplement de pin sylvestre en Belgique. Nous avons étudié le stockage du carbone dans le sol, dans la litière et dans différents compartiments de la phytomasse d'une plantation de pins sylvestre (Pinus sylvestris L.), âgés de 69 ans, localisée à Brasschaat, région de la Campine, Belgique. La quantité totale de carbone stockée au niveau de cette plantation était de 248,9 t ha-1. 47 % étaient localisés dans la matière organique du sol, 11 % dans la litière, et 42 % dans la phytomasse. Plus de 60 % de la quantité totale de carbone se trouvait dans le sous-sol. La quantité de carbone contenue dans la phytomasse était de 212 t ha-1. La plus grande partie de ce dernier a été trouvé dans les tiges (70 %). Le système racinaire était très superficiel et ne contenait que 14 % du carbone de la phytomasse, principalement localisé dans les grosses racines. Bien que la longueur des racines fines et vivantes ait été importante (3,3 km m-2), elles ne représentaient que 2 % de la phytomasse totale. (© Inra/Elsevier, Paris.

Widespread tree species must show physiological and structural plasticity to deal with contrasting water balance conditions. To investigate these plasticity mechanisms, a meta-analysis of Pinus sylvestris L. sap flow and its response to environmental variables was conducted using datasets from across its whole geographical range. For each site, a Jarvis-type, multiplicative model was used to fit the relationship between sap flow and photosynthetically active radiation, vapour pressure deficit (D) and soil moisture deficit (SMD); and a logarithmic function was used to characterize the response of stomatal conductance (G(s)) to D. The fitted parameters of those models were regressed against climatic variables to study the acclimation of Scots pine to dry/warm conditions. The absolute value of sap flow and its sensitivity to D and SMD increased with the average summer evaporative demand. However, relative sensitivity of G(s) to D (m/G (s,ref), where m is the slope and G(s,ref) is reference G(s) at D = 1 kPa) did not increase with evaporative demand across populations, and transpiration per unit leaf area at a given D increased accordingly in drier/warmer climates. This physiological plasticity was linked to the previously reported climate- and size-related structural acclimation of leaf to sapwood area ratios. G (s,ref), and its absolute sensitivity to D(m), tended to decrease with age/height of the trees as previously reported for other pine species. It is unclear why Scots pines have higher transpiration rates at drier/warmer sites, at the expense of lower water-use efficiency. In any case, our results suggest that these structural adjustments may not be enough to prevent lower xylem tensions at the driest sites.

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