Kulmala, L., Pumpanen, J., Kolari, P., Muukkonen, P., Hari, P., and Vesala, T.
The photosynthesis of ground vegetation is dependent on species composition and environmental factors that are extremely diverse during forest succession. However, present in situ measurements over the gross primary production (GPP) of ground vegetation are unable to cover this variability. The primary objective of the present study was to estimate the GPP of ground vegetation in five differently aged Scots pine (Pinus sylvestris L.) forests in southern Finland during the growing season of 2006 by using temperature, soil moisture, photosynthetically active radiation, and biomass of the ground vegetation to run a known process-based model. The GPP of ground vegetation was ~350 g*[m.sub.-2] at the 6- and 12-year-old sites and 168, 146, and 41 g x [m.sub.-2] thereafter at the 20-, 45-, and 120-year-old sites, respectively. The values decreased with stand age, because as the stand ages, light availability decreases, the dominant species below the canopy show lower rates of photosynthesis than species in open areas, and the biomass of the ground vegetation decreases. Grasses and herbs took up nearly half of the value at the youngest site but their role decreased thereafter, whereas low shrubs were responsible for most of the GPP of ground vegetation below closed canopies. La photosynthese de la vegetation au sol depend de la composition en especes et de facteurs environnementaux qui varient beaucoup tout au long de la succession forestiere. Cependant, les mesures in situ actuelles de la production photosynthetique (PPB) de la vegetation au sol sont incapables de couvrir cette variabilite. Le principal objectif de cette etude consistait a estimer la PPB de la vegetation au sol dans cinq for x ts de pin sylvestre (Pinus sylvestris L.) d'age different dans le sud de la Finlande durant la saison de croissance de 2006 en utilisant la temperature, l'humidite du sol, le rayonnement photosynthetiquement utilisable et la biomasse de la vegetation au sol pour alimenter un modele base sur les processus. La PPB de la vegetation au sol atteignait environ 350 g x [m.sub.-2] dans les stations agees de six et 12 ans et, par la suite, 168, 146 et 41 g x [m.sub.-2] respectivement dans les stations agees de 20, 45 et 120 ans. La valeur diminue avec l'age du peuplement parce qu'a mesure que le peuplement vieillit, la disponibilite de la lumiere diminue, les especes dominantes sous le couvert ont un taux de photosynthese plus faible que les especes dans un espace ouvert et la biomasse de la vegetation au sol diminue. Les graminees et les herbacees etaient responsables de la moitie de la valeur dans la station la plus jeune mais leur role diminuait par la suite alors que de petits arbustes assumaient presque toute la PPB de la vegetation au sol sous un couvert ferme. [Traduit par la Redaction], Introduction The capacity of forests to bind atmospheric carbon dioxide ([CO.sub.2]) has been widely studied, but still the quantitative estimate of gross primary production (GPP) has been a challenge in [...]