Scaling carbon uptake from leaves to canopies: insights from two forests with contrasting properties.

Abstract

AbstractA multilayer, one-dimensional model is used to explore the processes regulating photosynthesis in two forest canopies with contrasting structural and physiological properties. An evergreen coniferous forest dominated by Dacrydium, growing at a site with very low nutrient availability in New Zealand, is compared with a deciduous mixed Quercus forest, growing at a site with high nitrogen inputs from atmospheric deposition at almost the same latitude in the north-eastern USA. The canopy model integrates radiative transfer, energy balance, evaporation and photosynthesis, coupled with stomatal conductance, in 20 layers through the canopy and is combined with a simple water-balance model to introduce the effects of seasonal root-zone water balance and allow the estimation of net annual carbon uptake. Estimates of the vertical distribution of foliage area and measurements of physiological parameters for the two sites were made for the model. For Dacrydium, 54% of the foliage area occurred in the upper 11 layers of the canopy. In contrast, 52% of the foliage area occurred in the upper five layers of the Quercus canopy. At the top of the canopy, the maximum rates of carboxylation activity, Vcmax, for the Dacrydium and Quercus canopies (half-surface area basis) were 11.9 and 50.2 µmol/m2/s, respectively. In midsummer, this resulted in 50% of canopy photosynthesis taking place in layers 5 to 10 in the Dacrydium canopy and 97% of photosynthesis in layers 1 to 5 in the Quercus canopy. The annual net carbon uptake for the Quercus canopy (1.22 kg C/m2) was 14% greater than that for the Dacrydium canopy (1.07 kg C/m2), but the difference in the ratios of net annual carbon uptake to intercepted radiation was much larger (75%) for the Quercus canopy (0.96 g C/MJ (400-700 nm)) than that for the Dacrydium canopy (0.55 g C/MJ). Analysis of the component processes revealed that, at midday in typical midsummer conditions, photosynthesis for sunlit foliage throughout both canopies was limited by carboxylation activity. Shaded foliage in the Quercus canopy was strongly limited by the rate of electron transport, but, in contrast, photosynthesis in shaded foliage in the upper layers of the Dacrydium canopy was limited by carboxylation activity. The model is used to show that the consequence of the contrasting values of Vcmax and canopy structure is that carbon uptake in the Quercus canopy is much more sensitive to changes in irradiance than that in the Dacrydium canopy. Furthermore, at high incident irradiance, carbon uptake by the Quercus canopy is much more sensitive to increasing diffuse fraction of radiation than that by the Dacrydium canopy.