Penumbral and Foliage Distribution Effects on Pinus sylvestris Canopy Gas Exchange

Tree canopy water use and foliage net CO2 uptake (NPP) were simulated for a 31-year-old Pinus sylvestris (Scots pine) plantation near Hartheim, in the Upper Rhine Valley, Germany with a mechanistically-based, three-dimensional stand gas-exchange model (STANDFLUX) for a ten-day period during spring 1992. STANDFLUX was formulated to include the effects of penumbra caused by the fine structure of the needles on light distribution within crowns. Good correspondence was found between simulated rates of tree canopy water use when including penumbral effects and eddy-covariance ET and sap flow transpiration measurements. Water use was 8–13% lower and NPP was 10–17% lower in simulations for the ten-day period when penumbral effects were not included.