Radiative transfer modeling of direct and diffuse sunlight in a Siberian pine forest

We have expanded the Monte Carlo, ray‐tracing model FLIGHT in order to simulate photosynthesis within three‐dimensional, heterogeneous tree canopies. In contrast to the simple radiative transfer schemes adopted in many land‐surface models (e.g., the Big Leaf approximation), our simulation calculates explicitly the leaf irradiance at different heights within the canopy and thus produces an accurate scale‐up in photosynthesis from leaf to canopy level. We also account for both diffuse and direct sunlight. For a Siberian stand of Scots pine Pinus sylvestris , FLIGHT predicts observed carbon assimilation, across the full range of sky radiance, with an r.m.s. error of 12%. Our main findings for this sparse canopy, using both measurements and model, are as follows: (1) Observationally, we detect a light‐use efficiency (LUE) increase of only ≤10% for the canopy when the proportion of diffuse sky radiance is 75% rather than 25%. The corresponding enhancement predicted by our simulations is 10–20%. With such small increases in LUE, our site will not assimilate more carbon on overcast days compared to seasonally equivalent sunny days; (2) the scale‐up in photosynthesis from top‐leaf to canopy is less than unity. The Big Leaf approximation, based on Beer's law and light‐acclimated leaf nitrogen, overpredicts this scale‐up by ≥60% for low sky radiance (≤500 μmolPAR m −2 s −1 ); (3) when leaf nitrogen is distributed so as to maximize canopy photosynthesis, the increase in the canopy carbon assimilation, compared with a uniform nitrogen distribution, is small (≃4%). Maximum assimilation occurs when the vertical gradient of leaf nitrogen is slightly shallower than that of the light profile.