Structural acclimation and radiation regime of silver fir (Abies alba Mill.) shoots along a light gradient

Shoot architecture has been investigated using the ratio of mean shoot silhouette area to total needle area ( ) as a structural index of needle clumping in shoot space, and as the effective extinction coefficient of needle area. Although can be used effectively for the prediction of canopy gap fraction, it does not provide information about the within‐shoot radiative regime. For this purpose, the estimation of three architectural properties of the shoots is required: needle area density, angular distribution and spatial aggregation. To estimate these features, we developed a method based on the inversion of a Markov three‐dimensional interception model. This approach is based on the turbid medium approximation for needle area in the shoot volume, and assumes an ellipsoidal angular distribution of the normals to the needle area. Observed shoot dimensions and silhouette areas for different vertical and azimuth angles (A_S) are used as model inputs. The shape coefficient of the ellipsoidal distribution (c) and the Markov clumping index (λ₀) are estimated by a least square procedure, in order to minimize the differences between model prediction and measurements of A_S. This methodology was applied to silver fir (Abies alba Mill.) shoots collected in a mixed fir–beech–spruce forest in the Italian Alps. The model worked effectively over the entire range of shoot morphologies: c ranged from 1 to 8 and λ₀ from 0·3 to 1 moving from the top to the base of the canopy. Finally, the shoot model was applied to reconstruct the within‐shoot light regime, and the potential of this technique in upscaling photosynthesis to the canopy level is discussed.