Leaf area index estimation in mountain even-aged Pinus silvestris L. stands from hemispherical photographs

In this study a stand level approach is proposed for estimating leaf area index (LAI) in even-aged Scots pine stands using gap fraction data derived from hemispherical photographs. The approach includes both the effect of the spatial distribution of the foliage elements as well as the slope of the terrain. Two chronosequences consisting of 11 plots were established in two Scots pine (Pinus sylvestris L.) forests in the Central Mountain Range of Spain. 12 hemispherical photographs were taken in each plot and variables related to the stage of development, density, relative spacing and site index were calculated. The gap frequency Poisson model was used to estimate the leaf area index. A new function is proposed to assess the effect of the foliage inclination on light transmittance for Scots pine, based on the assumption that the foliage elements show circular horizontal cross-sections. A new model was also developed to restrict the clumping effect on the vertical component in the Poisson model. The effect of the slope on LAI estimates was determined and included in the model. The results indicate that both the clumping effect correction and the slope correction improve the fitting of the Poisson model to the gap frequency experimental curves, and that the function proposed for the inclination angle correction may provide an acceptable approach for some coniferous species. The relationship between the parameters of the gap fraction model (the foliage inclination angle and the clumping coefficient) and stand level variables, that is, the stage of development, density, spacing of the stand and the site index, is analysed.