Variation of leaf angle distribution quantified by terrestrial LiDAR in natural European beech forest

Leaf inclination angle and leaf angle distribution (LAD) are important plant structural traits, influencing the flux of radiation, carbon and water. Although leaf angle distribution may vary spatially and temporally, its variation is often neglected in ecological models, due to difficulty in quantification. In this study, terrestrial LiDAR (TLS) was used to quantify the LAD variation in natural European beech (Fagus Sylvatica) forests. After extracting leaf points and reconstructing leaf surface, leaf inclination angle was calculated automatically. The mapping accuracy when discriminating between leaves and woody material was very high across all beech stands (overall accuracy = 87.59%). The calculation accuracy of leaf angles was evaluated using simulated point cloud and proved accurate generally (R-2 = 0.88, p < 0.001; RMSE = 8.37 degrees; nRMSE = 0.16). Then the mean (theta(mean)), mode (theta(mode)), and skewness of LAD were calculated to quantify LAD variation. Moderate variation of LAD was found in different successional status stands (theta(mean) is an element of [36.91 degrees, 46.141 degrees,theta(mode) is an element of [17 degrees, 43 degrees] skewness is an element of [0.07, 0.48]). Rather than the previously assumed spherical distribution or reported planophile distribution, here we find that LAD tended towards a uniform distribution in young and medium stands, and a planophile distribution in mature stands. A strong negative correlation was also found between plot theta(mean) and plot median canopy height, making it possible to estimate plot specific LAD from canopy height data. Larger variation of LAD was found on different canopy layers (theta(mean) is an element of [33.64 degrees, 52.97 degrees], theta(mode) is an element of [14 degrees, 64 degrees], skewness is an element of [-0.30, 0.71]). Beech leaves grow more vertically in the top layer, while more obliquely or horizontally in the middle and bottom layer. LAD variation quantified by TL