Measuring and modelling the crown and light transmission characteristics of juvenile aspen

Crown dimensions, crown leaf areas, and leaf area densities were determined on individual trees in 96 juvenile Populus tremuloides Michx. stands in central Alberta, Canada. Crown radius, height, leaf area, and leaf area density were well explained by stem diameter at 30 cm (D30). Leaf area index, estimated using the LAI-2000, reached a maximum of 4 m2·m–2 in some stands by age 9 and decreased after age 25. Leaf area indices for the same stands, estimated by allometric relationships, were greater than 7 m2·m–2. Horizontal overlap between adjacent tree crowns declined from 68 to 38% of crown width with increasing average D30 from 1.5 to 15.0 cm. These data were used to calibrate MIXLIGHT, a spatially explicit light transmission model, to predict the range of light conditions in the understory of juvenile stands. Light predictions were validated in 18 additional plots in which light transmission was measured. Measured light transmission ranged from 4 to 68% of above canopy light among sites and from 16 to 53% within a mapped site. MIXLIGHT predicted transmission well for average stand light levels (R2 = 0.80). For individual positions within the mapped stand there was a strong relationship between predicted and observed light (R2 = 0.92), but there was underprediction at high light and over at low light.