Assessment of seasonal change in a young aspen ( Populus tremuloides Michx.) canopy using digital imagery

Abstract

Direct and indirect measurements of leaf area distribution during the 2001-growing season were used to characterize the structure of a small aspen grove in relict grassland of the northern Great Plains. Located in the Kernen Prairie near Saskatoon, the 1.5 ha grove had last been burned in October 1986. Measurements included solar radiation inside and outside the grove; digital hemispheric images of the tree canopy taken bi-weekly, and mean areas of individual leaves at three canopy heights. The images were processed using Idrisi® digital-image processing software designed for geographical information system (GIS) applications. Leaf size averaged near 5 cm 2, varying little through the summer, although larger leaves were proportionally more abundant in the upper canopy early in the growing season. Hemispherical cover increased rapidly in early May from about 40% (no leaves) to about 80% (fully leafed), dropping back to the earlier value after leaf fall in late September and early October. Solar radiation under the canopy dropped from 55% of ambient before the first leaves appeared to 25% during most of the summer, increasing to near 50% by late October. Calculated plant (leaf plus stem) area indices increased from about 0.4 in the leafless phase to near 1.7 during the full-leaf stage in late June. Digital imagery analysis provided a rapid assessment of canopy structure. This may be useful for long-term monitoring of stand responses to different management techniques.