Variation in canopy openness and light transmission following selection cutting in northern hardwood stands: an assessment based on hemispherical photographs

The objective of this study was to determine how canopy openness (CO) and light transmission are affected by selection cutting, and how they vary over time following harvesting in northern hardwood stands. We sampled five sugar maple-yellow birch-beech (Acer saccharum-Betula alleghaniensis-Fagus grandifolia) stands in Quebec (Canada). The stands had been logged, using the selection system, at different times (2-14 years) before the study, and were used as a chronosequence. We also sampled portions of each stand which had been kept as uncut controls. Ten I ha plots were sampled (five cuts and five paired controls). We took 20 hemispherical photographs per plot, at 5 m above-ground, which was above most understory vegetation. The CO, light transmission (gap light index (GLI) sensu Canham, (1988)), sunflecks characteristics, and angular distribution of openings from the zenith were calculated for each photograph. Selection cutting increased CO, especially within 60degrees of the zenith. The greater CO in the cuts allowed a greater light transmission (GLI), longer sunflecks, and a longer cumulative daily sunflecks duration (CDSD). The differences observed between the cuts and the control plots in terms of CO, GLI, and CDSD were greatest in the more recent cuts, and decreased as a function of time since logging. The relationships were best described by negative logarithmic (CO) and negative exponential (GLI, CDSD) models. In the youngest cut (2 years old), the CO, GLI, and CDSD were on average 2.3-2,5 times higher than in the control, while in the oldest cut (14 years old), the same variables were 1.6-1.7 times higher than in the control. The results of this study emphasize the importance of taking into account the temporal variation in canopy openness and light transmission after canopy disturbances such as selection cutting because that variation will likely have an important influence on regeneration dynamics. (C) 2002 Elsevier Science B.V. All rights reserved.