Vertical point sampling with a digital camera: Slope correction and field evaluation

Vertical point sampling with a digital camera (VPSC) is a promising new forest sampling method that can be used to improve existing sampling protocols or rapidly assess forest structure over large areas. Previous research into VPSC has not accounted for the potential bias that can result from implementing this method on sloping terrain. Here, we present a modified method of conducting VPSC on sloping terrain that maintains unbiased estimates by implementing an automated computer program to adjust for slope at each sample point. This updated method is easily implemented and includes minimal alterations to the existing VPSC protocol, though there will likely be some situations where it is impractical or unnecessary. To address this, we quantified the bias incurred for ignoring slope altogether by conducting a field study in two separate forest types: mixed conifer and mixed deciduous. The coniferous plots showed no slope-related bias whereas the deciduous plots displayed bias on steeper slopes. This difference in bias between forest types is likely due to the difficulty identifying deciduous tree tops in the digital photographs. The lack of discernible bias on the lesser slopes and in the conifer forests was largely due to the slope-related bias being overwhelmed by the unavoidable variability inherent in VPSC. Overall, the slope-related bias should be negligible, regardless of forest type, provided the majority of the sample points fall on slopes of approximately 35° or less. These results further support the use of VPSC as a useful new method of monitoring forest conditions, conducting forest inventories, or assessing wildlife habitat.