Detection of small single trees in the forest–tundra ecotone using height values from airborne laser scanning

Abstract

Because of global warming, it is assumed that the arctic and alpine tree lines will advance northwards into the tundra and upwards into mountainous regions. Methods are needed to monitor these advances. Airborne laser scanning has recently been introduced for detection of small pioneer trees that form the advanced alpine tree line. The objective of this study was to analyze the capability of high-density airborne laser scanning data used for detecting such individual small trees in the transition between the mountain forest and the alpine zone, the forest–tundra ecotone. The study used field and laser data collected along a 1500 km transect stretching from northern Norway (69°3′ N) down to the southern part of the country (58°3′ N). In the field, 744 trees of mountain birch, Norway spruce, and Scots pine were geolocated with centimetre accuracy, and they were measured for height, root collar diameter, and crown diameter. Tree heights ranged between 0.02 and 7.80 m. The laser data were acquired in two separate acquisitions with mean pulse densities of 6.8 m−2 and 8.5 m−2, respectively. Laser echoes with relative height values greater than zero within the individual tree crown polygons were used as a criterion for a successful tree detection. The detection success for trees taller than 1 m was 90%; however, for trees shorter than 1 m, the corresponding value was 49%. The highest detection success was found for spruce. Generalized linear models and a generalized linear mixed model with binary responses (detected/not detected) were applied to evaluate the effects of tree height, tree crown area, tree species, geographic location along the latitude gradient, and region on successful detection. Although they were highly correlated, tree height and tree crown area turned out to be the variables showing high significance (p ≤0.001) in all of these models.