Mapping state and change of the Scots pine population in Abisko in subarctic Sweden

Abstract

In the face of accelerating climate change, poleward and elevational range expansion have been recorded in many species of the subarctic. In recent decades Scots pine (Pinus Sylvestris) has become more abundant in the mountain-birch dominated forests of subarctic Sweden. Monitoring this shift is vital to understanding the effects of climate change on the subarctic ecosystem. Finding a cost effective and time efficient method for mapping and monitoring the Scots pine population over larger areas would improve our knowledge of the Scots pines response to climate change. This study evaluates the feasibility of mapping the state and change of the local Scots pine population in Abisko national park in subarctic Sweden using three different remote sensing techniques. The aim is both to evaluate the methods and to analyse eventual changes in the local Scots pine population. The first task, mapping of the current Scots pine population, was successfully performed utilizing a Sentinel-2 image and a method of estimating the Scots pine crown coverage of each pixel using a natural difference water index (NDWI). The second task, the change analyses were done in two parts, the first one being a repeat aerial photograph analysis, done by comparing an aerial photo from 1959 with an aerial photo from 2018. The second part measured changes in height and was done by calculating the height of 20 trees from measurements of tree-shadows in an aerial photo from 1959. The measurements were compared to LIDAR data from 2015. The results of the first part show an 9- 34% increase in Scots pine abundancy in two 1km2 large study areas. Most of the increase was however confined to already existing stands and no significant range expansion could be detected. The height comparison showed no increase in height between 1959 and 2015.