Increase of tree abundance between 1960 and 2009 in the treeline of Luongastunturi in the northern Swedish Scandes : change detection based on aerial photographs

Abstract: Aerial photo interpretation shows a recent densification of trees has been occurring between
1960 and 2009 in the mountain birch (Betula pubescens ssp. czerepanovii) forest of the
treeline ecotone in northern Swedish mountains. The correlation between treeline position and
annual temperature is commonly accepted and advancing treelines are considered to be a
useful indicator for monitoring climate changes. Previous research show that in the stressful
environment that makes up the treeline ecotone, tree density often respond quicker to a
temperature rise than an altitudinal advance of the treeline. The project is executed by
counting trees and measuring the tree height within 50 systematically distributed sample plots.
Mapping is done in black and white 1960 aerial photos and colour infrared aerial photos from
2009 in stereo view by using a digital photogrammetric station. Tree density changes from
1960 to 2009 within the sample plots are analyzed statistically. The results show that during
the studied time period tree abundance have increased in the high elevation mountain birch
forest of the study area. Mean trees per hectare has increased from 88 to 126. Tree height
interpretation indicates a growth of previously existing trees. Densification is more significant
on the northern sides of the mountains and with slightly higher rates. The increased growth is
likely due to a warmer climate, but other factors like e.g. browsing by herbivores like reindeer
has to be examined before securely attributing the densification to climate change.