Fifty years of tree line change in the Khibiny Mountains, Russia: advantages of combined remote sensing and dendroecological approaches.

Questions Tree line ecotone regions are expected to respond swiftly to climate changes. In this paper, remote sensing- and ground-based tree population data are used to examine past and on-going changes of the tree line ecotone in a sub-arctic region characterized by precipitation increase. Questions addressed are: (1) at what rate has the tree line ecotone changed since the mid-20th century; (2) can specific temporal dynamics be identified; and (3) do combined remote sensing and tree population analyses add essential knowledge for the interpretation of tree line changes? Location Khibiny Mountains, Kola Peninsula, northwest Russia. Methods Aerial photos from 1958, high-resolution satellite imagery from 2006/2008 and age structure data for dominant tree line species (birch and pine) were used to analyse rate of change and temporal and species-specific tree line recruitment patterns. This was accomplished using digital elevation models, resolution-merging procedures, visual interpretation and dendroecological methods. Results Mean tree line advance for birch and pine was recorded as 29 and 27 altitudinal metres (0.6 and 0.5 m·yr−1), respectively. The advance was accompanied by an apparent infilling of pre-established tree populations and by recruitment beyond the tree line. Evident increased recruitment occurred in the late 1980s for birch and in the 1970s and 1990s for pine. Establishment showed no strong correlations with climate variables, but the importance of non-growing season variables was indicated. Conclusions The recorded tree line advance is modest compared to global model predictions for advance at high latitudes, but in accordance with results from a number of high-latitude areas. Concomitantly, the apparent increased recruitment is indicative of a more rapidly advancing tree line zone. Studies combining remote sensing and ground-based data minimize the risk of under- or overestimating potential tree line advance. Low detectability of small seedlings and saplings by remote sensing can cause underestimation of the current potential, while ground-based data used alone can overestimate potential advance. A balance between the two approaches is beneficial and enhances quality in production of change scenarios related to high latitudinal tree line areas at local to large regional scales. [ABSTRACT FROM AUTHOR]