It is expected that the ongoing climate change will have a strong influence on the Earth’s vegetation and cause the advancement of treelines towards the poles and up to higher elevations. In the Swedish mountains, changes in the positions of alpine treelines have already been reported, and major changes due to changing climate are predicted for the near future. Remote sensing techniques have considerable potential to improve the monitoring of spatially complex treeline ecotones, which are likely to show site dependent responses to changing climate. Aerial photos provide the longest temporal record of remote sensing data for studying the historical treeline changes. High spatial resolution and the possibility of interpreting photos in three-dimensions are the main strengths of aerial photos. The National Inventory of Landscapes in Sweden (NILS) is a nationwide environmental monitoring program, which provides sampling infrastructure for monitoring treelines over the Swedish mountains using high spatial resolution remote sensing data.