A global data set of realized treelines sampled from Google Earth aerial images

We sampled Google Earth aerial images to get a representative and globally
distributed dataset of treeline locations. Google Earth images are
available to everyone, but may not be automatically downloaded and
processed according to Google's license terms. Since we only wanted
to detect tree individuals, we evaluated the aerial images manually by
hand.   Doing so, we scaled Google Earth’s GUI interface to a
buffer size of approximately 6000 m from a perspective of 100 m (+/- 20 m)
above Earth’s surface. Within this buffer zone, we took coordinates and
elevation of the highest realized treeline locations. In some remote areas
of Russia and Canada, individual trees were not identifiable due to
insufficient image resolution. If this was the case, no treeline was
sampled, unless we detected another visible treeline within the 6,000 m
buffer and took this next highest treeline. We did not apply an automated
image processing approach. We calculated mass elevation effect as the
distance to the nearest mountain chain limits. Continentality was assessed
by the distance to the nearest coastline. Isolation was calculated by the
nearest distance of a mountain chain to another mountain chain within a
comparable elevational band.