To determine the global position of tree line isotherms, compare it with observed local tree limits on islands and mainlands, and disentangle the potential drivers of a difference between tree line and local tree limit.
Major taxa studied
We modelled the potential climatic tree line based on monthly temperatures and precipitation for the period 1979–2013. We then compared the potential tree line based on climate to observed tree limits at 26 oceanic islands, 55 continental islands and 382 mainland locations. The differences between potential tree line and observed tree limits was then analysed by regression with the islands’ maximum elevation, age, isolation, and area. Additionally, we estimated growing season temperature niches for 16,041 species known to occur in the vicinity of the studied tree lines, and compared them across mainlands, and islands of continental and oceanic origin.
Observed local tree limits differ up to 2,066 m from the potential tree line at the mainland on oceanic islands. Climatic effects are responsible for a difference of up to 1,296 m between tree lines of mainland regions and oceanic islands (but only for 756 m for continental islands). On oceanic islands, a remaining difference of up to 829 m correlates with the isolation and the maximum elevation of an island. Floras of oceanic islands are however depauperate with respect to potential tree line species and species show an affinity to higher growing season temperatures.
Climate can explain about half of the differences between observed local tree limits and potential tree lines between the mainland and continental and oceanic islands. The remaining difference can be attributed to the higher isolation of oceanic islands, especially in the tropics, and as a consequence, a more depauperate flora and a lack of tree species that are able to grow at the tree line.