What drives treeline elevation on islands?

Global distribution of oceanic island treelines (red), continental island treelines (blue) vs mainland treelines (grey). While the mainland treelines are characterized by a subtropical double-hump, island treelines produce a single hump in the tropics and are substantially lower.

Authors: Severin D. H. Irl, Fabien Anthelme, David E. V. Harter, Anke Jentsch, Elisabeth Lotter, Manuel J. Steinbauer, Carl Beierkuhnlein

Treelines are one of the most fundamental borders in nature, indicating a transition from the globally important life form ‘tree’ to non-forested systems. However, traditionally treeline science has focused on mainland mountain systems, while islands have been neglected in this respect. However, island treelines might behave differently than mainland treelines due to island-specific features such as isolation, endemism, geologic origin or young age. We would like to fill this research gap by asking the following questions: Are island treelines lower than on the mainland? How does the island type influence treeline elevation (continental vs oceanic islands)? Do local island characteristics affect island treeline elevation on the global scale?

To answer these questions we collected the largest dataset on island treeline elevations using GoogleEarth and the Global Island Database. We find a clear decrease of island treelines from mainland mountains through continental islands to oceanic islands. Also, while mainland treeline elevation shows a double-hump treeline-latitude relationship with maxima located in the subtropics, island treelines produce a single hump in the tropics. This fundamental difference is attributed to unfavorable climates at high elevations on islands (i.e. high cloudiness, aridity above trade wind inversions) and isolation-induced low species richness.

An example treeline from the island of La Palma (Canary Islands), an oceanic island of pure volcanic origin. The treeline is produced by the Canary endemic Pinus canariensis and is situated at around 2000 m above sea level. The treeline is situated slightly above the trade wind inversion as can be seen by the clouds in the background (picture facing NE). Picture taken by Severin D.H. Irl.

We expected latitude to be the main driver of treeline elevation on islands, surprisingly, however, maximum islands elevation resulted as the best explanatory variable. We interpret this maximum island elevation effect as an indicator for the presence of the mass elevation effect, even on the relatively small scale of islands. By using an innovative sampling method and ‘simple’ biogeographic variables, we are able to explain almost all variance in the dataset (around 90% depending on the dataset). We are also able to successfully quantify the mass elevation effect, which constitutes a major step forward in understanding global treeline patterns and is possibly also transferable to non-island systems.

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