Global controls of forest line elevation in the northern and southern hemispheres

Authors: Jobbágy EG, RB Jackson

We explored whether the independent evolutionary history of extratropical forests in the southern and northern hemispheres affects the temperature-elevation relationship of mountain forest lines. We compiled 115 forest line descriptions from the literature covering the major extratropical mountain ranges of the world. We related forest line elevation to thermal regimes using mean annual temperature adjusted to sea level (MAT) and seasonal thermal amplitude (STA, mean of the warmest month minus the mean of the coldest) obtained from a global climate database.

Thermal variables explained 79% of the global variability of forest line elevation, which increased approximately 130 m for every 1 K of MAT and for every 2 K of STA. After considering STA, there was no significant effect of "hemisphere" on forest line elevation. MAT-elevation relationships for sites with STA < 15 K were the same between hemispheres.

Sites dominated by different tree life forms had similar mean temperatures for the warmest month (differences < 3 K), but contrasting mean temperatures for the coldest (differences > 30 K). Along a gradient of increasing STA and decreasing winter temperatures, forest lines were first dominated by evergreen broadleaf trees, followed by deciduous broadleaf species, evergreen conifers, and finally deciduous conifers.

Our results suggest that temperatures during the warm part of the year are the main control of forest lines elevation in extratropical regions while temperatures during the cold part of the year affect the dominant life form of trees. There is a high degree of convergence in adaptation to mountain climates between trees species of both hemispheres. This convergence suggests that there is not a wide vacant altitudinal belt for introduced forest line species of the northern hemisphere into the southern hemisphere.
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