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

1. We explored whether the independent evolutionary history of extratropica l forests in the southern and northern hemispheres affects the temperature- elevation relationship of mountain forest lines. We compiled 115 forest lin e descriptions from the literature, covering the major extratropical mounta in ranges of the world. We related forest line elevation to thermal regimes using mean annual temperature adjusted to sea level (MAT) and seasonal the rmal amplitude (STA, mean of the warmest month minus the mean of the coldes t) obtained from a global climate database. 2. Thermal variables explained 79% of the global variability of forest line elevation, which increased approximately 130 m for every 1 K of MAT and fo r every 2 K of STA. After considering STA, there was no significant effect of 'hemisphere' on forest line elevation. MAT-elevation relationships for s ites with STA < 15 K were the same between hemispheres. 3. Sites dominated by different tree life forms had similar mean temperatur es for the warmest month (differences < 3 K), but contrasting mean temperat ures for the coldest (differences > 30 K). Along a gradient of increasing S TA and decreasing winter temperatures, forest lines were first dominated by evergreen broadleaf trees, followed by deciduous broadleaf species, evergr een conifers, and finally deciduous conifers. 4. Our results suggest that temperatures during the warm part of the year a re the main control of forest line elevation in extratropical regions while temperatures during the cold part of the year affect the dominant life for m of trees. There is a high degree of convergence in adaptation to mountain climates between trees species of both hemispheres. This convergence sugge sts that there is not a wide vacant altitudinal belt for introduced forest line species of the northern hemisphere into the southern hemisphere.