Drought as a Determinant of Tropical Montane Forest Line Position

Abstract:

Globally, the upper elevation margins of montane forests follow a latitudinal pattern, ranging from below 1000 m at high latitudes to above 3000 m on continental tropical mountains. Forest lines on oceanic islands are distinctly lower, but are also latitude dependent. While proposed mechanisms to explain forest line position are generally related to effects of low temperature, including tissue damage from freezing and slow growth, aridity has been suggested as a possible cause of the lower oceanic island forest lines. A relatively undisturbed forest line is found on Haleakalā Volcano, on the Island of Maui at an elevation of around 2000 m. Forest on Haleakalā does not appear to be limited by low temperature; mean annual temperature at the forest line (8.8-10.5°C) is well above the suggested threshold growing season temperature. Instead, the position of this ecotone coincides with the mean elevation of the trade wind inversion (TWI), which separates the moist marine atmospheric layer from arid upper air. Abrupt changes in moisture-related climate variables are found at the TWI, including one of the steepest mean annual rainfall gradients in the world. In recent work, microclimate during drought was shown to play a critical role in structuring the Haleakalā forest line, the position of which was most strongly related to relative humidity during a strong El Niño-induced drought. The relationship between El Niño-driven drought and ecotone position was tested in long-term paleoecological data. Reconstructions of the cloud forest’s upper limit and moisture balance over the past 3,300 years were strongly associated with paleorecords of El Niño event frequency. The forest ecotone may be particularly responsive to strong, short-duration drought events because taxa here, particularly the isohydric dominant canopy tree Metrosideros polymorpha, are near their physiological limits. Minimum leaf water potential in M. polymorpha near the forest line is close to the turgor loss point under non-drought conditions, indicating that drier conditions may cause moisture stress and may be an important driver of ecotone position. Overall, data suggest that the forest line is controlled by moisture limitation and its current position may be vulnerable to increasing drought frequency associated with climate warming.