Are high elevations in tropical mountains arid environments for plants?

Plants growing at high elevations in tropical mountains show a scleromorphic habit at the leaf and plant level, despite the fact that alpine landscapes are generally considered to be cool and humid environments. This paper tests the hypotheses (1) that aridity (potential transpiration E-P/rainfall P) in tropical alpine environments can be high despite low temperatures, and (2) that aridity in tropical alpine environments may be correlated with scleromorphic habit. A leaf energy balance model was developed that calculates the change in E-P for a standard leaf along elevational transects in equatorial and middle-latitude mountains by accounting for pressure (elevation) and temperature effects on transpiration. Modeled leaf-level transpiration in tropical alpine environments can be substantial and may even exceed transpiration at low elevations in middle latitudes. At 5000 m elevation, a high diffusion coefficient for water vapor under low air pressure and a high radiation load result in E-P rates 50-90% higher in tropical mountains than in middle-latitude ones. High E-P rates at high elevations are expected (1) in mountains of the dry tropics with low cloudiness and low rainfall but high radiation and, (2) in small oceanic mountain ranges with a strong advection of moist air and, thus, a reduced temperature lapse rate on the slope. Data from tropical Mount Haleakala (Maui, Hawaii) and from the middle-latitude Tyrolian Alps (Austria) were used to test the predicted E-P rates. The two mountains sharply contrast with respect to vertical distribution of E-P and rainfall. At high elevation on Maul, the calculated E-P was high (and was consistent with measured atmometer evaporation rates E-m), whereas rainfall was moderate to low. At high elevation in the Alps, both E-P and E-m were low compared to similar elevations on Maul, but rainfall was high. Worldwide mountain climate data sets show that rainfall tends to decrease upslope in many tropical mountains, while it increases with elevation in most middle-latitude mountains. I conclude that (1) high-elevation environments in tropical mountains are more arid than similar elevations in middle-latitude mountains as a result of both higher evaporative demand and lower rainfall, and (2) that aridity can significantly contribute to the scleromorphy of tropical alpine plants which had previously been attributed to low temperature or high radiation effects alone.