Altitudinal Changes in Diversity and Abundance of Non-Vascular Epiphytes in the Tropics—An Ecophysiological Explanation

Abstract

It has long been noted that there is a pronounced increase in the abundance of non-vascular organisms, i.e., macrolichens, mosses, and liverworts with altitude in the tropics. In montane regions, these poikilohydric organisms may account for a considerable biomass in tree crowns, while being quite inconspicuous in the lowlands. Some 20 years ago, Richards suggested that the apparent unsuitability of lowland tropical forest for non-vascular organisms was due to a combination of continuous high temperature, high relative humidity, and low light intensities. Although we still do not have unequivocal evidence that this explanation is indeed true, the results of most studies make it a highly probable scenario. Here, results are presented from a number of field and laboratory studies with both bryophytes and lichens. These studies indicate, that on one hand these organisms are very likely to dry out during the day, which strongly reduces their carbon gain, and on the other hand are often hydrated at night, which at high temperatures leads to considerable respiratory losses. In combination, this results in frequent negative 24-h carbon balances in the lowlands, making growth and survival very difficult, if not impossible. Surprisingly, crustose lichens do not show the same altitudinal pattern, but to date no physiological data are available to address this puzzle.