Factors controlling the lower elevational limits in tropical montane plants in the Andes and their implications under the current climatic change

Abstract

To date all experimental approaches on the mechanisms that determine the lower elevational limits of plants have been carried out in temperate regions. Despite of the concrete and relevant information from these studies it is uncertain that these information could be applied to tropical mountain communities since diversity and community structure in these areas are different. Through a series of experimental approaches we examine the importance of some factors on the lower elevational limits of tropical plants. Since the current global warming will principally affect lower elevational limits, we also evaluated how climate change could affect the distribution of tropical montane plants. In the first approach we assess the importance of biotic and abiotic factors in the determination of the elevational limits of tropical ferns by observing the symmetry of the distributions for each species calculating the skewness. We assumed that abiotic factors should determine truncated and biotic factors attenuated limits, and that species range limits were spatially more regular when they were determined by abiotic than by biotic factors. The species presented different distribution patterns, which ranged from symmetric- to asymmetric-distributions. However most of the species were asymmetricaly distributed and showed truncated upper limits and attenuated lower limits. This is consistent with the observation that upper elevational limits are determined mainly by climatic factors while lower elevational limits are determined by biotic interactions. In a second experimental approach we analyzed the effects of litterfall on two understory species of tropical montane forest by observing their growth at different elevations. After two years, we found that the species restricted to higher elevation were more sensitive to damage by litterfall than the wideranging species. Based on these results, it can be suggested that litterfall may be playing an important role in the determination of the lower elevational limits in plants of trop ical montane forests, and that the elevational distribution reflect the species plasticity and resiliance to litterfall disturbs. In the third approach we examined the role of surrounding vegetation on the elevational distribution and occupation patterns of five high Andean geophyte species. Studying the structure of vegetation and the abundance of the geophyte species we found that structural changes in the vegetation appeared to determine the presence/absence and abundance of the geophyte species along the elevational gradient. In the fourth experiment we described the changes in the biomass allocation patterns for seedlings of two puna species when growing outside their natural elevational ranges. According to our results, the ability or inability to change the resource allocation patterns of these species appear to be playing an important role in the present elevational distribution, and could be key factor in the reaction of these species to climate change. In the last experimental approach we assess the effect of a warmer and more seasonal environment on non-vascular epiphyte communities. For that we translocated complete non-vascular epiphyte communities from 3000 m to 2700 m and 2500 m. After two years, epiphyte communities translocated to lower elevations did not differ in total bryophyte cover from those left at 3000 m, but they were more dynamic, with higher cover changes of species composition. The response of epiphytes to the translocation was distinctly species-specific. According to this it could be postulated not only that epiphyte communities may be already reacting to climate change but also that communities will not necessary collapse under the new climatic scenaria, and that ecosystem functions mediated by epiphytes, such as nutrient cycling and water retention may only experience limited changes.With this worldwide first experimental approach on the elevational limits of tropical plants, it has been possible to determine some relevant factors for the elevational distribution of plants in the Andes, and how climate change could affect them. However it is necessary to indicate that many factors, such as germination, dispersal limitation, and the direct effect of herbivory and pathogens could not be included in this investigation. In this way a complete perspective of the factors that determine the lower elevational limits of tropical montane plants and the effect of the climatic change on them is still incomplete. Only knowing the importance of these factors will permit to establish the true effect of the climatic change on these areas and to predict in an adequate way the response of the vegetation to this scenario.