LOW-TEMPERATURE AND ALTITUDINAL LIMITS IN PLANT ECOSYSTEMS - SPECIES RESPONSES TO COLD IN TROPICAL AND SUBTROPICAL MOUNTAINS

The high mountain ecosystems are characterized by low air temperatures and nocturnal freezing, changing their occurrence with latitude, alti tude and topography. The minimum temperature is an important variable in the limits of vegetation distribution, due to the different sensibi lity and resistance capacity of the species. Thus, tropical and extrat ropical plants present different strategies to survive, furthermore re lated with the avoidance and/or tolerance mechanims against freezing. This review attempts to explain the role of low temperatures in the al titudinal limits of higher plants ecosystems, comparing the adaptative responses to low temperatures in tropical and subtropical high mounta in species. The higher limit of the forest in tropical mountains in th e east of Africa and the north of the Andes correspond with freezing-f ree altitude, contrasting with Mexico, where this environmental factor sets out the boundaries of mixed and coniferous forests. However, in tropical mountains below the altitude of treeline, the factors that se parate differents types of vegetation depend not only on the temperatu re. In contrast, in the temperate mountain regions of Northamerica, do minated by evergreen coniferous forests, the capacity of supercooling under low temperatures has been described as the explanation for the t reeline formation and the altitudinal distribution of tree species. On the other hand, the latitudinal patterns of forest in the high mounta in in the east and south of Asia suggest that the altitudinal zonation is related to seasonal temperature fluctuations, deferring from the n orth and south of 20 degrees N. in Japan. which has subtropical to sev ere cold climates from south to north, the temperature has been indica ted as the determinant factor of vegetation zones. In the South Hemisp here, with mild seasonal changes, the altitudinal distribution of tree s in forests of evergreen and coniferous species, are related with the ir resistance to Freezing. In these temperate zones, plant species hav e a transitional period from low to high level of resistance against l ow temperatures and freezing: this cold acclimation allows these speci es to face the climate of subtropical latitudes. In tropical high moun tain species in the Andes, Hawaii and Africa. low temperatures avoidan ce by supercooling and the freezing tolerance mechamisms would be rela ted. besides. with the plant life-forms: plants that grow at the soil level show tolerance, the shrubby ones avoidance and the intermediate stratum exhibits both mechanisms. This pattern between plant lift-form and low temperature resistance mechanism, also described for subtropi cal Andes in Chile, would be related to microclimatic conditions aroun d the species. Finally, the effect of freezing temperatures in higher plants seems to be a major factor, but not the only one, that limits t he latitudinal and altitudinal distribution of species.