Phenology of temperate trees in tropical climates.

Several North American broad-leaved tree species range from the northern United States at approximately 47 degrees N to moist tropical montane forests in Mexico and Central America at 15-20 degrees N. Along this gradient the average minimum temperatures of the coldest month (T (Jan)), which characterize annual variation in temperature, increase from -10 to 12 degrees C and tree phenology changes from deciduous to leaf-exchanging or evergreen in the southern range with a year-long growing season. Between 30 and 45 degrees N, the time of bud break is highly correlated with T (Jan) and bud break can be reliably predicted for the week in which mean minimum temperature rises to 7 degrees C. Temperature-dependent deciduous phenology-and hence the validity of temperature-driven phenology models-terminates in southern North America near 30 degrees N, where T (Jan)>7 degrees C enables growth of tropical trees and cultivation of frost-sensitive citrus fruits. In tropical climates most temperate broad-leaved species exchange old for new leaves within a few weeks in January-February, i.e., their phenology becomes similar to that of tropical leaf-exchanging species. Leaf buds of the southern ecotypes of these temperate species are therefore not winter-dormant and have no chilling requirement. As in many tropical trees, bud break of Celtis, Quercus and Fagus growing in warm climates is induced in early spring by increasing daylength. In tropical climates vegetative phenology is determined mainly by leaf longevity, seasonal variation in water stress and day length. As water stress during the dry season varies widely with soil water storage, climate-driven models cannot predict tree phenology in the tropics and tropical tree phenology does not constitute a useful indicator of global warming.