Global warming impacts on wild species will vary with latitude for at least two reasons. The first is widely recognized – temperatures are increasing fastest at higher latitudes. The second reason is perhaps less widely appreciated, involves latitudinal differences among species rather than climate, and acts in the opposite direction. Tropical species experience a sharply curtailed range of temperatures. A classic definition limits the tropics to areas where the seasonal temperature range is smaller than the diurnal temperature range. In addition, mean annual temperatures (MAT) in the lowlands range from just 24 to 30°C over 47 degrees of latitude between the Tropics of Cancer and Capricorn. Most tropical species experience a limited range of temperatures both seasonally and throughout their geographic ranges. This observation led Janzen (1967) to hypothesize that tropical species have evolved to function optimally within narrow temperature limits. If correct, tropical species might be particularly sensitive to global warming. I use GIS analyses, global vegetation cover from the Global Land Cover 2000 project, and global temperatures and precipitation from the IPCC to evaluate the modern climates occupied by major tropical biomes. I then use 20 to 25-yr records of tree growth, leaf fall, flowering and fruiting to evaluate long-term trends that might be related to temperature and other anthropogenic changes occurring on Barro Colorado Island (BCI), Panama.

Results/Conclusions

The GIS analysis indicates that observed temperature increases are already moving tropical forests outside their 20^th century climate envelope. BCI temperatures have already increased by 1.5°C, while precipitation decreased between 1950 and 1980 and then returned to values typical of the early 1900s. Long-term trends in old-growth forest include increased contributions of lianas to leaf and flower production, increased production of flowers by trees, and decreases in tree growth rates. The timing of leaf fall, flowering and fruiting varies by up to two months among years, but timing shows no long-term trend. The causes of these long-term trends are controversial because many environmental factors are changing simultaneously and the very evidence for a trend depends upon the implicit assumption that forests are in equilibrium. This assumption is doubtful.