In the humid tropics there is no obvious climatic stimulus to make a tree shed its leaves, but leaf-shedding occurs nevertheless. Some trees shed old leaves and produce new ones all through the year, good examples being the papaya, the oil palm and the coconut. The majority shed leaves all at one go and replace them simultaneously with new leaves. The switchover is usually so smooth that the trees never appear to be bare, so tropical humid forests appear evergreen. In most years there is little or no synchronization between trees, but once in a long while there is a baffling mass synchronization event.
In the 1930s, R.E. Holttum, then Director of the Botanic Gardens Singapore, had the idea that leaves have an inherent life span and are shed when their time is up. He monitored the leaf change cycles in many trees for 10 years or more and found that each species follows an approximate periodicity, with a variation of up to a couple of months. He could not explain the variation.
Recent studies have shown that leaves exposed to full sun have shorter life spans than leaves in shade. A just-published paper by Ms Tong Pei Sin (my student) and myself describes an experiment comparing 5 species (Acacia mangium, Shorea roxburghii, Dyera costulata, Eusideroxylon zwageri and Cinnamomum iners) under 5 light intensities (4, 7, 25, 50 and 100%). This study shows that leaf life span is shortened by a constant amount for every doubling of light intensity. For example, Acacia mangium leaves have a life span of about 300 days at 4% dropping to about 100 days at 100% losing about 40 days per doubling of light intensity. Shorea roxburghii leaves have a life span of about 550 days at 4% dropping to about 200 days at 100%, losing about 70 days for each doubling of light intensity.
Hence leaves do have a life span, but it is a service life span, not a calendar life span. We propose photosenescence of leaves as one of the drivers of phenology in the humid tropics.
Trees in nature, being subject to varying cloud cover, would vary in periodicity of leaf fall because no two periods would ever experience exactly the same combination of cloud cover conditions. A long period of cloudy weather would prolong the leaf life span of many trees and if this is followed by clear sunny days, many trees would trip over their tresholds and shed leaves simultaneously. This would be accompanied by production of new leaves and inflorescences in mass synchrony. The full paper is availble: