And you thought you knew that all trees were either deciduous or evergreen, didn’t you! Ha! Well it turns out that there is a bit more to it than that - at least as far as the “ever” in evergreen is concerned.
Anybody who grows tropical plants already knows that evergreen plants do shed their leaves from time to time - or all the time - and that it varies from species to species. Believe it or not, there is a branch of horticulture that studies the phenomonon of leaf change - Phenology.
Leaf-changing among evergreens in the humid tropics was the topic for the presentation by Dr. Francis Ng at the annual award ceremony at the Kampong. He is currently working in Malaysia but has an extensive resume including work outside the tropics in Europe.
His presentation focused on his efforts to see if he could find out what triggers leaf change in plants in the equatorial tropics. Why do they shed their leaves and flower at periodic intervals and, more particularly, what makes all flowering species do this en mass. This would seem obvious in temperate and sub-tropical areas where the climate changes from hot to cold or dry to wet periodically throughout the year. But even those of us in semi-tropical like South Florida, with obvious seasonal changes in day length and light intensity, often question what triggers some fairly extreme variations in timing and intensity of bloom as well. We may chalk it up to too much rain or dry conditions one year but no explanation ever quite seems to be perfectly predictable.
Even more curious is that there is often distinct synchronicity of bloom and leafing in the humid tropical areas along the equator where there is fairly uniform rainfall (wet or wetter) and little temperature variation throughout the year. So Dr. Ng’s study is searching for the cues that trigger trees in the humid tropics to shed their leaves and initiate flowers in flushes twice a year. Why are some flushes much heavier than others? Beyond the obvious flushes, plants in the humid tropics also produce smaller numbers of flowers and fruits continuously throughout the year. His hypothesis centers around changes in light intensity associated with more light at the semi-annual equinoxes and lower light when there is greater cloud cover during the monsoon periods.
Evergreen plants in Malaysia have two flowering peaks annually - the bigger flush in March/April and a second lesser peak in October/November. ALL plants flower synchronously during these peaks. Flowering is always associated with new leaves, but leaf change also occurs at times not associated with flowering. The intensity of the flush can be quite variable from year to year with occasional spectacular flushes once or twice in a decade. Botanists working in South America have referred to similar flushes in deciduous wet/dry forests as “big-bang” flowering, saying it is a sight to behold.
Leaves become damaged, thin and fragile over a period of time and have to be replenished with fresh leaves in order to continue producing the chlorophyll that sustains life. Dr. Ng’s studies have shown that the more accumulated light the shorter the lifespan of a leaf.
It may be revealing to gardeners to know that there are several ways that evergreen plants change or shed their leaves.
Continuous exchange leafing describes plants whose leaves are shed and replenished continuously. Few trees in the humid tropics are truly deciduous. Some trees will drop all their leaves at once but begin to releaf almost immediately. This may happen several times a year. Others may shed their old leaves periodically as the new leaves are coming out and never be completely bare. Still others shed their leaves continuously throughout the year.
Leaf changing is associated with the intensity and amount of light the leaf receives. The more light a leaf receives the shorter its life span. Light intensity relates to its position in the canopy of a tree or in the understory. Continuous leaf exchange occurs in 90% of canopy species as well as plants like palms, papaya, Muntingia. I think Cecropia would be another example most of us know.
Flush accumulation type leafing species tend to produce new leaves in flushes which are clustered near the tips of the branches while the older leaves fall further down the branch. This is the typical habit of 90% of the species in the understory. Flushes occur more frequently in juvenile plants - up to 5 or 6 times a year and decreases as a plant ages.
Understanding these different types of evergreen leaf exchange is useful to horticulturists for understanding the characteristics of different species. How many times have we seen a young tree shed all its leaves several times in a season and worry that the leaves are yellowing or falling due to overwatering, cold, disease or dryness when they are actually just turning over leaves normally for their particular species.
[Dr. Ng's personal profile and assorted topics on horticulture can be found in his blog at http://tropicalhorticulture.blogspot.com/]
Tags: Add new tag, equatorial rain forest, evergreen, humid tropics, leaf-change, uat
Recent Comments