by Professor Gene Wengert
Q.
I am a junior high industrial technology teacher. Recently a student asked me if topical hardwood trees lose their leaves and produce annual rings. I was not sure about the answer since I have never read or studied this particular question. Can you answer our question?
A.
Most hardwood tropical trees lose their leaves annually, but some keep them (just like holly keeps its leaves). (Just in case you are not aware: softwoods lose their needles, but usually they last for two years [except tamarack] and lose them gradually, so we don't notice it--there always are some green needles.)
There are growth rings in the tropics that reflect the wet/dry season contrast. In fact, in some areas where there are two rainy seasons, there will be two growth rings per year. With tropical species, the nutrients are in the leaves, so a gradual release of leaves means that the soil (which often is sterile from being leached out for centuries) has a new application of minerals/nutrients. Jumping ahead: As a result, it is important that tropical forests are not clear-cut or burned, as that will release all the nutrients so quickly, that they will be leached out forever and the soil cannot support a new forest. In fact, it cannot even support good grass growth for cattle grazing. Because most tropical species are not suited for lumber/veneer production, we seldom had lumber harvesting destroying a tropical forest. But the need for land today for cattle and the fact that with the ban on tropical species that the tropical forests have no economic value (If we hadn't banned tropical wood, the trees would have been so valuable that we probably would have to reconsidering burning for land clearing!), vast areas are being burned and converted to grass permanently. In fact, 96% of the deforestation is the result of burning and not timber harvesting.
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
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