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  • Writer's pictureMartin Ford

Trees can "dump" their leaves and regrow them the same season—how and why?

Defoliate is a more accurate word than "dump", but to the homeowner, watching a tree suddenly drop all its leaves is a mystery—especially when, soon after, the tree quickly regrows a new crop of nice, new leaves.

So why did the tree go to such extremes? Disease or insect infestation is typically the answer. The tree’s decision is for self-preservation—firstly, to remove the food source (its leaves) from the "predator" and secondly, to grow undamaged leaves with which to build up resources again for the coming season.

When a tree completely defoliates, no green structures remain—only woody bark and new buds buried below protective outer scales. This applies to evergreen trees such as the White pine pictured here, which would normally drop its two- or three-year-old needles, but not last year’s needles (hence evergreen). Every tree has the skill to grow new leaves and abort older foliage, and so when we see a sudden and unforeseen defoliation, it is simply a "speeding up" of the trees natural system because of a "crisis”—the most efficient way to stop a pest/disease infestation quickly.

This defoliation "skill" is rarely used by a tree, as it is a heavy use of its nutrient reserves and secondary buds. The regrowth from these secondary buds will produce smaller new growth and smaller leaves for this year since these secondary buds' cell structure was designed as a “reserve” in case there happened to be a need to replace the normal new extension growth and its leaves. As a result, the tree's canopy will look thinner for the season after the "unexpected" defoliation, but it is not because the tree is in trouble—it is simply due to the type of "reserve" buds which are available on the tree's structure and the need to form new leaves to mature quickly so they can begin creating sugars again to replenish the plant's health.

In the tropics, where growth is not seasonally bound as in the north or south, most trees defoliate for a period of time (often only two weeks) as a way of creating new vigorous foliage, and to remove leaves which may have disease. In other words, it is normal to drop leaves each year—the difference is whether it is done quickly and in a coordinated mass defoliation, or gradually as part of the normal cycle of health. Either way, the tree has the ability to recognize its circumstance and respond to both typical and atypical circumstances by "undoing" its food producing foliage in both normal and abnormal scenarios.

So even though trees do not have the ability to move away from changing circumstances that arrive at their "doorstep", as animal life has, they do have the ability recognize a problem and make choices—and then adapt. Dropping all their foliage is just one of the available choices.


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