Why do some trees lose their leaves in summer?

 Normally, every year in the heat of summer, when temperatures are HOT and rain is limited, we start receiving inquiries on why some leaves are prematurely dropping from trees. It always seems to coincide with high summer temperatures and low precipitation. Just recently, I have seen this phenomenon on our River Birch trees. It’s been noticed with tulip poplar, sycamore, elm, hackberry, willow, and redbud, too.

David Merker, UT Extension Forestry Specialist, said he had the same situation, and it became a personal challenge to determine what these species have in common. Other than all having yellow autumn color, was there another characteristic that contributes to this situation? With some discovery, I think he has found it.

A few years ago, Dr. Jennifer Franklin (UT Tree Physiologist) and David Merker developed a publication titled “Tree Growth Characteristics.” In that, readers gain a general understanding of how trees grow. One of the characteristics that was addressed was shoot growth patterns - - specifically that leaves are produced from the shoot (branch tip) either: (1) continuously throughout the growing season, or (2) in discreet growth bursts termed flushes. Trees with continuous growth, normally called sustained growth, continue to produce leaves as long as growing conditions are favorable. It seemed fitting, then, that such trees might abort leaves when conditions aren’t favorable. In other words, they “over produce” leaves, then in an effort to conserve limited moisture in harsh, hot times, they rebalance by aborting leaves. Normally the oldest (interior) leaves are the first ones aborted.

When the list of seven species, mentioned above, were compared to the list of species having sustained growth, all of them fit. Trees that most often lose leaves prematurely all have sustained growth. Here’s the telling message: if you have one of these seven species and leaves are prematurely falling, it’s likely normal. Watering the trees might be beneficial, but not necessary, because, again, this is normal.

 I hope these thoughts help answer the perennial question of why some leaves prematurely drop from trees.