Being first isn’t always a good thing. For example, trees that are first to have their leaves turn color are definitely losers. Premature autumn leaf color change is a reliable indicator of failing health, and the worse a tree’s condition, the sooner it begins to turn. Although the display of colors that our hardwoods produce each autumn never fails to fill me with awe and appreciation, when it starts in late July or early August, it worries me.
In the summer of 2016, soil moisture dropped to record low levels, partly due to scant rainfall, but also because of low humidity, high temperatures, a very high UV index, and frequent and persistent winds. 2018 was also a very dry season, and the spring of 2021 was rather parched as well. In such conditions, tree roots actually die back, beginning with the fine absorbing roots a few inches below the surface.
Most people, me included, are surprised when they first learn that ninety percent of tree roots reside in the top ten inches of soil, and that very few roots penetrate beyond eighteen inches deep. Knowing this, it’s easy to understand how tree roots can run out of moisture in a drought.
A sort of Catch-22 situation happens as a result of root dieback. All the starches stored in the affected portions of the root system root are lost, so the tree has less energy the following year. But that’s when it needs extra energy to re-grow its roots. And because its roots are compromised, the tree can’t get adequate water and nutrients.
Dr. George Hudler from the Plant Pathology Department at Cornell says that it can take a healthy tree three years to recover from a significant drought, as long as the tree is healthy and conditions remain favorable. If one of the ensuing three years is short on water, the tree might begin to decline.
But what about trees whose health was less than stellar to begin with? A lot of the premature leaf color change one sees in early August is on roadside trees. Those unfortunate specimens are subject to road salt, root zone restriction, reflected heat from pavement, and in most cases, turf grass, which gobbles up all but the heaviest rains. It’s no shock that these are the ones that turn color first, but it is jarring to see it a month earlier than normal. When forest hardwoods growing on thin rocky soils began showing early color, it’s unusual, and an indication that climate change is affecting whole forest communities.
Knowing the significance of early color can be disconcerting, but it’s also an opportunity to see how the trees around your home are doing. Those that have almost completely turned color by late August are probably not long for this world, and it’s time to consider their removal and replacement in the next few years. Color change of fifty percent or less indicates moderate decline, and certainly extreme stress.
Providing supplemental water (one inch over the root zone per week) to these tree over the next few years may help keep them around longer. Mulching the root zone, which is twice the branch length, two to four inches deep in lieu of grass will also help at least as much.
When fall does get here, I hope we can still enjoy the colors as much as we used to.
Paul Hetzler has been an ISA Certified Arborist since 1996, and is a former Cornell Cooperative Extension educator.
Photo of maple tree by Aplaster, CC BY-SA 3.0, via Wikimedia Commons