Ash trees are deciduous, which means they participate in the annual cycle of shedding their leaves, typically in the autumn, before entering a period of winter dormancy. This seasonal leaf drop is a natural survival mechanism that allows the tree to conserve energy and protect itself from harsh winter weather. The genus Fraxinus, which includes 45 to 65 species, is widespread across North America, Europe, and Asia, with most following this deciduous pattern. Understanding this characteristic is key to identifying ash trees and appreciating the challenges they face, particularly the threat posed by the Emerald Ash Borer.
Understanding Deciduous Trees
The term “deciduous” refers to plants that lose all of their foliage for a portion of the year, a process driven by changes in daylight and temperature. This annual shedding, known scientifically as abscission, is a highly functional process that enables the tree to prepare for the cold season. Before the leaves drop, the tree reabsorbs valuable nutrients, such as nitrogen and carbon, back into its inner bark and roots for storage.
The primary biological purpose of this leaf loss is to reduce water loss through transpiration, a process that is highly inefficient when liquid water is scarce or frozen during winter months. By dropping its broad leaves, the tree minimizes the surface area exposed to drying winds and prevents dehydration. Entering dormancy also conserves the energy that would otherwise be spent maintaining the leaves and defending them from cold damage. This cycle of shedding and regrowing foliage differentiates deciduous trees from evergreens, which retain their leaves throughout the year.
Key Identifying Features of Ash Trees
Identifying an ash tree is possible even during the winter when it is completely leafless, thanks to its distinct branching pattern and bark texture. The most reliable feature for identification is the opposite branching arrangement, where buds and branches grow directly across from one another on the stem, rather than being staggered. While not every branch will have a perfect opposite mate due to natural die-off, observing this symmetry along a twig is highly indicative of an ash tree.
When leaves are present, they are compound, meaning a single leaf stem holds multiple leaflets, typically ranging from five to nine leaflets per leaf. These leaflets are arranged in pairs opposite each other, with a single leaflet at the tip. The bark of a young ash tree is often smooth and gray-brown, but as the tree matures, it develops a distinctive pattern of diamond-shaped ridges and furrows. This diamond pattern on mature ash bark is a strong visual clue that helps distinguish it from other tree species.
The Threat of the Emerald Ash Borer
The Emerald Ash Borer (EAB), an invasive, metallic-green beetle native to Asia, poses the single most severe threat to ash populations across North America. First detected in the United States in 2002, the beetle’s larvae are responsible for killing tens of millions of ash trees. Adult beetles lay their eggs in the bark crevices of ash trees during the summer months.
Once the eggs hatch, the larvae burrow into the tree, feeding on the phloem and inner bark tissue, which are responsible for transporting nutrients and water. This feeding activity creates winding, S-shaped tunnels, known as galleries, that effectively girdle the tree and cut off its vascular system. The interruption of nutrient flow causes the tree to weaken progressively, typically resulting in death within two to four years of infestation.
Telltale signs of an EAB infestation include canopy dieback, often beginning at the top of the tree, and the presence of small, distinctive D-shaped exit holes left behind by emerging adult beetles. Increased activity from woodpeckers, which feed on the larvae, can also create patches of lighter “blonding” on the bark. Landowners who identify an ash tree showing signs of infestation must decide between removal or chemical treatment, as the infestation spreads rapidly and renders the tree a public safety hazard once it dies.