Saturday, December 10, 2016

Winter Leaves That Hang On by Donna Duffy


This Acer grandidentatum, Bigtooth Maple, is slow to drop its leaves in winter,  photo by Donna Duffy
Ever wondered why some deciduous trees hold on to their leaves through the winter and others go bare? Learn about marcescent leaves and why they might just help a tree out. The article below was written by Jim Finley, Penn State College of Agricultural Sciences, Department of Ecosystem Science and Management.

Winter trees, photo by Donna Duffy
We are near the Winter solstice, and hardwood trees are mostly bare, stark against the sky, without their leaves. The only hint of summer’s green trees are the conifers scattered about yards and forests. Here and there, though, brown, dried leaves clothe some hardwood trees. These trees rattle in the winter winds, holding fast to summer’s leaves.

On winter woodland sojourns, you may have noticed hardwood trees holding fast, sometimes all winter long, to their spent and dried leaves. Marcescence, the term used to describe leaf retention, is most common with many of the oak species, American beech, witch hazel, hornbeam (musclewood), and hophornbeam (ironwood). 

Normally, as deciduous trees (which include hardwoods and some conifers) prepare to shed their leafy summer coats, cells at the interface between the twig and the end of the leaf stem release enzymes and form an abscission layer that “unglues” the leaf – separating it from the vascular bundles, allowing it to fall free. All trees shed leaves, even conifers; however, they generally retain their needles for more than one year. Leaf drop benefits deciduous trees by reducing water loss and allows them to develop leaves that efficiently use available sunlight during warmer seasons. Sometimes, early cold weather or frosts may interrupt the abscission process or “kill” leaves quickly. In these cases, the occurrence of marcescent leaves may increase. Lacking killing frosts, why would trees “decide” to retain their leaves? It is impossible to ask the trees, but we can speculate.

Marcescent leaves are often more common with smaller trees or more apparent on lower branches of larger trees. In the case of smaller trees, which in forest conditions would be growing beneath taller trees, the reduced sunlight might slow the abscission process. By doing this, the understory tree leaves and the leaves on lower branches of larger trees would also have the opportunity to continue or even increase their photosynthetic process as upper leaves fall. Then, perhaps, leaves lower in the canopy are “caught” with cold temperatures and their leaves hang on.

Some people speculate that retained leaves may deter browsing animals, such as deer. The dried leaves may conceal buds from browsers or make them difficult to nip from the twig. Researchers have found that the dried leaves are less nutritious. At least one study, conducted in Denmark, found that deer offered hand-stripped twigs preferred those to marcescent twigs, especially of beech and hornbeam, but not so for oak. Nutrient analysis found the protein content of oak twigs was higher and the dead leaves had less lignin. The protein content of beech and hornbeam twigs was about equal to the leaves; however, the lignin content was nearly half again higher in the leaves. Maybe there is something to the leaves protecting the twigs. 

Photo by Donna Duffy

The other reason trees might give for holding onto their leaves relates to nutrient cycling. Leaves that fall in the autumn would join others on the forest floor and begin to decay. As they decay, released nutrients could leach away and be unavailable to “feed” trees the next growing season. This might be especially important to small understory trees with smaller root systems. By holding onto their leaves, they retain and recycle their nutrients to themselves.

Regardless the reason for marcescent leaves, when growth begins next spring the expanding buds will push them off and clothe the branches with new greenery. Until that happens, enjoy the waving brown leaves and the texture they add to forest and yards. Then, too, think about the bit of shelter they provide for wintering birds as they perch among the rattling leaves, away from winter’s wind.