This Blog post is courtesy of the PA Forest Stewardship News Release. This was written by Jim Finley and is a very informative article on that question many people have as they drive along the road and see trees hanging on to the brown leaves throughout the winter months!! For those of you who live in Pennsylvania and are interested in learning more about the PA Forest Stewards, I highly recommend the training and program.
It is a wonderful two weekend course where you meet many like minded individuals and have the opportunity to learn great things about managing your forest--this is especially great for landowners who have forest land, or land managers who manage forests!
Winter
Leaves that Hang On
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. Two small trees in our yard, a white oak and a
shingle oak, both in the white oak group, 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.
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.
Written By Jim Finley
The trees in this picture (taken at Stateline Woods Preserve) show beech trees with their leaves remaining.
If you aren't familiar with the Farmer's Almanac, I recommend picking one up. This is typically the most accurate source of weather which is printed once a year for the entire season and mostly focuses on astronomical occurrences and weather patterns to make it's predictions. This is the oldest and most continuously published periodical. The first almanac was published in 1792 during George Washington's presidency. The history of the Almanac is entertaining and has had its ebbs and flows..it was even considered being banned by the government after a German spy was captured off the coast of Long Island in 1942 with a Farmer's Almanac in his pocket. The US government speculated that the Germans were using the book for the weather predictions--therefore the book was "supplying information to the enemy". The editor at the time, talked the US Government off the publishing ban of the periodical. 
