Tuesday, January 19, 2016

Hanging on, Defying the Winter Season

These January days, as I wander through a small woods near my home, the horizons through the understory are much longer than they were this summer, now that leaves have been shed.  In this remnant of what was once a larger mixed deciduous forest, weak winter sunlight glances off tree trunks, naked branches, and a carpet of decomposing leaves.  That deciduous habit intrigues me and, in particular, those species of deciduous trees that don’t quite fully make up their minds about losing their leaves.

In his engaging and graceful The Trees in My Forest (1997), biologist Bernd Heinrich tells the story of his 300 acres of Maine forest, its logged-over past and its carefully nurtured present.  The book is natural history at its finest, a teaching conversation with a master.

Pondering why many trees lose their leaves in the autumn (and some don’t), Heinrich suggests that, at least in his area of New England, leaf loss reduces the trees’ surface area subject to “dangerous ice-loading” from ice storms and, thus, minimizes broken limbs.  Persuaded by his study of the effects of a specific spring ice storm on deciduous trees that had leafed out, he writes,
Leaves have evolved to stay on in the winter, under situations where snow and ice-loading does little damage.  In my forest and the bog near it, the ground-hugging lycopods, Labrador tea, bog rosemary, wintergreen, as well as spruces, firs, and pines, all keep their leaves for at least one winter. . . . On the other hand, winterberry (a holly) and mountain holly both shed their leaves each fall, while other more southerly holly trees and bushes keep theirs through the winter.  The red oaks in my forest lose their leaves, while many southern oaks don't.  My red oaks hang on to their leaves for much longer than all the other deciduous trees as if they had not yet perfected their technique of shedding.  (p. 66)
All leaves are eventually lost, even among the evergreens; it’s really a matter of timing.  The physiological process by which leaves on trees separate from twigs and fall to the earth is triggered by the release of enzymes from a layer of cells where leaf stem joins twig.  These enzymes create an abscission layer that, as Jim Finley, professor of forest resources at Penn State, puts it, “’unglues’ the leaf – separating it from the vascular bundles, allowing it to fall free.”  (Winter Leaves That Hang On, Forest Resources, Penn State Extension, December 17, 2012.)  The first deciduous trees evolved in response to the need to cope with a seasonal climate, perhaps as ranges expanded or climate changed.  (Frederick T. Addicott and Jessye Lorene Lyon, Physiological Ecology of Abscission, chapter 3 in Shedding of Plant Parts, edited by Theodore Thomas Kozlowski, 1973; Michael Snyder, Why Do Some Leaves Persist on Beech and Oak Trees Well Into Winter?, Northern Woodlands, Winter 2010.)  Though deciduous trees gave up the advantages of continuing photosynthesis and greater conservation of nutrients, they gained an effective way of minimizing damage from frost, ice, and snow, reducing water loss, and maximizing photosynthetic production during the spring and summer.

In the passage quoted above, Heinrich mentions, in particular, the red oaks in his forest.  Several types of deciduous trees share that behavior of longer leaf retention.  Forester Michael Snyder, Commissioner of the Vermont Department of Forests, Parks, and Recreation, in an engrossing essay on this phenomenon, writes,
Today in our woods, we still have several evergreens, like pine, spruce, hemlock, and fir.  And we've also got the relative newcomers with short-lived leaves - birch, maple, cherry, and aspen, for example.  But then we have a third class of tree in beech and oak that seems to represent a middle ground of sorts between evergreen and deciduous.  Their leaves die, but many don't fall when they die.  Botanists call this retention of dead plant material marcescence.
(Why Do Some Leaves Persist on Beech and Oak Trees Well Into Winter?, see earlier citation.)

In my local woods, the marcescent trees are, indeed, beech (the American beech, Fagus grandifolia) and oaks (various species of the genus Quercus).  Pity the poor oaks leaves because they violate my aesthetic sense; I have little nice to say about the blotches of dull brown oak leaves that cling to their twigs, for all the world like ugly scabs.  Rather, it’s the delicate tan-to-white beech leaves that gently shake in nearly imperceptible breezes that add sweeps of beauty and gentle sound to my bleak midwinter landscape.

The American beech is marcescent, retaining some of its leaves through winter and into early spring.  The habit may well be specially a juvenile trait.  On mature beeches, leaves may remain affixed to just lower branches, while, on the juvenile trees (as seen above), populating the understory, the lingering leaves are abundant enough to reflect the shape of the tree.

Hold a marcescent beech leaf between your fingers and be struck by how smooth and silk-like it is.  There is a fineness in this tenacious leaf that cannot be matched by oak leaves still on their trees.

Why are some species of trees marcescent?  There is no single, consensus answer.  Rather, there are, as with many natural history puzzles, some wonderful hypotheses.  Unless noted, those presented below are mentioned by Snyder and Finley in their works cited earlier.

The marcescent habit may allow deciduous trees living in less-than-optimal places to gain a small nutritional boost in the spring when those hanger-on leaves fall and decompose.  Indeed, beech and oak trees do particularly well in challenging environments.  Further, the retained leaves may serve as an obstacle that retains snow, and the accumulated snow may provide welcome moisture in the spring.

Perhaps the development of the abscission layer is retarded for oaks and beeches in the understory in order to prolong the trees’ photosynthetic activity when more sunlight reaches the forest floor after the towering adult trees have lost their leaves.

Possibly, the retained leaves may protect the trees’ critical twigs and buds in various ways.  The leaves, being mostly dead (the base of the leafstalk and the abscission zone are still alive) and bleached out, are particularly unappetizing to winter browsers such as deer, thereby encouraging them to dine elsewhere.  The authors of a study of spider diversity in the canopies of European beech trees (Fagus sylvatica) identify an interesting symbiotic relationship that benefits beeches.  They found that lingering leaves serve as shelter for certain spider species that winter over on the beech trees eating herbivorous insects during the winter and early spring.  (Yu-Lung Hsieh and Karl Eduard Linsenmair, Seasonal Dynamics of Arboreal Spider Diversity in a Temperate Forest, Ecology and Evolution, Volume 2, Number 4, 2012.)

In closing his article (Winter Leaves That Hang On, cited earlier), Finley makes a comment about marcescent leaves that warms my heart in this bitterly cold winter period:
[T]hink about the bit of shelter they provide for wintering birds as they perch among the rattling leaves, away from winter’s wind.
Nature Blog Network