Deciduous trees typically lose all of their leaves by late autumn. But a stroll through the Arboretum reveals a scattering of deciduous trees and shrubs that still have leaves (albeit dry and brown) clinging tightly to branches. These plants are exhibiting marcescence, the trait of retaining plant parts after they are dead and dry. Marcescence most often refers to persistent leaves but can also refer to other parts such as flower corollas.Leaf marcescence is most often seen on juvenile plants and may disappear as the tree matures. It also may not affect the entire tree; sometimes leaves persist only on scattered branches. Unlike a typical deciduous leaf, a marcescent leaf doesn’t develop an abscission zone, an area at the base of the petiole containing a separation layer (thin-walled cells that break readily, allowing leaf drop) and, on the twig side, a protective layer of corky cells. The evolutionary reasons for marcescence are not clear, though theories include defense against herbivory (e.g., browsing by deer), protection of leaf buds from winter desiccation, and as a delayed source of nutrients or moisture-conserving mulch when the leaves finally fall and decompose in the spring.
Some woody plant species are more likely to exhibit marcescense than others. One of the most striking marcescent tree species is American beech (Fagus grandifolia), whose papery, pale tan winter leaves provide an easy identification feature as well as adding a ghostly shimmer in snow-filled woodlands. Many oak (Quercus) species are notably marcescent, and some hornbeams (Carpinus) and hophornbeams [pdf] (Ostrya) also tend to hold their leaves. Some witch-hazels [pdf] (Hamamelis) may retain foliage, which unfortunately can detract from the floral display of these winter-blooming shrubs. And if you visit the Arboretum this winter, note the handsomely marcescent specimens of narrow-leaved spicebush (Lindera angustifolia 740-75) along Bussey Road across from the lilac collection.