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TURNING OVER AN OLD LEAF

Our second major eye surgery of the year coincided with autumnal change at Hilton Pond Center, so while recuperating and unable to work at the computer we had lots of time to think. Despite impaired vision, we were able to see trees changing colors and leaves cascading earthward, so one thing we cogitated about was why it might be advantageous for deciduous trees to drop foliage in fall after spending tremendous amounts of energy making leaves the previous spring. Although some tropical plants lose their leaves--usually during the dry season--the vast majority of deciduous trees are in temperate or sub-arctic zones, so it's apparent that "turning over an OLD leaf" each fall is related to climate.

All text & photos © Hilton Pond Center

Pear, Pyrus communis (above, an introduced species)

One of the most common explanations for deciduousness is that in colder regions precipitation comes as snow or ice--non-liquid forms of water that are inaccessible to trees. For a tree in northern latitudes, frozen water is like no water at all, so winter is a drought season no matter how much snow falls; thus, a leaf-laden tree might wither and die--primarily because leaves give off huge amounts of water through transpiration. Here at Hilton Pond, however, winters are seldom cold enough to retain water in its frozen state for substantial lengths of time, so this argument for being deciduous might not seem to be appropriate for the Carolina Piedmont. However, when we reflect upon geological time instead of recent history, we're reminded that during the last ice age glaciers ranged as far south as the Great Lakes--which would have meant the Piedmont and points north were much cooler then, and that deciduousness might simply be a holdover strategy from glacial times. Today, the Carolinas are just cool enough in winter that a tree's "Ice Age" deciduous tendencies are still advantageous.

As often happens in nature, there's more than one way to "skin a cat," and that's undoubtedly the case when trees deal with cold weather. Foliage of a broadleaf tree would indeed transpire valuable water in winter when replacement water would be tied up as ice. Evergreen trees such as pines and cedars avoid this problem by making leaves covered by waxy coatings that retain moisture. And, whereas thin, water-laden broadleaves might be prone to damage from freezing, stiff and narrow evergreen needles contain aromatic compounds that act like antifreeze. Interestingly, some broadleaf trees such as American Holly, Ilex opaca (above), have waxy leaves that don't freeze--allowing them to retain foliage all winter long. To confuse the issue, there are some enigmatic needle-leafed trees like swamp-lover Bald Cypress, Taxodium distichum (right), and Northern bog-dwelling Tamarack, Larix laricina, that don't bear waxy leaves and turn out to be deciduous.

It may be the case that the main function of deciduousness in temperate trees is to help prevent loss of water when it's locked up as ice, but as we pondered the adaptedness of leaf-dropping plants we came up with some other explanations that also may be valid. These include the following:

1) When the growing season in temperate zones gives way to autumn and then to winter, the days grow shorter and fewer hours of available light can no longer support the lush growth of summer. Since photosynthesis fuels a green plant's entire metabolism, less--and weaker--sunlight means insufficient light energy is available. If a large tree such as Sweetgum, Liquidambar styraciflua (left), retained its chlorophyllous leaves in winter, it still might not be able to produce enough food to keep the plant healthy and functioning. Add the fact that biochemical reactions slow down as temperature drop and it seems more advantageous for a plant to simply discard its leaves and settle down for a winter rest.

2) Even if a broadleaf tree were to maintain its leaves in winter and somehow take advantage of reduced sunlight, it might be that those leaves wouldn't do a very good job because of damage from insects and the elements. Compared to a tree's trunk and twigs, its leaves are relatively delicate and get battered about all spring and summer by gusts of wind. More important, we think, nearly every leaf we found this week at Hilton Pond showed damage from insects, fungi, or disease (see photo below of Tupelo Gum, Nyssa sylvatica). Some fallen leaves were missing huge sections of their blades--likely the work of a voracious caterpillar bulking up before it pupated. Perhaps it's more efficient for a tree to disconnect these heavily damaged leaves each fall and start out with fresh ones the following spring.

3) We seldom have heavy snow in the Carolina Piedmont, but we often suffer the effects of freezing rain. Unlike snow--which would tend to bounce off foliage of a broafleaf tree that held its leaves in winter--freezing rain sticks to every surface. If a broadleaf tree held its leaves all winter, its leaves would collect large amounts of ice that, in turn, would result in significant weight stress--sometimes so much that leaves, twigs, and branches are stripped from the tree. This is exactly what happened a few years ago to a huge Southern Red Oak just outside the old farmhouse at Hilton Pond. This particular tree species tends to hold its leathery leaves long after they die and turn brown; usually the foliage is not dropped until January or later. So, when we had an unseasonably early freezing rainstorm on 4 December 2002, the oak's still-attached leaves collected so much ice the tree lost perhaps a third of its terminal twigs, plus one major limb 30 feet long and a foot in diameter. Had this deciduous tree actually lost its leaves at autumn's end, we're certain storm damage would have been much less (see The Ice Storm Cometh).

4) Any poor souls who feel obligated to rake or blow their yards each autumn--and we certainly don't exhibit that vice at Hilton Pond Center--know from back-breaking experience that a good-sized tree can cast of hundreds of thousands of individual leaves that pile up beneath it. Despite winter zephyrs, the majority of those leaves remain where they fall--directly beneath the tree that produced them. By dropping leaves, a tree such as Flowering Dogwood, Cornus florida (below right), gives itself at least three advantages we can think of. For one, the leaves form a thick layer of mulch that eventually will decompose and release valuable nutrients the tree can re-use annually with each new crop of leaves. Second, fallen leaves form a blanket on the substrate that helps insulate a tree's roots from cold AND that impedes evaporative water loss from the ground itself. And third, a dense mat of leaves under a tree may choke out other plant species that try to sprout and compete for valuable resources. We've always found it curious that the deciduous American Beech, Fagus grandifolia, typically holds its dead, brown leaves all winter and then dumps them in spring as new leafbuds burst. The only rationale we've come up with for such a strategy is that the beech is indeed trying to smother its competitors at the beginning of the growing season.

Undoubtedly there are other explanations for why it's advantageous for deciduous trees in the Carolinas and elsewhere to "turn over an OLD leaf" each fall. If you think of any, please send them on to us at INFO for possible discussion in a future installment of "This Week at Hilton Pond."

All text, charts & photos © Hilton Pond Center

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SPECIES BANDED THIS WEEK:
Winter Wren--2 *
Eastern Phoebe--1
Magnolia Warbler--1
Yellow-rumped Warbler--2
House Finch--1
Red-eyed Vireo--1
Tufted Titmouse--2
Downy Woodpecker--1
Scarlet Tanager--1
Northern Cardinal--1
Eastern Towhee--2

* = New species for 2004

WEEKLY BANDING TOTAL
11 species
15 individuals

YEARLY BANDING TOTAL
(2004)
61 species
1,825 individuals

BANDING GRAND TOTAL
(since 28 June 1982)
123 species
45,130 individuals