For about three quarters of the year, the leaves on our area’s trees and shrubs are at work, absorbing sunlight, opening and closing pores to limit water loss and to take up carbon dioxide. Each species has its distinct leaf shape, size, even color — there really is no one uniquely ideal leaf shape, spatial arrangement or composition. Leaves are diverse in form and function, permitting adaptation to variation in climate, soils and weather.
What may be less apparent, though, is the diversity of leaves’ life histories — from birth in the bud at the tip of a branch, rapid expansion followed by light capture and starch-making in maturity, leading to retirement and ultimately return to the earth. Let’s explore aspects of some of these leaf life histories, with a look at what happens in autumn and winter.
By the time autumn arrives, you might assume that leaves’ main work is done for the year. After all, leaf color change announces the breakdown of the light-gathering machinery. Certainly leaf fall, an active process in which the tree shuts off water connections to the leaves, ends the active relationship between leaves and tree. Yet many of the woody plants in our region don’t do that; they retain green leaves over the winter.
Also, fallen leaves are not gone; they are merely changing form, becoming a very important part of the soil below the plants that grew them.
New England is renowned for spectacular fall colors. Color change starts in the red maples in swamps, spreads to birches and aspens, then reaches a flaming climax in sugar maples, followed by the scarlet and russet embers of red oaks. These leaves are the sprinters, lasting less than a year. Maple and birch leaves decompose rapidly, perhaps even releasing nutrients before soils freeze. This allows soil microorganisms to take up those nutrients, supporting microbe growth with readily available sugars and nitrogen. The blanket of new fallen leaves acts as insulation, so upper soil layers stay above freezing.
The leaves of alders, which are nitrogen-fixing shrubs found along streams, have a higher nitrogen content and form rich humus. Oaks, on the other hand, are the tough guys of the leaf world. Heavily lignified and coated with a thick waxy layer, oak leaves decompose slowly. They’re known to form deep “fluffy” litter layers. This deep dry oak litter can burn quite easily, spreading and intensifying ground fires. Oaks, with their thick bark, are rather resistant to ground fires, while maples and birches have a thinner bark, and are more prone to fire damage. Hence, the fallen oak leaves may be part of the tree’s “strategy” for out-competing its neighbors.
Oaks and beeches (which are in the same plant family) also sometimes don’t lose their leaves completely. Perhaps this is due to the evolutionary origin of oaks in more southern regions, where in fact many trees in this family are evergreen. Up north, cold temperatures and short day length don’t trigger the formation of the barrier between leaf and branch in time. Oak and beech leaves are essentially still alive and active when cold winds finally freeze and dry them out before they drop. They’re torn off by wind during the winter, or pushed off when new leaves expand in nearby buds in the spring.
We usually use the word “evergreen” to describe woody plants that retain their needles in winter, but the term isn’t completely accurate. These needles simply live more than one year — they are long-distance runners rather than sprinters. They’re capable of photosynthesis when other trees are dormant. However, a variety of stresses — cold and dry winter winds, intense sunlight in summer, and attacks by insects and microbes — requires a variety of protective adaptations. For example, spruces and firs have flexible, downward growing branches that shed snow and ice, increasing their chances of survival.
The range of leaf ages in evergreens is remarkably wide. Bristlecone pine needles live as long as 45 years. Pines in our area keep needles for 2 to 4 years, hence the accumulated “pine duff” under these trees. Spruces shed their needles at 4 to 10 years old, leaving behind the “pegs,” the short shoots that held the needles.
Older pine needles retain about 40% of their capacity for light and carbon capture in the full warmth and light of summer. So, other than getting a big jump on growth in spring, do evergreen leaves continue photosynthesis in the winter? Studies of red spruce and eastern hemlock suggest that winter photosynthesis can contribute to a buildup of starch in foliage and twigs, particularly during winter thaws. The main limitation seems to be the freezing of water in the twigs and the shutting of leaf pores. So when the sun is out and temperatures are above freezing, leaves are not just hanging on for dear life. They are still at work.
These glimpses into the diverse lives of leaves illustrate how crucial adaptive growth is for trees and shrubs. Environmental conditions continuously change, and plants, of course, can’t relocate to better climes. Instead they adjust, rebuilding new canopies of amazing leaves.