Heavy winds and severe thunderstorms can cause leaves to fall before they change colors. Hurricanes can destroy trees and their foliage. For example, Hurricane Irene deposited salt on trees many miles inland, causing cell and tissue damage to leaves.
At the point where the stem of the leaf is attached to the tree, a special layer of cells develops and gradually severs the tissues that support the leaf. At the same time, the tree seals the cut, so that when the leaf is finally blown off by the wind or falls from its own weight, it leaves behind a leaf scar.
Most of the broad-leaved trees in the North shed their leaves in the fall. However, the dead brown leaves of the oaks and a few other species may stay on the tree until growth starts again in the spring.
In the South, where the winters are mild, some of the broad-leaved trees are evergreen; that is, the leaves stay on the trees during winter and keep their green color. Most of the conifers - pines, spruces, firs, hemlocks, cedars, etc. The needle- or scale-like leaves remain green or greenish the year round, and individual leaves may stay on for two to four or more years. Temperature, light, and water supply have an influence on the degree and the duration of fall color. Low temperatures above freezing will favor anthocyanin formation producing bright reds in maples.
Year after year, daylength will come with great regularity at the same point in earth's orbit, reliably signaling the inevitable onset of colder weather. But temperatures can vary greatly at any time of the year due to variations in the weather like this year when temperatures were unusually cool all summer and so are less reliable cues for the coming of fall.
Trees use temperature only as a secondary cue; if the fall is cool they hasten the development of their fall color, while if it is warm, they procrastinate and delay it. If global warming results in warmer fall temperatures, either during the day or night, this would tend to delay fall colors to later in the season. Will it mute the colors? It might, especially if the disconnect between daylength and temperature becomes extreme, thereby confusing trees and disrupting the synchrony of color development, such that some trees attain peak color later while others are less susceptible to the temperature cues.
Warming will also extend the potential growing season during times when light levels are low due to the lower zenith angle of the sun in winter and because shorter days allow less time for trees to carry out photosynthesis. This would result in less photosynthesis relative to respiration, which would lower their sugar reserves, and sugars are necessary to stimulate synthesis of anthocyanins, which give leaves their bright red fall color.
Higher precipitation would lower the intensity of fall color, not because it washes out the colors a wives' tale but rather, because the concomitant cloud cover and low light levels reduce photosynthesis. But why would increased nitrogen reduce fall colors? As I noted in a column last year, trees produce anthocyanins to help withdraw nutrients back into their twigs so they can use them for next year's leaves.
The fewer nutrients a tree has, the more precious they are, and so the more necessary it would be to protect their withdrawal system, which is what the anthocyanins do. So, trees that are slightly nutrient stressed, particularly those with low nitrogen, often have enhanced red color.
Paul Schaberg and his associates in the U. Forest Service in Vermont, have shown this with sugar maple. The key here is that when trees are stressed by low nitrogen, they tend to accumulate sugars in their leaves, and high sugar content is a trigger for anthocyanin production.
So, in a future high nitrogen world, trees will be less stressed, and as a result, perhaps less red too. Conversely, if acidic deposition continues to leach nutrients from our soils, then this might counteract the additional nitrogen coming in from farming activities.
This could result in a stand-off between leaching and deposition, and perhaps no change in fall red coloration. The biggest unknown will be how global change will affect tree distributions. We have already seen hardwood trees migrate upslope nearly 40 m in Vermont in just the past half century, where mean temperatures have risen 1.
For trees on flatter terrain, warmer temperatures will force them to migrate north. How far north is still an active area of investigation. But where do these colors come from?
It all starts inside the leaf. Leaves have color because of chemicals called pigments, and there are four main types of pigment in each leaf:. Chlorophyll is important because it helps plants make energy from sunlight—a process called photosynthesis. The summer sunlight triggers the leaves to keep making more chlorophyll.
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