All biological processes may be interesting to natural resource scientists, but few stimulate as much mass-appeal, or as many road trips, as when leaves change colors in the fall. In the “Street Talk” segment of the Oct. 5, 2017 edition of the APG News, some Team APG members listed the fall colors and changing scenery as their favorite part of this time of year.
Fall is the season when we end vacations, go back to school, harvest crops, stack firewood, and start hunting seasons. It is also a time when many Americans drive hundreds of miles to watch the leaves change color in Maine, Vermont, West Virginia, and right here in Maryland.
Here at APG, we are privileged to witness bright fall colors extending from the piedmont hills right to the edge of the Atlantic coastal plain and Chesapeake Bay. During a brief window of time the greens of summer transition into an array of yellows, oranges, and reds that contrast nicely with the dark evergreens, crisp blue skies, white clouds, and glistening bay waters.
As soon as we take this leafy display for granted, it turns to brown or beige and (mostly) falls to the ground. The dropped leaves, if unlucky enough to be bagged and landfilled, dry and crumble and release their life and nutrients back to the bottom of the food chain for detritivores and autotrophs, such as next year’s oaks that are now only acorns. Such are the colorful highs and lows, then highs again, in the cycle of life.
Where does the green go?
Most folks have learned that the green in leaves is chlorophyll and that it has something to do with photosynthesis. Actually the green pigment is absorbing red and blue wavelengths of light and thus reflecting the green light that we see.
The leaf cells use that solar energy to convert carbon dioxide (which we, other animals, and our cars exhale) into sugars. Carbon dioxide molecules, with just one carbon and two oxygen atoms each, are converted with water (hydrogen and oxygen) into various sugars that contain long chains of many carbons, hydrogens and oxygens. One byproduct of this process, as we all know, is oxygen gas (two oxygen atoms bound together) which is pretty convenient for us oxygen breathers.
Each acre of trees provides enough oxygen for about 18 people. So plants and plant-like organisms from aquatic algae to terrestrial trees are the foundation of life for most living things on earth … including us. They capture carbon dioxide, make sugar, and provide oxygen. One essential bit of chemistry for life that’s missing though is the fixing of nitrogen from the gas in our atmosphere into forms that are required for all living things including the plants. The nitrogen must be combined with hydrogen (e.g., into ammonium ion) or oxygen (e.g., into nitrates) so that it can ultimately be used to make amino acids for proteins and nucleotides for ATP, RNA, and DNA (all essential for life). That molecular magic is performed primarily by microorganisms like bacteria, sometimes working right next to, or even inside, the roots of plants.
Thus the combination of plants, soil, and organisms in our forests and other natural areas are literally life support systems for our planet.
As our green leaves temporarily go away locally we continue to breathe easily through the winter as evergreen plants here, trees and other plants in the southern hemisphere, and ocean phytoplankton, are still making new chlorophyll, and thus oxygen. We know the green leaves here will be back in the spring as our deciduous trees start producing chlorophyll again after bud-burst and leaf-out. The chlorophyll is contained in a leaf cell’s chloroplasts, which are organelles thought to be of bacterial origin. They have their own DNA and can divide and replicate by fission.
Chlorophyll is a complex molecule and takes a lot of energy to make so trees disassemble it in the fall, move it from the leaves back into the stems, and save the molecular parts for reassembly in the spring.
Fashion week for nature’s runway
The prevalence of different tree species and other factors affect the timing and vibrancy of the fall color displays we enjoy. Temperature and moisture are important factors.
According to the U.S. Forest Service, droughts can delay fall colors; warm fall weather can decrease color intensity; and wet springs and cool fall nights can increase color intensity. Situated near wetlands, streams, and the Chesapeake Bay, here at APG we often maintain the most vibrant colors locally while popular mountain foliage areas may be experiencing more drab years.
As the green chlorophyll is reabsorbed in the fall, the other less plentiful pigments are revealed. The yellow comes from xanthophyll, the oranges from carotenoids (as in carrots), and the reds from anthocyanin.
Apart from seeing these colors in many vegetables, fruits and flowers, you may also have encountered them in an Asian restaurant. The “seaweed” wrapped around sushi, or mixed into a stir fry, is actually multicellular algal colonies that grow in the ocean. Different colors of light have different wavelengths which affect how far they can penetrate the murky depths. Depending on what depth they grow at, various algal species have different pigments, and thus colors, to capture light energy for photosynthesis. Thus various species of seaweed display year-round colors that we only see in the fall in our trees. These algal cells not only make sushi wraps more colorful but also contain glutamates (e.g., monosodium glutamate or MSG) that enhance flavor.
Who sticks around in the winter months?
Evergreens like pines and hollies don’t change color all at once and drop their needles or leaves in the fall. They spread their wardrobe change over the entire year.
But some deciduous (fall leaf-dropping) trees also hang on to their leaves throughout the winter, if just in a less colorful form. My botany professor used to call those oaks and beeches “60 mph trees” because in the winter you can easily identify them while driving down the highway. Oaks and beeches change colors in the fall but retain their old leaves even after they’ve turned light beige (beeches) or dark brown (oaks) and can no longer photosynthesize.
Botanical physiologists are not entirely sure if there is some advantage to this strategy or if it is simply an evolutionary “half way point” between fully evergreen trees that drop and regrow needles or leaves year-round, and fully deciduous trees that drop all leaves in colder and drier fall weather.
The gift of leaves now and in the spring
There is no reason to mourn the loss of leaves in the fall. As long as a tree is healthy, it will regain that refreshing bright green outfit in the spring to help us out of the winter grays and blues. Enjoy their final fashion show and appreciate this colorful cycle of life.
If you must rake leaves, try to help complete their natural cycle by placing them in composters with grass clippings or food scraps that can return all those nutrients to your garden or lawn next year.
APG natural resources managers have a long history of continually coordinating the natural infrastructure stewardship that supports and sustains the Army’s testing and training mission every day. Working with partners from APG tenant organizations, the U.S. Fish and Wildlife Service and the Maryland Department of Natural Resources, APG DPW personnel developed and coordinated our Integrated Natural Resources Management Plan (INRMP) as a roadmap for a sustainable mission landscape that is created through proactive management of APG’s natural resources.