Seeing the Forests Through the Trees

Figure 1 Click on image for larger version

Healthy Forests Mean a Healthy Bay

From an ecological standpoint, forest cover is very important to the health of the Bay. For example, although forests now comprise a significant percentage of the land area in the Bay watershed, they only generate four percent of its nutrient load and produce an even lower percentage of other pollutants that can harm the Bay. Forests also help build soils and keep them in place.

Figure 2 Click on image for larger version

The most important forests for ensuring the health of the Bay's waters are the ones that buffer its streams and rivers. Stream-side forests, which comprise about five percent of the watershed, are also known as riparian forests. These play an essential role in the ecology of running waters. These riparian buffers filter runoff, absorb flood waters, store nutrients, stabilize stream banks, cool water, provide food for stream life, and are critical habitats for hundreds of species of wildlife (Figure 2). Regrettably, nearly half of the 100,000 miles of streams and rivers that drain to the Bay lack an adequate forest buffer along their banks. Reforesting these missing buffers is one of the most important and inexpensive strategies for helping to restore the health of the Bay.

Autumn Leaf Fall in the Watershed

Autumn leaf fall signals three major changes in the watershed. The first is that the “leaf pump” is turned off across the watershed. During the growing season, trees are quite thirsty. For example, an acre of mature forest can take up more than 1,800 gallons of water every day from the soil to drive the photosynthesis process. Consequently, groundwater and stream flows usually drop sharply during the growing season while trees are busy pumping water from the ground. Even in normal rainfall years, stream flow and groundwater levels drop to their lowest levels of the year by the end of the growing season. Once the leaves drop from the forest, the "pump" is effectively turned off until spring. In most years, the fall and winter are the primary seasons for replenishing groundwater and stream and river flows.

Figure 3 Click on image for larger version

The second major change associated with autumn leaf fall is the movement of leaves into small streams. Streams depend on autumn leaves to supply the energy needed to support stream life throughout the rest of the year. When leaves blow or flow into streams, they accumulate into “packs” that gradually break down over time, thanks to colonization by fungi (Figure 3). As the fungi do their work, the leaf pack mass decreases by about one to two percent each day.  The nutrients produced in the process enter the food chain of the stream.

The fungi on leaf packs are a major food source for many aquatic insects such as caddis flies, stoneflies and crane flies. These insects shred or collect fine leaf particles, and in turn, are eaten by small fish and other aquatic life. Thus, fallen leaves, which provide as much as 75% of the energy used in the food chain, really are the base of the food chain in small streams. At a time of year when growth is dormant elsewhere, the first few months after leaf fall usually triggers the biggest pulse of growth and activity in the small streams of the Chesapeake Bay.

The third major change associated with autumn leaf fall is the shift of carbon from the forest to the forest floor. At this time of year, a typical acre of mature forest in the watershed will drop between two and three tons of leaves, twigs and branches. This fresh supply of organic matter is gradually composted throughout the winter and spring and helps drive the forest ecosystem during the next growing season. A single acre of the forest floor can support more than a million earthworms, 40 million mites, 12 million spring tails and countless numbers of bacteria and fungi. These organisms, in turn, help support thousands of salamanders per acre. Indeed, salamanders collectively have more biomass than either birds or small mammals in many eastern forests. Thus, the autumn leaf fall supports most of the forest food chain.

You can also take advantage of the autumn leaf fall in your neighborhood to improve the quality of your soils by composting your leaves in your backyard. While, leaf waste accounts for about 10% of the annual trash flow in most communities, this rises to almost 70% in the fall. By composting your leaves, you can create your own supply of humus and, in the process, help preserve landfill space in your community.