Part 1—Understand How Forests Are Studied and Modeled
Fences in the Forest
Walking through the woods of New England, you might find something that seems out of place. In many areas, long stone fences run straight through the woods, with no obvious differences in the trees on either side of them. You might wonder who built these fences or what purpose they served.
In most cases, these fences (usually called stone walls) are older than the forests that surround them. In the mid-1800's, large sections of New England had been cleared of trees so the land could be used for growing crops and grazing animals, particularly sheep. Farmers marked the boundaries of their fields and pastures with these long stone fences. Since that time, most farmers have abandoned their fields and pastures in New England and the trees have grown back, and now only the stone fences remain.
For many of these fields, historical records supplied the dates that New Englanders stopped farming the land and trees began growing there again. This information proved very valuable for scientists who study forests. As they knew the exact age of many similar forests across the region, foresters could examine young, middle-aged, and old forests all in the same area, without having to wait decades to see how the forests would change. Foresters have used this information and developed experiments based on this idea to develop a thorough understanding of how forests grow and change over time.
Forestry Science
New Hampshire, one of the six states that make up the New England region, is almost completely covered in trees, with over 80% of its land classified as forest. Because it has so many trees, forestry scientists have done a lot of research there. Some portions of the state have been set aside as experimental forests, where researchers try different forestry management methods on specific plots of land. They gather data over many years and analyze it for interesting patterns of growth and change. Foresters have used these plots to document such things as how forests grow back after clear cutting (harvesting all the trees in a large area at once) compared to selective cutting (harvesting only large trees, for instance), how the application of nitrogen fertilizer affects tree growth, and how forests regenerate after fire damage.
Forestry science also advances through studies of forests across the rest of the United States. The U.S. Forest Service surveys forests from all 50 states plus three U.S. territories every five years. They go to the same sample plots and collect data to measure the growth and death of trees. They also record the conditions that the forests have experienced, including precipitation, temperatures, logging and wood collection activity, and if fires or insect infestations have occurred.
To measure growth of a forest, foresters select a representative sample of trees in each plot and measure them to estimate how much wood they have gained. They measure the height of each tree and the diameter of its trunk at breast height, 4.5 feet or 1.3 meters above the forest floor. To measure how much wood has been lost, they calculate the mass of dead trees plus branches and bark that have fallen off trees.
Nitrogen's Role in Forest Growth
Foresters also collect and study leaves or needles from each tree. They measure the concentration of nitrogen in the leaves, a parameter that is called foliar nitrogen. The word "foliar" means "of or relating to leaves". Foliar nitrogen is reported as the percent of the dry weight of leaves that is contributed by nitrogen. Nitrogen turns out to be a very important for tree growth—it is an essential component of the enzymes that facilitate photosynthesis.
When foresters compared measurements of the annual wood growth of any species of trees in New Hampshire with the foliar nitrogen levels in its leaves, they found a direct relationship. The higher the foliar nitrogen level, the more wood growth the trees experience. Researchers have documented this relationship so precisely that they can use tree's foliar nitrogen concentration to identify how much wood growth they experience each year. The discovery of this relationship is very fortuitous—it's already easier to collect leaves or needles from sample trees than it is to complete the physical measurements of their height and diameter, and researchers are now studying ways that foliar nitrogen can be measured by instruments on airplanes and satellites. This would allow forestry scientists to monitor changes in forests more frequently than they can be visited by teams of surveyors.
Show me more about nitrogen in plants
HideA Forest Model
Scientists have learned enough about how forests grow in different conditions that they have written and refined numerical equations that describe the forest system. These equations can also be used to predict how a forest will grow in the future. The set of equations represents a model of a forest system. A model is a tool that helps us understand, explain, and predict systems that are too complex or difficult to observe or comprehend on our own.
We often think that models must include complicated mathematical functions, but we can learn a great deal about systems, especially environmental systems, using simple models that require only the basic math functions such as addition, subtraction, multiplication and division.
Why Use Models?
Models are particularly useful in the study of environmental systems. Scientists use models to examine the fundamental behavior of a system, such as carbon storage in a forest. By knowing more about the system and understanding where knowledge is incomplete, scientists can generate hypotheses to guide future research. For example, the carbon storage in above-ground tree parts (leaves, branches, stems) has been successfully modeled based on millions of measurements. On the other hand, forest models have shown that there isn't enough detailed information available about the below-ground parts (coarse and fine roots) for scientists to fully understand how carbon storage below the ground changes over time.
Models are also useful to predict future conditions. Although such predictions are not necessarily what will happen, they can provide a range of possibilities that help scientists refine their research and help policy makers create action plans to prevent any undesirable outcomes. For example, we can use a forest model to investigate how carbon uptake and storage in forest ecosystems might change if there was an increase in temperature over the next twenty years.
