I propose that the total mass of all leaves that fall off of trees within the United States during a typical fall is 750 billion pounds. In order to answer this question, I considered the trees by my own house. The tree in my backyard is a large oak tree. Rather than attempt to approximate the number of leaves on the tree and the mass of one leaf, I considered how much a 13 gallon bag weighs when it is filled with leaves that have fallen off the tree. After raking the leaves, I do my best to fit as many leaves as possible into the bag, which means the leaves often crumble as I keep putting in more and more leaves. From my experience last fall, the bag weighs about 5 pounds. However, my estimate of the bag’s weight is based on perception; I know for certain that the bag is neither as light as two or three pounds nor as heavy as nine or ten pounds. Therefore, my best estimate of the weight of the bag is 5 pounds. Furthermore, when raking the leaves, I usually fill up three bags (I rake the leaves only once). As such, in one fall season, I collect about 15 pounds of leaves from a single tree.
However, because the tree is large and it is often windy, I am certain that many leaves either fly off into the lake, a neighbor’s house, the roof, and/or the front yard. In addition, it seems implausible to assume that I fit all the tree’s leaves into three garbage bags. Therefore, I estimate that I am only able to collect about half of all the leaves. With this assumption, I conclude that the tree in my yard loses about 30 pounds of leaves.
From this point on, it would be a simple matter to multiply 30 pounds by the number of trees in the United States to determine the total mass of all leaves that fall of trees. Unfortunately, it is much more difficult to determine the number of trees in the United States. In addition, there are certain species of trees that do not even lose their leaves in the fall. Without external information, such as the total forestland area or the average size of trees, I cannot provide an estimate that I know to be reasonably accurate. I can, however, deduce that there must be millions of acres of forestland in the United States, and that hundreds of trees can fit in one acre. If I assume that 500 trees can fit in one acre and that there is about 500 million acres of forestland, then there must be 25 billion trees in the United States. If, on average, each tree loses 30 pounds of leaves, then the total mass of all trees that fall within the United States is approximately 750 billion pounds. This amount, however, must be taken with skepticism because of the assumptions I have made. I assume that all trees are similar to the one in my backyard, and I also assume the area of forestland in the United States and the number of trees that can fit into that area.