A Closer Look at Pennsylvania’s Forests -- Leave Them Be, or Lend a Hand?
Leslie Horner, July 27, 2015
If we want to continue to enjoy Pennsylvania’s forests and wildlife, we’ll need to think about how we react to changes taking place—shifting trees species and age classes.
Pennsylvania’s woods provide us with numerous benefits -- among them are a variety of recreational opportunities, clean water, and habitat for a wide range of wildlife. These forests, like others in the eastern United States, have returned with vigor since the late 1800s and early 1900s. Then, land clearing and the use of wood for building homes, fueling factories and building the nation’s transportation infrastructure (i.e., roads, canals, and railroads) were so extensive and fast-paced that the Pennsylvania’s forests were reduced to less than half of the forest cover we see today. Today, with over 16.5 million acres of forest, which cover more than half the state, it seems hard to imagine that the forest could be anything but healthy and robust. While that is true, a closer look shows that Pennsylvania’s forests are changing, and some of those changes are cause for our concern and attention.
In forests statewide, the number of trees with large diameters has been increasing, and trees in smaller diameter groupings have been declining since the 1980s. While size does not always indicate a tree’s age, studies of Pennsylvania’s forests have shown that our trees are aging. In many of our woodlands the bigger trees are about the same age, creating what is known as an “even-aged forest.” Like people, trees have different life expectancies that vary quite broadly, but do not have an infinite lifespan. Eventually all trees die. Since many of our trees are about the same age, we could see many trees reaching the end of their lifespan around the same time.
Why should we be concerned? Won’t the forest just come back again? That is what we would expect, but Pennsylvania’s forest now contains far fewer tree seedlings and saplings than one would have seen in the same woods two or three decades ago. These tree seedlings and saplings (also referred to as “advanced regeneration”), which should be the next generation forest, are absent in many forest stands. One major cause of the decline in regeneration is the increased competition from some plants that are growing where they did not used to be found. These “invasive plants” grow so quickly that they out-compete tree seedlings and other plants in the struggle to access water and nutrients in the soil, space for roots to stretch out, and room for leaves to access sunlight. Tree seedlings that don’t die are stunted in their growth, leaving them small and not very hardy.
Another change in the state’s forests is a shift in species—the species that used to be less common are trading places with species that used to be more common. Red maples are more than twice as common as any other tree species, while the number of oaks is declining. Red maples are native to Pennsylvania’s woods and are not out of place; however, they are competitive. They grow faster than oaks; so in a forest opening where oaks would normally thrive, red maples beat them to the sunlight and slow the growth of oak seedlings, or other species that used to more numerous in our woods.
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