Growing the Next Generation of Forests in Pennsylvania

Posted: September 30, 2015

If you want to regenerate a forest with naturally occurring native tree species it takes planning and the recognition that you will likely have to undertake a harvest to allow the conditions for regeneration to occur. Research focused on forest regeneration provides some guidelines to help assess the potential for success in growing the next forest.

Pennsylvania’s forests face an uncertain future. The health of our forests is at risk. Invasive insects are threatening ash, hemlock, black walnut, and elm. Invasive plants such as honeysuckle, autumn and Russian olive, oriental bittersweet, Japanese stiltgrass, garlic mustard, and almost too many species to list challenge native species in many niches. Unrestrained parcelization and development are breaking woodlands into smaller and smaller properties that make it difficult to manage many of these threats as neighbors fail to cooperate or to understand how their individual decisions lead to increasingly depauperate forest conditions. There are myriad threats that must be considered or controlled when faced with trying to retain or regenerate (start anew) the forests. Well-informed landowners can make decisions that will move forests to a better place. They just have to know what to understand and how to create a plan to implement their goals.

For more than 40 years, research and inventory data has raised questions about forest regeneration in Pennsylvania. A recently published forest inventory of Pennsylvania’s public and private forests by the USDA Forest Service finds that forest regeneration is a major concern. In those woodlands where canopy disturbance, from either tree mortality or harvesting, has changed light conditions that should spur natural regeneration, only about 4 in 10 acres has sufficient desirable tree seedlings to ensure the next forest. Desirable tree species include those important for timber production or wildlife food. If the list of species expands to include nearly all of our native tree species, the situation does not improve that much, as only about 5 of 10 acres are on a positive trajectory.

What is causing this to happen? As suggested earlier, non-native competitive plant species are benefiting from natural- or human-caused disturbance and winning the competition for light in our woodlands. Some of these competitors have an upper hand because they leaf out earlier in the spring and extend their growing season later into the fall and thus get a jump on native species. Adding to the struggle, white-tailed deer, which have exceeded ecological thresholds in some parts of the state for 50 to 70 years, have selectively browsed species they find palatable and given the advantage to species they feed on less frequently. For example, three species of ferns that spread by underground root-like structures and are not eaten by deer cover about 1 in 5 acres of forests in the state and exceed guidelines for achieving competitive tree regeneration.

If you want to regenerate a forest with naturally occurring native tree species it takes planning and the recognition that you will likely have to undertake a harvest to allow the conditions for regeneration to occur. Light has to reach the forest floor, but there are also other threats which have to be accounted for. Contrary to what many people believe, successful forest regeneration does not just happen following a harvest or disturbance. It did seem to work that way when today’s forests came along, but that was 90 or 130 years ago. Things have changed – invasive insects, plants, and deer. Now, achieving adequate desirable tree regeneration is a process requiring time and investments before removing any of the bigger trees extending into the canopy.

Research focused on forest regeneration provides some guidelines to help assess the potential for success in growing the next forest. In some ways, assessing forest regeneration is intuitive: are there tree seedlings and are there lots of them? Most guidelines combine the size and number of seedlings and you might find the suggested numbers for success overwhelming. To achieve adequate competition between trees as they develop and to ensure that enough trees will eventually reach the canopy, estimated numbers of seedlings might range from 10,000 to more than 50,000 per acre.

If woodland owners want to assess their regeneration, the first step is to identify what is already growing. Many people struggle to identify mature trees. Identifying seedlings adds another level of complexity; however, it does become easier over time and with repetition. In most cases, the species growing in the canopy should reflect the seedling composition and this becomes an identification aide. Of course, you may also find species brought into the area by wind, water, gravity, and wildlife.

If you want to improve forest health and want to improve conditions for natural regeneration, recognize that this will require a controlled disturbance, in the form of tree removals. It is important to consider how any disturbance in the forest canopy affects light distribution in the understory and on the forest floor. When trees die from cutting or insects, disease, weather, or competition, light conditions below the canopy change and that light begins to drive change, which can be either beneficial (e.g., spurs young tree seedlings to grow) or detrimental (e.g., fosters expansion of competitive plants, either exotic or native). To plan for successful forest regeneration after harvesting there are seven questions to answer before any tree removals occur:

  • How much will the planned disturbance increase light below the canopy? If when looking up at the summer canopy there is 40 to 60 percent or more blue sky, the remaining tree crowns will likely lack the capacity to attain full closure in the near future and you are facing a regeneration situation .If the goal is to establish or release regeneration, then the cutting intensity maybe correct.
  • Do you have adequate desirable regeneration already in place? With regeneration in place, the species you hope to become the next forest will have a head start on the competition. Desirable regeneration refers to tree species that are important to wildlife and for any other goals you may have for your woodland, such as firewood production or producing timber to harvest. In today’s forests, it is seldom the case that we have sufficient regeneration in place unless there was prior planning, efforts to control deer browsing, and a lack of competitive plants.
  • Are competitive native or non-native plants covering 30 percent of the area of interest? When competition is covering 30 percent or more of the area, studies suggest that it will expand quickly and dominate the area once light conditions change. If this is the case, it is likely prudent to invest in practices to control their spread – some form of forest vegetation management.
  • Is there an adequate seed source of desirable tree species? When light conditions are right (see question 1), it is important to have adequate numbers of desirable seed producing trees. The answer to this question depends on the species, residual tree spacing, and ability of the trees to disperse their seed in the area. Light winged seeds such as red maple naturally disperse further than acorns, for example.
  • Will the planned disturbance eliminate or greatly reduce the occurrence of a desirable species in the woodlot? If so, can anything be done to retain that species or is it a valid decision to allow it to go away? For example, trying to retain ash in Pennsylvania woodlots is likely a lost cause. Contrarily, failing to retain red and white oak seed sources is not a good idea in most areas where they naturally occur.

The final two questions relate mostly to disturbance caused by timber harvesting where the woodlot owner or the manager has more control over what they retain than what they remove.

  • Are the trees planned to be retained healthier and with fewer defects than the trees cut? Tree health and quality among the trees kept should improve. This guideline relates to harvesting or removing the worst trees first in timber harvests, which are a controlled disturbance. Too often, timber harvests focus on removing the valuable trees with the hope that the little poorer quality trees will improve over time. This is seldom the case.
  • Will the average tree diameter in the woodlot increase, stay the same, or decrease? There are few scientifically sound harvesting practices that result in the average diameter decreasing. Other than a clearcut, which if regeneration is in place and not stifled by competition is valid too, the average tree diameter should stay the same or increase some.

To adequately assess forest regeneration, you might find it necessary to involve a forester who has experience conducting inventories. As well, a forester should have the ability to evaluate competing vegetation, make suggestions for controlling or reducing invasive species, and lay out a plan for ensuring that you have adequate regeneration. Unfortunately, as suggested earlier, many of our woodlands are rapidly declining in health as owners and managers fail to recognize threats to forest health today and the implications for tomorrow.

To find a forester to help you with the management of your woodlands, visit the Bureau of Forestry website and in the left-hand menu click on “Your Woods.” There you will find a listing of service foresters by Pennsylvania counties who can provide some on-site advice or help you find a consulting forester working in your county. When choosing a professional forester to work with you and your land, keep the above seven questions in mind and don’t be afraid to articulate your vision of what your value about your land. Find a professional who shares your interests and your trust to steward your land with you.

Contact Information

James Finley, Ph.D.
  • Professor Emeritus of Forest Resources
Phone: 814-863-0402