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Weather, Woodlands, and Preparing for Uncertainty

Posted: July 22, 2016

Looking around the places where we live and work, it does not take long to find at least one sign of trees that are stressed by a shortage of rainfall across Pennsylvania.

Trees frequently demonstrate their ability to adapt to challenging places or conditions in which they grow. A walk through the woods quickly reveals some examples of this adaptability: roots stretching across rocks, tree seedlings growing atop a boulder, or roots creating a sort of scaffolding along a stream bank where water has created new flow patterns and eroded away soil. Individual trees can be tremendously resilient to harsh conditions.

On the other hand, looking around the places where we live and work, it does not take long to find at least one sign of trees that are stressed by a shortage of rainfall across Pennsylvania. In Centre County, the signs include tulip poplars with leaves turning yellow and falling, leaves of river birches losing turgor pressure and falling, azaleas and other shrubs common to yards drooping and requiring watering intervention to stay alive. Even the little-leaf lindens—a tree species that normally can handle adverse conditions like a dry spell—are showing signs of distress.

In the eastern United States, we are accustomed to a wide range of weather patterns which can often be a source of humor. What meteorologists and other scientists in our region have noticed, though, is that precipitation levels have become even less predictable in recent years. Rainfall and snowfall totals for the year-to-date in Pennsylvania are below average in every county to varying degrees. After heavier rainfall in June, the state Department of Environmental Protection lifted the drought watch that had been in place for 27 counties in the state. However, every county in Pennsylvania remains behind in average precipitation levels for the year, according to data provided by the National Weather Service.

Since we know individual trees can grow in stressful environmental conditions, it seems logical that we could expect the same adaptability of woodlands to make it through challenging circumstances. Drought has always existed. One of the main differences now is that the observed precipitation trends show less frequent rain and snow throughout the year each season, and when it does come, it happens in more intense storm events. Much like the soil in a houseplant container, the soil beneath our lawns and forests must retain a certain degree of moisture to be able to absorb more water. When soil becomes too dry, water flows over rather than into the soil. This change in precipitation and soil moisture, when combined with the rise in the average yearly temperatures, will have direct and indirect long-term effects on forests.

Forests in the western United States are generally made up of tree species that are more tolerant of hot, dry conditions. Eastern forests are more temperate environments, with more moisture. In drier soils and hotter temperatures, woodland trees become stressed in the same ways that yard trees do. Stomata are pores on the underside of leaves which allow plants to “breathe” by taking in carbon dioxide and also transpiring water vapor. Stomata close when a tree becomes stressed under extreme dry conditions. Long dry periods—especially when repeated over the course of several years—lead to a loss of a tree’s ability to draw water throughout its structure and eventually cause the tree to die.

As trees become more stressed by weather, their natural ability to respond to damage by insects and diseases is weakened. When many trees within a forest are stressed, disease and insect pests spread more quickly throughout the forest. One recent example of this increased susceptibility is the western pine bark beetle, which spread quickly through western forests already stressed by drought. Another factor in that scenario is that those forests were “overstocked,” meaning there were too many trees in competition with each other for the same growing space. Like plants in an overcrowded garden, trees in an overstocked forest are competing for the same limited space, water, and nutrients and thus are not as healthy or vigorous as they would be in a less dense forest. This, in turn, leaves the trees more vulnerable to stress and more susceptible to death.

There are specific actions we can take to make our forests more ready to handle changing weather patterns. We can reduce the risk of widespread tree death by making sure our woodlands contain a diversity of tree species. All tree species have light and moisture conditions they are best suited to—some are more tolerant to extremely dry conditions. If we manage our woodlands so that a wide range of tree species grow, including drought-tolerant species, the number of individual trees that will be able to survive weather extremes will increase. Secondly, we can improve the overall health of our woodlands by making sure individual trees have room to grow, so they can fully develop and function. Reducing competition for space, water, and nutrients will allow healthier individual trees that are able to withstand stress.

Woodland owners play an important role in helping to ensure that our forests will be able to be resilient in the face of changing weather patterns. If you are a woodland owner, contact a service forester from Pennsylvania DCNR or a consulting forester to ask for help in identifying what actions you can take in order to improve species diversity and enhance the overall function of your woods.

Contact Information

Leslie Horner
  • Forest Stewardship Program Associate
Phone: 814-867-5982