Disturbance – from fire and subsistence living to widespread exploitative logging – enabled the growth of oak (Quercus) forests across the eastern U.S. These disturbances are not common today. Reduced disturbance, coupled with a long-term increase in moisture availability has been good for non-oak trees, which establish and grow under the older oak canopy – and inhibit oak seedling growth.
Prescribed fire is sometimes used to promote oak regeneration and recruitment. As efforts to re-introduce fire as a land management tool in oak forests expand, it is increasingly important to understand the intricacies of how different trees, oak and non-oak, will respond to it.
USDA Forest Service research forester Tara Keyser set out to understand those intricacies by examining the resprouting of seedlings following experimental burns. Keyser studied seedlings from four species common in oak-dominated forests. The results were published in the journal Oecologia.
Keyser chose white oak (Q. alba) and northern red oak (Q. rubra) because these two species are widely distributed across the eastern U.S.
Two other species, red maple (Acer rubrum) and yellow poplar (Liriodendron tulipifera), were chosen because they are competitors in oak forests that often interfere with the growth of new oak trees. Red maple is of particular interest because it is a shade tolerant tree that often occupies the subcanopy. “A well-developed subcanopy can inhibit oak seedling growth,” says Keyser. Oaks are only somewhat tolerant to shade.
Keyser conducted the study at Bent Creek Experimental Forest in Asheville, North Carolina. She planted 36 seedlings of each species 0.75 m apart and took special care to ensure that the root collar of all seedlings was located slightly below groundline. Placement of the root collar can influence whether a seedling resprouts post-fire. When the root collar is located above the groundline, buds are more susceptible to damage from fire.
The seedlings were all in direct, unobstructed sunlight.
The main differentiator between Keyser’s study and others with similar goals is the uniform conditions all seedlings were subject to. Rather than studying seedlings in a forest where different seedlings are subject to variable conditions, Keyser studied seedlings growing in a controlled environment.
Keyser and collaborating technicians planted one-year-old seedlings in March 2015 and subjected them to experimental burns lasting four minutes in June 2016. Keyser observed deadening of stems and complete loss of foliage for all trees one week after the burning. Keyser logged the total number of shoots sprouting from the base of each topkilled stem and the height of the tallest shoot every two weeks until October. Next June, they repeated the same process with the surviving trees.
After the first burn, there was an 82 percent chance that a red oak seedling would resprout by the end of the growing season in October. The probability for the other trees hovered around 50 percent. Almost all trees remaining survived the second burning.
While it was expected that red oak would have high resprout rates due to its more developed root structure, it was surprising that white oak – having a similar root structure – had the lowest resprout rate of all the species tested. However, the comparatively lower survival rate of white oak as compared to red oak is consistent with another study comparing resprout rate of seedlings in mature oak forests in Kentucky after a prescribed burn.
“We often lump the oaks together and now we are beginning to understand that white and red oaks are more different than we previously thought,” says Keyser, who notes that traditionally, all oaks are known for being good at resprouting after fire.
This research suggests that fire alone may not be enough to create a forest environment where white oak seedlings can outcompete shade tolerant species. “To aid oak regeneration in historically oak-dominated forests, we need to consider the whole suite of treatments that silviculturists have,” says Keyser.
For more information, email Tara Keyser at email@example.com