Type and seasonal timing of damage influence numbers of resprouting stems and biomass of juvenile Quercus nigra (water oak)

Background/Question/Methods

Hardwood species in pine savannas of the southeastern United States resprout following damages that cause “top-kill” of the aboveground portion of the plant. Historically, hardwoods were top-killed by frequent fire, but on the modern landscape mowing is sometimes used as an alternative to fire for hardwood control. We hypothesized that fire would cause greater damage to dormant buds on root crowns and result in fewer resprouting stems compared to mowing. Seasonal timing of top-kill can also influence hardwood resprouting. Root reserves are utilized for new aboveground growth in the spring, reducing belowground storage and resprout potential. We hypothesized hardwoods top-killed when root reserves are reduced in the spring would produce smaller resprouts compared with plants top-killed prior to spring growth. To test these hypotheses, we collected Quercus nigra seedlings and planted them in one-gallon nursery pots in 2008. Three top-kill treatments (burn, clip, undamaged) were applied at three times during the year (February, April, July) in 2010. Prior to treatments, stems of each genet were measured. In 2011 resprouts were measured, and the plants destructively sampled to determine above and belowground biomass. We performed a two-way ANOVA to test for differences in biomass among top-kill and seasonal treatments.

Results/Conclusions

Results support the hypothesis that top-kill by fire results in reduced numbers of resprouting stems. Quercus nigra top-killed by fire had significantly fewer stems than clipped genets, suggesting clipping does less damage to dormant buds at the base of the plant. Our second hypothesis, that season of treatment would affect resprout biomass, was also supported. The average one-year post-treatment aboveground biomass of individuals top-killed in April and July was significantly less than the plants top-killed in February. The average one-year post-treatment root biomass of top-killed plants was less than undamaged plants; however, there was not a significant difference among treatments. Differences in aboveground biomass of Q. nigra damaged in April and July reinforce the concept that conducting prescribed fires when root reserves are low should be more effective at reducing resprouting sizes of woody plants in southeastern pine savannas than prescribed fires at other times of the year. Understanding how hardwood species respond to the type and timing of top-kill is important for evaluating different management actions. Based on these results, we conclude that fire surrogates in experiments or management are not equivalent to fire in their effect on woody plants.