Estimating Heat Tolerance of Tree Buds in the Southeastern United States
McClure, Adam Blake
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Tree diameter, bark thickness and texture, litter bulk density and chemistry, and sprouting ability are a few, species-specific adaptations and properties that may be related to an individual tree stem's potential fire resistance. Based upon these features and others, trees exhibiting similar characteristics have been broadly classified as pyrophytic or pyrophobic. To our knowledge, few if any, research studies have been conducted to determine if tree buds may exhibit speciesspecific fire or heat tolerance. Understanding potential relationships between bud characteristics and fire tolerance may assist prescribed fire managers as they target control of specific undesired tree species while promoting desired tree species. Buds of six common hardwood species in the southeastern U.S. were harvested and exposed to different heat dosages using a propane gas tube burner: red maple (Acer rubrum L.), yellow-poplar (Liriodendron tulipifera L.), American beech (Fagus grandifolia Ehrh.), mockernut hickory (Carya tomentosa Lam.), scarlet oak (Quercus coccinea Münchh.), and chestnut oak (Quercus montana Willd.). These species are commonly categorized as pyrophobic (American beech, red maple, yellow-poplar) and pyrophytic (chestnut oak, mockernut hickory, scarlet oak). Using electrolyte leakage of heated and unheated buds, the mean difference in bud percent mortality between heated and unheated buds was compared by species, functional group, and differing heat dosages. The mean difference in bud percent mortality differed between the heated and unheated buds by species (p < 0.0001). Heated buds of yellow-poplar, mockernut hickory, and chestnut oak displayed similar percent mortality; however, the presumed pyrophytic species had greater heated bud percent mortality (34.0 %) than the pyrophobic species (26.8 %) (p = 0.0003). Heated bud percent mortality differed based upon differing heat dosage levels (height above the propane burner and heat exposure time) and their species-specific interactions. Fire tolerance, as assigned and assessed by features such as bark texture and thickness, may not be directly related to mid-story tree bud physical properties (i.e. length, mass, and diameter) or fire tolerance. Research including additional species, heat dosages, and different sampling times (i.e. late fall vs. late winter) would be valuable for investigating these dynamics further. Deploying a similar, field-scale experiment before and after prescribed burns would be useful to determine how tree buds may respond to different heat dosages that could be exacted during prescribed burns.
General Audience Abstract
Prescribed fire is utilized on millions of acres per year in the southeastern United States. These burns are conducted for a variety of reasons, including: wildlife habitat establishment, restoration, and maintenance; aesthetics; hazardous fuel reduction; and the control of undesired vegetative species. Factors that may influence an individual species' response to fire include, but are not limited to: bark thickness and texture, litter bulk density and chemistry, and resprouting strategy. These traits may differ by species and by the age of a given stem. Few studies have investigated these characteristics in younger tree stems within the southeastern United States. Additionally, few research studies in any ecosystem have investigated potential species-specific tree bud responses to increased heat. Determining if undesired tree species respond differently to heating than desired tree species may provide additional information to assist prescribed fire managers in many locations where vegetation control is a management objective. As a result of this knowledge gap, a research study was designed to investigate these dynamics in the southeastern United States for six common, hardwood tree species: red maple (Acer rubrum L.), yellow-poplar (Liriodendron tulipifera L.), American beech (Fagus grandifolia Ehrh.), mockernut hickory (Carya tomentosa Lam.), scarlet oak (Quercus coccinea Münchh.), and chestnut oak (Quercus montana Willd.). The results suggested that percent bud mortality may be species-specific, but few relationships appeared to be present to relate bud physical characteristics to bud mortality. Additionally, supposed heat tolerance groups assigned to older tree stems based upon traits, such as bark thickness and texture, did not provide a valid assessment of bud mortality. Additional research is needed to understand these dynamics further, including burning under different heat dosages, evaluating additional tree species, and assessing bud mortality resulting from in-the-field prescribed fires.
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