1994 Proceedings
North American Conference on Savannas and Barrens

Fire is an integral component of change in many types of ecosystems.. Past studies of the specific effects of fire on trees have focused mainly on coniferous forest species in regions with high frequency of natural fires (Hare 1965; Kayll 1963; Gill and Ashton 1968; Vines 1968; Martin and Christ 1968). Most of this research has been conducted to determine heat limits tolerable in prescribed or natural fires.

In eastern North America, fire has been implicated as a factor in maintaining oak savannas which were commonly intermixed with presettlement tallgrass prairie (Abrams 1992). Upon settlement and transformation of land to urban and agricultural uses, fires occurred with decreased frequency and intensity. However, exclusion of fire from post-settlement lands has been implicated in the gradual replacement of dominant early to mid-successional Quercus species with late-successional, more shade tolerant Acer species (Wuenscher and Valiunas 1967; Reich et al. 1990). Pallardy et al. (1991) noted significant changes in species composition over a 22 year period for three distinct forest types in Missouri. Increases in the prevalence of sugar maple (Acer saccharum) and decreases in Quercus spp. were attributed to exclusion of fire and succession. The continuing trend toward late successional stages has led many resource managers to reconsider fire as a management tool to control forest composition.

The consensus among many researchers is that the vascular cambium of trees will be killed at temperatures exceeding 60o C (Hare 1961). Recent studies by Hengst and Dawson (1994) indicate that bark thickness and thermal conductivity influence moderation of cambial temperature increases resulting from artificial burning of several species native to the oak-hickory association of the eastern United States Bark has long been recognized as a factor which affords fire resistance to trees (Starker 1934; Hare 1965; Martin and Christ1968; Vines 1968). Numerous air-filled cells in bark provide insulation and prevent rapid fluctuation of temperatures at the vascular cambium. Martin (1968) conducted laboratory experiments examining the thermal conductivity of the bark of several species. His results demonstrated the good insulating qualities of bark and identified some factors contributing to variation in heat conductance among the bark of different species.

Density is a factor which affects thermal characteristics of various bark types (Martin and Christ 1968). There is a positive correlation between bark density and thermal conductivity. A trend towards higher bark density for thin barked species of the central hardwood region was observed by Hengst and Dawson (1994). This observation coincided with greater thermal conductance and higher peak cambial temperatures. When a heat source is applied to the bark surface, fluctuation of temperature varies with bark thickness. Cambial temperatures have been observed to continue to rise after heat sources were removed in simulated fires (Vines 1968; Kayll 1963; Hengst and Dawson 1994). Thick-barked species require longer times to reach peak temperature and return to ambient conditions. Thin-barked species peak higher and earlier than thin barked species and start to cool as soon as the heat source is removed. The objectives of this study were to determine the effects of a simulated burn on cambial mortality and subsequent wound wood production one year after an artificial burn conducted by Hengst and Dawson (1994).

STUDY AREA

Study trees were located in the Illini Forest Plantations (40o04 No Lat. 88o12 Wo Long.) in Urbana, Illinois. The plantation contains hardwood and softwood species approximately 40 years old. The species examined included silver maple (Acer saccharinum), white ash (Fraxinus americana), black walnut (Juglans nigra), sweetgum (Liquidambar styraciflua), yellow poplar (Liriodendron tulipifera), eastern cottonwood (Populus deltoides), sycamore (Platanus occidentalis), black cherry (Prunus serotina), burr oak (Quercus macrocarpa), American basswood (Tilia americana), and eastern white pine (Pinus strobus).

METHODS

Burn Simulation Technique

A modified wick-fire technique was employed by Hengst and Dawson (1994) in the fall of 1992 to measure heat transfer through bark. A 1.5-cm-diameter piece of kerosene-soaked cotton rope was tacked to the north side of each tree to serve as the heat source for the simulated burns. The length of each piece of rope corresponded to one half of the diameter of each tree to insure uniform heat levels per unit surface area for trees of different diameters. Thermocouples were positioned 10-cm above the rope inside the bark at the cambium as well as on the outer bark surface. Ambient cambial and air temperatures were recorded prior to ignition. Measurement of cambial and bark surface temperatures were taken at 10 second intervals until surface temperatures stabilized at ambient levels. Bark samples were obtained from each species to determine seasonal moisture content, thickness, specific gravity, and thermal conductivity.

Determination Of Cambial Damage

Bark was removed from the area affected by the simulated burn during the winter of 1993, one full growing season after experimental burning. Acetate sheets were tacked to areas subjected to experimental burn to trace the border of dead cambium. Boundaries of wound wood were traced to examine relative rates of sealing. Extent of cambial damage was determined by transferring inner and outer wound wood boundaries onto paper and measuring the area with a LiCor model LI-3000 area meter. Average radial wound wood growth was measured by transforming the areas delineated by inner and outer radii into circumferences of idealized circles and subtracting inner radii from outer radii.

RESULTS AND DISCUSSION

Stepwise regression was used to explore relationships between the dependent variable outer wound wood boundary and independent variables maximum cambial temperature, bark thickness, bark specific gravity, and thermal conductivity. Maximum cambial temperature was the only variable that contributed significantly to cambial mortality represented as outer wound wood boundaries. Species means for dbh, bark thickness, wound wood area, outer and inner wound wood boundaries, maximum cambial temperatures, bark specific gravity and thermal conductivity, and radial wound wood growth are listed in Table 1. A correlation matrix was constructed to examine relationships between individual variables (Table 2). All statistical analyses were conducted using SAS version 6.0 (SAS Institute Inc. 1989).

Results suggest that damage to the vascular cambium by fire is most influenced by bark thickness and thermal conductivity, which determine maximum cambial temperatures. Quercus macrocarpa and Pinus strobus had the lowest cambial mortality probably a result of thick bark and low maximum cambial temperatures achieved in the burn.

Ecologically, thinner barked species of the central hardwood region may have been restricted to moist environments near natural fire breaks. The extensive cambial necrosis of floodplain- inhabiting A. saccharinum is consistent with this idea. Platanus occidentalis was observed to sustain relatively low rates of cambial mortality among thin barked bottomland species with high bark thermal conductivity. This observation may be attributable to cambial tissues which can tolerate temperatures in excess of 60oC. Populus deltoides is a riparian species which has very thick bark and thus sustained only non-lethal cambial temperatures. However, cambial damage in P. deltoides was greater than that observed for 6 of the 11 species in this study. Cambial mortality in P. deltoides may be attributed to the deeply furrowed bark which did not provide uniform insulation from heat. Quercus macrocarpa is a species which was once common in prairie savannas. The observed fire resistance of this species may explain its ability to persist in areas with a high frequency of natural fires.

In past studies, rates of wound closure have been directly correlated with radial growth of trees (Neely 1970). A general trend of fast growing bottomland species having greater radial wound wood growth was observed in this study. However, one exception to the trend was J. nigra which has only moderate growth rates yet had the second highest amount of radial wound wood growth among all 11 species. Populus deltoides had the greatest radial growth of wound wood following fire among all species examined. This finding suggests that an alternative, little considered quality could contribute to fire resistance: rapid wound sealing.

Results may offer an explanation of the occurrence of Q. macrocarpa and P. deltoides along rivers further west into the fire prone prairies of the great plains and beyond the range of many other eastern deciduous species. Juglans nigra exhibited strong fire resistance characteristics that have not been previously noted.

ACKNOWLEDGMENTS

Appreciation is extended to Eric O Connor for his help in gathering data during the winter of 1993.

LITERATURE CITED

Abrams, M. D. 1992. Fire and the development of oak forests. Bioscience 42(5):346-352.

Gill, A. M. and D. H. Ashton. 1968. The role of bark in relative tolerance to fire of three central victorian Eucalypts. Australian Journal of Botany 16:491-498.

Hare, R. C. 1961. Heat effects on living plants. In: U. S. D. A. Forest Service Service. Southern Forest Experiment Station SO-183. 32p.

• 1965. Contribution of bark to fire resistance of southern trees. Journal of Forestry 63:248-251.

Hengst G. and J. O. Dawson. 1994. Bark properties and fire resistance of selected tree species from the central hardwood region of North America. Canadian Journal of Forest Research 24:688-696.

Kayll, A. J. 1963. Thermal properties of Bark. Forest Products Journal 18:419-426.

Martin, R. E. and J. B. Christ. 1968. Selected physical-mechanical properties of eastern tree barks. Forest Products Journal 18:54-60.

Neely, D. 1970. Healing of wounds on trees. Journal of the American Society of Horticultural Science 95(5):536-540.

Pallardy, S. G., T. A. Nigh, and E. Garrett. 1991. Sugar maple invasion in oak forests of Missouri. In: G. V. Burger, J. E. Ebinger, and G. S. Wilhelm, eds. Proceedings Oak Woods Management Workshop. Eastern Illinois University. Charelston, IL. pp 21-30.

Reich, P. B., M. D. Abrams, D. S. Ellsworth, E. L. Kruger, and T. J. Tabone. 1990. Fire affects ecophysiology and community dynamics of central Wisconsin oak forest regeneration. Ecology 71:2179-2190.

SAS Institute Inc. 1989. SAS/STAT User's Guide, Version 6, Forth Edition, Volume 1. Cary, NC. 943pp.

Starker, T. J. 1934. Fire resistance in the forest. Journal of Forestry 32:462-467.

Vines, R. G. 1968. Heat transfer through bark, and the resistance of trees to fire. Australian Journal of Botany 16:499-514.

Wuenscher, J. E. and A. J. Valiunas. 1967. Presettlement forest composition of the River Hills Region of Missouri. American Midland Naturalist 78:487-495.

Table 1. Species means for stem DBH and for bark and wound characteristics. Sample size n=3 observations per species.