Differential relative bark thickness and aboveground growth discriminates fire resistance among hardwood sprouts in the southern Cascades, California
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Bark thickness relative to stem diameter and radial stemwood and height growth patterns provided a more accurate discrimination of fire resistance than absolute bark thickness alone in juvenile stems.
Thick bark provides thermal protection to cambial tissues and adventitious buds during fires, and thus, is a functional trait associated with more fire-resistant species and species persisting in fire-prone environments. However, bark thickness alone may not provide the best indicator of fire resistance due to differences among species-dependent growth, reproductive, and survival strategies. We measured bark thickness relative to radial stemwood and height growth among five juvenile co-occurring hardwood species following the Chips Fire (2012) in the southern Cascades, California. We found canyon live oak (Quercus chrysolepis) and Oregon white oak (Quercus garryana), the most fire-resistant, did not have the greatest bark thickness (0.7–0.4 mm; 0.8–0.3 mm), but rather had significantly greater relative (to stem diameter) bark thickness (8–18%) at all heights along the stem (p < 0.001). These species grew at slower rates: 48 and 41% shorter than the more fire-sensitive bigleaf maple (Acer macrophyllum; 2.9 m) and had 53 and 41% less diameter inside bark (DIB) than faster-growing, but less fire-resistant California black oak (Quercus kelloggii; 3.4 cm). Two discriminant functions (explaining 90% of variance) confirmed that separation of species was most strongly driven by DIB and relative bark thickness, accurately depicting species placement along a fire resistance spectrum (where Q. chrysolepis > Q. garryana > Q. kelloggii > Pacific dogwood, Cornus nuttallii > A. macrophyllum). Our findings highlight the importance of considering bark thickness relative to stem diameter and aboveground growth patterns in assessing species fire resistance.
KeywordsBark thickness Fire Functional traits Radial growth Regeneration Tradeoffs
Funding was provided by the USDA Forest Service Pacific Southwest Region and Pacific Southwest Research Station and the McIntire-Stennis Cooperative Forestry Research Program. We appreciate the assistance of Lassen and Plumas National Forest personnel, particularly P. Doyle and M. Hennessey. D. Nemens provided field assistance. Lab assistance was provided by E. Barker and D. Walker and laboratory equipment by C. Copenheaver at Virginia Tech. Discussions with B. Wing, J. Kreye, and D. Nemens helped shape the research. Earlier reviews by T. Shearman improved the manuscript.
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Conflict of interest
The authors declare that they have no conflict of interest.
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