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Differential relative bark thickness and aboveground growth discriminates fire resistance among hardwood sprouts in the southern Cascades, California

Kidd, Kathryn R., Varner, J. Morgan
Trees 2019 v.33 no.1 pp. 267-277
Acer macrophyllum, Cornus nuttallii, Quercus chrysolepis, Quercus garryana, Quercus kelloggii, bark, buds, cambium, fire resistance, fires, hardwood, juveniles, stems, stemwood, variance, California
KEY MESSAGE: 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.