Variation in Bark Allocation and Rugosity Across Seven Co-occurring Southeastern US Tree Species

Timothy M. Shearman,J. Morgan Varner

doi:10.3389/ffgc.2021.731020

Timothy M. Shearman, J. Morgan Varner

Open Access

https://doi.org/10.3389/ffgc.2021.731020

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Abstract

Bark is a complex multifunctional structure of woody plants that varies widely among species. Thick bark is a primary trait that can protect trees from heat generated in surface fires. Outer bark on species that allocate resources to thick bark also tends to be rugose, with bark being thickest at the ridges and thinnest in the furrows. Tree diameter or wood diameter is often used as a predictor for bark thickness but little attention has been made on other factors that might affect bark development and allocation. Here we test multiple mixed effect models to evaluate additional factors (height growth rate, measure height) that correlate with bark allocation and present a method to quantify bark rugosity. We focused on seven co-occurring native tree species in the Tallahatchie Experimental Forest in north Mississippi. Approximately ten saplings of Carya tomentosa, Nyssa sylvatica, Prunus serotina, Pinus echinata, Pinus taeda, Quercus marilandica, and Quercus falcata were destructively sampled for stem analyses. Outer bark thickness (OBT) ranged from 0.01 to 0.77 cm with the thickest maximum outer bark occurring on P. taeda (0.77 cm) and the thinnest maximum outer bark occurring on P. serotina (0.17 cm). Our outer bark allocation models suggest that some individuals with rapid height growth allocate less to outer bark in C. tomentosa, N. sylvatica, P. taeda, and P. serotina, but not for P. echinata or either oak species. All species except for C. tomentosa and N. sylvatica showed evidence for outer bark taper, allocating more outer bark at the base of the bole. Inner bark also was tapered in Carya and the oaks. Bark rugosity varied among species from 0.00 (very smooth) to 0.17 (very rugose) with P. Serotina and C. tomentosa having the smoothest bark. OBT was the best fixed effect for all species. Aside from providing data for several important yet understudied species, our rugosity measures offer promise for incorporating into fluid dynamics fire behavior models.

Highlights

The development of thick bark in woody plants has widely been cited as a trait selected for by frequent fire regimes as it protects trees from the heat generated in surface fires (Spalt and Reifsnyder, 1962; Hare, 1965; Vines, 1968; Pausas, 2015)
Multiple comparisons showed that C. tomentosa had significantly thinner bark than P. echinata (P = 0.01), P. taeda (P < 0.001), and Q. marilandica (P = 0.003, Table 2)
Previous studies on bark in relation to fire have overwhelmingly found that bark thickness is a highly significant predictor in post-fire tree mortality and that bark thickness increases with stem diameter (e.g., Hare, 1965; Lawes et al, 2011; Pausas, 2015)

Summary

Introduction

The development of thick bark in woody plants has widely been cited as a trait selected for by frequent fire regimes as it protects trees from the heat generated in surface fires (Spalt and Reifsnyder, 1962; Hare, 1965; Vines, 1968; Pausas, 2015). Tree species that occur in frequently burned ecosystems (e.g., savannas and woodlands) generally develop thicker bark than those found in less flammable environments (Hoffmann et al, 2003; Lawes et al, 2011; Schafer et al, 2015). Outer bark is suggested to have multiple functions including insulation from heat of fire, protection from herbivory, and structural support (Vines, 1968; Rosell, 2019). Inner bark is not generally considered a primary fire defense, its high density and water content may provide protection from fire (van Mantgem and Schwartz, 2003; Schafer et al, 2015), several studies suggest that moisture content in bark decreases heat resistance due to high heat conductivity (Vines, 1968; Odhiambo et al, 2014)

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