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Comparing bark thickness: testing methods with bark-stem data from two South African fire-prone biomes

Hempson, Gareth P., Midgley, Jeremy J., Lawes, Michael J., Vickers, Karen J. and Kruger, Laurence M. (2014). Comparing bark thickness: testing methods with bark-stem data from two South African fire-prone biomes. Journal of Vegetation Science,25(5):1247-1256.

Document type: Journal Article
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IRMA ID 84376995xPUB18
Title Comparing bark thickness: testing methods with bark-stem data from two South African fire-prone biomes
Author Hempson, Gareth P.
Midgley, Jeremy J.
Lawes, Michael J.
Vickers, Karen J.
Kruger, Laurence M.
Journal Name Journal of Vegetation Science
Publication Date 2014
Volume Number 25
Issue Number 5
ISSN 1100-9233   (check CDU catalogue  open catalogue search in new window)
Scopus ID 2-s2.0-84895852812
Start Page 1247
End Page 1256
Total Pages 10
Place of Publication United Kingdom
Publisher John Wiley & Sons Ltd.
HERDC Category C1 - Journal Article (DIISR)
Abstract Aims
Bark thickness–stem diameter relationships are non-linear above a stem diameter threshold in many woody species, which makes relative bark thickness measures dependent on the range of stem diameters that are sampled. This influences the appropriateness of different methods for comparing fire responses of woody plants across studies. Here we develop a framework for bark thickness comparisons by evaluating relative bark thickness estimates and bark thickness predictions obtained from linear and curved models fitted to raw and log-transformed bark–stem data. We use this framework to contrast bark thickness among fynbos and savanna plant functional groups.
Location
Fynbos (17 species) and savanna (21 species) systems in South Africa.
Methods
The linear subset of bark–stem data was identified using a three-step procedure. Linear regressions (with and without an intercept) and curved models (allometric and modified exponential models) were fitted to the linear subset and complete raw bark–stem data set, respectively. In addition, linear regression models were fitted to the log-transformed complete bark–stem data set. Regression slopes and bark thickness predictions obtained from these different approaches were compared, to determine which method provides the most robust metric for comparing bark thickness. Bark thickness was compared among fynbos resprouter guilds and acacias from low- and high-fire savannas.
Results
The slope of the regression model fitted to the linear subset of raw bark–stem data provides a reliable metric for general comparisons of relative bark thickness. Bark thickness predictions from the curved and log models were comparable at 20-cm stem diameter, but the log model underestimated bark thickness at 5 cm for certain species. Relative bark thickness was significantly higher in: (1) fynbos fire resisters and epicormic resprouters than in non-resprouters; and (2) acacias from high- vs low-fire savannas.
Conclusions
The slope of the regression model fitted to the linear subset of raw bark–stem data is a useful metric for bark thickness comparisons across studies, and compares favourably with bark thickness predictions derived from models fitted to the complete bark–stem data set. Fynbos and savanna trends support the proposition that relative bark thickness reflects differences in woody plant responses to fire and indicate the modal fire regime of ecosystems.
Keywords Allometic model
Fynbos
Linear regression
Modified exponential model
Relative bark thickness
Savanna
Stem diameter
DOI http://dx.doi.org/10.1111/jvs.12171   (check subscription with CDU E-Gateway service for CDU Staff and Students  check subscription with CDU E-Gateway in new window)
 
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