SCIENTIFIC CORRESPONDENCE

Basal stem area is a better measure of woodiness than canopy cover in the savannas of the Kruger National Park

Anthony J. Mills^I; Tercia Strydom^II

^IDepartment of Soil Science, Stellenbosch University, Stellenbosch, South Africa
^IIAbiotic Processes, South African National Parks, Skukuza, South Africa

Correspondence

Keywords: woody cover, basal stem diameter, South Africa, allometry

Water availability, soil nutrient availability, fire and herbivory are all known to affect the abundance of trees in savannas; yet the strength of each factor is often puzzlingly variable between sites.^1-3 Woody cover in African savannas, for example, varies from <1% to >80% under similar rainfall regimes.⁴ We hypothesised that part of the problem in determining how environmental factors affect woodiness in savannas relates to the difficulty of quantifying woodiness effectively.

One of the most common ways of measuring woodiness is by estimating the extent to which the canopy of woody plants covers the ground in a plot.⁵ We have found, however, that canopy cover estimates frequently differ by up to 25% amongst observers in the same plot. Another method of measuring woodiness is allometry. This method is also, unfortunately, fraught with difficulty, mainly because obtaining a sufficient sample size to develop accurate allometric equations for all woody plant species in a landscape is usually not practical.⁶

In a recent study on the effects of soil nutrients on woodiness in the savannas of the Kruger National Park of South Africa, we examined two ways of measuring woodiness in 10 m by 10 m plots: firstly by visual estimation of canopy cover (%), and secondly by measuring the basal stem diameter (cm) of each woody stem. Basal stem diameter measurements were used to calculate basal stem area (πr²) and then summed for each plot to provide a cumulative measure of total woody area (cm²). The methods of soil sampling and analysis are described in Mills et al.⁷. All data were analysed using R statistical software.⁸

Relationships between the two different methods of quantifying woodiness and soil nutrients are shown in Figure 1 and Table 1. Both measures of woodiness were positively correlated with a wide range of soil nutrients, with the strength of correlations varying between sampling sites. Such positive correlations have been attributed to effects of nutrient availability on the growth rate, and therefore establishment, of trees.^9,10 Where growth rates are faster, it is more likely that trees will establish and not succumb to pressures such as fire or herbivory. Assuming that such causal mechanisms are taking place, it is noteworthy that there were more, as well as stronger, correlations between basal stem area and soil nutrients than between canopy cover and soil nutrients. This finding suggests that basal stem area is more appropriate than canopy cover as an index of woodiness at our study sites in the Kruger National Park. If the same applies in other savannas, measurements of basal stem diameter could be of considerable value for isolating which environmental factors have the greatest influence on tree abundance. Such information is likely to be of practical value for land managers wanting to alter the ratio of trees versus grass in a savanna environment.

Acknowledgements

We gratefully acknowledge the South African Department of Environmental Affairs, Natural Resources Management Programme and the National Research Foundation of South Africa (grant number FA2005040700027) for funding this research; and Zurelda le Roux, Jessica Allen and Ruan de Wet for technical assistance.

References

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Correspondence:
Anthony Mills
mills@sun.ac.za

Published: 29 January 2020