Soil clay influences Acacia encroachment in a South African grassland

Abstract

Because of technical difficulties in measuring soil properties at a large scale, little is known about the effect of soil properties on the spatial distribution of trees in grasslands. We were interested in the associations of soil properties with the phenomenon of tree encroachment, where trees increase in density at the expense of grasses. The spatial variation of soil properties and especially soil texture may modify the properties of hydraulic conductivity, and the availability of soil water and mineral nutrients, which in turn may affect the spatial distribution of encroaching trees. Through the development of a geophysical method (Slingram) using an electromagnetic device EM38 and Bayesian inversion, we were able to accurately map soil electrical conductivity (EC) of a Luvisol in a grassland of South Africa. EC measured at the 0·8 to 2 m depth on a 1·5 ha area is a proxy for clay content and was correlated with the spatial distribution of four size classes of the encroaching Acacia sieberiana. Tree location (all sizes considered) was significantly correlated with EC. Tall acacias (>3m height) were totally absent from patches with EC >24 mS m−1. For all other size classes from medium trees to seedlings, tree density decreased with increasing EC. This suggests that high clay contents at depth associated with high EC values may prevent the establishment and/or survival of trees and influence the spatial distribution of A. sieberiana. This result also shows that geophysical tools may be useful for demonstrating important ecological processes. Copyright © 2014 John Wiley & Sons, Ltd.

PioLaG: a piosphere landscape generator for savanna rangeland modelling

“…This can be expected as the establishment of woody vegetation in arid to semi-arid savannas is primarily limited by moisture availability (Sankaran et al 2005). Our results are also in accordance with studies showing that higher soil clay contents can counteract the positive effect of high precipitation on recruitment and growth of woody vegetation (Kgosikoma et al 2012;Grellier et al 2014).…”

“…Likewise, palatable perennial grasses were negatively affected by MAP but also by the number of bushthickened camps, as both increase the woody vegetation at the expense of this vegetation type. Clay content counteracted increases in woody vegetation [as described by Kgosikoma and Mogotsi (2013) and Grellier et al (2014)] and consequently had a positive effect on (palatable and unpalatable) perennial grasses. In addition to competition also disturbance played a role: the higher the overall grazing pressure (stocking rate), the fewer 'palatable perennial grass' cells occurred (Fig.…”

“…We use clay content as an indicator of soil texture as it was shown to be the most important soil property influencing bush thickening dynamics. Finetextured clay soils have smaller pores and lower rain water infiltration compared to sandy soils, thus diminishing growth of trees and counteracting an increase in density or cover of woody species even under otherwise favorable conditions and especially during high rainfall years (Kgosikoma et al 2012;Grellier et al 2014). Accordingly, MAP and soil clay content are parameters used in PioLaG to set upper bounds of possible woody vegetation cell frequency.…”

How does clay constrain woody biomass in drylands?

“…For any given moisture value, small deviations in clay content may have important implications for moisture availability during drought, which provides a primary explanation for the association between clay and tree mortality demonstrated by our analysis. The subtlety with which soil clay influences tree mortality in our study is reflected in a recent study of tree recruitment within South African grassland (Grellier et al ., ), where larger trees were negatively associated with soil clay content at depth.…”

Mechanisms of lateral and linear extension of gullies (dongas) in a subhumid grassland of South Africa

“…It has a very well‐developed angular blocky structure. Some previous geophysical studies conducted in the lower part of the watershed (Figure 1) showed the variability in the depth of appearance of the Bt horizon (Grellier et al ., 2013, 2014). The BC horizon is often a Pleistocene colluvial layer that has a strong brown colour (7.5YR 4/3 to 7.5YR 6/8) with a massive structure that is very prone to dispersion.…”