Evaluating Ecohydrological Theories of Woody Root Distribution in the Kalahari

The contribution of savannas to global carbon storage is poorly understood, in part due to lack of knowledge of the amount of belowground biomass. In these ecosystems, the coexistence of woody and herbaceous life forms is often explained on the basis of belowground interactions among roots. However, the distribution of root biomass in savannas has seldom been investigated, and the dependence of root biomass on rainfall regime remains unclear, particularly for woody plants. Here we investigate patterns of belowground woody biomass along a rainfall gradient in the Kalahari of southern Africa, a region with consistent sandy soils. We test the hypotheses that (1) the root depth increases with mean annual precipitation (root optimality and plant hydrotropism hypothesis), and (2) the root-to-shoot ratio increases with decreasing mean annual rainfall (functional equilibrium hypothesis). Both hypotheses have been previously assessed for herbaceous vegetation using global root data sets. Our data do not support these hypotheses for the case of woody plants in savannas. We find that in the Kalahari, the root profiles of woody plants do not become deeper with increasing mean annual precipitation, whereas the root-to-shoot ratios decrease along a gradient of increasing aridity

Left: Fire frequency from 2000 to 2011 calculated from the MODIS Burned Area Product (MCD45) [<b>72</b>] in yr⁻¹.

<p>White areas experienced no fires during this period. Right: Average fire frequency (in yr⁻¹) calculated along a longitudinal transect (21.3°) using a moving box of approximately 100×100 km.</p

Root diameter distribution along the soil profile across the Kalahari's rainfall gradient.

<p>The error bars indicate the minimum and maximum data values, unless outliers are present (shown as circles). The black line indicates the median, while the box boundaries are the lower and upper quartiles. Based on a set of 60 soil profiles sampled at each site.</p

Coordinates, mean annual precipitation (MAP), and plant community composition of the study sites.

<p>Information on precipitation includes mean annual precipitation (MAP) ± the standard deviation of annual precipitation and (in parentheses) the minimum annual precipitation recorded in 1971–2006. Because rainfall data from Kuke do not exist, the values reported are from Ghanzi.</p

Grain size analysis of the soil samples was conducted using a particle size analyzer (LS 13 320, Beckman Coulter®).

<p>Soil samples were collected from depths 0–0.1 m, 0.1–0.3 m, 0.3–0.7 m, and 0.7–120 m at all sites except for the Kuke/Ghanzi area, where we have used only soils from the top 10 cm.</p

Linear root density (average root lengths per volume) across the Kalahari rainfall gradient in Botswana.

<p>Kuke site exhibits the highest length/volume readings compared to the other sites. The error bars indicate the minimum and maximum data values, unless outliers are present (shown as circles). The black line indicates the median, while the box boundaries are the lower and upper quartiles. Based on a set of 60 soil profiles sampled at each site.</p

Results of the analysis of covariance performed on the wet to dry biomass ratio, <i>k</i>, using tissue type (i.e., above ground or below ground biomass) as a factor and size of the wet sample as a covariate.

<p>Results of the analysis of covariance performed on the wet to dry biomass ratio, <i>k</i>, using tissue type (i.e., above ground or below ground biomass) as a factor and size of the wet sample as a covariate.</p

Geographic location of the study region and of the research sites.

<p>Geographic location of the study region and of the research sites.</p

Measured average woody plant biomass (above and belowground) per unit area (top 1.1 m) across the Kalahari transect aridity gradient.

<p>Inset: ratio of below to above ground biomass. The error bars represent ± standard deviation calculated for a set of 3 plot replicates at each site.</p

Parameters, <i>a</i> and <i>b</i>, and associated statistics from the fitting of an exponential distribution <i>r</i>(<i>z</i>) = <i>a</i> e^−<i>bz</i> to the average root profile in the vertical (<i>z</i>) direction.

<p>The parameters <i>a_L</i>, <i>b_L</i>, and associated statistics from the fitting of an exponential distribution <i>r_L</i>(<i>z</i>) = <i>a_L</i> e<i>^−b_L^z</i> to the average root density (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0033996#pone-0033996-g003" target="_blank">figure 3</a>) are shown in italic fonts. The distribution is calculated as an average of three plots (20 soil pits per plot) at five depths.</p