Evaluating Ecohydrological Theories of Woody Root Distribution in the Kalahari

Abstract

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.

Biomass and Its Allocation in Relation to Temperature, Precipitation, and Soil Nutrients in Inner Mongolia Grasslands, China

“…This disagreement can be resulted from several reasons. First, despite the large spatial scale in our study and other similar studies in dry regions [10], [11], [42], the climatic variations in these studies represent only a small fraction of the global climatic variations that are considered in global meta-analyses [12], [17]. Therefore, the differences in ecosystem-specific results and global syntheses reflect different ecological scales.…”

“…The biomass in dry and warm sites may not be sufficient to support frequent fires, whereas sites with more precipitation may have high fire frequencies due to more frequent lightning ignitions and available biomass fuels. As hypothesized by Bhattachan et al [42], high allocation to belowground due to the need for additional storage as the risk of fire increases may be an important life-history strategy for sites with high percipitation and high fire frequencies.…”

A quantitative description of the interspecies diversity of belowground structure in savanna woody plants

“…Following the method of Bhattachan et al (2012) for analyzing the depth distribution of roots in biomass sampling pits, we fit a cumulative exponential distribution of root biomass abundance with depth to the individual root systems…”

“…It is the only site with sufficient biomass to support a fire regime, burning once every 5-10 years (Bhattachan et al 2012). The dominant woody species are Acacia mellifera, Acacia erioloba, Rhigozum trichotomum, and Boscia albitrunca.…”

From facilitative to competitive interactions between woody plants and plants with crassulacean acid metabolism (CAM): The role of hydraulic descent

“…Crassulacean acid metabolism plants are predominant in arid and semiarid environments (Withgott, ; Ogburn & Edwards, ), and thus, we parameterize this model with respect to environmental conditions with low (λ = 0.2/day and h = 5 mm) and moderate (λ = 0.2/dayand h = 10 mm) total rainfall amounts. Because this study investigates the facilitation or competition of CAM plants by woody plants, we assume that the growing seasons of trees and CAM plants coincide and last 210 days each year (e.g., Bhattachan et al, ). CAM plants typically have very shallow roots (e.g., Franco & Nobel, , ; Ogburn & Edwards, ), and thus, the root depth of CAM plants is taken to be constant ( Z 1 = 10 cm) in all the simulations.…”