Contrasting rooting patterns of some arid-zone fruit tree species from Botswana – II. Coarse root distribution

Spatial coarse root distribution of the in situ grown species Strychnos cocculoides Bak., Strychnos spinosa Lam. (Loganiaceae), Vangueria infausta Burch. (Rubiaceae) and Grewia flava DC. (Tiliaceae) was investigated. The woody roots provide the scaffolding for fine roots, and thus underpin potential competition with fine roots of other species. We developed a method for quantitative description of spatial patterns of coarse roots and correlated fine root distribution with the spatial arrangement of the coarse root systems. In order to estimate different exploration and exploitation strategies, we used the spatial distribution of structural roots within the individual soil volumes (ISV) of each root system, and compared the results with other published parameters. We defined a new parameter, “generalized efficiency of exploitation” GEE(φ), unifying different notions from the literature. Among all the investigated species and dependent on the considered parameter, either Vangueria infausta or Grewia flava had the highest mean values of exploration, hence they could clearly be separated from both Strychnos species. For the exploitation indices, no significant differences were found. However, the generalized efficiency of exploitation GEE(φ) was again different between the species. For correlations between coarse and fine roots, the restriction to the ISV was useful to increase the strength of correlations.