Aspects of root architecture, including topology, link length, diameter and scaling relations, were analyzed in excavated coarse root systems of three field-grown fruit tree species (Strychnos cocculoides Bak., Strychnos spinosa Lam. and Vangueria infausta Burch) and the fruit-bearing shrub Grewia flava DC. We investigated the root systems using semi-automatic digitizing and computer-based 3-D reconstruction techniques. Topological analysis was carried out to investigate branching patterns as basic determinants of root architecture. New topological indices were developed and revealed significant differences among the species. The different architectural strategies can be explained in terms of cost-benefit relations and efficiency in soil resource exploration and exploitation. In addition, some well-known hypotheses about geometry and scaling, most of them previously unverified by empirical observations on root systems, were tested. For practical applications, the main emphasis is on the relationship between proximal root diameter, an easily determined parameter, and several parameters describing the size of the whole root system. We also tested the "pipe stem" theory, essentially dating back to Leonardo da Vinci, which underlies many models and which we found conformed to our measurement data with reasonable accuracy. A physiological consequence of the "constant cross-sectional area rule" may be a certain homogeneity of hydraulic architecture throughout root systems.

-Coarse root systems of four different fruit tree species from southern Africa were completely excavated and semi-automatically digitized. Spatial distributions of root length were determined from the digitally-reconstructed branching systems. Furthermore, the fractal characteristic of the coarse root systems was shown by determining the box-counting dimensions. These quantitative methods revealed architectural differences between the species, probably due to different ecophysiological strategies. For fine root samples, which were taken before digging out the whole systems, fractal analysis of the planar projections showed no significant inter-species differences. Methodologically, the study underlines the usefulness of digital 3-D reconstruction in root research. root / digital reconstruction / fractal / architectural analysis / coarse root Résumé -Structures et dimensions fractales des systèmes racinaires provenant de quatre espèces d'arbres fruitiers de Botswana. Des systèmes de grosses racines, provenant de quatre espèces différentes d'arbres fruitiers d'Afrique du Sud, ont été complètement déterrés et digitalisés semi-automatiquement. Les distributions spatiales des longueurs de racines ont été calculées à partir des maquettes informatiques reconstituées. En outre, le caractère fractal des systèmes de grosses racines a été prouvé par une déter-mination de dimensions utilisant la méthode du comptage de boites. Ces méthodes quantitatives révèlent des différences architecturales entre les espèces, résultant probablement de différentes stratégies écophysiologiques. Pour les échantillons de racines fines, obtenus avant l'excavation des systèmes complets, l'analyse fractale des projections planes n'a pas montré de différences significatives entre les espèces. Concernant la méthode, l'étude fait apparaître l'apport de la reconstruction digitale 3-D dans le domaine de la recherche sur les appareils racinaires. racine / reconstruction digitale / fractal / analyse architectural / racine gros

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(/), 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(/) 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.

To assess the possible degree of root competition from fruit trees which could potentially be used in agroforestry systems, fine root density of fruit trees Strychnos cocculoides BAK., Strychnos spinosa LAM. (Loganiaceae) and Vangueria infausta BURCH. (Rubiaceae), as well as from the shrubby species, Grewia flava DC. (Tiliaceae) was investigated. Vangueria infausta had the highest fine root densities in both vertical and horizontal extensions. In Vangueria infausta fine root density decreased with increasing soil depth. For the other species in the 80 cm soil profile investigated, no significant changes in fine root density with soil depth were found. For Strychnos cocculoides almost no fine roots were detected in the upper soil horizon (0-20 cm). Using fine root surface area densities, exploration and exploitation indices were calculated. Vangueria infausta had the highest value of the exploration index compared to the other species. For use in agroforestry systems Vangueria infausta was estimated to be the most competitive of the investigated species, whereas Strychnos cocculoides seems to be the less competitive. Strychnos cocculoides has additionally spatial arrangements of fine roots favourable for agroforestry, slowly increasing with depth and additionally low concentrations in upper soil layers.

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