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Accueil du site → Doctorat → Allemagne → 2003 → Root morphology of co-occurring African fruit tree species with contrasting strategies of exploration and exploitation

Georg-August-Universität Göttingen (2003)

Root morphology of co-occurring African fruit tree species with contrasting strategies of exploration and exploitation

Oppelt, Armin

Titre : Root morphology of co-occurring African fruit tree species with contrasting strategies of exploration and exploitation

Wurzelmorphologie koexistierender afrikanischer Obstbäume mit unterschiedlichen Strategien der Exploration und Exploitation

Auteur : Oppelt, Armin

Université de soutenance : Georg-August-Universität Göttingen

Grade : Doktorgrade 2003

Investigations of completely excavated coarse root systems from Grewia flava DC (Tiliaceae), Strychnos cocculoides BAK., Strychnos spinosa LAM. (Loganiaceae) and Vangueria infausta BURCH. (Rubiaceae) were conducted to detect species-specific rooting patterns. Additionally, systematic soil sampling was carried out to achieve information about morphological patterns of fine roots. Attributes of both entities, also including their spatial arrangement, are compared. Semi-automatic digitising was used to obtain 3-D models from coarse root systems. Analysis of fine roots was performed with digital image analysis, to gain morphological parameters and to correlate them with dry weight for approximation of whole fine root distribution.Box counting dimension of coarse root systems was determined for three-dimensional (D) and two-dimensional (Dxy) considerations, while fractal behaviour of fine root samples was only analysed under the assumption of planarity. Two new indices, qa and qb, related to altitude a and mean topological depth b, were developed. Additionally, topological analysis was complemented with link length analysis. All these parameters are furthermore compared with species-dependent capabilities of "exploration" and "exploitation". For this purpose, already established parameters for fine roots, e.g. exploration index E(0) and exploitation index E(φ), were applied to coarse root systems and fine root samples. The parameters number of explored cells NEC, exploration efficiency EE and a new parameter, generalized efficiency of exploitation GEE(φ), were used to quantify explorative and exploitative behaviour of coarse root systems. Thus, definitions from the literature were further extended and generalized to clarify commonly used notions. The determination of an individual soil volume ISV for each coarse root system, taking varying spatial extensions into account, was beneficial to detect differing fine root dispersal inside and outside the restricted soil volume.Clear distinctions between the investigated species are obvious. Strychnos cocculoides, with relatively weak coarse root systems and also scarce development of fine roots, was most contrasting with root systems from Vangueria infausta. In terms of competition, Strychnos cocculoides as least and Vangueria infausta as most explorative conspecific among the investigated species are most contrasting. Grewia flava and Strychnos spinosa lie between both extremes. For fine root characteristics, the same ranking occurs for exploitative parameters. However, for exploitation by coarse root systems, separation of the species was only possible with the parameter GEE(φ) : here, Grewia flava, as the most exploitative species, was most opposed to Strychnos cocculoides. In this context, a link between deep rooting, less explorative and "herringbone" coarse root systems seems to exist. The other extreme, with intensive branching, is associated with shallow rooting, indicating a high exploration potential with a "dichotomous" configuration. Also the external link length (le) was well correlated with topological parameters. Affinities between characteristics of coarse and fine roots hint at a ’common strategy’ of the plant for the construction of both entities. Empirical tests of Leonardo s rule confirmed the pipe stem theory with reasonable accuracy. Although the relation between root collar diameter and other metrical parameters (root length, root volume) yielded good correlations, the test of a supposed exponential tapering rule did not show convincing results.A combination of complete excavation of coarse roots with systematic sampling of fine roots was a suitable method to obtain detailed information about whole root system composition, and to detect species-specific differences in potential competition. The results suggest that co-occurring species develop contrasting rooting patterns to maximise their acquisition of soil resources as an effective means of spatial and/or temporal sharing.


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