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Roles of Root Symbioses in African Woodland and Forest: Evidence from $^{15}\mathrm{N}$ Abundance and Foliar Analysis

Peter Hogberg and Ian J. Alexander
Journal of Ecology
Vol. 83, No. 2 (Apr., 1995), pp. 217-224
DOI: 10.2307/2261560
Stable URL: http://www.jstor.org/stable/2261560
Page Count: 8
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Roles of Root Symbioses in African Woodland and Forest: Evidence from $^{15}\mathrm{N}$ Abundance and Foliar Analysis
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Abstract

1 To gain insight into the nutrition of trees with ectomycorrhizas (ECM), VA-mycorrhizas (VAM) or VAM plus $\mathrm{N}_2$-fixing legume root nodule symbioses (NOD) in tropical woodland and forest ecosystems, we measured foliar nutrient concentrations and $^{15}\mathrm{N}$ abundances in miombo woodland in Zambia (22 spp.) and in lowland rain forest in Cameroon (20 spp.). 2 In miombo woodland, confirmed NOD species had low $^{15}N$ abundances (mean $^{15}N = 0.2%$), high % N and high N:P ratios. Baphia bequaertii, a species thought likely to be NOD, shared these characteristics, but so did Cassia abbreviata which is less likely to be NOD. 3 Among VAM and ECM species in miombo there were, in general, positive correlations between $^{15}N$ abundance and % N, and between % N and % P. Dominant ECM species (Caesalpinioideae-Dipterocarpaceae) had slightly higher % N, but not higher $^{15}N$ abundance than VAM species. The $^{15}N$ data do not agree with data on ECM and VAM species previously obtained from Tanzanian miombo. 4 In the rain forest, there were no large differences between the three symbiotic groups. For NOD species $\delta^{15}N$ was almost 4% above that of atmospheric $N_2$ and only slightly lower than that of non-NOD species. NOD species also had relatively high foliar % N. 5 Data on species composition, foliar $^{15}N$ abundance and N:P ratios support the idea that $N_2$-fixation carried out by $N_2$-fixing trees is more important in the woodland than in the rain forest.

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