Roles of Root Symbioses in African Woodland and Forest: Evidence from 15 N Abundance and Foliar Analysis

1 To gain insight into the nutrition of trees with ectomycorrhizas (ECM), VAmycorrhizas (VAM) or VAM plus N2-fixing legume root nodule symbioses (NOD) in tropical woodland and forest ecosystems, we measured foliar nutrient concentrations and 15N 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 15N abundances (mean 15N = 0.2%o), 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 15N abundance and % N, and between % N and % P. Dominant ECM species (Caesalpinioideae-Dipterocarpaceae) had slightly higher % N, but not higher 15N abundance than VAM species. The 15N 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 15N was almost 4%o above that of atmospheric N2 and only slightly lower than that of non-NOD species. NOD species also had relatively high foliar % N. 5 Data on species composition, foliar 15N abundance and N:P ratios support the idea that N2-fixation carried out by N2-fixing trees is more important in the woodland than in the rain forest.