P. Hogberg et Ij. Alexander, ROLES OF ROOT SYMBIOSES IN AFRICAN WOODLAND AND FOREST - EVIDENCE FROM N-15 ABUNDANCE AND FOLIAR ANALYSIS, Journal of Ecology, 83(2), 1995, pp. 217-224
1 To gain insight into the nutrition of trees with ectomycorrhizas (EC
M), VA-mycorrhizas (VAM) or VAM plus N,-fixing legume root nodule symb
ioses (NOD) in tropical woodland and forest ecosystems, we measured fo
liar nutrient concentrations and N-15 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 N-15 abundances (mea
n N-15 = 0.2 parts per thousand), high % N and high N:P ratios. Baphia
bequaertii, a species thought likely to be NOD, shared these characte
ristics, 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 N-15 abundance and % N, and between % N and % P.
Dominant ECM species (Caes alpinioideae-Dip tero carpaceae) had sligh
tly higher % N, but not higher N-15 abundance than VAM species. The N-
15 data do not agree with data on ECM and VAM species previously obtai
ned from Tanzanian miombo. 4 In the rain forest, there were no large d
ifferences between the three symbiotic groups. For NOD species delta(1
5)N was almost 4 parts per thousand 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 N-1
5 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.