Abstract
In the ectomycorrhizal caesalpiniaceous groves of southern Korup National Park, the dominant tree species, Microberlinia bisulcata, displays very poor insitu recruitment compared with its codominant, Tetraberlinia bifoliolata. The reported ex situ experiment tested whether availabilities of soil potassium and magnesium play a role. Seedlings of the two species received applications of K and Mg fertilizer in potted native soil in a local shade house, and their responses in terms of growth and nutrient concentrations were recorded over 2years. Amended soil concentrations were also determined. Microberlinia responded strongly and positively in its growth to Mg, but less to K; Tetraberlinia responded weakly to both. Added Mg led to strongly increased Mg concentration for Microberlinia while added K changed that concentration only slightly; Tetraberlinia strongly increased its concentration of K with added K, but only somewhat its Mg concentration with added Mg. Additions of Mg and K had small but important antagonistic effects. Microberlinia is Mg-demanding and apparently Mg-limited in Korup soil; Tetraberlinia, whilst K-demanding, appeared not to be K-limited (for growth). Added K enhanced plant P concentrations of both species. Extra applied Mg may also be alleviating soil aluminum toxicity, and hence improving growth indirectly and especially to the benefit of Microberlinia. Mg appears to be essential for Microberlinia seedling growth and its low soil availability in grove soils at Korup may be an important contributing factor to its poor recruitment. Microberlinia is highly shade-intolerant and strongly light-responding, whilst Tetraberlinia is more shade-tolerant and moderately light-responding, which affords an interesting contrast with respect to their differing responses to Mg supply. The study revealed novel aspects of functional traits and likely niche-partitioning among ectomycorrhizal caesalps in African rain forests. Identifying the direct and interacting indirect effects of essential elements on tropical tree seedling growth presents a considerable challenge due the complex nexus of causes involved.
Highlights
The Atlantic Coastal Forest of Central Africa is a spatially and temporally complex forest type characterized by patches of varying extent in which species of the subfamily Caesalpiniaceae dominate compositional abundance (Letouzey 1968, 1985).A great majority of these species are in the tribe Amherstieae and they are ectomycorrhizal (EM) (Newbery et al 1988)
Ecology and Evolution published by John Wiley & Sons Ltd
Whilst height and branch number continued to rise steadily until the time of harvest 3 (Fig. 1B and D), leaf number for Microberlinia decreased after harvest 1, and for Tetraberlinia it became constant after midway between harvests 1 and 2
Summary
Litter-bag trials at Korup (Chuyong et al 2002) indicated that among the five macronutrients, just Mg showed significantly much higher mineralization rates in grove than nongrove soils, indicative of a large demand by predominantly adult M. bisulcata trees for Mg, and in turn that available levels of Mg would be reduced for seedlings around adults. This new insight led to the hypothesis that Mg could be a potentially limiting factor for M. bisulcata regeneration. The two selected species will be referred to from here on by just their generic names
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