Density-dependent dynamics of a dominant rain forest tree change with juvenile stage and time of masting

Although negative density dependence (NDD) can facilitate tree species coexistence in forests, the underlying mechanisms can differ, and rarely are the dynamics of seedlings and saplings studied together. Herein we present and discuss a novel mechanism based on our investigation of NDD predictions for the large, grove-forming ectomycorrhizal mast fruiting tree, Microberlinia bisulcata (Caesalpiniaceae), in an 82.5-ha plot at Korup, Cameroon. We tested whether juvenile density, size, growth and survival decreases with increasing conspecific adult basal area for 3245 ‘new’ seedlings and 540 ‘old’ seedlings (< 75-cm tall) during an approximately 4-year study period (2008–2012) and for 234 ‘saplings’ (≥ 75-cm tall) during an approximately 6-year study period (2008–2014). We found that the respective densities of new seedlings, old seedlings and saplings were positively, not and negatively related to increasing BA. Maximum leaf numbers and heights of old seedlings were negatively correlated with increasing basal areas, as were sapling heights and stem diameters. Whereas survivorship of new seedlings decreased by more than one-half with increasing basal area over its range in 2010–2012, that of old seedlings decreased by almost two-thirds, but only in 2008–2010, and was generally unrelated to conspecific seedling density. In 2010–2012 relative growth rates in new seedlings’ heights decreased with increasing basal area, as well as with increasing seedling density, together with increasing leaf numbers, whereas old seedlings’ growth was unrelated to either conspecific density or basal area. Saplings of below-average height had reduced survivorship with increasing basal area (probability decreasing from approx. 0.4 to 0.05 over the basal area range tested), but only sapling growth in terms of leaf numbers decreased with increasing basal area. These static and dynamic results indicate that NDD is operating within this system, possibly stabilizing the M. bisulcata population. However, these NDD patterns are unlikely to be caused by symmetric competition or by consumers. Instead, an alternative mechanism for conspecific adult–juvenile negative feedback is proposed, one which involves the interaction between tree phenology and ectomycorrhizal linkages.

Limitation of seedling growth by potassium and magnesium supply for two ectomycorrhizal tree species of a Central African rain forest and its implication for their recruitment

In the ectomycorrhizal caesalpiniaceous groves of southern Korup National Park, the dominant tree species, Microberlinia bisulcata, displays very poor in situ 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 2 years. 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.

Litter nutrients and retranslocation in a central African rain forest dominated by ectomycorrhizal trees

In the strongly seasonal, but annually very wet, parts of the tropics, low-water availability in the short dry season leads to a semi-deciduous forest, one which is also highly susceptible to nutrient loss from leaching in the long wet season. Patterns in litterfall were compared between forest with low (LEM) and high (HEM) abundances of ectomycorrhizal trees in Korup National Park, Cameroon, over 26 months in 1990–92. Leaf litter was sorted into 26 abundant species which included six ectomycorrhizal species, and of these three were the large grove-forming trees Microberlinia bisulcata, Tetraberlinia bifoliolata and Tetraberlinia moreliana. Larger-tree species shed their leaves with pronounced peaks in the dry season, whereas other species had either weaker dependence, showed several peaks per year, or were wet-season shedders. Although total annual litterfall differed little between forest types, in the HEM forest (dominated by M. bisulcata) the dry-season peak was more pronounced and earlier than that in the LEMforest. Species differed greatly in their mean leaf litterfall nutrient concentrations, with an approx. twofold range for nitrogen and phosphorus, and 2.5–3.5-fold for potassium, magnesium and calcium. In the dry season, LEM and HEM litter showed similar declines in P and N concentration, and increases in K and Mg; some species, especially M. bisculcata, showed strong dry-wet season differences. The concentration of P (but not N) was higher in the leaf litter of ectomycorrhizal than nonectomycorrhizal species. Retranslocation of N and P was lower among the ectomycorrhizal than nonectomycorrhizal species by approx. twofold. It is suggested that, within ectomycorrhizal groves on this soil low in P, a fast decomposition rate with minimal loss of mineralized P is possible due to the relatively high litter P not limiting the cycle at this stage, combined with an efficient recapture of released P by the surface organic layer of ectomycorrhizas and fine roots. This points to a feedback between two essential controlling steps (retranslocation and mineralization) in a tropical rain forest ecosystem dominated by ectomycorrhizal trees.

Tree architecture in a Bornean lowland rain forest: intraspecific and interspecific patterns

Intraspecific and interspecific architectural patterns were studied for eight tree species of a Bornean rain forest. Trees 5--19 m tall in two 4-ha permanent sample plots in primary forest were selected, and three light descriptors and seven architectural traits for each tree were measured. Two general predictions were made: (1) Slow growing individuals (or short ones) encounter lower light, and have flatter crowns, fewer leaf layers, and thinner stems, than do fast growing individuals (or tall ones). (2) Species with higher shade-tolerance receive less light and have flatter crowns, fewer leaf layers, and thinner stems, than do species with lower shade-tolerance. Shade-tolerance is assumed to decrease with maximum growth rate, mortality rate, and adult stature of a species.

Does proximity to conspecific adults influence the establishment of ectomycorrhizal trees in rain forest?

Three ectomycorrhizal legume trees, Microberlinia bisulcata, Tetraberlinia bifoliolata and T. moreliana, form discrete groves in the southern part of Korup National Park, in southwest Cameroon and contribute c. 45–70% of stand basal area locally in a matrix of otherwise species-rich arbuscular mycorrhizal forest. A transplant experiment was performed to assess the importance of ectomycorrhizal infection associated with proximity to parents in seedling establishment of the grove-forming species. Nonectomycorrhizal seedlings of the three species were transplanted into plots of two forest types, one of high (HEM, within-grove) and one of very low (LEM, outside the grove) abundance of all three species as adult trees. For two species (T. moreliana and M. bisulcata) there was no difference in survival over 16 months, but for the third (T. bifoliolata) survival was best in HEM forest, and correlated with the basal area of adult trees of ectomycorrhizal species. Only one species (T. moreliana) increased in biomass over the experimental period; the others declined. There was no effect of forest type on overall growth of any species, but the survivors of two (T. moreliana and M. bisulcata) had heavier stems in the HEM forest. Differences in survival and growth of transplants between the three species were in accord with the ecology of the species as inferred from the frequency distributions of adult tree size in the forest. Seedlings became infected with ectomycorrhizas in both forest types; where there was a difference in extent of infection (T. moreliana) this was not related to survival or growth; and where there was a difference in survival (T. bifoliolata) this was not related to extent of infection. These results confirm that mycorrhizal inoculum associated with conspecific adults is neither a prerequisite nor a guarantee of seedling establishment, but indicates that in some circumstances there might be benefits of being close to parents. Further research is required to unravel the complexities of ectomycorrhizal community structure in this spatially and temporally heterogeneous forest, and to clarify the extent to which the various hosts share ectomycorrhizal partners.