Micronutrients may influence the efficacy of ectomycorrhizas to support tree seedlings in a lowland African rain forest

In the lowland rain forest of SW Cameroon, a field experiment tested whether ectomycorrhizal hyphal connections might affect the growth and survival of seedlings of a principal tree species, Microberlinia bisulcata, close to its adults. Nursery-raised seedlings were planted into fine-, medium-, and coarse-mesh root bags, and as no-bag controls, in replicate subplots. The bags prevented fungal hyphae, and fine- and medium-sized roots, respectively, entering from the outside forest floor root mat. Harvests were taken after 1 and 2 yr, with non-destructive recording in between. Seedlings grew in typically lowlight locations. Survivorship did not differ between treatments (33%), but seedlings grew significantly better in terms of stem dry mass by harvest 2 in the medium-mesh compared with other treatments. Treatment 1 to 3 seedlings had stem masses 25, 44, and 5% higher than controls, respectively. Using a method of differences across treatments, the positive effect of ectomycorrhizas on growth was 13.6%, while the negative effect of root competition (RCM) was 31.2% (net outcome = 17.6%). Adjustment was made to account for root penetration damaging some mesh bags. Differences in growth in replicate subplots were, however, much larger than those for treatments. Elemental analysis of seedling plant parts showed few differences between treatments, but phosphorus was high in stems, aluminum and iron were very high in roots, and copper was deficiently low in leaves. Soil analyses revealed very low copper levels, suggesting with the seedling results that this element was critically limiting for seedlings. Ectomycorrhizas are probably important for copper uptake (as for phosphorus), so roots may have been competing for this element. Because seedlings were growing in the shade and the soil was inhibitory to roots, they could not form network connections enough to positively out-balance root competition. The efficacy of ectomycorrhizal networks for at least seedling establishment in this forest is low.