Recruitment dynamics of the grove-dominant tree Microberlinia bisulcata in African rain forest: extending the light response versus adult longevity trade-off concept

In groves of ectomycorrhizal caesalpiniaceous species in the Atlantic coastal forest of Central Africa the dominant tree Microberlinia bisulcata, which is shade-intolerant as a seedling but highly light-responding as a sapling, shows very limited regeneration. M. bisulcata saplings were mapped in an 82.5-ha plot at Korup and found to be located significantly far (>40 m) away from adults, a result confirmed by direct testing in a second 56-ha plot. Sapling growth over 6 years, the distribution of newly emerging seedlings around adults, recruitment of saplings in a large opening and the outward extent of seedlings at the grove edge were also investigated. Two processes appear to have been operating: (1) a very strong and consistent restriction of the very numerous seedlings establishing after masting close to adults, and (2) a strong but highly spatially variable promotion of distant survivors by increased light from the deaths of large trees of species other than M. bisulcata (which itself has very low mortality rate). This leads to an apparent escape-from-adults effect. To maintain saplings in the shade between multiple short periods of release ectomycorrhizal connections to other co-occurring caesalp species may enable a rachet-type mechanism. The recorded sapling dynamics currently contribute an essential part of the long-term cycling of the groves. M. bisulcata is an interesting example of an important group of tropical trees, particularly in Africa, which are both highly light-demanding when young yet capable also of forming very large forest emergents. To more comprehensively explain tropical tree responses, the case is made for adding a new dimension to the trade-off concept of early tree light-response versus adult longevity.

Do fungal pathogens drive density-dependent mortality in established seedlings of two dominant African rain forest trees?

Where one or a few tree species reach local high abundance, different ecological factors may variously facilitate or hinder their regeneration. Plant pathogens are thought to be one of those possible agents which drive intraspecific density-dependent mortality of tree seedlings in tropical forests. Experimental evidence for this is scarce, however. In an African rain forest at Korup, we manipulated the density of recently established seedlings (~5–8 wk old; low vs. high-density) of two dominant species of contrasting recruitment potential, and altered their exposure to pathogens using a broad-spectrum fungicide. Seedling mortality of the abundantly recruiting subcanopy tree Oubanguia alata was strongly density-dependent after 7 mo, yet fungicide-treated seedlings had slightly higher mortality than controls. By contrast, seedling mortality of the poorly recruiting large canopy-emergent tree Microberlinia bisulcata was unaffected by density or fungicide. Ectomycorrhizal colonization of M. bisulcata was not affected by density or fungicide either. For O. alata, adverse effects of fungicide on its vesicular arbuscular mycorrhizas may have offset any possible benefit of pathogen removal. We tentatively conclude that fungal pathogens are not a likely major cause of density dependence in O. alata, or of early post-establishment mortality in M. bisulcata. They do not explain the latter's currently very low recruitment rate at Korup.

Stable N isotope composition of nitrate reflects N transformations during the passage of water through a montane rain forest in Ecuador

Knowledge of the fate of deposited N in the possibly N-limited, highly biodiverse north Andean forests is important because of the possible effects of N inputs on plant performance and species composition. We analyzed concentrations and fluxes of NO3 −–N, NH4 +–N and dissolved organic N (DON) in rainfall, throughfall, litter leachate, mineral soil solutions (0.15–0.30 m depths) and stream water in a montane forest in Ecuador during four consecutive quarters and used the natural 15N abundance in NO3 − during the passage of rain water through the ecosystem and bulk δ15N values in soil to detect N transformations. Depletion of 15N in NO3 − and increased NO3 −–N fluxes during the passage through the canopy and the organic layer indicated nitrification in these compartments. During leaching from the organic layer to mineral soil and stream, NO3 − concentrations progressively decreased and were enriched in 15N but did not reach the δ15N values of solid phase organic matter (δ15N = 5.6–6.7‰). This suggested a combination of nitrification and denitrification in mineral soil. In the wettest quarter, the δ15N value of NO3 − in litter leachate was smaller (δ15N = −1.58‰) than in the other quarters (δ15N = −9.38 ± SE 0.46‰) probably because of reduced mineralization and associated fractionation against 15N. Nitrogen isotope fractionation of NO3 − between litter leachate and stream water was smaller in the wettest period than in the other periods probably because of a higher rate of denitrification and continuous dilution by isotopically lighter NO3 −–N from throughfall and nitrification in the organic layer during the wettest period. The stable N isotope composition of NO3 − gave valuable indications of N transformations during the passage of water through the forest ecosystem from rainfall to the stream.

Seed fate and seedling dynamics after masting in two African rain forest trees

How the effects of biotic factors are moderated by abiotic factors, and their consequences for species interactions, is generally understudied in ecology. A key abiotic feature of forests is regular canopy disturbances that create temporary patches, or “gaps,” of above-average light availability. Co-occurring in lowland primary forest of Korup National Park (Cameroon), Microberlinia bisulcata and Tetraberlinia bifoliolata are locally dominant, ectomycorrhizal trees whose seeds share predator guilds in masting years. Here, we experimentally tested the impact of small mammal predators upon seedling abundance, growth, and survivorship. In 2007, we added a fixed density of seeds of each species to exclosures at 48 gap–understory locations across 82.5 ha within a large Microberlinia grove, and at 15 locations outside it. For both species, small mammals removed more seeds in gaps than in understory, whereas this was reversed for seeds killed by invertebrates. Nonetheless, Microberlinia lost twice as many seeds to small mammals, and more to invertebrates in exclosures, than Tetraberlinia, which was more prone to a pathogenic white fungus. After six weeks, both species had greater seedling establishment in gaps than understory, and in exclosures outside compared to exclosures inside the grove. In the subsequent two-year period, seedling growth and survivorship peaked in exclosures in gaps, but Microberlinia had more seedlings' stems clipped by animals than Tetraberlinia, and more than twice the percentage of leaf area damaged. Whereas Microberlinia seedling performance in gaps was inferior to Tetraberlinia inside the grove, outside it Microberlinia had reduced leaf damage, grew taller, and had many more leaves than Tetraberlinia. No evidence was found for “apparent mutualism” in the understory as seedling establishment of both species increased away from (>25 m) large stems of either species, pointing to “apparent competition” instead. In gaps, Microberlinia seedling establishment was lower near Tetraberlinia than conspecific adults because of context-dependent small mammal satiation. Stage-matrix analysis suggested that protecting Microberlinia from small mammals could increase its population growth rate by 0.06. In the light of prior research we conclude that small mammals and canopy gaps play an important role in promoting species coexistence in this forest, and that their strong interaction contributes to Microberlinia's currently very poor regeneration.

Mast fruiting of large ectomycorrhizal African rain forest trees: importance of dry season intensity, and the resource limitation hypothesis

Mast fruiting is a distinctive reproductive trait in trees. This rain forest study, at a nutrient-poor site with a seasonal climate in tropical Africa, provides new insights into the causes of this mode of phenological patterning. •  At Korup, Cameroon, 150 trees of the large, ectomycorrhizal caesalp, Microberlinia bisulcata, were recorded almost monthly for leafing, flowering and fruiting during 1995–2000. The series was extended to 1988–2004 with less detailed data. Individual transitions in phenology were analysed. •  Masting occurred when the dry season before fruiting was drier, and the one before that was wetter, than average. Intervals between events were usually 2 or 3 yr. Masting was associated with early leaf exchange, followed by mass flowering, and was highly synchronous in the population. Trees at higher elevation showed more fruiting. Output declined between 1995 and 2000. •  Mast fruiting in M. bisulcata appears to be driven by climate variation and is regulated by internal tree processes. The resource-limitation hypothesis was supported. An ‘alternative bearing’ system seems to underlie masting. That ectomycorrhizal habit facilitates masting in trees is strongly implied.

Seedling survival and growth of three ectomycorrhizal caesalpiniaceous tree species in a Central African rain forest

Tree recruitment is determined in part by the survivorship and growth of seedlings. Two seedling cohorts of the three most abundant caesalpiniaceous species forming groves at Korup, Cameroon, were followed from 1995/1997 to 2002, to investigate why Microberlinia bisulcata, the most abundant species, currently has very few recruits compared with Tetraberlinia korupensis and T. bifoliolata. Numbers of seedlings dying, and the heights and leaf numbers of survivors, were recorded on 30 occasions. Survivorship after 2.5 y was 30% for M. bisulcata and 59% for the similar Tetraberlinia spp. together. After 7 y the corresponding values were 4 and 21%. Growth of all species was slow for the first 4 y; but survivors of T. korupensis became 63% taller, as the other species stagnated, by 7 y. The poor recruitment of M. bisulcata was the result of its very low seedling survival. Within species, the tallest seedlings of M. bisulcata and T. bifoliolata, but medium-height ones of T. korupensis, survived longest. This was likely due to higher root allocation in T. korupensis. Seedling dynamics of M. bisulcata and T. korupensis over 7 y accorded well with relative abundances of adult trees; T. bifoliolata is predicted to recruit later.

Buttress form of the central African rain forest tree Microberlinia bisulcata, and its possible role in nutrient acquisition

Buttressing is a trait special to tropical trees but explanations for its occurrence remain inconclusive. The two main hypotheses are that they provide structural support and/or promote nutrient acquisition. Studies of the first are common but the second has received much less attention. Architectural measurements were made on adult and juvenile trees of the ectomycorrhizal species Microberlinia bisulcata, in Korup (Cameroon). Buttressing on this species is highly distinctive with strong lateral extension of surface roots of the juveniles leading to a mature buttress system of a shallow spreading form on adults. This contrasts with more vertical buttresses, closer to the stem, found on many other tropical tree species. No clear relationship between main buttress and large branch distribution was found. Whilst this does not argue against the essential structural role of buttresses for these very large tropical trees, the form on M. bisulcata does suggest a likely second role, that of aiding nutrient acquisition. At the Korup site, with its deep sandy soils of very low phosphorus status, and where most nutrient cycling takes place in a thin surface layer of fine roots and mycorrhizas, it appears that buttress form could develop from soil-surface root exploration for nutrients by juvenile trees. It may accordingly allow M. bisulcata to attain the higher greater competitive ability, faster growth rate, and maximum tree size that it does compared with other co-occurring tree species. For sites across the tropics in general, the degree of shallowness and spatial extension of buttresses of the dominant species is hypothesized to increase with decreasing nutrient availability.

Plurality of tree species responses to drought perturbation in Bornean tropical rain forest

Drought perturbation driven by the El Niño Southern Oscillation (ENSO) is a principal stochastic variable determining the dynamics of lowland rain forest in S.E. Asia. Mortality, recruitment and stem growth rates at Danum in Sabah (Malaysian Borneo) were recorded in two 4-ha plots (trees ≥ 10 cm gbh) for two periods, 1986–1996 and 1996–2001. Mortality and growth were also recorded in a sample of subplots for small trees (10 to <50 cm gbh) in two sub-periods, 1996–1999 and 1999–2001. Dynamics variables were employed to build indices of drought response for each of the 34 most abundant plot-level species (22 at the subplot level), these being interval-weighted percentage changes between periods and sub-periods. A significant yet complex effect of the strong 1997/1998 drought at the forest community level was shown by randomization procedures followed by multiple hypothesis testing. Despite a general resistance of the forest to drought, large and significant differences in short-term responses were apparent for several species. Using a diagrammatic form of stability analysis, different species showed immediate or lagged effects, high or low degrees of resilience or even oscillatory dynamics. In the context of the local topographic gradient, species’ responses define the newly termed perturbation response niche. The largest responses, particularly for recruitment and growth, were among the small trees, many of which are members of understorey taxa. The results bring with them a novel approach to understanding community dynamics: the kaleidoscopic complexity of idiosyncratic responses to stochastic perturbations suggests that plurality, rather than neutrality, of responses may be essential to understanding these tropical forests. The basis to the various responses lies with the mechanisms of tree-soil water relations which are physiologically predictable: the timing and intensity of the next drought, however, is not. To date, environmental stochasticity has been insufficiently incorporated into models of tropical forest dynamics, a step that might considerably improve the reality of theories about these globally important ecosystems.