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.

Canopy gaps promote selective stem-cutting by small mammals of two dominant tree species in an African lowland forest: the importance of seedling chemistry

Small mammals can impede tree regeneration by injuring seedlings and saplings in several ways. One fatal way is by severing their stems, but apparently this type of predation is not well-studied in tropical rain forest. Here, we report on the incidence of 'stem-cutting' to new, wild seedlings of two locally dominant, canopy tree species monitored in 40 paired forest understorey and gap-habitat areas in Korup, Cameroon following a 2007 masting event. In gap areas, which are required for the upward growth and sapling recruitment of both species, 137 seedlings of the long-lived, light-demanding, fast-growing large tropical tree (Microberlinia bisulcata) were highly susceptible to stem-cutting (83% of deaths) - it killed 39% of all seedlings over a c. 2-y period. In stark contrast, seedlings of the more shade-tolerant, slower-growing tree species (Tetraberlinia bifoliolata) were hardly attacked (4.3%). In the understorey, however, stem-cutting was virtually absent. Across the gap areas, the incidence of stem-cutting of M. bisulcata seedlings showed significant spatial variation that could not be explained significantly by either canopy openness or Janzen-Connell type effects (proximity and basal area of conspecific adult trees). To examine physical and chemical traits that might explain the species difference to being cut, bark and wood tissues were collected from a separate sample of seedlings in gaps (i.e. not monitored for stem-cutting). These analyses suggested that, compared with T. bifoliolata, the lower stem density, higher Mg and K and fatty acid concentrations in bark, and fewer phenolic and terpene compounds in M. bisulcata seedlings made them more palatable and attractive to small-mammal predators, likely rodents. We conclude that selective stem-cutting is a potent countervailing force to the current local canopy dominance of the grove-forming M. bisulcata by limiting the recruitment and abundance of its saplings. Given the ubiquity of gaps and ground-dwelling rodents in pantropical forests, it would be surprising if this form of lethal browsing was restricted to Korup.

Transitionen im Jugend- und jungen Erwachsenenalter: Ergebnisse der Schweizer Längsschnittstudie TREE

Nach dem ersten, 2011 erschienenen Band versammelt der vorliegende zweite wiederum eine Auswahl von Beiträgen, die auf Analysen der Daten der Schweizer Längsschnittstudie TREE (Transitionen von der Erstausbildung ins Erwerbsleben) basieren. Die kritischen Übergänge im Jugend- und jungen Erwachsenenalter werden aus soziologischen, ökonomischen, psychologischen und erziehungswissenschaftlichen Blickwinkeln beleuchtet. Die Beiträge widerspiegeln damit eindrücklich das analytische und interdisziplinäre Potenzial der TREE-Daten. Thematisch steht der langfristige Einfluss der sozialen Herkunft auf Bildungs- und Erwerbsverläufe, insbesondere auf den Zugang zu höherer Bildung, im Zentrum.

Light and seed size affect establishment of grove-forming ectomycorrhizal rain forest tree species

• Regeneration of the dominant ectomycorrhizal tree Microberlinia bisulcata in groves in Korup, Central Africa, is very poor. The hypothesis was tested that this species is more shade intolerant than other co-occurring species. • In two 1-yr trials, each with M. bisulcata and four other species at a nursery close to Korup, growth was measured under five PAR levels, with ± added P and ± watering in the dry season. In parallel experiments the effects of PAR with two R : FR ratios were investigated. • Increasing PAR had a consistent effect on the rates of increase in plant mass and on changes in the other variables. Doubling soil P, watering and halving the R : FR ratio had almost no effect. However, across species, mass at low PAR and relative growth rate related positively and negatively, respectively, to seed mass. • One contributing factor for the poor recruitment of M. bisulcata is therefore its low survival and slow growth at low PAR, due to its small seed size. The two codominant ectomycorrhizal grove species of Tetraberlinia, with larger seeds, were less affected by low PAR.

Neighbourhood abundance and small-tree survival in a lowland Bornean rainforest

Conspecific effects of neighbours on small-tree survival may have a role in tree population dynamics and community composition of tropical forests. This notion was tested with data from two 4-ha plots in lowland forest at Danum, Sabah (Borneo), for a 21-year interval (censuses at 1986, 1996, 2001, 2007). Species with ≥45 focal trees 10 to <100 cm stem girth per plot in 1986 were selected. Logistic regressions fitted mean focal tree size and mean inverse-distance-weighted basal area abundance of neighbours (within 20 m), for the periods over which each focus tree was alive. Coefficients of variation of neighbourhood basal area abundance, both spatially and temporally, quantified the changing environment of each focus tree. Fits were critically and individually evaluated, with corrections for spatial autocorrelation. Conspecific effects at Danum was generally very weak or non-existent: species’ mortality rates varied also across plots. The main reasons appear to be that (1) species were not dense enough to interact despite frequent although weak spatial aggregation, and their neighbourhoods were highly differing in species composition; and (2) these neighbourhoods were highly variable temporally, meaning that focus trees experienced stochastically fluctuating neighbourhood environments. Only one species, Dimorphocalyx muricatus, showed strong conspecific effects (varying between plots) which can be explained by its distinct ecology. This understorey species is highly aggregated on ridges and is drought-tolerant. That this functionally and habitat-specialized species, has implied intraspecific density-dependent feedback in its dynamics is a remarkable indication of the overall processes maintaining stability of the Danum forest.