Tropical rainforest gaps and tree species diversity

Annual Review of Ecology and Systematics 11/1987; 18:431-451. DOI: 10.1146/annurev.ecolsys.18.1.431
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    ABSTRACT: Gaps created 10 years ago by clearing of conifer plantations in Kibale National Park, Uganda, still experience very little tree regeneration. We studied the arrested succession by examining the survival of Neoboutonia macrocalyx Pax in different sized canopy gaps. We examined whether Neoboutonia seedlings can survive in gaps, which the tree rarely colonizes naturally and whether survival is limited by the size of the canopy opening, herbaceous climbers or insect folivory. A total of 384 seedlings were planted in 24 gaps, and survival was monitored for 19 months. Survival was dependent on canopy openness, being lowest in small gaps and highest in large gaps. We found a significant negative relationship between survival and folivory and a positive relationship between survival and climber infestation. Because 53% of the seedlings survived, regeneration could be limited by seed dispersal, rather than high seedling mortality. However, considering its high survival rate in arrested succession gaps, Neoboutonia seems to have good potential for forest restoration. Résumé Les trous créés il y a dix ans en éclaircissant des plantations de conifères dans le Parc National de Kibale, en Ouganda, ne connaissent que très peu de régénération d'arbres. Nous avons étudié la succession stoppée en examinant la survie de Neoboutonia macrocalyx Pax dans des trous de taille différente de la canopée. Nous avons voulu savoir si les jeunes plants de Neoboutonia peuvent survivre dans des clairières que l'arbre colonise rarement naturellement, et si leur survie est limitée par la taille de l'ouverture de la canopée, par des plantes herbacées grimpantes ou par des insectes folivores. Nous avons planté un total de 384 jeunes plants dans 24 trous, et nous avons suivi leur survie pendant 19 mois. La survie dépendait de l'ouverture dans la canopée, plus faible dans les petites ouvertures et plus élevée dans les plus grandes. Nous avons relevé une corrélation négative significative entre la survie et l'activité des insectes folivores et une relation positive entre la survie et l'envahissement de plantes grimpantes. Comme 53% des jeunes plants ont survécu, il est possible que la régénération soit limitée plutôt par la dispersion des semences que par une forte mortalité des jeunes plants. Cependant, vu son taux de survie élevé dans des trous de succession stoppée, Neoboutonia semble avoir un bon potentiel en matière de restauration forestière.
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    ABSTRACT: Understorey gaps are common in subtropical forests where pole-size trees are harvested for building materials. The creation of artificial understorey gaps potentially affects tree regeneration and the maintenance of forest tree diversity by locally increasing resource availability (light, nutrients, moisture). Understorey tree gaps had significantly lower foliage density in the 3-10 m stratum and higher soil moisture. However, fine root biomass and nutrient availability (N, P, K, C org) were not different to controls under intact understorey. Light intensity at the forest floor was a function of the age of the understorey gap, being greater in recent gaps but did not differ between older gaps and controls. The early regeneration community in gaps differed in composition from intact understorey. Understorey gaps promoted the regeneration of understorey species (density and species richness) probably by increasing community-wide seedling establishment of understorey species. However, understorey gaps had no effect on the early regeneration of canopy species. We argue that under low harvesting intensity (11.6 % of the available pole-size stems) understorey gaps have a neutral role in the long-term maintenance of understorey species diversity and no apparent role in the long-term maintenance of canopy tree species diversity. Our findings suggest that low levels of subsistence harvesting of understorey species will not detrimentally affect the overall species composition of scarp forest.
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    ABSTRACT: A life-history trade-off between low mortality in the dark and rapid growth in the light is one of the most widely accepted mechanisms underlying plant eco-logical strategies in tropical forests. Differences in plant functional traits are thought to underlie these distinct ecological strategies; however, very few stud-ies have shown relationships between functional traits and demographic rates within a functional group. We present 8 years of growth and mortality data from saplings of 15 species of Dipterocarpaceae planted into logged-over forest in Malaysian Borneo, and the relationships between these demographic rates and four key functional traits: wood density, specific leaf area (SLA), seed mass, and leaf C:N ratio. Species-specific differences in growth rates were separated from seedling size effects by fitting nonlinear mixed-effects models, to repeated measurements taken on individuals at multiple time points. Mortality data were analyzed using binary logistic regressions in a mixed-effects models framework. Growth increased and mortality decreased with increasing light availability. Spe-cies differed in both their growth and mortality rates, yet there was little evi-dence for a statistical interaction between species and light for either response. There was a positive relationship between growth rate and the predicted proba-bility of mortality regardless of light environment, suggesting that this relation-ship may be driven by a general trade-off between traits that maximize growth and traits that minimize mortality, rather than through differential species responses to light. Our results indicate that wood density is an important trait that indicates both the ability of species to grow and resistance to mortality, but no other trait was correlated with either growth or mortality. Therefore, the growth mortality trade-off among species of dipterocarp appears to be gen-eral in being independent of species crossovers in performance in different light environments. Introduction Light is generally accepted to be the most limiting resource in tropical rain forests (Whitmore and Brown 1996) and has long been hypothesized to be important in the maintenance of tree species diversity (Denslow 1987). Two mechanisms have been proposed to explain how dif-ferences in species-specific responses to light availability may maintain species diversity in tropical forests. First, species that grow well in one light environment have ª
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