Artificial canopy gaps and the establishment of planted dipterocarp seedlings in Macaranga spp. dominated secondary tropical rain forests of Sabah, Borneo

The continued losses of primary tropical rain forests have increased the pressure on secondary tropical rain forests and led to additional logging and changes to other land uses. A requirement for a secondary tropical forest to recover the main traits of old-growth forests is the regeneration of non-pioneer (climax) species. To accelerate the recovery of non-pioneer species where natural regeneration is insufficient, enrichment planting can be used in artificially created gaps or lines. The studies underlying this thesis evaluated several approaches to canopy gap creation in Macaranga spp. -dominated secondary tropical rain forests in Sabah, Borneo, and their effects on light conditions close to the forest floor and both survival and relative growth rates among under-planted seedlings of four dipterocarp (Dipterocarpaceae) species (Dipterocarpus applanatus, D. caudiferus, Shorea argentifolia and S. pauciflora; all non-pioneers). On average the total basal area of trees with diameters at breast height (DBH) >0.1 m in the Macaranga-dominated experimental sites was 35 m2 ha-1 and the mean number of stems of this size was 480 per ha. Three canopy treatments (selective girdling or selective felling of canopy trees and control (untreated)) were combined with two sub-canopy treatments (slashing smaller woody pioneer stems or untreated control) in a randomized split-plot block design. Hemispherical photographs (showing canopy openness) and recordings of above-canopy and forest floor photosynthetic photon flux densities (PPFD) were simultaneously taken in four of the seven blocks before the treatments and 0, 6, 18 and 30 months later. Seedling survival was registered every third month and seedling height every six months in all blocks, and a sample of seedlings was selected for destructive measurements (fresh and dry weight determination) both at the start of the experiments and at the final revision after 30 months. Analyses of variance (ANOVA) using general linear models (GLM) were used to evaluate treatment effects. The basal area ratio of canopy trees selected for girdling or felling averaged 31.2 % of the initial basal area. The results showed that the artificial gap creation, by means of canopy and sub-canopy treatments, had positive effects on both light conditions above the forest floor and the establishment of under-planted dipterocarp seedlings. The light intensities under the closed canopies and in the gaps after the treatments averaged ca. 2 % and 10-30% of above-canopy levels, respectively. Canopy openness on the forest floor means were ca. 10 % before and 10-15 % after the treatments. Sub-canopy slashing of pioneer saplings and smaller trees significantly increased light intensities, measured as both canopy openness and relative PPFD (PPFDR-values were 3 – 6 percent units higher than in the control) as well as seedling survival rates (for three out of the four species) and their relative height and biomass increments. These positive sub-canopy treatment effects persisted throughout the study period. Canopy treatments (felling or girdling of selected canopy trees) also had significantly positive effects on light conditions and seedling relative growth, but did not significantly affect seedling survival. Felling caused immediate, strong positive effects on light conditions and seedling height growth, but these effects gradually disappeared, while the effects of girdling were weaker but more persistent. After two years, no significant effects of canopy treatments on light conditions and seedling relative height growth were detected. Felling canopy trees and sub-canopy slashing resulted in the highest relative biomass increments during the study period. The survival rates averaged 73-86 %, after 30 months, for the four dipterocarp species. There were also significant between-species differences in seedling survival and growth rates.