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. ...