Enrichment planting of dipterocarps in logged-over secondary forests: effect of width, direction and maintenance method of planting line on selected Shorea species

Abstract

Natural dipterocarp seedling stock is usually very scarce or totally absent from heavily logged dipterocarp forests which have been subjected to shifting cultivation. Instead, a dense cover of secondary forest of low commercial or ecological value prevails. These logged-over low-volume forests can either be converted into plantations or restocked by local, valuable timber. The aim of this study was to obtain information on appropriate silvicultural methods for restocking such secondary forests with dipterocarps, with particular emphasis on the most commonly applied line planting method. We studied the main factors affecting line planting success, i.e. planting direction, line width and method of maintenance during a 2 year critical period of seedling establishment. The species used, Shorea johorensis, Shorea leprosula and Shorea parvifolia, are of the light red meranti timber group. This type of timber is the main source of veneer and plywood. The experiments were established within an active logging concession area, in a secondary forest previously subjected to shifting cultivation. Planting stock was produced utilising locally collected wildings which were hardened at a nursery before planting. The trials were arranged in a randomised complete block design with four replications. Seedlings were planted in a line opened up in the secondary forest in order to allow overhead light to reach the forest floor. Four line directions (N-S, E-W, SW-NE and SE-NW), four line widths (0, 1, 2 and 3 m) and four line maintenance methods (vertical, horizontal, vertical + horizontal and control) were applied. Survival, height and diameter increment were measured for a period of 2 years after planting and compared using analysis of variance (ANOVA) and the associated aposteriori comparison of means (LSD-test). Line direction seems to have little effect on survival or growth rate of the species studied. Regarding the growth rate, SE-NW was the best planting direction for S. johorensis. Line width did not affect the survival of the species. However, its effect on growth was very clear: growth depends heavily on the amount of overhead light, which in turn can be controlled by line width. The growth of S. johorensis and S. parvifolia benefitted from line maintenance. Horizontal maintenance is better than vertical. No effect on survival was found. We suggest Shorea species should not be planted under the canopy as this clearly retards growth. A 2-m-wide planting line is recommended. Sufficient overhead light should be maintained by means of horizontal tending. Vertical tending did not give a positive reaction. The three tested species reacted differently; differences between closely related species should be studied more in the future. Our results indicate good prospects for line planting of dipterocarps in secondary low-volume forests. With the observed annual volume increment of 8–17 m 3 ha −1 year −1 and current high prices of red meranti timber, planting of dipterocarps is much more profitable than the commonly applied practice to convert low-volume forests into plantations of fast-growing exotic tree species.