The vegetative propagation & early development of dipterocarp cuttings

Abstract

Many of the lowland rainforests of SE Asia have been degraded by logging and shifting cultivation, are lacking in natural recruitment (particularly among the dipterocarps) and are in critical need of rehabilitation. Enrichment planting is an established method of rainforest rehabilitation but this depends upon a reliable supply of dipterocarp seedlings. However, due to their habit of mass flowering, the supply of dipterocarp seed, and hence planting material, is sporadic. It is therefore of critical importance that alternative methods for the large-scale production of dipterocarps are developed. Vegetative propagation by cuttings would in theory be ideal but few rehabilitation projects are propagating dipterocarps by this method. The reasons for this are two-fold: questions remain at the propagation phase (particularly the influence of light and applied hormones on rooting) and, more importantly, there is almost no evidence to indicate how dipterocarp cuttings develop after planting. The role of light and plant growth regulators on rooting in cuttings of Dryobalanops lanceolata, Parashorea malaanonan and Shorea leprosula was investigated. The responses of these species to the level of irradiance were plastic and there were no effects on cutting survival, rooting percentage or root development. Previous research on the use of plant growth regulators to promote rooting in dipterocarp cuttings has been inconclusive. Several concentrations of indole-butyric acid (IBA) were applied to cuttings of the same species for various durations. High concentration IBA combined with long exposure duration treatments resulted in high cutting mortality. Application of IBA did not significantly improve either root initiation or subsequent development. Cuttings showed higher mortality than seedlings up to 20 months after planting though, for both plant types, survival was similar to that reported in previous research on enrichment-planted and naturally-recruited dipterocarp seedlings. Relative growth rates were higher in cuttings than seedlings. Cuttings had a lower root:shoot ratio at planting, and lower above- and below-ground biomass, but after 20 months these values had converged towards those of seedlings. After eight years cuttings and seedlings of D. lanceolata had similar above- and below-ground biomass. Cuttings tended to have a higher root:shoot ratio but there were no differences in rooting depth or root distribution down the soil profile. Cuttings produced a ‘pseudo-taproot’ of similar form and extent to the taproot produced by seedlings. In conclusion, the propagation of dipterocarps by cuttings could provide a viable alternative for the large-scale production of planting material. Cuttings showed similar development to seedlings after planting and it can be reported with some confidence that the root systems of dipterocarp cuttings would likely be capable of supporting the tree to maturity.