Early effects of four fast-growing tree species and their planting density on ground vegetation in Imperata grasslands

Ground vegetation development was studied under young plantations of Acacia mangium, A. crassicarpa, Gmelina arborea and Paraserianthes falcataria on an Imperata grassland site in South Kalimantan, Indonesia. The study was based on two separate trials: 1) a species trial with a spacing of 3 x 3 m, and 2) a spacing trial with progressively decreasing planting density. Both trials were established by use of intensive site preparation, NPK-fertilization, and weeding during the first year. Two years after planting and one year after the last weeding, ground vegetation in the first trial was dominated by Imperata cylindrica grass and shrubs Chromolaena odorata and Clibadium surinamense. In the open area (control) only Imperata grass appeared. Significant differences in total ground vegetation biomass (t ha(-)1) between the tree species were as follows: G. arborea (0.2) less than or equal to A. mangium (1.4) less than or equal to A. crassicarpa (4.2) < P. falcataria (9.6) less than or equal to open area (11.4). The early results from the spacing trial showed similar trends in the hardwood species' abilities in Imperata suppression. Highly significant linear relationships existed in regression analysis between the distance between planted trees and the ground vegetation biomass as follows: G. arborea (r(2) = 0.75), A. mangium (r(2) = 0.95), A. crassicarpa (r(2) = 0.97), P. falcataria (r(2) = 0.96). Tree species and their planting density should thus be critically considered in forest plantation establishment on Imperata grasslands, since ground vegetation development plays a decisive role in fire susceptibility, maintenance requirements and the promotion of native species in plantations.