Competition in a Mixed-Species Planting with Four Contrasting Tree Species

Mixed-species systems are well-suited to smallholder and community forestry, but reliable evidence regarding and procedures to assess species performance in such systems is scarce. This field trial uses a pair of clinal plots with varying spacing and species composition to gain insights into competition between four species proposed for landscape rehabilitation in the Philippines. We examine the efficiency of this compact clinal trial in revealing growth traits of four species (Paraserianthes falcataria, Pterocarpus indicus, Shorea palosapis, Swietenia macrophylla) in mixed plantings. The use of a size-distance competition index allowed an assessment of the competitive and collaborative influences between four tree species. Within the expected general trend for diameter increment to decline with increasing competition, there were indications that Shorea palosapis is a benign competitor that may stimulate the growth of neighbouring individuals. Paraserianthes falcataria is a strong competitor that is also strongly impacted by competition, especially through antagonistic intraspecific competition. Paraserianthes falcataria appears well suited as a solitary tree in a field or village, whereas Shorea palosapis seems ideal for plantations, in both monoculture and mixed plantings. The clinal spacing and mixture trial, when examined using a size-distance competition index based on diameter increment, revealed useful insights into species performance. Pterocarpus indicus exhibits strong intraspecific, but low inter-specific competition, so appears well-suited for polyculture plantings. Of the four species trialled, Swietenia macrophylla appears to be best-suited as a monoculture species as it exhibits the lowest intraspecific competition.