Establishment and heteroblasty of Acacia koa in canopy gaps

Abstract

Abstract Acacia spp. are important tropical forest trees but their commercial value may be limited by expression of poor apical dominance and extensive lateral branching. Multiple-aged and gap silviculture may harness the developmental plasticity of phyllodinous Acacia spp. to improve regeneration establishment, without sacrificing survival rates, by delaying heteroblasty (transition early in development from true leaves to modified petioles called phyllodes). However, successful development of this system is dependent on knowledge of seedling shade tolerance and ability to establish within canopy gaps. We planted Hawaiian Acacia koa (koa) beneath an existing restoration planting with a monospecific overstory canopy of koa on a dry site within koa’s natural range. Seedlings originated from a local putative drought-adapted population and a non-local mesic-adapted population, allowing evaluation of responses to a continuous light availability gradient in two populations. Leaf transition rates and the growth and survival impacts of rust fungus infection introduced from the existing canopy were also assessed. We detected significant and positive responses to light availability. Koa exhibited low shade tolerance and cannot sustain overtopping; both height and basal diameter growth increased exponentially with increasing canopy openness. Transition to high-light, drought-adapted mature foliage occurred more rapidly with increasing light availability. High developmental plasticity occurred in response to shading, but we did not find evidence of the adaptive value in that plasticity because of low survival rates and reduced growth in low-light microsites. We detected evidence for a growth-survival trade-off in which the non-local population grew faster overall, yet had reduced survival. Rust infection reduced survival and growth, limiting the possibility of converting degraded koa plantations into multiple-aged, gap silviculture systems. High rust infection rates and impacts on field performance found in this study reflect potential for pathogen-limited growth and survival in gap silviculture.