Comparative morphology and physiology of gap and understorey seedling leaves from a mixed dipterocarp forest

Abstract

Leaf morphology and rates of stomatal conductance, net photosynthesis and dark respiration were determined for seedlings associated with a gap-understorey situation in the mixed dipterocarp rainforest of the Batu Apoi Forest Reserve in the Temburong District of Brunei Darussalam. The species investigated included four emergent tree species, Shorea falciferoides Foxw., Shorea leprosula Miq., Parashorea macrophylla Wyatt-Sm. ex Ashton, and Dryobalanops lanceolata Burck., two canopy species Aporosa lunata (Miq.) Kurz and Drypetes sp. and two pioneer species, Macaranga trachyphylla Airy Shaw and Musa sp. The pioneer species were confined to the gap while all other species were present in both the gap and understorey. Comparative morphological studies indicated that leaves of understorey seedlings are thinner, have a lower stomatal density and higher specific leaf area than leaves of the same species growing in the gap. Stomatal conductances for leaves of gap seedlings fall into two categories; high rates for the pioneers and more conservative rates for the non-pioneers. In the understorey low photosynthetically active photon flux density values are associated with relatively high stomatal conductance. Rates of light saturated net photosynthesis were significantly higher for pioneer species than non-pioneers. Based on apparent quantum yields, understorey seedlings tended to be photosynthetically more efficient than gap plants; also, light compensation points and rates of dark respiration were lower for understorey plants than for those of the same species growing in the gap. These features may be important adaptations which allow seedlings to establish in the very low light of the understorey.