Population dynamics and spatial patterns of Dipterocarp seedlings in a tropical rain forest

Abstract

Population dynamics and spatial pattern of dipterocarp seedlings were investigated in lowland dipterocarp forest in Danum Valley Conservation Area, Sabah, East Malaysia. Seedlings (<10 cm gbh) were enumerated in two areas (2.0 and 0.48 ha) within the tree enumeration plots established by the University of Stirling project, and surveyed over 22 months.

Seedlings of the major canopy and emergent dipterocarps in the area were common: Shorea johorensis (Red Meranti); S. argentifolia, S. leprosula, S. parvifolia (Light Red Merantis, LRM) and Parashorea maleanonan, all light demanding species; S. fallax,S. pauciflora (Dark Red Meranti, DRM), more shade tolerant emergent species; and Hopea nervosa, Vatica dulitensis and V. sarawakensis, shade tolerant canopy species. Total seedling densities were 2000-2500 ha-1.

Mortality rates varied from 0 to 16% yr-1 in different species, and were highest in the LRMs and lowest in the canopy species. Temporal and spatial variation in mortality rates was greatest in the LRMs. In both plots, seedling mortality rates were significantly positively correlated with basal area of conspecific trees ≥10cm gbh.

Net growth rates were very variable, even within size classes in the same species. Median growth rates were highest in the LRMs and lowest in canopy species. Frequency distributions of growth rates were strongly leptokurtic in slow-growing species, with most seedlings having growth rates around zero, although individual seedlings could produce large increments. In fast-growing species, more seedlings achieved high growth rates. A significant proportion of seedlings suffered height loss due to falling debris, and almost half the seedlings showed evidence of previous stem damage. Large growth increments were recorded in most species in response to canopy openings, usually very small gaps caused by branch falls. Individual increments exceeded 1 m yr-1 in seven species.

Growth and mortality rates were significantly positively correlated across species in Plot 1, but not in Plot 2.

Seedling spatial patterns were examined in eleven species in Plot 1, and covered a wide range of degrees of aggregation. Seedlings of the light-demanding emergent species were we1l-distributed throughout the study area, though some showed aggregation at a small scale. DRM seedlings were strongly clumped around adult trees. Seedlings of two of the canopy species were very strongly aggregated around adult trees, while the third species, V. sarawakensis, had randomly distributed seedlings.