The effects of mother trees and site conditions on the distribution of natural regeneration establishment in a Bornean rainforest disturbed by logging and fire

Abstract: In the tropics, logging and wild fire can result in degraded secondary forests with lower biodiversity than in the natural forest. One way to limit forest degradation is to rehabilitate the present large areas of secondary forests. However, there is a lack of knowledge concerning the ecology of tropical tree species and further studies would be advantageous for future rehabilitation efforts of degraded rainforest. The objective of this study was to investigate how mother trees in secondary forest and different site factors affect the abundance and spatial distribution of natural regeneration of non-pioneer tree species in a tropical rainforest in Sabah, Malaysia. The study took place in the INIKEA Forest Rehabilitation Project area, which suffered from wild fire in 1983 and has been selectively logged over both before and after the fire. Natural regeneration and site variables were inventoried in plots, while mother trees were inventoried in the whole survey area. Species were divided into groups used in the INIKEA enrichment planting; Dipterocarps, Non-Dipterocarps and Fruit trees while remaining species were set as Other timber. ArcGIS was used for positioning natural regeneration and mother trees while the Spearman Rank Correlation Coefficient was used for the statistical analysis. All groups analyzed had a positive correlation between the number of naturally regenerated plants per hectare and the number of mother trees per hectare. The species group Dipterocarps had a generally lower regeneration capability than Other timber. Shorea generally had a lower regeneration capability compared to other Dipterocarp genera analyzed in the study. Dryobalanops spp. and Dryobalanops keithii Sym had fewer naturally regenerated plants on sites which had a lot of natural regeneration from other species. D. keithii also had more natural regeneration located to creeks or areas were water flow during heavy rains than on dryer locations, indicating adaptation for more open areas or disturbed locations. Parashorea tomentella (Sym) Meijer on the other hand had more natural regeneration located to sites which had a lot of natural regeneration from other species. This may indicate that P. tomentella has more shade tolerant properties. Dryobalanops indicated longer dispersal ability for natural regeneration than earlier studies have shown. Parashorea spp. and Shorea spp. showed short dispersal abilities for natural regeneration, a pattern in conformity with earlier studies. An exception was P. tomentella, showing dispersal ability up to 75 m. Comparing seed weights and dispersal abilities indicated that shorter dispersal capability is not due to heavier seeds. An indication that the regeneration process in Sabah´s rainforests are worthy of further studies is that in an area of nearly 20 ha 119 species were recorded, which had various regeneration success. Of these, 47 species were found only as mother trees which probably had not regenerated after the wild fire of 1983 and 21 species that had established without any mother trees found within the area of inventory. The result of the present study has contributed to better understanding of the regeneration dynamics of tropical tree species and this new knowledge can be used for improvement of rehabilitation methods.