Integrating genetic factors into management of tropical Asian production forests: A review of current knowledge

• Genetically viable tree populations are a precondition to sustained timber yields.
• Lower adult population densities increased inbreeding in most studied dipterocarps.
• Minimum population densities for logging may help avoid genetic erosion.
• Research should identify species prone to genetic erosion and ways to manage them.

Maintaining the genetic viability of timber species is especially important to sustaining the productivity and quality of timber in tropical production forests because the diversity of tree species is high, their population densities are low, and pollen dispersal of many species is limited to short distances. Conservation of the genetic diversity of timber species also enhances the conservation value of production forests and contributes to landscape connectivity. Tropical forests are commonly managed under selection logging regimes with minimum diameter cutting limits. These typically result in systematic removal of the largest and often the most fecund trees. Very little guidance is available to forest managers on the measures needed to maintain viable populations of timber species in production forests. We reviewed current knowledge regarding the impacts of logging on genetic processes in tropical Asian tree species to find foundations for developing practical guidelines for managing these impacts and to identify research needs. The review focuses on mixed dipterocarp forests, where the canopy layer is dominated by trees of the Dipterocarpaceae family, one of the most globally important timber families. We examined the impacts of practices including logging intensity, minimum diameter cutting limits and spatial patterns of tree removal with regards to biological parameters such as effective breeding units, size at reproductive maturity, fine-scale spatial genetic structure and the importance of general flowering in producing diverse seed. Research conducted to date shows that because of characteristics such as naturally low population densities, limited pollen dispersal and fine-scale spatial genetic structure, many tree species in mixed dipterocarp forests are sensitive to reductions in the density of reproductive conspecifics; in most cases tree extraction resulted in increased inbreeding. Although genetic research in these forests is still limited, enough knowledge exists to initiate the development of practical management guidelines e.g. on minimum population densities as a precondition for logging or documentation of sources of propagation material used in enrichment planting. Future research should focus on studying species characteristics which can indicate vulnerability to genetic erosion and the prevalence of these characteristics among species in mixed dipterocarp forests, to identify species or species groups which may require specific measures to maintain genetically viable populations in logged forests and in the larger landscape. Such characteristics may include large size at reproductive maturity or slow increase in fecundity with size, fine-scale spatial genetic structure, low population densities, limited pollen dispersal, or high juvenile mortality.