Optimum light transmittance for seed germination and early seedling recruitment of Pinus koraiensis: implications for natural regeneration

Light transmittance regulated by canopy openness influences the microsite conditions for natural regeneration. The successful transition from seed germination to subsequent seedling recruitment (i.e., early seedling survival and growth) determines the natural regeneration potential. However, there is little information on the effect of varying light transmittance on seed germination and seedling recruitment of Pinus koraiensis Siebold & Zucc. (Korean pine). We aimed to determine the optimum light requirements for this transition process in P. koraiensis to propose practical measures for improving its natural regeneration. The transition process was studied under five light transmittance regimes (100%, 60%, 30%, 15% and 5% of full light) over two consecutive years (2010 and 2011). The highest germination percentage in both years occurred at 30% light transmittance. Generally, mean germination time (MGT) declined with increased light transmittance. Seedling survival exhibited no significant differences between treatments for 1-year-old seedlings, but was higher at 30% than at 5% light transmittance for the 2-year-old seedlings. In contrast, seedling height, root collar diameter and total biomass were highest at 60%-100% light transmittance for both 1- and 2-year-old seedlings. Furthermore, the light transmittance also influenced the growth characteristics of P. koraiensis seedlings through regulating MGT. These results suggest that growth of P. koraiensis seedling requires a higher light transmittance (60-100%) than that required for seed germination, even though 30% light transmittance was favorable to the earlier emergence with larger specific leaf area. Silvicultural measures such as thinning are recommended to increase light irradiance in the forest understorey with the aim of improving the natural regeneration of P. koraiensis.