*2.5. Resin Collection Season*

To determine the resin collection season, the phases of cell development in the outmost tree rings were observed under stereoscopic microscope. In the Republic of Korea, red pines growing at low altitude begin to form the annual ring in March and end between October and November [26,27]. In the temperate zone, the latewood formation in conifer trees begins in June/July [28]. Based on the past reports, the resin collection season was determined as follows: for those where only earlywood was observed in the outermost tree rings (Figure 3A), the resin collection was done between spring and early summer; for those where incomplete latewood formation was observed (Figure 3B), between late summer and autumn; and for those where complete latewood formation was observed (Figure 3C), between autumn of the current year and spring of the next year.

**Figure 3.** Stereoscopic observation of the outermost annual ring. (**A**) Earlywood (EW); (**B**) uncompleted latewood (UCLW); (**C**) completed latewood (CLW); PTR: previous annual ring.
