Later I found out: A drill is used to take an approximately ten-centimeter core sample from the base of a trunk. From counting the number of rings in this section, an estimate can be made of the age of the whole tree.
打聽以後才知道，林業試驗所的專家，用鑽子從樹幹隱蔽處鑽出大約十公分深的「切片」，算出這段長度的年輪數，再按比例估計整棵樹的半徑有多少年輪。