The number of rings on a stump indicates the number of years the tree lived. You’ll see rings of darker and lighter bands; 1 year of growth is composed of both a dark and light band. Since they’re easier to distinguish, count the dark bands to estimate the age. The rings can also tell you about the environmental conditions for a particular year. Thinner rings represent colder or dryer years, and thicker rings represent better growing conditions. If the rings are hard to make out, start by sanding them with coarse, 60-grit sandpaper. Finish with a very fine sandpaper, such as 400-grit. Spraying the surface lightly with water can also make the rings easier to see. You might find that some rings are too close together to see clearly. If necessary, use a magnifying glass to get a better view. Find the pith, or the small circle at the center of the concentric rings. Start counting from the first dark band around the pith. Continue counting until you’ve reached the bark. The last ring is pressed up against the bark and hard to see, so make sure to include it in your count. If you have trouble keeping track, try writing a number or making a mark every 10 rings with a pencil.

TL;DR in Vietnamese:
Kiểm tra các vòng tròn trên mặt cắt của gốc cây. Chà nhám gốc cây để nhìn các vòng cho rõ hơn. Đếm các vòng tròn từ lõi đến vỏ.