The researchers studied pine trees in Poland to see how the size of their leafy parts relates to their growth. They found that the bigger the leafy parts, the more the tree grows. This means that by measuring the leafy parts, we can predict how fast the tree will grow. Specifically, the size of the leafy parts is strongly linked to how much the tree's diameter and area increase over time. This suggests that measuring the tree's diameter can help us understand how well it's using sunlight to grow.