The dots in the middle of the circles are the trunks. Next, every 5th year, the Trees compete for light. Neighboring Trees whose crowns are within 45 degrees of straight overhead are capable of reducing light input at this Tree's crown. Light is calculated at crown top. The Tree goes through all candidate neighbors and asks for their shadows at this Tree's height.
Shadows have shape and lightness.
The shape is crossed with the current Tree's crown area for overlap. Each Tree species has a parameter lightCoeff which says how much light filters through their crowns. A Hemlock casts a dark shadow, with only 6 percent of daylight eeking through. Sugar Maple and Yellow Birch allow 40 percent of daylight through. It's cast at high noon, at a solar angle of 26 degrees, which is an estimate of the growing season average effective sun angle at 42N lattitude.