concept

Source–sink dynamics

The principle that material tends to flow from where it is abundant (a source) toward where it is scarce (a sink), down a concentration gradient. In a forest, a sunlit tree photosynthesising hard is a carbon source; a shaded, starving seedling is a sink. When two trees share a fungal network, carbon can flow net from source to sink - exactly what Simard's isotope experiment measured. This is the physics of a gradient, not kindness: the flow follows the slope, not an intention.