The photosynthetic "machinery" inside each leaf is becoming established for the season. That machinery resides in the chloroplasts, bodies that are descended from once free-living bacteria that live in every photosynthetic plant cell. The cells of plants were invaded by these bacteria eons ago, but in a sense they re-infect the plant (and the forest) with the beginning of each photosynthetic season.
So in a way when we observe a mountainside with its steadily greening trees we are seeing a wave of bacteria (beneficial ones) spreading across the population. Photosynthesis is a partnership between the leaf and the chloroplasts, between the host plant and its endosymbiotic bacteria. It's the spread of these bacteria, the chloroplasts, as they invade in a wave over the weeks of greening, that transform a northern mountainside into a shimmering green solar collector.