As trees grow they change their own climate

At polar treelines, the cooling of soil by tree canopies can cause a rise of permafrost which can kill the trees, causing a return of the open tundra thus warming of the ground with permafrost retreating and consequently new encroachment by trees ... and so on endlessly in centennial-long cycles. It is unknown whether such feedback by soil temperatures may also cause treeline vegetation to cycle between forest closure and forest opening.

1 - As seedlings emerge from the warm boundary layer near the ground they enter cool air and start shading their own root zone.