They create shade for the grass.
They make the food for the plant.
They make green pigment from chlorophyll.
They provide food for herbivores.
The sugar production is cut in half.
The tree has stunted growth.
The tree will "sleep" until it is added back.
Photosynthesis can't happen at all.
The leaves would freeze.
To stop transpiration.
There is not enough rain in the winter.
There is enough water in the ground in the winter.