Posted by Dan Gouvea on December 16, 2005

Q: Do evergreen plants in cold winter months continue to photosynthesize? If they do, how do they obtain the water to do so, since most of it is presumably in a solid (and unavailable) state?

Isn't the lack of available water the principal reason broad-leafed trees drop their leaves and metabolically shut down in cold winter months?

Thank you for your response.

A: Most conifers do indeed photosynthesize in the winter. However, subfreezing temperatures reduce this essentially to zero when it occurs. Conifers living in areas that have both freezing and thawing are able to photosynthesize during the times when the temperature climbs above freezing and the plant's roots have access to water. Conifers do transpire (lose water from their needles) in the winter and if this occurs too long and they're not able to absorb moisture from the ground, they'll "burn". Although the top layer of soil may be frozen solid, there is usually enough unfrozen soil deeper down so roots can obtain moisture to keep the plant hydrated. You may have seen pines that had been newly planted in the fall, brown up as the winter progresses until they're just an overall reddish brown - their reduced root mass couldn't keep pace with transpiration. Especially cold winters where the frost pushes deep into the ground can also cause this to occur in conifers that have been in the ground for a long time. It should also be pointed out that conifer needles go through a "hardening" process in the fall that prepares them for subfreezing temperatures so they aren't damaged. Deciduous trees have very delicate leaves that would be damaged in the winter if kept on the tree, so it isn't the lack of access to water but more of the quality of the leaf structure.

One of the big advantages for conifirs in keeping their needles all year is that in the spring as soon as it warms above freezing, these trees can begin making their own food - they don't have to wait for buds to open and grow.

John Wiessinger ( December 17, 2005)