Nature's Clocks: HOW Do Trees Sense The Coming of Spring?

[ralph124c]

I went for a walk in the woods yesterday, and I was amazed to see the surface of the snow littered with the sheaths of tree buds (oak, birch, butternut,elm, etc.). Obviously,the buds are beginning to swell, and the trees are preparing to set their leaves for the approach of spring. My question: how do the trees sense the coming of spring? Are there light receptors of some kind that sense the lengthening of the daylight hours? Or,is there some internal clock, which counts the days and activates the growth of the buds at some point?
It's hard to see what could be going on..it has been sub-freezing here for the past 3 weeks..so there can't be much fluid movemnt within the bark, since it has been so cold.
Nature is wonderful..does anybody knowhow trees do this?

[An Arky]

... Are there light receptors of some kind that sense the lengthening of the daylight hours? Nature is wonderful..does anybody knowhow trees do this?

Well, I know for sure that under normal conditions, the trees leaf turning/dropping is triggered by shortening days, so I believe that the buds and leaves emergence would be triggered by the lengthening days.