Trees go dormant in the winter, kind of like hibernating. In the yard of the home where I used to live, my black walnut trees took a long time to leaf out in the spring. People who didn't know that might think they were dead. But they just stayed dormant in the spring longer than most trees.
When deciduous trees (the ones with leaves) go dormant, their metabolism and energy consumption slow down, according to an excellent article by Eileen Campbell in Treehugger News. Until I read her article, I didn't realize there were two types of dormancy.
One is called endo-dormancy. In that kind of dormancy, the trees will not grow even when they experience good, warm, growing conditions. Something inside the trees keep them from growing.
The other is called eco-dormancy. That's when the days get shorter and weather gets colder, usually below the mid-40 degrees Fahrenheit, about the time trees start to lose their leaves.
Each tree produces a chemical called abscisic acid (ABA) at the tip of the stem where the stem and the leaf connect. That acid is also produced in coniferous trees, the ones that usually have needles and pine cones. The acid temporarily stops growth. It also prevents cells from dividing. ABA helps the trees to survive in winter by reducing the amount of energy they need to produce. But even in dormancy, evergreen trees don't usually lose their needles unless they are under stress or are getting older.
If people force a tree not to go into eco-dormancy by bringing it inside where it's warmer, that reduces the tree's lifespan. To remain healthy, trees need to go dormant for a while each year.
When I see trees with bare limbs after their leaves have fallen, I am glad they are slowing down and taking a rest in the winter. I enjoy a break from gardening and other summer activities too. I will be happy to see trees start to leaf out in the spring when it gets warmer outside. That's about the time I start to think about getting my garden ready to plant!