Why do trees go dormant during winter?
From the tree's perspective, it's to protect it from freezing
and breaking. It redirects it's water and nutrients into the roots
so that they are protected from the cold and available in the
spring. It also reduces the water in the trunk and branches so that
when the freeze does occur the water doesn't expand in the wood and
burst the cells and break the branches.
From our perspective it's because of a combination of shorter
daylight hours and colder temperatures. The plants somehow sense
this occurring and make the necessary preperations for the above
Even evergreens go through a (mostly) dormant period where
growth will cease and only enough nutrients to maitain the tree
will be sent upwards.