I have lived in Minnesota all my life and I never thought about it before. Why don't evergreen trees or conifers don't drop their leaves like other trees? Remember a needle is really a modified type of leaf. Actually it is a different way for evergreens trees to survive a very cold winter. It also explains why evergreen trees can grow on mountains at high elevations with very cold temperatures when deciduous trees cannot. Really, evergreen trees are more energy efficient!

It is all about the energy balance and return on investment in leaves. It takes a lot of energy for deciduous trees to develop leaves every year. The first part of the summer the trees are trying to rebuild the energy reserves that were depleted because of the leaf development. Evergreen trees have their needles or leaves for many years!

When fall approaches deciduous trees move the chlorophyll from the leaves into the trunk to store it for the next spring. When the green chlorophyll is moved out of the leaf you see the "fall colors." The chlorophyll stays in the evergreen needles thus they are green all winter long. On cold winter days evergreen trees preform on more photosynthesis than the trees that have dropped their leaves.

When warmer weather returns in the spring evergreen trees do begin photosynthesis much sooner than trees that are still developing leaves. Another interesting fact about evergreen trees is they make their own antifreeze, called cryoprotectants that prevents the cells from freezing without interfering with metabolism. I now have more appreciation for evergreen trees!