When the needle drops
By Claire Hafdahl
HTF Contributor
Winter presents many an inconvenience for us humans. The dry air makes us go through several tubes of lip balm. We break our backs shoveling heavy snow pile-ups. The low temperatures and wind make us put on our long johns. But imagine how winter would be if you couldn’t move; if you were rooted in place, exposed to the elements. How would you survive? Look no further than your back yard, and you may find an organism or two specialized for just those conditions: the conifers.
Possibly the largest challenge that trees need to overcome in winter is water loss. Sub-freezing temperatures make it so that trees cannot take up water; they need to conserve what little water they have. Leaves are the biggest source of water loss, so why not follow the lead of the deciduous trees, like oak and maple, and get rid of them? There is one deciduous conifer in Minnesota- the tamarack, or larch - which turns bright yellow right before their needles drop each fall. The others only lose about 1/3 of their needles each year, a much more reasonable replacement cost for trees with limited resources. It takes a lot of energy to produce leaves every spring. By hanging on to most of their needles over winter, conifers are able
to conserve energy that is then used for other functions, such as growth, reproduction, or defense.
Conifers have had to modify their leaves to deal with the dryness and cold. Most have either needles (pines and spruce), or dense, scaly leaves (cedars) instead of broad leaves. These modified leaves have fewer pores for gas exchange, called stomata, than a broad leaf, which reduces water loss. They also are covered in a waxy cuticle that acts essentially as the needles’ lip balm.
But hanging on to needles creates a lot of space for snow to land. In fact, one reason conifers are shaped in that Christmas-tree fashion to minimize snow pile-up. The angle that the branches protrude from the trunk can support heavy snow without breaking. Dry snow can weigh 5-7 pounds per cubic foot, so imagine supporting a 6-inch snowfall over a few hundred square feet ...we’re talking a few thousand pounds!
Conifers also undergo many changes at the cellular level, somewhat analogous to hibernation, that help them survive winter. For example, like in insects, the location of water in and between cells is manipulated
to prevent ice crystals from rupturing cell membranes. Because of these mechanisms, Minnesota forests contain trees with some of the lowest kill temperatures, or the temperature at which their cells freeze and die. Some examples include northern white cedar, red pine, balsam fir, quaking aspen, black spruce, and paper birch.
It’s hard to imagine being rooted to one place all winter…without long johns…where many conifers thrive. It is their suite of adaptations that we have to thank for the unique species composition of the forests we know and love.
Claire Hafdahl is a native of Virginia, MN, a St. Kate's graduate, and is currently working on her M.S. in biology at UMD.
