The scent of snow fills the air, and the hissing plops of wet snow falling off the trees fills my ears. The woods, and its inhabitants, are transitioning between fall and winter. It’s the same for me, as I jog down the road in my summer hat and winter gloves, wishing for ski season to arrive.
Early fall was a time of vibrant colors and lots of action. Colors have faded a bit now. If you have lived in the north for a while, you may have come to appreciate the subtle gold of a tamarack swamp, or the rich browns in an grove of oaks as they extend the fall color season. But have you ever stopped to think about how weird those two trees are?
Tamaracks are conifers, bearing their seeds in cones just like their relatives the pines, spruces, and firs. But conifer isn’t our first choice for describing pines – we’d rather call them evergreens. When we do that, though, tamarack doesn’t fit. It is the only deciduous (losing its leaves seasonally) conifer in Wisconsin. Oaks, in contrast, are in a group known as broad-leaf trees, most of whom are deciduous. Yet oaks cling to their leaves.
Why would a tamarack lose its needles? Why would a pine keep its needles? And why does the oak keep its dead leaves?
There is adaptive value in each strategy, otherwise they would not persist. Needles are really just modified leaves, better suited to low nutrient, low moisture situations. They have basically the same parts as a maple leaf, but everything is more tightly packed and protected. The stomata (pores for gas exchange) hide in a groove, protected from dry winds. A waxy outer layer helps to prevent water loss. By retaining green, chlorophyll-filled leaves all year, evergreen trees can take advantage of any warm days to photosynthesize, and save themselves the trouble and nutrient expense of growing new leaves each spring. They replace only about a third of their needles per year.
On the other hand, broad-leafed deciduous trees, like maples, grow large leaves with a lot of surface area for photosynthesis. The broad leaves also result in a lot of water loss. This is fine when it is raining, but not when it is frozen. Although trees use enzymes to protect leaves from freezing while they are still photosynthesizing, that only works for so long. Then, frost-damaged leaves would be a liability as an entrance for disease.
Why would tamarack combine the two strategies and lose its needles?
Well, we don’t know for sure, but my favorite theory is that it has something to do with how far north the tamarack’s range extends. On the Winter Solstice this year, Duluth, MN, will only have 8 hours and 32 minutes of sun. In Fairbanks, Alaska, near the northern edge of the tamarack’s range, the sun will shine weakly for 3 hours and 42 minutes. Most of the tamarack’s habitat is in the middle of that range. What good are green needles if there is little sunshine? By building more delicate needles that don’t have to withstand harsh winter conditions, tamaracks can save a little energy.
Likewise, what good are the dead, brown leaves of an oak, even with sunshine? Oaks are a broad-leaf tree, but, oddly, they hang onto their leaves until heavy snow knocks them off, or until new leaves push them out. Most deciduous trees (including tamaracks) cut their leaves off by growing a protective abscission layer on the end of the twig, and then encouraging the leaf to skedaddle with digestive enzymes or a new layer of cells.
In contrast, oak leaves start to grow an abscission layer soon after new leaves form, but do not finish the process until the next spring. Scientists call this retention of dead stuff “marcescence.”
Plant physiologists agree that marcescence is a juvenile trait, associated with young trees and newer branches. This makes sense, since the young aspens in the field near my house are still holding onto their leaves. And understory trees, which tend to be younger, always seem to change colors later in the fall.
Marcescence also may be juvenile in terms of evolutionary history. In southern regions, some oaks are evergreen. Our northern oaks may be in transition from being fully evergreen to being fully deciduous. Maybe they are not done yet…or maybe they like where they’ve paused!
Although there are tasty new buds waiting to come out in the spring, this year’s dead, dry, crinkly oak leaves are not very palatable, and that may deter deer and moose from nibbling on the new growth. The tardily deciduous aspens probably gain that benefit, too.
Another hypothesis is that the oaks are saving their leaves until spring. When the leaves fall, they will provide the tree with nutrient-rich mulch for the growing season, instead of the leaves decomposing throughout the winter. The leaves dangling from lower branches may also act as a snow fence, trapping extra moisture for the tree.
Of course, there is no way for us to know for sure just what the oak is “thinking” as it rustles its skirt of leaves in the middle of a blizzard. Nor do we understand what the tamarack is “planning” when it turns golden, and then bares its knobby twigs for the winter.
As with humans, the weirdest organisms are often the most interesting. At least, that’s what I hope, since some of you probably think I’m weird to be wishing for ski season in October!
For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI, at 13470 County Highway M. The current exhibit, “Deer Camp: A Natural and Cultural History of White-tailed Deer,” opened in May 2013 and will remain open until April 2014.
Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com/.