Musing on backyard trees....

So this is what winter looks like here in Lake County:

Blue Spruce (Picea pungens) and Silver Maple (Acer saccharinum), Lake County, IL, 1/6/14

Lots of snow, which makes it hard to hike anywhere. But even in this shot, you can see something interesting -- the tree in front is completely missing it's leaves, while the one in back is fully leafed out.

Now, you may be saying, well of course -- one's a deciduous tree, the other one's an evergreen. But... why should one be evergreen, and the other deciduous? Let's explore that for a moment...

As a tree, the obvious problem with losing your leaves is this: you can't photosynthesize without them, which means no food until you can grow new ones. (Yes, deciduous trees fast over the winter -- fortunately they don't need much food when the temps are this low.) So, by hanging onto its leaves (well, we call them needles, but they're just modified leaves), the spruce can feed all winter long, which should give it a noticeable advantage over the maple. Simple.

Except -- most of the eastern US is dominated by deciduous trees. So what gives?

The answer lies in a bit of chemistry - chemical reactions (including photosynthesis) slow down in the cold. (To be precise, the dark reactions do.) So, if the weather's going to be cold enough, keeping your leaves only provides an advantage in the spring, when the deciduous trees have to regrow theirs. That's still a fair bit of an advantage in the long run (remember, trees may live for several centuries), so what else is going on?

Turns out leaves, in order to capture Carbon Dioxide for photosynthesis, have to be open to losing water. In the winter, when water is frozen and therefore hard to get ahold of, this can be a problem, and in fact researchers have noticed winter drying of conifers on a number of occasions. (1,2) Trees that undergo this process will typically show reduced growth or even loss of parts of the tree, so it's not a trivial issue.

So, when should we expect to see an area dominated by evergreens? When the advantages of year-round leaves outweigh the disadvantages. So, when the growing season is short (so that re-growing your leaves takes too much of the season), or when there are significant, unpredictable warm periods during the winter. One of the neat things about living in Lake County is that we're right where this transition used to start, with White Oak, Shagbark Hickory, and Sugar Maple today, and a southbound extension of the Great Lakes White Pine forest formerly reaching us.

(1) Curry, John R.; Church, Thomas W. (1952). "Observations on Winter Drying of Conifers in the Adirondacks." Journal of Forestry,  50 (2): 114-116
 

(2) Herrick, Graham T.; Friedland, Andrew J. (1991). "Winter desiccation and injury of subalpine red spruce." Tree Physiol 8 (1): 23-36 doi:10.1093/treephys/8.1.23