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NatureSpeak Articles

Why evergreen?

Other than overgrown logging roads and the odd patch here and there, our trees are pretty much the same: darkish green foliage, conical in shape and some brown bark. That the forest is mostly made up of evergreens is pretty self-evident. But what is it about evergreens that make them so pervasive? Glad you asked.

First, sorry to disillusion you, but the difference between deciduous and evergreen trees isn’t that one loses its leaves while the other one doesn’t. Oh sure, from a distance evergreens don’t seem to change much (except if they’re infested with parasites like mountain pine beetle or European rust fungus) but their leaves, needles, do fall regularly. Each lasts about two-three years. The difference is that as needles fall out, new ones take their place, kind of like the teeth of a shark. We don’t notice the loss, hence our perception of those trees as evergreen.

If you take a look at the forest floor, it’s got a good covering of needles. All those needles aren’t going to waste though. In some tree species, like pines, the needles are allelopathic (allelo=other), that is, they’re toxic to other plant species, thus pines create a biohazard zone around themselves to stave off competition for sunlight, water and nutrients. This also means pines are a poor choice for controlling erosion, as they’ll interfere with the establishment of other species. So don’t count on pines to help stabilize the bank over your driveway.

The main difference between our evergreens, or more accurately, conifers (from ‘cone’ bearing) and deciduous trees is in the details of the leaf structure. Conifer leaves are xeromorphic (xero=dry), that is, they are drought adapted while deciduous leaves are less so. I know you must be thinking that I’m off my head, this being the wet coast and all, but let me explain. In winter, water freezes, and when it freezes, two things happen: it stops flowing (equivalent to drought) and it expands. Both are lethal to plants that don’t have specialized structures to deal with those effects. Conifers have the structures that allow them to keep their leaves and function, while deciduous trees don’t. They shut down photosynthesis until spring.

The most important adaptations of conifer needles are surface coating, size and cellular response to cold. Each needle is coated with a layer of wax, which prevents water loss. In addition, stomata, cells that control the passage of air and water, are located in deep pits, providing even greater protection from dehydration.

The small size of needles prevents snow load. With a small surface area for snow to accumulate on, the tree doesn’t have to build elaborate structures to support each needle and thus the needles stay on the branch all winter. The conical, snow-shedding shape of the whole tree is part of this strategy as well.

This brings us to the last, clever bit of engineering. What’s the point of keeping small hydrated leaves on your branch if they’re frozen? Indeed. Well, they’re not frozen. Every tree species has a ‘killing temperature’, the temperature at which the trees’ internal structures develop ice crystals and cellular function stops. In sub-zero temperatures, conifer needle cells expel water and increase lipid concentrations inside. They ‘fatten up’. This lowers their killing temperature. Then, their water transport tubes kick in. When water freezes, it expands. This destroys the tubes of many plants. Not so for conifers. They have specialized valves along the tubes that can tolerate internal pressures of up to 900 psi. So, when a little sunlight hits a conifer in mid-winter, they’re ready to go and photosynthesis takes off. That’s the secret of conifer success.

Upcoming Events:

Christmas Bird Count – Dec. 14. Field partiers will set out at 8:30 a.m. and will wind up the day at 4 p.m.

Monthly Bird Walk – Saturday, Jan. 3, 2004. 8 a.m. Meet at the bottom of Lorimer Road at the Catholic church. Join Whistler experts in the monthly update of our feathered locals and migrants.

Written by: Karl Ricker


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