Well, the experts on tree growth will chime in here, but my unsophisticated answer, which has helped me understand tree growth somewhat, is that there are two things that influence the direction in which a tree grows--geotropism and phototropism. These are just two dollar words for simple ideas. The first is the tendancy a tree has to grow directly away from the earth's gravity, which means straight up. The second is the tendancy a tree has to grow towards light.

In general conifers tend to be more strongly geotropic--many will grow directly upward regardless of the light direction. Often they will grow up into the crown of a tree overhead even if it could find more room and light if its growth were deflected only slightly. In areas where there is a lot of ice and snow, this geotropic tendency helps trees survive by being well balanced against the added weight. In other areas, especially where there is competition from trees that develop broad crowns, this adaptation does not work so well.

Many hardwoods, especially some of the oaks, are strongly phototropic. If overhead light is not available, they will often grow towads the light at a fairly strong angle away from verticle. Eastern white oak is one of the best examples of this ability. But some other hardwoods, such as tuliptree, are more geotropic and will not grow towards the light so strongly.

--Spruce

"geotropism .... is the tendancy a tree has to grow directly away from the earth's gravity, which means straight up"

Bit of minor pedanticism . . . the lead shoots of trees of course exhibit negative geotropism ;-)
(i.e., away from gravity; geotropism is towards gravity)

Resin

Resin:

Yes, of course--I hate to admit it but that word in this context always bothered me. I remember reading somewhere about geotropism of plants and knew something was wrong, at least in regards to their growing upward. I am laughing. But then could we say that negative geotropism is in fact a kind of geotropism, if we can understand that to mean something that would include both positive and negative geotropism? My brain can be easily addled by such simple matters! And sometimes language logic is trumped by traditional usages, and in this context I am not familiar enough with traditional usages to know.

And then roots must be truly, or positively geotropic, although some may, more or less, be more strongly hydrotropic!

Well, I can say for sure that I am dendrotropic.

--Spruce

What luck that I just learned (or perhaps relearned) the word dendrology in one of my new tree books. And I can even add that you might be dendrophilic, as opposed to dendrotropic. But I am definitely enlightened on the subject of geotropism - thank you.

KarinL

I've seen many old Arborvitae growing away from buildings that were planted maybe 2 feet (60 cm) at the most from a house foundation. Growing with a curve away then up over decades. Makes them more prone to ice damage thou.

You can see all kinds of odd growths of conifers in old neighboorhoods that are poorer or around apartment bulidings. Nobody bothers to cut them down.

Not exactly in response to Karins initial thread-starter., but I think the champion among trees for phototropism is the box elder. These things are always swooping out from the edge of tree groves, able, as a maple, to arise in the shade of the trees along the edge, but then veering up and out to get some sunshine.

Perhaps in other zones there are species that exhibit this behavior similarly, but here in North-temperate regions, Acer negundo is the phototrope king.

+oM

"...growing against house walls in an apparently even fashion".
While it may be that conifers do not lean, they are nevertheless greatly effected when planted with one side in the shade. We see it all the time, poodles, pom-poms and D. Alberta Spruce planted up next to the house. While at the nursery, these plants were grown in full sun. When planted too close to the house the shaded foliage eventually dies out.
Sam

Noki, you're right that one does see different responses in the plants, which is why I asked whether I was observing correctly. Perhaps the plant's response to stimuli changes with age.

Sam, you're also correct that the foliage directly facing the wall is often dead. However, it's the live foliage immediately adjacent to that dead patch, if there is one, that fascinates me. It is sometimes growing up the wall as if it had suction feet.

KarinL