Snow load

This winter has been pretty light on snowfall so far, and both last night’s snowfall as well as the previous snowfall last month came when the temperatures were hovering around 32ºF, which meant that the wet snow was sticky and eagerly clung to the branches of our trees. Unless it’s freezing rain, this isn’t much of a problem for our deciduous trees as the absence of leaves means that only a negligible amount of snow accumulates on the branches. This is partly because there just isn’t much tree for the snow to hold onto, but also because the snow doesn’t stay on the branches long. The open canopies allow wind to blow freely through the canopy and the branches create turbulence which quickly pulls snow free of the branches. But for conifers, snow load can be a significant problem in the winter.

A snow loaded hemlock with bent branches (Centennial Woods, Burlington)

The shape of conifers

The ability of conifers to tolerate tons of snow accumulating on their branches is largely due to their shape. If you were to ask a kid to draw a tree, they would likely draw one of two forms: the round, bushy (decurrent) crown of a maple or the spired conical (excurrent) shape of a conifer. In excurrent conifers, the lead shoot exerts strong apical control over the lateral branches. As the lead shoot grows up both against gravity and towards the sun, it sends the plant hormone auxin down to the lower branches and roots. In lateral branches, auxin slows growth and essentially overrides their vertical growth proclivities, forcing them to grow outward rather than upward. In roots, it stimulates growth, which helps feed nutrients and water back up to the lead shoot, giving it a competitive advantage over the lateral branches. When a lead shoot is damaged, the tree becomes split-trunked as the lateral branches begin to grow upward and compete for dominance (this is readily seen on white pines that have been parasitized by white pine weevils).

Lots of white pine weevil damaged stems of white pine in the Adder's Tongue (Centennial Woods)

The conifer advantage

There are two major advantages of this excurrent shape for trees growing in the north. Snow and rime can accumulate on the crowns of trees in the winter, weighing down branches and causing mechanical damage (mechanical breakage from snow load is thought to be the major limiting factor to timberline: source). While the long thin flexible needles of white pine readily shed snow (this becomes a real liability, however, during ice storms), the stiff, brush-like branches of open grown spruces and balsam fir tend to hold snow quite well (source). Snow load may help to insulate the tree from extreme cold temperatures, but too much snow and the tree risks losing branches.

Norway spruce after a light snowfall

After a particularly snowy winter (1993), researchers in Finland calculated the total weight of snow accumulation on Norway spruces along an elevation gradient. A 60’ tall tree at the top of the gradient (at an elevation 1,150’) was burdened by an astonishing 7,253 pounds of snow (source). And yes researchers actually weighed all this snow. A more useful measurement than total weight of the snow is the weight of snow relative to the height of the tree (typically in kg/m, but I’ll give as lb/ft), which is called packed snow. The aforementioned 60’ spruce had the highest packed snow at 125 lb/ft, while the low end of the range was closer to 35 lb/ft (which was in spruces that were around 15’ tall).

Norway spruce boughs laden with snow; look at that droop (Centennial Woods, Burlington)

The lateral branches of spruce and fir extend perpendicular to the trunk and bend gracefully under the snow’s weight, bowing down towards the ground to efficiently shed the heavy weight of the snow. Lower branches support upper branches under tension, and so even when coated in rime that isn’t as easily shed, this growth pattern reduces the likelihood of a branch snapping. Additionally, black spruce frequently reproduces by layering. Snow loaded branches can retain their bent shape even after the snow is shed. If the branch remains in contact with the soil long enough, it can develop adventitious roots and send up a new vertical lead shoot. The repeated bending of the branches under the weight of snow may actually facilitate this form of reproduction.

Heavy wet snow weighing down white pine branches (My Neighbors' Woods)

Conical shape helps absorb winter sunlight

The steep conical shape of many northern conifers also helps the tree capture light from the sun, especially in the winter at high latitudes when the sun is at a lower altitude (or elevation). In dense coniferous forests, the incoming sunlight that isn’t absorbed by the needles on a branch, ricochets off and hits an adjacent tree, which absorbs some of this reflected light and again reflects the rest. The light ping pongs back and forth until most of it is absorbed (much like how the pyramid-shaped foam of soundproofing panels trap soundwaves). This is less effective when the sun is right overhead, but quite effective at higher latitudes where the sun’s altitude is much lower (as you’ll see below, this is primarily an advantage in the spring and fall). So effective is this growth form at catching and absorbing light, that an area populated by conifers may be 5-10ºC warmer than adjacent barren patch. Note, however, that this growth form would have been significantly less of an advantage in the more southern, late-Pleistocene boreal forests than it is in today’s boreal forests (because the sun’s elevation is significantly higher throughout the year; compare the elevation angles in the table below). In contrast, the decurrent shape is more advantageous in cloudier areas where incoming light is more diffuse and scattered.

City Latitude Approx Elevation Angle of the Sun
Summer Solstice Winter Solstice
Miami (25.7617ºN) 85º 41º
Vermont (44.5588ºN) 69º 22º
Greenland (71.7069ºN) 52º
The conical shape of conifers reflects and absorbs more sunlight, particularly during the winter when the sun’s angle is at its lowest (Image from Peter Thomas’s Trees: Their natural history)
A boreal forest of mostly white spruce forest in Soldotna, Alaska

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