Larix Lyallii | Derivations on a survival regime
The alpine tundra of the Pasayten Wilderness is characterized by small hummocks decorated with diminutive heaths and grasses with the much taller conifers surviving on only the fringes of this landscape.
Subalpine larch (Larix lyallii) has a range restricted to the North Cascades and Northern Rockies where they are locally common on exposed rocky areas as well as pioneers on disturbed sites and more recently in snowfields. They do not compete well with nearby conifer associates because they are shade intolerant. For this reason, over time they have been chased upward in an attempt to stay away from other cold loving conifers of the region like Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa). Life on the edge of the alpine tundra includes a deciduous disposition, which appears to be the antithesis of the cold-loving-conifer survival strategy. It was mysterious to me that this tree would evolve deciduous leaves in subalpine conditions. Few other plants here are deciduous and those that are are about 3 inches tall. During our week-long trek we spent 4 days at high elevations—giving me ample time to ponder this unique species.
What follows are my interpretations—anthropocentric and otherwise—as to why this survival regime is effective.
Subalpine larch on the edge of the alpine tundra.
1. Deciduous leaves
At any given place on the tree, the subalpine larch allocates needles—how long or how many—to optimize the energy balance based on availability of resources.
The thin needles must not require much energy to make but are surely efficient and feisty photosynthesizers during their short (<4 month) life.
If resources begin to run low, the tree can shut down needles for the season. This senescence allows resources, like water, to be conserved by limiting loss to transpiration. Resources are therefore sent to other regions to maximize net photosynthesis.
2. Time needle growth and pioneer disturbed sites
Notice the larches in the background are fully leafed out. The larches in the foreground are still in a snow drift and have just begun to leaf out. Though their growing season will be shorter these trees are able to pioneer snowfields in the mountains as they begin to grow with snow melt.
Rocky north-facing slopes are the habitat of choice for the subalpine larch.
Subalpine larch is also moving upslope (along with whitebark pine and Englemann spruce) onto the alpine tundra as the climate warms and snow melts faster—lengthening the growing season. Young trees also have specialized leaves that survive through one winter and a second summer2—possibly a neotenic trait carried from an ancient pine ancestor?
3. Winter snow will rarely weigh down and break branches
Because these conifers are deciduous they do not have to maintain the typical model conforming growth form other evergreen subalpine conifers do (see Massart's Model below). This "Christmas tree" form is common at high elevations because it is effective in sloughing heavy snow and limiting branch breakage as snow accumulates on needles. Subalpine larch have no needles in winter so I can only assume snow will collect in limited amounts—generally not enough to break branches. This is also an effective strategy during the growing season for a shade intolerant species. As seen in Ruah's model below, the larch is able to maintain a growth form more typical of a temperate oak or even a tropical species while in the subalpine—spreading and splaying branches to maximize net photosynthesis for this shade intolerant species.
Evergreens at high elevation begin life using a predictable growth form defined by Massart while temperate deciduous trees use the form defined by Ruah. Follow this link and click the first result to read more.
Another example of the freedom enjoyed by the larch (center) to grow as it pleases, leaning at a rakish angle out over a slope, and hence maximize photosynthesis. This tree has grown out and around the spruce and hence receives direct sunlight from sunrise to sunset (I watched this tree all day and without this growth form would have missed direct light for about the first 2 hours each day—being blocked by the spruce. An evergreen conifer at high elevation would risk toppling in winter at an angle like this.
4. Reiterations in response to damage
If a tree is damaged by snow or wind they respond with reiterated trunks as seen in this image—taken at treeline. This is a response one might expect in temperate regions, but not in the subalpine. Again, this is effective because subalpine larch do not have needles in the winter and do not risk branch breakage because of snow accumulation. (see Tomlinson image above)
5. Epicormic sprouting
Epicormic sprouting occurs from dormant buds embedded at the base of branches and allows new branches, and ultimately leaves, to be produced quickly after injury or theoretically in a year with abundant water.
More thoughts on the deciduous disposition from Northwest Trees1
- Since leaves are not exposed to desiccating wind and cold in the winter, like evergreen conifers, the moisture in needles does not have to be replaced by water in the soil—which often remains frozen and inaccessible into late spring for many other conifers.
- Buds on the larch protect shoots in a tough woody layer and these are not vulnerable to winter damage like evergreen conifer needles.
Jay Smith, Carl Arnatt and I will surely do some plant explorations again next summer...
1. Arno, Stephen., Hammerly, Ramona. 2010. Northwest Trees. Mountaineers Books, Seattle, Washington.
2. Tomlinson, P.B. 1983. Tree Architecture. American Scientist. 71:141-149