Why Is the Treeline at a Higher Elevation in the Tetons than in the White Mountains?

Why Is the Treeline at a Higher Elevation in the Tetons than in the White Mountains? Image

Photo by Thomas Jones
Treeline in the East is lower than out West

Trees grow where they can. And they are surpassingly capable, making do in an impressive array of conditions around the globe, including many harsh places. But there are limits to their versatility. There are some natural environments where trees simply do not grow, thrive, or even survive, such as near the tops of the bigger mountains.

As you hike up a big one, you can see many gradual changes in the composition of the forest. Eventually the diversity of species plummets, the trees are much shorter and more scattered, and at last they look more like shrubs. Then, at some elevational limit (in any mountain locale), the trees stop. Fact is, most don’t even start. Above here, even if a Bicknell’s thrush dropped a viable mountain ash seed, it would have a slim chance of survival, much less germination. This is the alpine elevational treeline, although it is very rarely a distinct line.

Treeline is more commonly an uneven zone of transition from scattered shrubby trees to treeless areas with but a few specialized – and very tough – herbs and a hardy crust of lichens. The transition occurs unevenly because there may be sheltered and sunnier local conditions – various crevices, coves, and crannies – at higher elevations where trees can still make it, thus extending the upper limit of the forest here and there. Indeed, topography, soils, precipitation, and exposure to sun, wind, snow, and ice all play important roles in determining which trees grow where and how well. But temperature generally decreases with increasing elevation (and latitude), and many ecologists suggest that low temperatures – which limit growth in general – seem to be the single most critical factor in determining the upper elevational (and latitudinal) limit of trees.

Alpine (and arctic) environments are marked by seriously harsh winters. And while trees there do take a beating that severely limits their growth, some species have made amazing adaptations to withstand such winters. But to do so, they need a sufficiently long summer growing season – free of snow on the ground and with adequate temperatures – to earn a living that they spend all winter long. The problem at higher elevations (and latitudes) is that the growing season is not only colder but also shorter. It starts later at higher elevations because spring bud break is so closely tied to the temperature of the soil, which takes longer there to warm up.

The elevational limit of such suitable summer conditions varies by latitude. In Mexico, for example, treeline occurs somewhere around 13,000 feet, whereas farther north, in the Tetons, for instance, it occurs lower, at approximately 10,000 feet. Again, it’s a ragged line that may vary by hundreds of feet on any mountain, depending largely on shelter and exposure.

But how then to explain the elevation of treeline in the White Mountains of New Hampshire? Mount Washington sits at a latitude comparable to the Tetons, but its treeline is at roughly 4,500 feet. That’s less than half the elevation. Is it because the Presidential Range has frighteningly bad winters that blast needles right off trees with screaming bits
of ice? Sure, wind plays a part in limiting tree growth, by desiccating needles, scouring away thin soils, and directly damaging limbs and twigs. But winter is brutally hard in the Tetons, too, and nearly as windy.

The difference, instead, may have more to do with summer conditions, when trees have their only chance to germinate, grow, and reproduce. Because the Tetons are inland – and are so much more massive – their summers are warmer and drier than in the White Mountains, which are closer to the coast and therefore have cooler and cloudier summers. On Mt. Washington, in fact, there has never been a temperature recorded higher than 70 degrees.

Because the elevational treeline is so closely tied to temperature, many suggest that it could be a particularly sensitive indicator of global climate change. Presumably, rising temperatures would increase the elevation of treeline in any locale, altering forest distribution and potentially ousting rare plant communities – and their inhabitants – that now exist above treeline. Although the specific physiological mechanism of treeline formation is not fully understood, there is growing photographic and other evidence of upward shifts in treelines worldwide.

Michael Snyder is the Chittenden (Vermont) County Forester.

 
Discussion
  1. Kimmie → in Burlington, VT
    Nov 09, 2008

    Why does it take longer at treeline for the soil to warm up? Is it a difference in the soil? Soil moisture? Snowpack? Air temperature?

  2. Dave → in Northern Woodlands, Corinth, VT
    Nov 20, 2008

    I’d guess all of those things, Kimmie. Or at least 3 of 4. Snowpack certainly would have an effect (more snow means longer snow means colder soil). Cooler ambient air temperature certainly contributes. I’d buy the thinner soil argument, based on the fact there’s not as much insulating detritus to stave off hard frost. I’m not sure about moisture, though.

  3. Jeff Taylor
    Dec 14, 2008

    His explanation about is not correct. The reasons for the summers being moister in New Hampshire than Wyoming have nothing to do with proximity to the coast, since summers are also drier on the Pacific Northwest coast and every place else in the west. I could go into the climatology, but I’ll focus on treeline. The answer is which elevation results in a minimum winter temperature of -40C. That is the temperature that causes ice to form in plant cells and kills trees. Because of the arctic fronts that move down from Canada, those temperatures occur more in the Northeast than the west. It takes one rising to 10,000 feet in Wyoming to routinely achieve those temperatures.

    By the way, treeline is only 5,000 feet in mild Washington, where temperatures rarely drop below 0F even in the mountains. Why? Huge deep snowpacks that bury trees till July.

  4. Robert → in Waterville, VT
    Oct 13, 2009

    Jeff- I’m not sure that’s correct either. I grew up in interior Alaska where temps went well below -40C and treeline is higher than along the coast which has a more moderate temperature. My guess would be it’s a combo of minimum temp, summer temp, moisture and wind that determines elevation of treeline.

  5. Sean → in Orono, ME
    Jun 09, 2010

    And the answer is…large scale atmospheric circulation. The westerly winds coming of the Pacific Ocean are deflected over and around the western mountains forming a mean-annual standing wave across North America. Although the overall configuration of the wave pattern changes somewhat with the seasons, it is generally characterized by a ridge of warm air over the west, and a trough of cold air over the east. Treeline altitude is mostly dependent upon summer conditions (it occurs over a zone where average summer temperature is near 5 degrees C).* In summer, the western U.S. lies directly below persistent subtropical high pressure associated with the descending limb of the equatorial Hadley circulation.  This affords hot, dry air masses. If the land areas of the interior west were near sea level, summer temperatures would range from 30 to 40 degrees C; but given the high mountains and plateaus, temperatures are more typically between 20-30 degrees C. The northeast sees only a few bouts of subtropical highs each summer (also part of the Hadley circulation, but humid due to moisture from the Gulf of Mexico and the North Atlantic Ocean), with the circulation instead being dominated by subpolar flow driven by a low pressure often forming over Quebec (associated with the polar frontal zone; the low pressure drives the blessed northwest wind bringing cool, dry, clean air from Canada). Summer temperatures near sea level in the northeast are on average around 20 degrees C. Ten to twenty degrees C cooler than expect near sea level temperatures for Wyoming and Colorado…  And THAT is the fundamental reason why treeline is so low in the east relative to in the west.

    * Note that treeline for most tree species is not dependent upon winter cold. Proof is abundant: The boreal forest biome extends into inland Alaska and *Siberia*. These places experience brutally cold winters, but mild (or even hot!) summers.

  6. Charles Burroughs
    Mar 20, 2012

    I’ve often heard that Mt. Monadnock in southern New Hampshire has a “false treeline,” noticeably lower than the Whites, because the top was burned off back in the 19th century to kill off marauding wolves that were killing the farmers’ livestock at lower elevations.  The top of Mt. Monadnock has an elevation of 3,165’ and the treeline is some 100’ to 200’ below that.

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