Krummholz: life at the edge

How extreme can environments be before life can no longer hold on? The quaking aspen (Populus tremuloides) trees I study seem to cope well with all sorts of extremes – they are found as far north as boreal Canada and as far south as Mexico. Normally when I see an average aspen clone, it looks happy and healthy, as in this subalpine Colorado meadow.

low elevation

But averages are boring. The interesting biology happens at the extremes. We’re interested in carbon economics, and want to understand if aspen clones in very extreme environments are almost at the point of negative carbon economics, where more carbon is lost through respiration than is gained through photosynthesis. So off we went in search of a very unhappy aspen clone – myself and a post-baccalaureate student in charge of this project named Richard.


A few years ago I climbed Mt. Bellview, the brown peak in the background, and recalled seeing some very small trees growing near its summit. So off we went to find them, and then measure them.


About 1500′ of scrambling later, Richard and I found ourselves on a dry scree field, covered in small and sharp rocks. Shifting these rocks revealed bedrock just a few centimeters down. Yet somehow, more than a few aspens were growing out of this substrate.

find aspens

They didn’t look very happy. In good conditions, aspens will grow easily ten meters high, with lush dark green leaves; here, each stem was no more than knee-high, with small and tortured looking stems. This kind of vegetation is typical of alpine krummholz where plants are exposed to extreme temperature and wind. The plants have no choice but to hunker down.

aspen roots

The slope was loose, but we measured some aspects of growth rate, and then installed light-loggers in the trees’ upper branches.

leaf length

light loggers

In a few weeks we’ll climb the mountain again, this time with a portable gas analyzer able to measure photosynthetic rates in these plants. By merging these measurements with the growth rate and light data we are now collecting, Richard will be able to understand how well these plants are growing at this upper elevational extreme. To me it doesn’t look like that great of a place to live – we were happy to downclimb the slope and head home – but maybe the aspens see it differently. We’ll find out soon.

bellview bottom

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