Snow, hail, and mortality on the mountain

A lot of change can happen in just one year. This field season was unlike any other I have seen in the years I have been monitoring alpine plants. A lot of dynamics happen on a mountaintop, even when the overall rhythm of life – snow, snowmelt, growth, snow – remains the same.

The year started out as a wet one, with a deep snowpack that persisted into the summer. Even in late June it was difficult to access our site in the Rocky Mountains at 11600′ elevation because of the need to traverse a number of snowfields on the hike in.

And even after arriving at the site, there was very little to do – two weeks after the solstice we found a meter or two of snow covering most of our long-term plots, the remnants of a deep cornice. I had seen snow at the site this late in previous years, but not nearly this much – and not in the same place. Typically cornices form in reliable locations, but the wind or snowfall patterns must have been very different this past winter.

As the weeks passed by and the snow finished melting, a little life began to return to the mountain, and a few of the plants began to grow again. But weeks passed with no rain, and growing conditions were challenging.

By the end of July things were far enough along that we were able to initiate our annual census of all the plants.

The results of the census were striking, and evident enough from a cursory walk through the plots. We give a permanent metal tag to every single plant, seedlings and adult alike. Usually new plants appear, and existing plants re-appear next to their tags – the familiar rhythm of new life and new growth. But this year we found many tags with no plants. In some plots, more than ninety percent of the plants were missing or dead. We were left with bare scree and metal, with no life at all.

If there is no growth at a tag over two years, we assume the plant is dead, and pull the tag. This year, we pulled 511 tags – more than a third of the total number of plants at the site. Only a few dozen new plants appeared, compared to hundreds in previous years. And nearly all of the dead were lupines, though many other species saw comparable losses. Below you can see some of the many tag that we pulled and retired.

Traditionally, population dynamics in alpine plants is thought to be very slow and steady, with many plants persisting for decades to centuries. I was very surprised to see such fast dynamics here. Whether they are a one-time aberration or the beginning of a long-term trend is hard to say, but they are certainly atypical in the several years I have walked these sites. Only this kind of detailed demography makes these types of observations possible – this year’s data has made me into a much more careful observer of some of the other environments that I repeatedly encounter in my daily life.

So what is going on – why are so many plants dead? Perhaps they are simply dormant, and are waiting for a better year to return to aboveground growth. I don’t think this is likely, given that several species do not exhibit dormancy. Perhaps the summer was too hot and dry. Or perhaps the winter was too cold and long. Or perhaps last year’s summer was too hot and dry. I don’t know yet, but we have all the data in place to be able to ask and answer these types of questions. Doing so may help us better predict how these landscapes will respond to changing climates. I hope that the answer is not simply that they will die back, and not return.

Contemplating these questions is typically a pleasant experience on the hike down to the valley. This year the field season ended more dramatically. Pulling tags for the last dead plants took longer than expected, and we found ourselves stuck in a thunderstorm dropping painfully large hail on us. But the work is done, the data are assembled. I have had enough snow and storms and death for the season. It is time to sit at a desk again – and think.

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