Very Chilly Cushions – A Common Plant Adaptation to Life in the Cold
I am not sure why, but lately I’ve been thinking a lot about how plants adapt to extremes of cold, both in the arctic and the alpine regions. I’m sure it has nothing to do with the fact that it’s been between 14-17 °F the past few nights, making our uninsulated bedroom a toasty 35 °F.
There are a bunch of interesting and unique adaptations of arctic and alpine plants (which, in fact, are often the same thing), and I plan to provide an overview of those in an upcoming post. Today, however, I just want to cover one adaptation that is extremely common in the tundra, especially in the very harshest parts. Cushion plants.
A cushion plant is not a plant from any particularly family or genus – cushion refers to the growth form the plant takes. And species of plants over a very wide range of groups (at least 30 families) have adapted this growth mode in cold, short seasoned climates, in and around alpine regions worldwide, as well as the arctic and antarctic. If you get high up almost any mountain range, both above and approaching timberline, you will notice that many of the small forbs you begin to see tend to grow very close to each other in clumps. These clumps are usually vaguely circular and ofter appear puffy, like a pillow, though not more than a few inches above the soil. While there are plenty of mat-forming plants in warmer climes, these unique “cushions” are found almost exclusively in alpine/subalpine environments (or arctic/subarctic ones). Cushions typically consist of many smaller plants, which may be vegetative clones of each other or may be genetically distinct, but are often treated as a single organism.
Clearly, the plants derive some serious benefit from this behavior. But what on earth could crowding each other in a resource-limited habitat do to help them? They aren’t like penguins, who generate their own heat and huddle together to share warmth. A cold plant plus another cold plant generally equals two cold plants. (Note: Some plants actually do generate heat, but they are not alpine plants. Rather they tend to be early-spring plants and, oddly, tropical plants). But their genius is in their shape and density.
The dome-like shape which the cushions tend to take (made possible by an adaptation that makes all the plants in the clump grow upward at the same rate, so no one plant is high above all the others), and the closeness with which those plants grow, makes these clumps perfect heat traps. The temperature on or inside a cushion can be up to 15 °C more than the air temperature above it. The cushions are able to retain heat radiating up from the soil, as well as absorbing heat from the sun (a very dense, large, clump of green can get surprisingly warm on a sunny day at high altitude). Add to that the fact that the wind speed in and around a cushion can be cut by up to 98% from open areas, you have a perfect recipe to prevent heat loss. Many alpine cushion plants also have very hairy leaves, which trap even more heat within. This allows the plants to maintain a relatively stable, warmer than average microclimate that is resistant to sudden changes in weather and temperature outside (such as freezing temperatures at night or sudden storms). Interestingly enough, this stabilizing effect can also be a benefit when it gets too hot out, maintaining lower temperatures against baking sunshine.
The benefits of a cushion-like growth form at high altitudes go beyond just heat retention and wind protection – they also extend to increasing soil moisture and nutrition in the area beneath the cushion. Cushion plants, like most alpine plants, have a large, deep taproot. Because alpine soils often drain quickly and are poor in nutrients, a long root system is important for the plant to be able to reach enough water and nutrition. Most alpine plants are perennial and die back to their roots in fall, having to exist on stored resources throughout the 10 month dormant season, and a thick taproot can help with that. In addition, in a place with high winds, snow, and not a lot of other plants around, a good anchor is vital. Cushion plants have the advantage of having a root system that is not just deep, but also extensive, consisting of one or more taproots and a network of entangled smaller roots closer to the surface. These help trap water and nutrients where the plant can get at them, and the dense cover provided by the cushion drastically cuts evaporation. This is not an insignificant effect; some research has shown that soil moisture in cushion sites can be increased up to 70% above non-cushion sites, and available nitrogen up to 90%.
Cushion plants often start out by colonizing completely bare soil, in the harshest of climates. As you can imagine, the process of getting started without any existing buffering cushion is a little bit problematic, so new cushion establishment is actually rather rare. That’s okay though, because depending on the species a single cushion can reach up 350 years old (and some Andean species are claimed to have reached 3,000). The cushion grows very slowly, much more so than lower-elevation plants, but can eventually reach up 3 m in diameter. Cushion species also tend to have very large showy flowers, often strongly smelling, for their small size, which may bloom all at once, attracting a greater number of pollinators than if they had smaller flowers or grew more sparsely.
As you can imagine, all these factors contribute to making these cushions ideal microclimates in a very inhospitable habitat. What’s really interesting is that although cushions of the same plant may reach from the subalpine or even montane habitat to well past timberline and onto nearly bare, windswept ridges, the general conditions inside the cushion remain fairly constant. So at a lower altitude the relative difference in conditions from open ground may not be very large, but as the environment becomes higher and colder and windier, conditions in the cushion will be vastly different, while still resembling conditions inside the lower-altitude cushion of the same species.
The desirable microclimate created by the cushion is not just desirable for the plant itself – in fact these plants have been shown to provide habitat for other plants, which shelter under the canopy of the cushion, as well as a variety of microorganisms and arthropods. Studies on various species of cushion plant around the world have shown that, depending on the type of plant and the location, cushion plants may improve species diversity, richness, or evenness (often all three!) when compared to a similar site with no cushion. This has led to cushion plants being variously known as “nurse plants“, “ecosystem engineers“, and “foundation species“. All these terms have different exact meanings, but in general refer to a species that alters its environment in such a way that is beneficial or even vital for a number of other species or individuals in that ecosystem. In fact, the higher up slope you go and the more relative difference between conditions in and outside of the cushion, the more important to other species cushion plants seem to be.
Badano, E. and Cavieres, L. (2006). Impacts of ecosystem engineers on community attributes: effects of cushion plants at different elevations of the Chilean Andes. Diversity and Distributions, 12(4): 388–396.
Cavieres, L., Arroyo, M. T.K., Peñaloza, A., Molina-Montenegro, M. and Torres, C. (2002), Nurse effect of Bolax gummifera cushion plants in the alpine vegetation of the Chilean Patagonian Andes. Journal of Vegetation Science, 13: 547–554.
Cavieres, L., Badano, E., Sierra-Almieda, A., and Molina-Montenegro, M. (2007). Microclimatic Modifications of Cushion Plants and Their Consequences for Seedling Survival of Native and Non-native Herbaceous Species in the High Andes of Central Chile. Arctic, Antarctic, and Alpine Research, 39(2): 229-236
Morris, W. and Doak, D. (1998). Life history of the long-lived gynodioecious cushion plant Silene acaulis (Caryophyllaceae), inferred from size-based population projection matrices. American Journal of Botany, 85 (6): 784-793