Finan Rodinson gets down to the roots of alpine botany...
If you're an altitude enthusiast, you've probably prepared a pack or two with a thought to the extreme daily temperature variations, lack of water, strong winds and high UV exposure that the unforgiving mountains inflict upon our fragile frames. These hostile forces form part of a relationship of fear, respect and intrigue associated with the mountains. They are also largely responsible for the diverse shapes and colours of an interesting group of plant species: alpine plants, who have adapted to these conditions in some surprising ways.
Join me as we develop some of the ideas presented in the documentary "Les Jardins du Vertige (Vertigo Gardens)" and give you an introduction to plant life in the mountains. Who knows, perhaps you'll never see a "mossy" or "overgrown" hold in the same way again!
One of the most striking morphologies among certain high altitude (or latitude) plants is known as the "cushion". Though these green globular masses of small, tight leaves and stems can appear soft to the touch, they are cushions in name alone and provide a surprising resistance to anyone wishing to squash them. Their rigidity comes from the incredible density of plant material which forms a stiff inner matrix of dead tissues giving the plant its spherical shape as well as creating a climactic "bubble", trapping air and humidity. Much like a big puffy jacket, the trapped air and dead plant matter insulates the plant and mitigates rapid temperature changes. Meanwhile the spherical shape of cushions ensures the lowest surface area per volume possible, reducing the ability of the wind to chill, or dry out the plant.
The capacity of these plants to create their own microclimates of favourable conditions has been demonstrated by researchers using temperature and humidity probes. Often, without any equipment, an attentive passer-by can come to the same conclusion by observing the number of different flowering plants all growing out of the same cushion! From the point of view of a small seed, these tight green globes are small oases in a desert of rock and those lucky enough fall onto the cushion have far better chances of germinating. Once they begin to grow, these opportunists can exploit all the benefits the cushion has created for itself in a phenomenon known as the "nurse effect", or "facilitation".
You might wonder if the relationship between the host plant and its uninvited guests is parasitic (the guests profiting at the cost of the host) or mutualistic (both the guest and the host gaining something). Well, it depends. As a host plant, there is a cost associated with having guests; they take up room, and if they take up too much room they can block out sunlight or even hide the cushion's flowers from the pollinators it's desperately trying to attract. However, up to a point, a good variety of different species of guest plants can have positive effects on the diversity of nutrient-producing bacteria and mini-arthropods living inside the cushion and its soil. In cushion pink for example, although a number of guests can reduce the amount of flowers produced by the plant, they can also increase the quality of the seeds it produces.
Remarkably, cushion plants do not belong to any one family, nor do they all share a common "cushion ancestor". Instead cushion plants are an example of evolutionary convergence. The similar alpine environmental conditions have selected for the same traits in different families of plants, even those from different continents, resulting in plants which not only look alike, but occupy a similar ecological niche.
Hairs (or trichromes to the initiated) might seem like an obvious adaptation in an alpine environment. Comparing a forget-me-not and its alpine equivalent may well evoke images of an elephant standing next to a woolly mammoth. However, these hairs aren't simply to stave off the cold, they also do a good job of reflecting both ultra violet and infra-red radiation reducing damage to the photosynthetic cells of the plant. The layer of air created by these hairs also protects the plants from winds which would otherwise rob them of their moisture.
In ecological parlance the "Alpine zone" is considered to begin at the treeline: the altitude at which trees disappear from the landscape due to environmental constraints, giving way to alpine meadows. As is often the case in the living world, exceptional organisms have risen to the challenges set for them by the climate. Even at altitudes of up to 3000m certain "trees" persist, though their diminutive form ensures they remain discrete and often unnoticed.
A handful of these "dwarf trees" belong to the willow or Salix genus including the four most common alpine species herbacea, reticulata, retusa and serpyllifolia. Often described as natural bonsai trees, it's interesting to note that the surprising morphology of these plants remains constant despite changes to the surrounding conditions. Dwarf willows planted below the treeline may have a slightly faster metabolism than those perched atop a summit, but they will retain their low, flat shape all the same. Comparing this shape to those of cushion plants is apt and confers the same advantages of added temperature buffering and reduced wind desiccation.
Reproduction in the mountains
Alpine plants, be they cushions, tiny trees or otherwise, have a tricky time reproducing. As altitude increases, pollinators become increasingly scarce. Combined with the short yearly window of optimal reproductive conditions, strategies to mitigate these constraints come into play.
Although sexual reproduction is important in maintaining the genetic diversity of a species, higher up the mountain, more and more plants invest in vegetative reproduction. On scree, tendril like "stolons" emerge above ground from the main body of certain plants to establish new clones, should the mother plant fall victim to shifting rocks. Dwarf tree branches perform a similar action, producing roots when a branch creeps over a patch of suitable ground. Alternatively, "viviparity" can be an effective form of multiplication in which a small bulbil develops on the adult plant before falling to the ground and (hopefully) taking root.
One of the most important adaptations is longevity. Slow growth and intermittent reproductive success can be alleviated by staying around for a long time. It isn't uncommon for most cushion plants to live for centuries, ever so gradually expanding in size to reach football, or rarely, exercise ball-like proportions. Even the smallest of cushions however, could be older than the enthusiastic climbers with whom they share the rock!
Respect through learning
As humans, we often seek to protect what we understand. What we don't understand is sometimes relegated to the position of "unimportant", but it's important to be practical. Is every climber going to become a lichen expert to avert the potential harm to these chlorophyllian-fungal symbionts? Probably not. What we can try is to learn a little more about the life that inhabits our vertical playgrounds and makes them even more amazing than we may have considered. So whether it's just keeping an eye out as we climb, or breaking out an illustrated flora to determine a species we've never seen before, we have everything to gain from spicing up our climbing with an appreciation of the living world.
Aubert. S et al, 2014, A revised worldwide catalogue of cushion plants 100 years after Hauri and Schroter, Alp Botany (2014) 124:59–70
Anthelme.F et al, 2011, Unexpected mechanisms sustain the stress gradient hypothesis in a tropical alpine environment, Journal of Vegetation Science 23 (2012) 62–72
Bonanomi. G et al, 2016, Cushion plant morphology controls biogenic capability and facilitation effects of Silene acaulis along an elevation gradient, Functional Ecology 2016, 30, 1216–1226
Minor. M.A et al, 2016, Effects of cushion plants on high-altitude soil microarthropod communities: cushions increase abundance and diversity of mites (Acari), but not springtails (Collembola), Arctic, Antarctic, and Alpine Research, Vol. 48, No. 3, 2016, pp. 485–500
Reid, A. M., and C. J. Lortie. 2012. Cushion plants are foundation species with positive effects extending to higher trophic levels. Ecosphere 3(11):96Rodrıguez-Echeverrıa. S et al, 2016, Influence of soil microbiota in nurse plant systems, Functional Ecology 2016, 30, 30–40
Cavieres. L et al, 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, Vol. 39, No. 2, 2007, pp. 229–236
Cavieres. L et al, 2013, Facilitative plant interactions and climate simultaneously drive alpine plant diversity, Ecology Letters 2013
Cavieres. Let al, 2015, Facilitation among plants as an insurance policy for diversity in Alpine communities, Functional Ecology 2016, 30, 52–59
Cavieres. L et al, 2008, Facilitation of the non-native Taraxacum officinale by native nurse cushion species in the high Andes of central Chile: are there differences between nurses?, Functional Ecology 2008, 22, 148–156
Michalet. R et al, 2016, Beneficiary feedback effects on alpine cushion benefactors become more negative with increasing cover of graminoids and in dry conditions, Functional Ecology 2016, 30, 79–87
Schob. C et al, 2013, A global analysis of bidirectional interactions in alpine plant communities shows facilitators experiencing strong reciprocal fitness costs, New Phytologist (2014) 202: 95–105