Cooperation in the Face of Glacier Retreat: The Plants’ Story

As new areas become exposed by glacier retreat, plants begin to colonize them. Do the different species support or compete with one another? A recent study in the Journal of Vegetation Science follows the interactions of the circumpolar moss Silene acaulis, a type of cushion plant, with other secondary species like the buckwheat Bistorta vivipara in a southern Norway glacier, which has been retreating.

Five transact sites chosen for the study (Source: Klanderud et al.)

To answer this question, Kari Klanderud and her colleagues from the Norwegian University of Life Sciences demarcated five transact areas that were of increasing distance from the Midtdalsbreen glacier. Areas further away from the glacier represent better environmental conditions for growth as abiotic stress decreases, showing an environment gradient. The further from the glacier, the longer the area has been exposed, resulting in more advanced colonization. The abundance of buckwheat and the number of species of secondary plants within and outside of cushion plants were analyzed by conscientiously photographing the plant species in those areas. Soil temperature, moisture, organic content and pH measurements were also taken to examine if cushion plants modify the abiotic environment.

“We chose to work on S. Acaulis because it is common in alpine and arctic areas worldwide,” lead author Klanderud told Glacierhub. The cushion plant, a fascinating species that can survive in harsh climates, is commonly found in exposed habitats such as alpine tundra and places of cold-air drainage such as glacial moraines. It resembles a large green mat that can grow up to three meters in diameter. As a pioneer species in alpine habitats, the cushion plant nobly optimizes environmental conditions to facilitate growth of secondary plants like the buckwheat. This kickstarts the process of primary succession. These ungrateful secondary plants will continue to shamelessly grow, dominate and ultimately replace their pioneers so that eventually a community with larger species variety is achieved.

Cushion Plant, Silene acaulis which is also known as Moss Campion (Source: Wikipedia Commons)

Why is the cushion plant able to survive the harsh conditions in the first place? Lawrence Walker, a professor at the University of Nevada who specializes in plant ecology, told Glacierhub, “Their compact growth form preserves heat, which leads to a longer growing season and minimal frost damage during summer months. The cushion growth form also avoids breakage of stems from strong winds.” The benefits of being compact is not only self-serving, its structure also facilitates secondary plants’ growth and survival by buffering extreme soil temperatures. “Even pollinating insects may find refuge in the cushion,” says Walker.

In an environment with limited space and resources for growth, it is every plant for itself. The cushion plant allows other plants to grow within them and in turn compete for nutrients. There must be a threshold for its altruism if the cushion plant wants to survive in the face of the buckwheat and other secondary plants. As Walker explains, “It is common that nurse plants (ones providing protection for small individuals of other plants) can later be outcompeted by the plant that they nursed.”

Must the cushion plant really engage in negative interactions to impede the growth of its ‘child’, the buckwheat to survive? Biologists have discovered that relations of plants vary depending on the level of stress. Coined the stress-gradient hypothesis, this means that competition between plant species is strongest during favorable environmental conditions, but these species will support one another when the going gets tough. With decreasing biotic stress, plant interactions tend to shift from facilitative to competitive.

Buckwheat, Bistorta vivipara (Source: Pinterest).

Indeed, for sites close to the glacier that represent the harshest abiotic conditions, the buckwheat performed better, as shown by bigger leaves when it is grown between the stems of the cushion plant. These sites are characterized by fewer organisms which accentuates the harsh conditions. Soils closer to the glacier contain less organic matter due to a shorter lifespan of the ecosystem present. The plants also lack support and shelter from one another to moderate the environmental conditions. “The very dense and dome-shaped cushion modifies the microclimate and thus the growing condition for other plants,” Klanderud explained.

There was limited difference in buckwheat growth performance in more favourable environments further from the glacier. In this case, the cushions support the theory above. However, during conditions of low abiotic stress, it still has a “conscience,” shifting only from facilitative to neutral interactions instead of negatively affecting the performance of the buckwheat. In terms of secondary plant species diversity, a trend of higher species richness was observed within the cushions across all the sites, with cushions buffering extreme soil temperatures as the main abiotic reason.

While glacier retreat often has a negative connotation, it can also represent new opportunities for plants and other species to build communities in newfound lands. Nonetheless, one thing is for sure – survival is key, prompting cooperation when needed but also knowing when to draw the line, just like humans.

Where Can Alpine Plants Hide from Global Warming?

Androsace alpine living on rock glaciers as well as moraine ridges and deglaciated forelands (Source: Apollonio Tottoli/Flickr).

Environmental conditions, including climate, strongly influence the distribution of plant species. As temperatures continue to rise around the world, many people are concerned about the possible shifts in distribution of plant species, since plants are immobile, and many of them have a limited ability to disperse. These restrictions to changes in their distribution are particularly severe for plants that are adapted to cold conditions, such as those found in high mountain regions.

Studies by Valenti Rull and others have shown that during interglacial periods in the geological past, alpine plants were able to disperse to microrefugia, small-scale sites which allowed species to persist when most of their ranges became unsuitable for them. Thus, in the current era of warming, such sites, with locally favorable climate, could once again prove to be important for the survival of cold-adapted alpine species. A newly published study by Rodolfo Gentili of the Department of Environmental Sciences at the University of Milan and several co-authors in Ecological Complexity establishes a fresh approach to the study  of microrefugia. The authors examined the geomorphological and ecological features of microrefugia during earlier interglacial stages and used these features to identify potential microrefugia areas for alpine plants in and near glaciers, in both the present and the near future.

Leucanthemopsis alpine living on mountain summits (Source: Apollonio Tottoli)
Leucanthemopsis alpine living on mountain summits (Source: Apollonio Tottoli/Flickr).

In general, there are three recognized strategies which alpine plants can adapt to survive under a warming climate. They can migrate to higher elevation, remain at local microrefugia or evolve through genetic differentiation to adapt to new climate. However, there had been no overview to date of how plants in the Alps and other high mountains of Europe could respond to future warming. Gentili and his co-authors conducted  a thorough literature review, focusing in particular on geomorphological processes and landforms associated with plant communities in alpine environment. (They found only one study which addressed the genetic evolution of an alpine plant.)

The authors developed a typology of alpine landforms and characterized each one according to its “vegetation features, climatic controls, microclimate features of active landforms and microrefugium functions.” They recognized eight landform types, which differ in terms of the processes that generate them. These landforms are mountain summits, debris-covered glaciers, moraine ridges and deglaciated forelands, nivation niches or snow patches,rock glaciers, alpine composite debris cones (debris slopes and scree), alpine corridors (composite channels, including avalanche channels and tracks), and ice caves.

Saxifraga oppositifolia living on alpine corridors (Source: Alastair Rae/Flickr).
Saxifraga oppositifolia living on alpine corridors (Source: Alastair Rae/Flickr).

Taken individually, all of these eight landforms have been documented in the published literature as serving currently as microrefugia, except for the debris-covered glaciers, which nonetheless are promising as future microrefugia because of their relatively cool temperatures which result from the presence of sub-surface ice. The other landforms all have been shown to function as microrefugia. They offer a number of advantages, including suitable sites for colonization (moraine ridges and deglaciated forelands), cooler temperatures (debris-covered glaciers, rock glaciers, nivation niches or snow patches, ice caves), a vertical range that facilitates dispersal (alpine corridors) and a large variety of niches (alpine composite debris cones). Taken together, these landforms provide a very wide range of habitats, increasing the likelihood that any given alpine species could have a favorable spot to which it could disperse. These relations are indicated in the figure from the paper, shown below, which demonstrates that the geomorphological heterogeneity—the diversity of habitats within and across landforms—promotes the survival of species.

The relation of geomorphological diversity to species survival (Source: Gentili et al./Ecological Complexity).
The relation of geomorphological diversity to species survival (Source: Gentili et al./Ecological Complexity).

The researchers note that these glacial and pre-glacial landforms are potential microrefugia for alpine plants under warming conditions. They recognize that human intervention—purposive translocation of plants—may assist in the survival of species. In addition, they point out that the plant species themselves may adapt genetically to changing environmental conditions. They conclude by suggesting that researchers could profitably direct their attention to evolutionary processes within this geomorphologically complex and climatically dynamic environment, seeing whether species, pressed by climate change, can adapt, or even evolve into new species.

Saxigrada bryoides living on debris-covered glaciers (Source: /Flickr).
Saxigrada bryoides living on debris-covered glaciers (Source: Benoit Deniaud/Flickr).

How Invertebrates Colonize Deglaciated Sites

Mitopus morio (Source: Javier Díaz Barrera/Flickr).
Mitopus morio (Source: Javier Díaz Barrera/Flickr).

Scientists have long wondered how species colonize sites after deglaciation. A recent study by Amber Vater and John Matthews in the journal The Holocene of invertebrates–animals without backbones—on a number of sites in Norway advances the understanding of this colonization. It pays particular attention to succession, the processes of change in the species composition of ecological communities over time. The invertebrate groups which were studied include insects, spiders and mites, as well as harvestmen, also known as daddy longlegs.

To study the process of succession, Amber and Matthews collected invertebrate samples from pitfall traps in 171 locations across eight glacier forelands, which deglaciated over the last few centuries, in the Jotunheimen (high altitude) and Jostedalsbreen (low altitude) subregions in southern Norway. Jotunheimen is the highest mountain in Europe north of the Alps and west of the Urals, while Jostedalsbreen is the largest ice-cap in Europe outside Iceland. These forelands represent different ecological regions and areas that have been deglaciated for periods of different length. A variety of geological and biological evidence allowed the researchers to establish the precise timing of glacier retreat across their sites. The researchers identified the organisms by taxa—the species, genus or family to which they belong—since species identification was difficult in some cases.

The location of the eight glacier forelands in southern Norway (Source: Vater and Matthews/Sage Journals).
The location of the eight glacier forelands in southern Norway (Source: Vater and Matthews/Sage Journals).

Several major findings were derived from this study. Firstly, invertebrates arrive fairly quickly after the retreat of glaciers, within a decade or two. In particular, initial colonization is faster and dispersal is more effective at high altitudes, where glacier forelands are small, reducing the distance from established communities to new sites; in addition, the strong winds in such areas can carry organisms further. The flying insects, such as flies, aphids, bees, wasps, stoneflies, caddisflies and flying beetles, arrived earlier than the ground-active non-flying species, such as spiders, harvestmen, mites, ants, and non-flying beetles. Moreover, the communities grow more complex over time. In the first stage, lasting about 20 years, 11-31 taxa were found; this number increased to 21-55 in the fourth and final stage, over two centuries later. The authors found as well that invertebrate communities tend to be more diverse at low altitudes, where environmental conditions are more favorable.

Jotunheimen from southern Norway (Source: Thomas Mues/Flickr).
Jotunheimen from southern Norway (Source: Thomas Mues/Flickr).

Vater and Matthews summarize their findings by stating “invertebrate succession on the glacier forelands is viewed as driven primarily by individualistic behavior of the highly mobile species with short life-cycles responding to regional and local abiotic environmental gradients”.

Amara quenseli (Source: Chris Moody/Flickr).
Amara quenseli (Source: Chris Moody/Flickr).

This research calls into question earlier studies of succession. Previous studies, often based on plant species rather than invertebrates, have emphasized that nearly all taxa occur only in some of the stages of succession. By contrast, Vater and Matthews find that most of the taxa that first appear remain all the way till the final stage—65-86%, depending on the site. The authors describe their results as an ‘addition and persistence’ model (because taxa remain, once they arrive) rather than the more established ‘replacement-change’ model, in which different taxa replace each other over time. This ‘addition and persistence’ model seems to be more applicable in severe environments.

This research offers some insights into the regions that will become exposed as glacier retreat continues. It brings the positive finding that lands that appear after glacier retreat will not remain barren for long, since invertebrates are likely to colonize these sites soon. However, the new areas at higher elevations may have only a small number of specialized invertebrate taxa instead of a wide range of them.

For more details on invertebrates living on glaciers, look here.