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Will Antarctic Oases Remain Green?

A green oasis of Antarctic mosses in the Windmill Islands. Zbyněk Malenovský

A green oasis of Antarctic mosses in the Windmill Islands. Credit: Zbyněk Malenovský

By Zbyněk Malenovský

Antarctic mosses are threatened by climatic change and human activity, but researchers can now detect their health by analysing spectral patterns imaged from the ground or remotely by drones.

When we think of Antarctica, most of us imagine vast, pristine snowfields, frozen lakes, glaciers, mountains and nunataks, a freezing sea full of icebergs and, indeed, penguins, seals and whales. Arriving to Antarctica, enthusiasts of raw nature will never be disappointed. The silent scenery is an artistically designed collage of simple landscape features such as rocks, water bodies, snow and ice patches “dressed” mostly in grey, blue and white.

But the colours of Antarctica can change very dramatically. Walking along the coast of Eastern Antarctica, one can encounter bright and dark greens decorated with yellow, orange and red spots. These unexpectedly contrasting colours belong to one of the most cold- and drought-resistant plant species of our planet, Antarctic mosses.

Mosses in Antarctica grow prevailingly along the snow-free rocky coast. Although they are well-adapted to survive inhospitable Antarctic conditions, polar biologists are concerned about the future of these green oases of polar deserts. Temperature and wind anomalies caused by the depletion of stratospheric ozone and global climate change are likely to trigger their decline and potential extinction.

More than 100 species of mosses and liverworts have been found in the Antarctic. Most of them grow along the Antarctic Peninsula and its associated islands, but a few are found in coastal regions around the edge of the Antarctic continent. Green mosses require a stable source of fresh water, originating from seasonal snowmelt, and nutrition supplied mostly by penguins and other polar birds that have been fertilising Antarctica for thousands of years. At locations with favourable growth conditions, mosses form large beds tens of metres in size, whose simple and peaceful nature reminds us of neatly designed Japanese zen gardens.

Since Earth’s climate is recently changing due to global warming and the stratospheric ozone hole, these Antarctic green oases have started to suffer from water shortage and high levels of ultraviolet (UV) radiation. As a consequence, they may contract and slowly die out once they cross the tipping point of their stress resiliency. Flourishing moss, for example, turns from a fluffy green turf into a stress-resisting yellow-red-brown pack as soon as it is forced to move out of its ecological comfort zone. If this stress persists, moss becomes a dry, compact, black and lifeless mat.

Drying and high UV radiation are not the only threats. Shoots that form moss turfs are quite fragile, and thus highly susceptible to any mechanical damage. Short but extreme snowmelt episodes, which could occur more frequently due to global warming, produce intensive local floods. The physical force of these “flush” events is strong enough to remove moss from its original habitat. Moss chunks often get damaged as they are tumbled around and bounced off rocks by water currents. Moreover, strong flood streams might deposit them at unfavourable growing locations, where scarce life resources and high stress are lethal.

Antarctic moss might be a survival champion that can regrow and colonise new sites, even after being torn away and placed upside down, but its full regeneration takes several years due to the short Antarctic summers, which are full of harsh weather events such as strong wind gusts and blizzards.

The past century introduced a new disturbance – humankind. New polar stations established in the vicinity of regular melt lakes often pump fresh water in an unregulated manner, which diminishes liquid water supplies for the mosses. Additionally, careless human visitors may damage moss beds by trampling them and engaging in other harmful activities.

Why is it important to protect Antarctic moss beds? Mosses have established stable ecosystems of simple but unique plants and animals. Nations that ratified the Antarctic Treaty agreed to protect this biodiversity. Equally important to biodiversity is the potential of Antarctic mosses to reveal the secrets of their stress tolerance. Understanding how these plants live in conditions similar to a household freezer might help us to produce new crops that would cope better with cold and dry environments. This is an important contribution towards our future food security, which is challenged by more frequently occurring weather extremes.

However, the most direct value of Antarctic mosses is their bio-indicative behaviour. Living at the edge of inhabitable lands, any shift in climate tests their survival capability. As a consequence, their stress reactions, such as rapid discolouration, can be regarded as sensitive biomarkers flagging almost unnoticeable changes in Antarctic climate. Although the climate of continental Antarctica is not changing as fast as in Arctic maritime regions, we need reliable indicators of the first subtle changes that take place before large-scale irreversible climate shifts occur with catastrophic consequences.

To exploit Antarctic mosses as climate indicators, their actual health must be first diagnosed and then monitored at regular intervals. Because the moss beds are fragile and the weather highly erratic, the method must be non-intrusive and capable of surveying large areas of fragmented landscapes in the “blink of an eye”.

Research from the universities of Wollongong and Tasmania has now developed a short-range remote sensing approach that provides a fast, non-destructive and spatially explicit assessment of Antarctic moss health. The approach, published recently in New Phytologist, uses a portable scanner to capture a spectrally discrete image for each wavelength of the visible and near-infrared electromagnetic spectrum. The device, which looks like a digital camera, is attached to a computer-controlled rotating platform mounted on a tripod about 2–3 metres above the ground. Being rotated around its centre point, it records photons of sunlight reflected from the moss surface and splits them into hundreds of images corresponding to single wavelengths, just like taking a snapshot of a rainbow.

Chronically stressed moss plants gradually lose their green chlorophyll pigments, which absorb the sunlight required for photosynthesis. This loss of chlorophyll slows down photosynthesis, which in turn lowers the plant’s production of energy for both growth and stress protection. It also induces moss turf to change colour from dark green to light green.

Antarctic mosses also produce pigments of other colours. Some of these act as “sunscreens” that help to protect plant cells from damage caused by excessive solar energy, including UV radiation. The imaging spectral scanner records this pigment composition and quantity changes as different colour intensities of corresponding visible wavelengths.

A shortage of liquid water causes “curling” of moss leaves. These leaf shape changes are similar to the wilting of other higher plants, except they happen faster, occur at the microscopic scale, and modulate interactions of light with moss canopies in a very specific way. Curled leaves allow light to penetrate deeper into moss turf, which increases the chance that it will be absorbed.

Greater light absorption reduces the pool of near infrared rays that can be reflected from moss surfaces back to the detector. The diminished reflectance of incoming sunlight is then registered in the spectral imagery of water-stressed mosses as a near-infrared signal of low intensity. In other words, the infrared image of the moss canopy, which normally looks as bright as the surface of a full moon, becomes darker.

We studied the stress responses of three Antarctic moss species near Australia’s Casey Station during the summer of 2012–13. The physical relationships discovered between moss reflectance and moss stress reactions were applied to spectral images of moss ecosystems scanned from a tripod in the field. These experimental image analyses produced the first map of moss vigour, indicating the actual moss health between zero (very sick) and 100% (absolutely healthy).

The major advantage of this method is its applicability to both ground-collected and remotely sensed airborne images. In fact, the field experiment was conducted as a trial for future work with low-altitude flying unmanned aerial systems. Drone-based spectral images of moss beds located near the Casey polar station were collected in the same season as the ground measurements. Even though analyses of the aerial images are still in progress, their intermediate outcomes look promising.

An airborne approach could enable broad-scale mapping and monitoring of moss populations across several Antarctic coastal regions. For the first time ever, this would enable polar biologists to diagnose the health of Antarctic moss ecosystems in a consistent manner and separate mosses maintaining good health from those in danger of losing their energising green colour and disappearing from their biotopes.

Zbyněk Malenovský is a research fellow at The University of Wollongong and an adjunct researcher at The University of Tasmania.