Getting to the roots: Alpine vegetation under climate change
At 2500 m elevation in the central Alps, researchers from the University of Basel maintain a unique experimental infrastructure to explore how alpine vegetation responds to climate change. For the first time, this project provides data on the interplay between above- and below-ground processes in a high elevation ecosystem — filling a huge knowledge gap as roots are often neglected in ecological research.
As part of my PhD program in physiological plant ecology, I contribute to the Bel-Alp project. Funded by the Swiss National Science Foundation, this project assesses the root dynamics of alpine plants under changing climatic conditions. While vegetation above the tree line may seem barren at first glance, it harbors a vast diversity of plants with extensive root systems. These root systems are important, because they function like screws or nails that hold together the steep and erosion-prone soils for almost a quarter of Switzerland, safeguarding villages, roads and power lines.
Simulating global warming: Snow manipulation
Once a year since 2016, four to six of us gather in the mountain village Realp in early June. From there, a helicopter takes us to alpine wilderness at 2500 m, which is then still snow-covered. For the next few days, we are busy shoveling away snow from selected parcels and pilling it up in other parcels. What might sound like a childish folly, is actually an effective way to study how alpine vegetation has been responding to climate change.
Alpine grassland above the natural tree line is covered by snow for two thirds of the year. Because of global warming, snowmelt has advanced by 2.5 weeks since the late fifties, considerably prolonging the total snow-free period. It is precisely this shifting snowmelt timing that we are trying to simulate by manipulating the snow cover. The parcels where snow was shoveled away (down to 50 cm) will become snow-free earlier than untouched spots, while snow piles will take longer to melt. This then gives us the possibility to study timing effects on the alpine vegetation.
Inducing drought conditions
A few weeks later, we take up residence in the nearby research station ALPFOR. Once the experimental site is fully snow free, the simulation of climate change continues: we install 30 rainout-shelters, which look like tents made of UV-B permeable foil. As the name suggests, rainout-shelters keep the rain away and induce drought conditions on the underlying vegetation and soil. Summer drought has become more frequent in Switzerland and elsewhere, and is expected to become even more prevalent in the future. Our experiment studies two scenarios: drought conditions for five and for ten weeks after snowmelt.
Whether and how climate change, drought in particular, affects alpine grassland is largely controlled by its hidden organs: roots and rhizomes. But studying roots in the field is much more difficult than studying its green parts. As a result, there is considerably less knowledge about the physiology and ecology of roots than about stems, leaves and flowers. Hence, this project contributes to a facts-based projection of climatic change effects on alpine vegetation.
Observing root growth
In 2019, we forced transparent tubes into the soil that serve as windows to observe root growth. These durable tubes are inserted so that the uppermost centimeters remain above the soil surface and stay accessible. Once in place, an optical root scanner captures the tubes’ outer surface by means of 360°-images.
From snowmelt in June to snowfall in October, we repeat the scanning weekly for a total of 90 tubes, leaving us with more than 2500 root images to this day. Fortunately, recent advances in machine learning allow for a speedy recognition of root structures. Once we know which pixels belong to roots, we can calculate root growth.
Our preliminary data suggest that the strongest effect on roots comes from the shorter, five-week-drought. As soon as the burden of water shortage is removed, root growth seems to increase substantially, clearly outperforming those parcels that either never experienced drought (controls) or were still under drought. As a result of our drought timing, this effect appears in late summer — at a time, when most leaves have long stopped growing and become already senescent. This illustrates a previously vaguely known pattern: root growth extends into later summer and autumn, while leaf growth occurs in the first weeks after snowmelt.
Timing of drought is crucial
A pattern is slowly emerging that has been observed in other ecosystems: the timing and duration of droughts seem crucial to understanding the way plants respond. If weeks of water shortage are followed by wet conditions, root growth may even accelerate in alpine grassland. Given the projections by climate models, this is a likely scenario. At least in the short term, the already immense root fraction may further increase, with cascading effects on soil microbes and nutrient cycling. Whether the observed pattern persists will be revealed in the season of 2021 — starting this June in Realp, waiting for the helicopter.
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