Arctic Sea ice decline and plant growth
One may think that Greenland will become greener in the future. Not the least because of a longer and warmer growing season and a potential increase in nutrients. However, not all conditions are optimal for increased growth.
Center for Permafrost (CENPERM) at the University of Copenhagen has been involved in a new study that shows that plant growth in the ice-free areas in Greenland and in the rest of the Arctic is affected very differently as a result of current climate changes. The results shows that water availability is a major key to predict Arctic plant growth in the future.
The sea ice extent plays a major role for the stability of the climate on land. It also affects regional precipitation, cloud formation and energy budget and thereby potential plant growth. Year after year, the extent of sea ice in the Arctic has been decreasing, and the part that remains becomes thinner. But it remains unclear how the sea ice changes affect life on land, in the past as well as in the future.
To study this, researchers from Center for Permafrost (CENPERM) have been involved in a large-scale study led by Dr. Agata Buchwal from Adam Mickiewicz University in Poznan (Poland). In the project, two widespread tree species, namely birch and willow, were studied from locations all across the Arctic. These two plant groups are important to study as they play a major role to which extent the Arctic will become "greener" in the future. The study is the largest circumpolar dendrochronological study (study of tree annual rings - see fact box) of willow and birch in the Arctic so far, and has just been published in the journal Proceedings of the National Academy of Sciences.
Bo Elberling, head of the Center for Permafrost and professor at the Department of Geosciences and Nature Management at the University of Copenhagen, has been responsible for collecting samples from parts of Greenland. He explains:
“In Greenland, you find areas that have become markedly greener in the last 20-30 years, and you see changes in the species composition of plants. Here, warmer temperatures seem to play a major role for the observed changes. But availability of water and nutrients will probably be very important for how wide-spread the greening might be – even though it will be warmer and the sea ice will continue to retreat."
Greening is water-depended
In many places in the Arctic, rainfall is so limited that one can speak of a cold desert. There is so little rainfall that salt precipitates on the soil surface. Both the lack of water and high salt contents cause stress to plants. It is therefore a valid question whether a warming of this type of habitat will actually lead to an increased plant growth - or whether the stress levels are simply increased, and plant growth therefore reduced. The issue is important because the most significant climate change has been observed in the Arctic, and because increased plant growth may contribute to increased sequestration of carbon dioxide from the atmosphere.
The new study shows that plant growth in the ice-free areas of Greenland and in the rest of the Arctic is affected very differently by the retreat of sea ice. Where there is enough water, in the form of precipitation or meltwater, a clear connection is seen between a decrease in sea ice and a faster plant growth. However, in places where there is not enough water, a reduced sea ice extent is correlated with a reduction in plant growth.
In this study, birch and willow have been collected from 19 localities throughout the Arctic. The plants have been cut up so that the thickness of annual rings could be measured. Annual growth rings can be dated from the year of collection. Growth patterns over time can then be compared with local climatic conditions, soil conditions and the distribution of sea ice, etc. The method of measuring the thickness of annual rings is called dendrochronology (see fact box).
The 19 locations can be divided into two main groups
At two thirds of the localities, an increase in plant growth can been seen over the last 30 years. These places are characterized by there being enough water in the soil. The water content does not have to come from precipitation, but can also come from meltwater from local snow drifts or glaciers. In some places, the local reduction of sea ice is limited, and here it can be difficult to distinguish between local effects due to more open water near the coast and a link to a reduction of sea ice on a large scale. This applies, for example, to locations in North Greenland.
Conversely, at one third of the localities, plant growth has been reduced as the sea ice extent has declined. These localities are characterized by a dry climate and a further warming can therefore contribute to additional water stress for the plants.
“The new conclusions are important for future work on clarifying whether the Arctic as a whole is affected by climate change, and not the least which positive and negative effects can be expected. An increased plant growth will be able to capture more carbon dioxide from the atmosphere. However, this study suggests that plants in dry Arctic regions will only grow more where there is enough water” explains Bo Elberling.
Buchwal, A., Sullivan, P. F., Macias-Fauria, M., Post, E., Myers-Smith, I. H., Stroeve, J.C., Blok, D., Tape, K.D., Forbes, B. C., Ropars, P., Lévesque, E., Elberling, B., Angers-Blondin, S., Boyle, J. S., Boudreau,S., Boulanger-Lapointe, N., Gamm, C., Hallinger, M., Rachlewicz, G., Young, A., Zetterberg, P., Welker, J. M. (2020). Divergence of Arctic shrub growth associated with sea ice decline. Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2013311117
Trees are perennials with one growth period per year. In the Arctic, birch and willow grow in length (in fact creeping along the ground) but also in width. Every spring, a new annual ring is formed, consisting partly of a light spring-wood, because the structure is open and provides space for water transport, followed by a darker autumn-wood. This makes it possible to measure the thickness of single year's growth and thereby going back in time to study how growth conditions have changed.
When you know the year for when the sample was taken, you can count backwards and link a single year's growth to a specific growing season. Hot and humid years typically produce the thickest annual rings. It sounds simple, but the work is complicated because the annual rings in the Arctic are very thin and therefore can only be measured under a microscope. Further complications include that trees can form false annual rings or lack an annual ring e.g., in a year with massive frost damage. Therefore, a lot of plants and many sections must be counted for each plant to get a robust dataset.
A cross section of an 80-year-old Salix arctica (arrow) from Brønlundhus in North Greenland with a maximum thickness of just under 2 cm and a plant height of 8 cm. This type of dwarf growth is typical in the north, but to the south, where it is warmer and the availability of water and nutrients is greater, the same species can easily grow 2 m tall. Microscopy image: Agata Buchwal, first author of the new study.
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