Cassiope tetragona as thermometer
Posted by Ari Jokimäki on October 12, 2010
There are not much temperature series from Arctic and it also has been difficult to obtain reliable proxy based temperature reconstructions from the region. Recently published study has made a 131-year temperature reconstruction from Svalbard based on the changes in the annual growth of Cassiope tetragona.
Arctic regions have been estimated to be most affected by the climate change, and therefore it is important to have a good knowledge of the current and past climatic conditions in the region. There aren’t much temperature series from Arctic regions, and even the few series don’t extend very far to the past. There is a need for other accurate temperature indicators, which could also be used to interpret the past temperatures from the region. Here we encounter another problem. There aren’t really growing trees in the Arctic and smaller shrubs of the region tend to have very thin annual rings, which they don’t necessarily even produce every year. Thus, the annual tree-ring based methods are of no help in the problem.
In 1989 there was one method presented for determining temperatures from vegetation changes in the Arctic areas (Callaghan et al., 1989). The method is based on annual growth of Cassiope tetragona. Cassiope tetragona is an evergreen plant with white bell-shaped flowers and scale-like opposite leaves. Cassiope tetragona belongs to the heather plants. Cassiope tetragona is a perennial plant and is found in the arctic regions of Europe, North America, and Asia.
Cassiope tetragona grows a lot of leaves. It may have more than 230 in a twenty year old shoot. The leaves remain green for a long time and they do photosynthesis at least four years. Subsequently, leaves turn first yellow or red, and after couple of additional years they turn brown.
The new leaves of Cassiope tetragona are shorter in spring and autumn than in summer. This creates a wavy pattern which makes it possible to identify the growth of a single year and thus offers an opportunity to examine the association of the annual growth length and the number of new leaves to the climate.
Earlier studies have focused mainly on the details of the Cassiope tetragona annual growth association to the climate. These studies have shown that it is possible to use Cassiope tetragona for making temperature reconstructions. So far there’s not many actual temperature reconstructions based on Cassiope tetragona. Previous reconstructions have been up to hundred years in length (Rayback & Henry, 2006).
In a new study from Svalbard, a 131-year temperature reconstruction based on Cassiope tetragona annual growth was made (Weijers et al., 2010). The annual growth in length was measured from 32 plant samples. This resulted in a 169-year chronology of the annual growth length. This is the longest chronology so far, and the temperature reconstruction made from the chronology is also the longest so far.
The chronology’s connection to temperature and other climatic parameters was studied using different statistical methods. It turned out that the July temperature describes the annual growth length best. Behind that, other important climatic factors associated to the annual growth are the previous year’s precipitation in September and the average air temperature.
A reconstruction of July temperatures was made from the chronology. The reconstruction covered the period from 1876 to 2007. Reconstruction was calibrated with the measured temperatures from the region (which exist between 1912-2007). Reconstruction correlated quite well with the measured July temperature (R2 = 0.34 for calibration period 1912-1959 and R2 = 0.47 for the reference period 1960-2007). Temperature reconstruction together with the measured temperatures reveals that Svalbard has experienced significant warming after the year 1876. The amount of warming on average has been 0.07 degrees celsius per decade.
Temperature reconstructions from Cassiope tetragona thus appears to offer little relief to the sparseness of climatic data in the Arctic. Especially good aspects are the availability of Cassiope tetragona in the Arctic tundra also as fossilized, and the wide spread of Cassiope tetragona throughout the Arctic region. These factors together with good association between Cassiope tetragona annual growth and climate means that in the future we can expect to see temperature reconstructions reaching longer into the past and covering the entire Arctic vegetation region.
Stef Weijers, Rob Broekman and Jelte Rozema, Quaternary Science Reviews, 2010, Dendrochronology in the High Arctic: July air temperatures reconstructed from annual shoot length growth of the circumarctic dwarf shrub Cassiope tetragona, doi:10.1016/j.quascirev.2010.09.003. [abstract]
Shelly A. Rayback, Gregory H. R. Henry, Reconstruction of Summer Temperature for a Canadian High Arctic Site from Retrospective Analysis of the Dwarf Shrub, Cassiope tetragona, Arctic, Antarctic, and Alpine Research, Volume 38, Number 2 / May 2006, Pages 228-238, DOI 10.1657/1523-0430(2006)38[228:ROSTFA]2.0.CO;2. [abstract]
See also my list of papers on Cassiope tetragona as a climate proxy