Detecting Changes in a Permafrost Peatland from 1943 to 2013 with Historical and Recent Remote Sensing Data.

Peatlands or mires contain about one third of the global terrestrial carbon pool and are located on between 3-6% of the global land area. In boreal and sub-arctic permafrost peatlands the soil organic carbon (SOC) pools are stable and decomposition is suspended only as long as the soil is frozen. Climate warming is projected to be greater in the high latitudes, observed mean annual air temperatures in northern Sweden have increased by 2-3oC since the 1950s. Thawing permafrost leads to new hydrological regimes potentially leading to increased production of methane. In this study, two sets of data were analysed: (i) a stereo-pair of black and white aerial photographs acquired in August 1943 by the Swedish Airforce, with a spatial resolution of 50cm, and (ii) a geo-rectified Worldview2 (WV2) multispectral image acquired on the 24th of July, 2013. The aerial photographs were digitized using a very high resolution camera, georeferenced and incorporated into a geodatabase. The analysis of image areas was performed by heads-up visual interpretation both on a computer monitor and through stereoscopes. The aim was to identify wet and dry areas in the palsa peatland. Feature Analyst (FA) object oriented image analysis (OBIA) was used with the WV2 dataset to extract features that are related to the hydrological state of the mire. Feature Analyst is an extension to ArcGIS. The method uses a black box algorithm that can be adjusted with several parameters to aid classification and feature extraction in an image. Previous studies that analysed aerial photographs from 1970 and 2000 showed that there was an increase in the amount of wet areas on the Swedish palsa bog mire Stordalen. In this study we determine the change in wet areas over a seventy-year period. The central part of the palsa mire has been extensively studied as it has been presumed that it has collapsed due to warmer temperatures in recent decades. However, our analysis shows that much of the internal hydrological patterns on this part of the palsa bog seem to be temporally stable, at least since 1943. Macro changes not identified in previous studies are observed here where it can be seen that the extent of the palsa has retreated, in areas contiguous to streamflow, possibly in response to contact with relatively warmer streamflow.