Monitoring of Swedish Palsas using very High-Resolution Satellite Data
Wester, Kjell1; Per, Wramner1; Backe, Susanne2; Hedvall, Thomas1; Gunnarson, Urban3; Abenius, Johan4
1Brockmann Geomatics Sweden AB, SWEDEN; 2County Adm. Norrbotten, SWEDEN; 3County Adm. Dalarna, SWEDEN; 4Swedish Environment Protection Agency, SWEDEN

Palsas are peat mounds or plateaus in mires with a core of permanently frozen peat and/or mineral soil found in the discontinuous permafrost zone (figure 1). In Sweden, they mainly occur in a zone delimited by an annual mean temperature of -2 to -3° C and winter precipitation of less than 300 mm. Palsas go through a cycle of stages. Initially, a local creation of permafrost with associated frost heave causes the bog surface to rise, a process which can be self-accelerating and result in the growth of a palsa. The mire vegetation is successively replaced by plant communities adapted to drier conditions. The palsa raises to a mature stage, which can last for a long period, but finally a degradation phase usually starts. Lateral so called block erosion of palsa slopes, melting and collapse linked to ponds on palsas, deflation etc. take over and result in a final complete degradation of the palsa. Only a water body more or less surrounded by unfrozen palsa remnants remains. It is gradually filled by vegetation resulting in a new mire surface where palsa formation can start again.

Figure 1. Five meter high dome-shaped palsas in Tavvavuoma, northern Sweden (photo: Susanne Backe).

Palsa mires belong to the so called Natura 2000 habitats (habitat 7320). That means that Sweden has a specific responsibility, according to EU legislation, to monitor the development of and preserve ("keep in a favourable conservation status") all palsa mires which are pointed out as Natura 2000 sites. The main threat to palsa mires, according to literature and results of the ongoing project, is climate change which has resulted in a substantive increase of palsa degradation (figure 2). Therefore, monitoring of such degradation has to be a major component of all efforts to preserve palsa mires.

Therefore, in order to fulfill its obligations vis-a-vis the EU, Sweden has to set up palsa monitoring programmes with an emphasis on palsa degradation. The tremendous amount of work needed to monitor Swedish palsas through field work alone, together with the concentration of such mires to remote parts of Lappland, mean that any monitoring programme for palsa mires has to a large extent to be based on remote sensing techniques. The necessity to pay great attention to degradation of palsas means that there is a strong need for remote sensing techniques which make it possible to monitor degradation processes (figure 2). Also development of new palsas should be considered in this context.

Figure 2. Degradation of palsas.

The overall purpose of the project is to develop an operational method, with very high-resolution satellite data as a main component, to specifically monitor degradation of palsas and include it in the SEPA sub-program "climate-related monitoring", thereby making it publicly available.

Water is a key agent for the decay of palsas, primarily block erosion but also other types of degradation, mainly because dry peat isolates the frozen palsa core during summer much better than wet peat. An open water body is also the final result of degradation of palsas. Therefore, different sizes and forms of water bodies on and adjacent to palsas could be used as indicators for the palsa status, particularly as regards degradation. The questions to be answered using high-resolution satellite imagery include frequency, location, size and form of water bodies, amount of vegetation in the water and water characteristics (e.g. humus content). Figure 3 shows an example of combining very-high resolution satellite data with laser data for mapping of palsas.

Figure 3. Mapping of palsas using laser scanning together with very-high resolution satellite data.

A method that can be used in a cost-benefit way for monitoring palsa mires is relevant for follow-up of both environmental goals and the Habitat Directive and thus of interest for the Environmental Protection Agency as well as the County Administrations, since traditional methods are both time- and cost consuming and will not give a full-cover aerial result. A method for monitoring palsa mires in a cost-benefit way over vast areas is also of interest for a broader community in terms of contributing with information on climatic change. At the national level, our results will be operationalized by being used in the national monitoring program for palsa mires. Our intention is also, at a later stage, to apply for additional funds for international cooperation to promote export of operational monitoring knowledge/experience.