## 8 Challenges and Promising Approaches
Even in the case of ten (surficial material) classes ([PERSON], 2016), a lower number is eventually relevant for permafrost-related classes, as they represented also non-permafrost types. This demonstrates that the potential of satellite data to conclude on sub-ground thermal properties is limited. Only general classes can be derived, and applicability has been proven in permafrost transition zones only. The relevant types are peatlands (with differentiation by vegetation cover) ([PERSON], 2016) and ecotypes that distinguish between low and tall shrubs, as well as their characteristic moss and organic layers ([PERSON], 2016). This demand for details on shrubs coincides with statements in studies for numerical permafrost modeling ([PERSON], 2016; [PERSON], 2016). They are important for the modeling of snow distribution (and subsequent impact on ground thermal conditions). A height threshold of 0.5 m is suggested for permafrost modeling purposes ([PERSON], 2016). A range of recent publications on the use of polarimetric SAR data ([PERSON], 2016; [PERSON], 2016; [PERSON], 2016) has shown the potential of such data for shrub mapping, but the applicability remains limited since SAR data are only locally acquired in fully-polarimetric mode. Current acquisition strategies of SAR missions only provide equal and dual polarized data. Shrub mapping in general needs to consider phenological stages ([PERSON], 2016). There is no generally agreed upon classification scheme for the northern high latitudes. [PERSON] et al. (2016) have also pointed this out as an issue for in situ mapping across the Arctic recently. Classes of regional maps for multiple purposes, such as up-scaling of carbon pools and fluxes, represent shrub physiognomy (dwarf species or higher) and wetness patterns. Shrubs are included as a class in global land cover maps, but are either not present in their Arctic parts or patterns differ among them. Wetlands or wetness descriptions are in general under-represented in global maps. They can have differing vegetation coverage. The same applies to lichens. They are omnipresent. Vegetation classes need to be reviewed and described for their associated lichens. The class 'lichens and mosses' in the CCI land cover can be interpreted as either cryptogam crust or graminoid dwarf shrub tundra. When lichen species are dominant and not covered by the canopy of higher vegetation, they are part of cryptogam crusts or develop within vegetation communities with graminoids. This should be labeled accordingly to avoid confusion with fruitcose lichen heaths in boreal environments. Alternatively, lichens can be addressed by a vegetation community approach or by quantification of their bias on reflectance. A good example for thematic content is the scheme developed by [PERSON] et al. [108] for classification of Landsat data for Canada. Several graminoid and growth from dependent shrub classes are included. They specifically also address PFTs. In total, 17 PFTs are suggested for north of the treeline. The suitability of the suggested scheme for high latitudes, as well as the classification for Northern Alaska by [PERSON] et al. [46] has been recently pointed out in a review by [PERSON] et al. [137]. Classes for high Arctic tundra are also suggested by [PERSON] et al. [138] (graminoid types, lichens, mosses and forb). They are however derived using high spatial resolution (WorldView) data. The heterogeneous distribution of the PFTs can be captured at the local scale only. Time series representing phenology have been applied in all three cases [108; 138; 46]. Strategies for classifications on global and regional level differ regarding the type of input data. Acquisitions from several points in time are used also regionally, but seasonality is in general not considered. Global classification schemes make use of mosaics of different time intervals and comply with international classification schemes. These schemes do not reflect the diversity of landscape types in high latitudes, which are required in permafrost regions for common applications.