Spectral Characteristics of Tundra

3.1. Unique Characteristics of Tundra Surfaces

Lichens, bryophytes, and vascular plants occur in different proportions along gradients of climate, soil properties, and landscape history in the Arctic (CAVM Team, 2003; Epstein et al., 2008, 2020) and possess different physiologies and spectral reflectance patterns. This variability poses unique challenges for remote sensing of tundra vegetation properties, but an understanding of geographic patterns of vegetation structure and function can help interpret such measurements. (Walker et al., 2005) provide a framework to characterize the central tendencies of Arctic tundra structure and composition by dividing the biome into five bioclimatic subzones (A–E) distributed along gradients of summer temperature. The subzones range from the coldest Subzone A, found in coastal areas of the High Arctic with persistent summer sea ice, to the warmest Subzone E, generally found in continental areas near the northern limit of tree establishment. Subzone A, occasionally termed “polar desert” (Matveyeva, 1998) is characterized by discontinuous vegetation cover that is typically dominated by nonvascular vegetation; shrubs and sedges are usually absent, vascular plant diversity is very low, and a large proportion of the ground surface is unvegetated. In Subzone B, lichens and bryophytes dominate the cover and shrubs are generally limited to only a few species (e.g., Salix arctica, Dryas spp.) with a prostrate growth form (< 5 cm height). From Subzone C southward, vascular plants - particularly shrubs - occur at greater abundances and species richness, and of higher stature. In Subzone E, vegetation is typically continuous and forms a multi-layered canopy, with shrubs commonly reaching heights of > 80 cm. Near the southern boundary of Subzone E, broadleaf and needleleaf trees are often present. The tundra-taiga ecotone (TTE) is typically a diffuse transition zone where trees first occur as isolated patches within the tundra matrix and become more abundant and spatially dense southward and at lower elevations. In North American and European ecotones, tree cover is generally dominated by evergreen species (e.g., Picea, Pinus ), whereas deciduous needleleaf species (Larix ) are dominant in Siberian TTE. Within each bioclimatic subzone, there is a great deal of heterogeneity in the relative abundance of plant functional types along landscape-scale gradients of moisture, topography, permafrost, and soil properties. Thus, IS applications must consider the relative abundance of plant functional types along both circumpolar-scale climate gradients and landscape-scale environmental gradients.
Furthermore, although plant functional types are expected to share suites of similar traits, within plant functional types there can still be enormous variation among traits that are important for ecosystem function (Table 1). This trait diversity corresponds to spectral variation within individual plant functional types in (Figure 3).
Table 1 . Summary of heights, patch sizes, dominant taxa, and distributional patterns of plant functional types in Arctic tundra ecosystems and forest-tundra ecotones. For patch sizes, minimum values refer to typical individual plants, and maximum values refer to contiguous areas in which the functional type forms the top of the canopy.