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.