Figure 3 . Median (black), 75% (dark ribbon), and 90% (grey ribbon) quartiles of spectral reflectance for eight plant functional types from the Arctic tundra biome. Sample size (n ) is shown parenthetically. Sentinel-2 bandpasses are indicated with vertical bars to illustrate the advantage of imaging spectrometers with contiguous bands over multispectral instruments. Spectra were collected in the field with leaf clip or contact probe and illumination source across Alaska between 2010-2019, primarily 2017-2019. Most of the data were collected with a Spectral Evolution PSR+3500 under AVIRIS-NG flight lines +/- 14 days of flight in most cases. Spectra were collected at 1 nm resolution and trimmed to 450-2400 nm to remove sensor artifacts.

3.2. Lichens

Lichens reach high diversity, cover, and biomass in certain tundra ecosystems and play a significant role in biogeochemical and physical processes, such as land-atmosphere radiative exchange, hydrological buffering, and nitrogen (N) cycling (Cornelissen et al., 2007). The genus Cladonia (reindeer lichens) create dominant carpets across the Arctic that likely represent the majority of lichen cover and biomass. Other genera do contribute significant biomass and cover, such as Cetraria , Flavocetraria and Stereocaulon all which grow mostly upright and intermixed with bryophytes, lichens and other plants. However, talus slopes and other rock surfaces are often covered with very different genera (eg. Rhizocarpon andAspcilia , both crustose or stain-like growth forms that can cover boulders and talus fields), which creates complexity in estimating the total cover of lichens. Lichens contribute substantial ground cover in periglacial environments, stabilizing soils (Makoto & Klaminder, 2012). Albedo varies widely among lichen groups, with implications for heat exchange with fractional cover variability (Aartsma et al., 2021). A large fraction of biodiversity of terrestrial vegetation in the tundra is composed of lichen species. Most caribou and reindeer survive in northern climates, in part, by eating mostly lichens throughout winter months (Heggberget et al., 2002; Joly et al., 2007). A major opportunity for SBG to enhance wildlife habitat mapping will be to use the unique spectral signatures to separate lichen groups (Macander et al., 2020; Nelson et al., 2013; Petzold & Goward, 1988; Rees et al., 2004).
Physiological differences between lichens and vascular plants affect their spectral reflectances. Lichens have more broadly different cellular structure than vascular plants. The upper surfaces of most lichens, composed of fungal cells of one or sometimes two fungi (Spribille et al., 2016), often with pigments, protect the next inner layer of cells, usually composed of the photobiont (algae, cyanobacteria, or both). The upper cortical cells of lichens are usually dense and have high concentrations of pigments produced by one or both fungi that are attributed to photoprotection. These fungal pigments protect the algal photosynthesis machinery by dealing with reactive oxygen species produced by high irradiance by dissipating excess energy as thermal wavelengths (Beckett et al., 2021). Under the cortex, a thin layer of photobiont (algae, cyanobacteria, or both) receives sufficient light for photosynthesis. The parts of the spectral signature of lichens similar to vascular plants belies the presence of the photobiont(s). After the photobiont, little if any light likely penetrates in the fungal structural backbone of a lichen body, the medulla, which is often thick, white or pale. Amongst the > 12,000 species of lichens, there is a diversity of mixtures of cortical cell structure, chemistry and photobiont that contribute to the spectral signatures of lichens.
Lichens are spectrally variable both within and among species, but compared with vascular plants, tend to have higher reflectance in the visible range and lower reflectance in the NIR (Figure 3). Hundreds of compounds, many with pigments detectable in the visible range, can be found across the diversity of tundra lichens. These complex molecules aid in differentiating lichens from vascular plants but also make modeling lichens as a group difficult. However, most mapping efforts have treated lichens as a monolithic group, focused on one relatively homogenous color group (e.g., light) (Macander et al., 2020) or at most treated lichens in a few color groups (Nelson et al., 2013). Lichen spectral signatures indicate high degrees of variability within and among species (Kuusinen et al., 2020; Petzold & Goward, 1988; Rees et al., 2004). Lichens have no true vascular tissue therefore hydration is based on short term meteorological conditions which in turn drives short term metabolic activity of lichens (Lange et al., 1996). Nonvascular plants, including lichens and bryophytes (i.e., mosses, hornworts, and liverworts), lack true vascular tissue (parenchyma) and therefore passively dessicate and rehydrate (poikilohydry) (Walter, 1931). The hydration status of lichens greatly influences the overall magnitude of reflectance as well as spectrum shape (Kuusinen et al., 2020; Rees et al., 2004) but the difference between dry and wet lichen spectra varies both across wavelengths and species. Water content can be estimated for lichens (Granlund et al., 2018) but uses wavelengths beyond those proposed for SBG (i.e., > 5000 nm). A key challenge for SBG in the Arctic will be accounting for water content in spectral profiles of the lichen (and bryophyte) mat since photosynthesis and respiration are both tied to hydration. Rapid changes in hydration make observations of productivity fleeting and unstable in non-vascular plants. To address the impact of hydration state on the reflectance profiles of non-vascular plant communities, diurnal and seasonal spectral measurements with high temporal density colocated with in situ moisture probes are needed.
Lichens tend to be very small organisms but in the tundra can form confluent patches of varying sizes and mixtures of patches with different species and other organisms. Studies of tundra with coincident imagery of different spatial resolutions suggest pixels smaller than 3 m are needed to accurately classify patches (Räsänen & Virtanen, 2019) with a loss of 30% absolute accuracy associated with declining resolution (2-20 m) (Virtanen & Ek, 2014). Another key challenge for leveraging observations from SBG will be the fact that the composition of surfaces in 30 m pixels will have a wide range of pure patch sizes, from centimeters to meters.
There are few measurements on the phenology of pure lichen patches. Measurements of tundra mixtures with abundant lichens display limited seasonal variability (John A. Gamon et al., 2013) with spectral changes mostly associated with moisture status. This may be one of the few positive features of lichens for remote sensing and SBG. To take advantage of this, SBG could use observations after snow melt but before green up and then after leaf-off but before first snow to observe lichen (and bryophyte) dynamics in more detail. At those times, non-vascular vegetation would have less over-topping vegetation, reducing occlusion from nadir-viewing sensors.

3.3. Bryophytes

One of the main features of the tundra are the bryophytes, which can be found growing on most surfaces and conditions, from fully immersed in water to exposed rock or bare soil. Bryophytes (i.e., mosses, hornworts, and liverworts) usually appear as mats or patches of miniature plants formed by multiple individuals. Bryophytes can form the primary understory vegetation in many tundra plant communities, from wet, acidic bogs where Sphagnum spp. dominate to the fine matrix of moist tundra where numerous species of bryophyte form dense mats interspersed with lichens and vascular plants. In wet environments, Sphagnumspp. can create large colonies with deep accumulation of senescent material storing carbon as peat. In less hydric sites, Hylocomium splendens (stair step moss) and Pleurozium schreberi (big red stem) are dominant. They have exceptional hydrologic and thermal buffering qualities and are tied to the formation and stability of permafrost (Blok et al., 2011; Shur & Jorgenson, 2007). Bryophytes such as Polytrichum spp. and Ceratadon purpureus can also form short-lived but extensive colonies post-fire which aid in stabilizing carbon recovery. They are crucial to carbon sequestration and storage, protecting the permafrost layer while also forming a living layer beneath a sparse vascular plant canopy. Despite their obvious importance to Arctic ecosystems, bryophytes have been largely neglected in remote sensing except for narrow cases like Sphagnum spp. (Angela Harris & Bryant, 2009; Huemmrich et al., 2013).
Bryophyte physiology differs vastly from vascular plants, primarily due to reduced-to-absent vascular tissue. By virtue of this, bryophytes can absorb large amounts of water, but are not able to actively regulate moisture content via a root system like vascular plants. Instead, bryophytes form colonies, sometimes only with one species but often with many species, which together determine hydration through water holding capacity of the living layer. As a result, bryophytes may hydrate or dessicate quickly. Similar to lichens, bryophyte hydration status is known to significantly influence spectral reflectance, with many changes observable in the visible to short-wave infrared spectra (Van Gaalen et al., 2007; Vogelmann & Moss, 1993) (Figure 4).