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).