Figure
1: The Alphabet Hills fire, burned in 2004 for habitat management, is
located west of Glenallen, AK. The map on the left shows the fire’s
location in relation to the total fire history of Alaska while the map
on the right shows the burn perimeter, the fire intensity readings
(MTBS.gov), and the sampling points in purple. The legend shows the
range in severity from high to low, including areas that were not
processed or masked, as well as the acreage considered to be within each
category.
Although moose in this area have access to large amounts of habitat,
they often show indications of nutritional stress. Twinning rates and
age of first parturition are significantly lower than their reproductive
capacity (Testa 2004, Boertje et al. 2007). This led to an intensive
management plan that includes predator controls and habitat management
strategies such as prescribed burns. The Alphabet Hills, within the
Nelchina Basin, was burned in 2004 as part of a management plan to
increase moose habitat. The anticipated increase in moose productivity
was expected to increase harvest opportunity, and to buffer losses to
predation.
Stand succession, and stand quality after fire, is highly dependent on
the pre-fire characteristics of the area as well as fire severity.
Higher intensity fires can volatilize nitrogen that otherwise would be
available to growth of forages and ultimately to the quality of moose
diets (Boerner 1982, Bayley et al. 1992). The volatilization of nitrogen
could be especially important in boreal regions due to the natural
limitation of nitrogen in boreal soils. In addition, the response of
plants to allocating carbon and nitrogen to growth vs defense is likely
dependent upon browsing severity, climate and site conditions, nutrient
availability, and plant competition. Taken together, it is difficult to
predict the impact of natural or prescribed fire on moose populations or
their productivity.
Timing since burn is itself an important factor for moose forage
selection (Brown et al. 2018). Moose are considered near-obligate
browsers, consuming the leaves and terminal twigs of a variety of
deciduous woody trees and shrubs, particularly of the genus Salix.
Hence, they are often associated with successional and riparian
communities where such plants are abundant. In the boreal forest biome,
where moose are most abundant, the climax community is typically
coniferous, and in North America, dominated by black (Picea
mariana ) and white spruce (Picea glauca ). However, early
successional stages following wildfire in spruce forests are generally
dominated by deciduous browses. Previous studies (Vitousek and Reiners
1975, Regelin et al. 1987, Julianus 2016) have shown that moose appear
to select successional stands that are between 10 to 20 years in age
post disturbance compared to stands that are less than 10 years or
greater than 25 years post disturbance. For younger stands, preferred
species could be small and harder to reach by moose, but, primarily,
snow depth could also limit their use during the winter by burial of
stems and inhibiting movement through an area (Vitousek and Reiners
1975). Deeper snow also correlates with increased selection of riparian
areas and areas at lower elevations that could increase localized
predation risk (Joly et al. 2016). Stand replacement occurs between 30
and 40 years when the dominant species shift from deciduous shrubs to
spruce, and competition with spruce limits the amount of forage in the
area (Regelin et al. 1987, Brown et al. 2018). Studies focusing on
habitat selection show that during the winter, moose did not select for
areas until 10 to 25 years after a disturbance because deciduous stems
were buried by the snow prior to that (Regelin et al. 1987, Joly et al.
2016).
The Alphabet Hills area of the Nelchina Basin is classified as an open
spruce forest/shrub/bog or as an open and closed spruce forest (Viereck
1992, Markon and Walker 1998). These stands are typically characterized
by cold, poorly drained organic soils, and overall low productivity
(Landhausser and Wein 1993, Crevoisier et al. 2007). Alaskan black
spruce fires typically occur every 70 to 125 years, with
replacement/high severity fires that kill 75% of the upper canopy
occurring in 45 to 85% of fires (Landhausser and Wein 1993, Crevoisier
et al. 2007). Fire can reduce the overall organic layer thereby
increasing water drainage and limiting water available to plants (Yi et
al. 2009). Post-fire environmental conditions, such as increased UV
exposure, water stress, and nutrient leaching/loss may place additional
stresses on plant growth, which may result in an increase in secondary
defense compounds such as condensed tannins. Condensed tannins are
thought to have originally evolved to protect the plant from water
stress, but they are also made to protect the plant from herbivores
(Herms and Mattson 1992). Tannin concentration is higher in the early
growth stages of browses important for moose which may be a result of
induced chemical defenses of the newest, most productive tissues (Bryant
and Kuropat 1980, Bryant et al. 1983, Herms and Mattson 1992, Spalinger
et al. 2010). High condensed tannin concentrations in conjunction with
relatively low protein concentrations in some plants can result in net
negative protein gain for an herbivore due to its ability to bind both
plant proteins and animal/microbial proteins endogenous to the GI tract
(Spalinger et al. 2010, Tharayil et al. 2011, Adamczyk et al. 2017).
In burned habitats, defensive compounds may also increase because of
environmental stresses that change the resource allocation patterns of
plants. For example, increased solar radiation and more xeric soil
conditions can induce water stress in summer (Herms and Mattson 1992,
Gundale et al. 2010), leading to increases in oxidative stress
responses, including polyphenolic concentrations (Chapin, 1991, Rau et
al. 2008, Tharayil et al. 2011). Heightened solar radiation can also
slow overall photosynthetic rate due to a build-up in sucrose-levels
that limits the movement of carbon dioxide (Sweet and Wareing 1966). The
diminished photosynthetic rate limits the productivity of the tissues
which reduces the overall quality available for herbivores (Sweet and
Wareing 1966). Increased tannin concentration may affect the microbiome
of the rumen which ruminant species rely on to digest woody browse
(Solden et al. 2017). This may result in lower nutritional quality,
particularly nitrogen availability, to large herbivores, potentially
limiting animal productivity (McArt et al. 2009). High concentrations of
condensed tannins can also further limit nitrogen availability for
plants in boreal forests by binding to amino acids and/or soil microbial
exoenzymes in the soil once deposited by fire, creating a positive
feedback loop (Gundale et al. 2010). This therefore limits uptake by
microorganisms of both proteins, and potentially carbon as well (Gundale
et al. 2010).
Development of seral vegetation following fire may attract an
overabundance of moose, leading to a reduction in the overall quality of
those habitats through selective removal of the most nutritional
species. Fire can also redistribute moose to early seral stands, in turn
leading to a reduction of the overall quality of those habitats through
the removal of the most nutritional species (Collins and Schwartz 1998,
Shipley 2010, Collins et al. 2011). Over browsing by moose can result in
a shift in dominant species from preferred species like Salix spp. to
the less nutritious Betula spp. (Christie et al. 2015). Reduction
in forage diversity because of over browsing can also reduce the
opportunity for the animal to balance toxin loads. Browsing by moose
also has the potential to keep shrubs within browsing height, making
them easily outcompeted by spruce (Pastor et al. 1988).
The objectives of this study were (1) to test the hypothesis that
wildfire creates compositional and chemical changes in plants in burned
habitats that are beneficial to moose and (2) to examine the potential
tradeoffs between food abundance and quality on nutritional status of
moose.