Discussion
We found a significant difference in the canopy cover of the browses
between the burned and unburned area of the Alphabet Hills. Burned sites
had a higher species diversity, which could prove to be more beneficial
to moose if forage diversity lowers the effect of toxins on digestive
function, a hypothesis referred to as the detoxification limitation
hypothesis (Marsh et al. 2006). This hypothesis states that reliance on
a single food source with high amounts of PSMs can limit the overall
detoxification system of an animal (Freeland and Janzen 1974). An
overwhelming amount of PSMs can negatively affect the gut microbiome
which can further limit intake and digestion (Freeland and Janzen 1974,
Dearing et al. 2000, Marsh et al. 2006). Although the forest had higher
canopy cover and higher bite frequency, the overall diversity of the
burn plots may have more of a significant role in the tradeoff between
quantity vs quality of available forage in an area.
Our findings suggest that browse availability in our study site, as
measured by ground cover, was greater in the forest than in the burn,
contrary to the findings of others (Hobbs and Spowart 1984, Blair 1997,
Greene et al. 2012). We attribute this to several potential factors that
could influence the recovery of a site after fire, including the burn
severity, the site conditions prior to the fire, and that many of the
unburned sites were within the total burn perimeter suggesting they were
protected from the fire. The Alphabet Hills area is classified as an
open spruce forest/shrub/bog or as an open and closed spruce forest
(Landhausser and Wein 1993, Crevoisier et al. 2007), which are
characterized by cold, poorly drained organic soils, and overall low
productivity. The forest sites may have been protected from the burn due
to their location of the landscape near more riparian areas or areas
where moisture could accumulate due to the area’s topography. Because of
their potential exposure to higher moisture levels as well as retention
trees, or trees that remain alive and standing after a fire (Den Herder
et al. 2009), the browse in forest sites may have had higher
productivity due to reduced stress. The fire history of the area, as
well as the soil characteristics, could explain why there was greater
average canopy cover in the forested sites over the burned sites. The
burn characteristics of the Alphabet Hills fire are more aligned with
the establishment stage of a younger fire which would also explain why
we did not see a significant increase in browse availability in the
burned sites vs the unburned sites.
One potential influence that was not included in this study is the
effect of fire se-verity on nutritional quality and quantity. Intense,
high severity fires could either promote the growth of aspen and other
moose preferred browse due to their ability to bud from their roots, or
they can favor lower quality species that are wind dispersed like birch
(Joly et al. 2016, Wan et al. in press). Although we have relative scale
of severity and how our sampling points align with the fire severity, we
were unable to attribute the variation in plant chemistry to fire
severity. We chose to group plots into burned and unburned to just look
at the two major habitat groupings; this also groups riparian habitats
and habitats with varying sun exposure that were found inside and
outside the burn perimeter. Future work would need to be done to expand
the number and location of sampling sites, as well as adding other
characteristics that indicate changes in severity, such as soil
temperatures at various depths and depth of fuel consumption, to
quantify the effect fire severity has on browse species and therefore
moose.
The significant decrease in digestible energy in the burn over the
forest could be due to a higher desiccation rate in the burn due to the
lack of cover. Shaded plants have been found to have higher levels of
chlorophyll to take advantage of reduced sunlight and thinner cuticle
layers which reduces the amount of indigestible material for a moose (Bø
and Hjeljord 1991, Gratzer et al. 2004, Hagemeier and Leuschner 2019).
Differences in light availability will also change the distribution of
nutrients and moisture in the soil, further affecting growth and
allocation of resources (Bø and Hjeljord 1991, Gratzer et al. 2004).
Warmer temperatures, both ambient and soil, have been shown to increase
fiber deposition for browse species, lowering the quality for moose (Bø
and Hjeljord 1991, Barboza et al. 2018). For immature plants with
extremely low cell wall concentrations (low structural carbohydrate
concentrations), we may have underestimated DDM and DE. The equations
for digestibility derived in moose by Spalinger et al (2010) were based
on forages of much higher structural carbohydrate concentration, and
plants below approximately 20% NDF fall outside the domain of those
equations (Robbins et al. 1987, McArt et al. 2006, Spalinger et al.
2010).
Moose are not immobile, although we found that the forested sites had
higher abundance and quality of browse, the moose found within the
Alphabet Hills and the Nelchina basin can move between habitats and in
and out of the area to maintain healthy body condition, promote growth,
and avoid predators. In the summer, females may choose areas that
provide more cover for calving (Oehlers et al. 2011, Joly et al. 2016).
In the winter, after reducing intake rates during the mating season,
males may choose riskier habitat to prevent starvation (Oates et al.
2019). Habitat use is highly dependent on the season, this study chose
to focus on the changes in habitat quality as summer progresses because
that is a critical time for weight gain and growth of new tissues for
males and females (McArt et al. 2009). Because we found an increased
availability of biomass, as well as a higher quality of the browse in
the forest, we may expect that moose in the area will spend a majority
of their time within the forested areas (Searle et al. 2005, 2007).
However, because of the increased diversity of species in the burned
area, it could be a more important habitat for when the plants have the
highest deposition of PSMs. For a moose, the overall dietary value of a
forest stand is transient, often lasting less than 30 years, but moose
need access to a wide age range in forest stands for protection from
predators and available cover for their young (McMahon et al. 2000, Osko
et al. 2004, Dussault et al. 2006, Joly et al. 2016). Further work is
necessary to link movement of individuals throughout the area to the
nutritional quality and bite frequencies throughout the year in this
fire as well as other fires across the state.
Although others have shown a positive effect of wildfire for herbivore
populations, we found that some areas, including the Alphabet Hills
area, may not be suitably adapted to benefit from the quick release of
nutrients after fire and may not allocate more resources to biomass as
previously expected (Hobbs and Spowart 1984, Rea and Gillingham 2001).
These findings highlight the importance of quantifying both availability
and quality of food resources for herbivores, when addressing the
benefits of fire. We showed that a traditional method of increasing
available habitat for moose is not applicable for all habitats or areas.
In the future, habitat management strategies that include wildfire
should consider the history of the area and the pre-fire communities, as
well as how fire can shift resource availability. Field surveys of
pre-fire community characteristics such as understory community
composition and individual species densities with paired surveys of
plant chemistry is necessary to ensure that the fire is increasing
biomass reachable by moose, while also increasing the nutritional
quality. Soil characteristics, such as its water retention capacity and
microbial community can also have a significant effect on post fire
vegetation communities and their nutritional quality after wildfire and
should be assessed before prescribing fires for habitat enhancement.
Although logistically challenging, predictions of how fires may affect
wildlife should be based on assessments of previously burned stands
having similar physical and biological characteristics.