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.