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.