Introduction
Ecologists and wildlife managers
generally agree that throughout the world, large-scale disturbance, such
as wildfire, can be beneficial for species of large herbivores that are
adapted to seral plant communities. Burned plots in states like Colorado
and Nevada have been shown to increase crude protein intake for mule
deer (Odocoileus heminonus ) and bighorn sheep (Ovis
canadensis ) in grassland and montane shrub communities (Hobbs and
Spowart 1984). Higher abundances of bison (Bison bison ) were
found in recently burned watersheds within the Konza Prairie Research
Natural Area in Northern Kansas (Knapp et al. 1999). In summer, these
herbivores select grasses and herbaceous plants that respond rapidly to
the release of post-fire nutrient deposits (Boerner 1982). In boreal
forest ecosystems, post-fire stands are commonly assumed to support
higher populations of moose (Alces alces ) than adjacent unburned
stands, but quantification of the impacts of fire on moose population
productivity is limited. Fire, both natural and prescribed, is
considered beneficial to moose because deciduous woody browses and
herbaceous foods (e.g., Chamerion angustifolium ) respond rapidly
to post-fire conditions, increasing food abundance significantly
(Landhausser and Wein 1993). Studies focusing on vegetation after fire
show high variability with respect to both the biomass productivity and
the species composition that regenerates (Rau et al. 2008, Johnstone et
al. 2010).
Although there is significant evidence suggesting that browse biomass
density increases after fire, there is little regarding the effect of
fire on plant chemistry after fire (Bryant et al. 1983, Maier et al.
2005, Joly et al. 2016, Brown et al. 2018). Alteration of plant
chemistry after fire could have dramatic effects on the overall
palatability, and therefore the nutritional quality, of browse species.
Globally, herbivores are limited by the availability of nitrogen, the
element necessary for making protein and therefore building muscle.
Nitrogen concentration in boreal plants is a limiting factor for the
plants themselves and may be a limiting factor for herbivore growth and
reproductive success (McArt et al. 2009, Gundale et al. 2010). Low
resource environments, including the boreal forest, are dominated by
plants such as evergreens that are slow growing, even when provided with
an optimal supply of resources (Chapin, 1991, Gundale et al. 2010). Slow
growth in these plants is attributed to an inability to quickly uptake
nutrients, a higher allocation to survivorship traits such as chemical
defenses, and internal constraints to growth (Chapin, 1991). Due to
these growth limitations, nutrients may be leached out of a system
before the species has the chance to sequester them (Chapin, 1991,
Gundale et al. 2010), although fire has been found to increase nutrient
availability. Deciduous species located in nutrient-limited stands have
a greater proportion of secondary metabolites than in more productive
ecotones, making them less usable by associated herbivores (Gundale et
al. 2010). This defense strategy makes these plants less palatable to
herbivores.
Nutritional carrying capacity is limited by two elements of a habitat:
forage availability and forage quality. Both of which may be
significantly altered after a disturbance such as fire. This potential
tradeoff between higher availability of browse and potentially lower
quality could affect moose productivity and movement. Hobbs and Swift
(1985) showed that in high biomass areas nutrient intake is limited by
the nutritional quality of the food source. If browse availability
increases significantly in burned stands, but the nutritional quality
declines, then moose density may increase, at the cost of per capita
fitness (Hobbs and Swift 1985, 1988).
The response of moose to fire depends on browse biomass and diversity,
and nutritional quality, including plant defenses (Hobbs and Swift 1985,
Hobbs 2003). We measured these funda-mental characteristics in a
16-year-old prescribed burn in the Nelchina Basin and in its adjacent
unburned forests. The Nelchina Basin located west of Glenallen, AK,
supports one of the highest moose harvests in the state (Boertje et al.
2007), drawing in hunters from throughout south-central and interior
Alaska (Figure 1).