MAMMALIAN HERBIVORY SLOWS THE GROWTH OF BROADLEAF SPECIES IN POST-FIRE, EARLY SUCCESSIONAL FORESTS IN INTERIOR ALASKA

Boreal forest vegetation is likely to change in response to warming and associated changes in disturbance regimes. Recently, high severity fires have caused a shift from pre-fire mature black spruce stands to early assemblages of broadleaf trees. However, interactions between plants and animals may modify the outcomes of early forest succession. Selective herbivory by mammalian herbivores can alter the relative dominance of forest canopy species and influence successional pathways. However, woody plants have evolved multiple strategies to maximize chances of survival and long-term fitness after herbivory. In this thesis, I explore the dynamics of moose-tree interactions and their consequences for early succession in boreal forests of interior Alaska. I assessed the interaction of moose browsing and fire severity on tree growth and canopy composition and found that moose slow the rate of trembling aspen growth, but only in severely burned areas. Black spruce showed no direct or indirect growth responses to moose browsing, indicating that moose browsing on aspen did not alter the initial trajectory to an aspen-dominated canopy within my study area. In lightly burned areas, moose may benefit from longer durations of forage availability and accelerate the development of a mixed spruce-aspen canopy. Alaskan paper birch is the other dominant broadleaf species colonizing early post-fire forests and I determined saplings’ ability to tolerate simulated summer leaf stripping by moose. I found decreases in woody growth and carbon-based defenses while individual leaf area increased in response to simulated leaf stripping. My results are consistent with the carbon-nutrient balance hypothesis suggesting that the loss of growing points during leaf stripping decreased competition for nutrients, which were then available for leaf regrowth. Birch saplings have the potential to compensate for herbivory, but resilience of individuals will depend on the intensity, season, and frequency of herbivory. I set up long-term exclosures to test if mammalian herbivores can alter patterns of canopy succession in early post-fire sites that ranged in broadleaf dominance. Removal of natural moose herbivory inside exclosures led to species-specific responses after two years. Alaskan paper birch (dominant broadleaf species) grew larger when safe from herbivores while the opposite was true for black spruce. However, impacts of natural moose browsing on the dominant birch were insufficient to eliminate the competitive hierarchies that supported birch dominance of the canopy and suppressed growth of black spruce in the understory. I conclude that even with the negative impacts of moose on broadleaf species growth, their tolerance to browse damage enables their persistence as the dominant canopy species after large and severe fires in the boreal forest of Alaska