Colorado mountains
From Long-Term Data to Understanding: Toward a Predictive Ecology
2015 LTER ASM Estes Park, CO - August 30 - September 2, 2015

A three-pronged approach to understanding how fire and mammalian herbivory interact to influence post-fire forest growth and composition in interior Alaska

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Alexandra Conway
Jill Johnstone

In the western North American boreal forest, recent changes in fire severity might result in shifts from coniferous to deciduous dominated forests. Changes in dominant tree species will alter plant-herbivore interactions and we are currently unaware of the interacting effects of fire severity and mammalian herbivory. I aim to quantify the effects of mammalian herbivory on post-fire tree growth and composition throughout interior Alaska using (1) long-term exclosures, (2) simulated browse, and (3) tree-rings and dendroecology. In 2013, in collaboration with the Bonanza-Creek LTER, I installed six exclosures (144m2 each) with paired control plots at sites that burned in 2004; tree seedlings were tagged and re-measured in 2014 and 2015. Results from (1) suggest a positive effect of exclosures on paper birch (Betula neoalaskana) growth. To better understand the sensitivity and response of paper birch to browsing, in 2014 I simulated summer moose (Alces alces) browse by leaf-stripping (0-100% long-shoots) paper birch seedlings. Results from (2) are incomplete however I expect paper birch to respond with a change in the ratio of short- and long-shoots. Lastly, in 2014, I revisited 12 sites that experienced a low or high severity burn in 1994. I collected basal and stem disks from trembling aspen (Populus tremuloides) and black spruce (Picea mariana) at each site. Vertical growth of aspen was significantly greater (t=5.52, p<0.001) in high severity stands with some trees reaching a safe height (three meters) from apical meristem moose browsing within six years post-fire. High browsing rates had a positive effect on aspen growth in low severity sites (t=2.11, p=0.04) but a negative effect in high severity sites (t= -2.89, p<0.01). Results from (3) indicate that aspen is responding differently to browsing depending on fire severity. My study provides evidence of the negative effects of herbivory, the interactive effect of fire severity, and has important implications for the health of the boreal forest and habitat management for herbivores.

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