Interactions between herbivory by snowshoe hare (Lepus americanus) and the establishment of white spruce (Picea

glauca) on the Tanana River floodplain, AK

Proposed research byJustin Olnes

M.S. Biological Sciences University of Alaska-Fairbanks

2/14/2014

Outline

Theoretical framework-Plant-herbivore interactions

Background-Snowshoe hares-White spruce

Preliminary data and hypothesesSignificance

Plant-herbivore interactionsThe impact of herbivory on dominant vegetation

• Density of the herbivore

• Density of the plant species—is it dominant or rare?

• Secondary compounds and forage selection

Plant-herbivore interactionsThe impact of herbivory on dominant vegetation

22-year record of changes in species composition (indexed by leaf litter biomass) in LTER FP1 sites

Snowshoe hares (Lepus americanus)

• Dominant herbivore of the boreal forest• Total biomass can exceed that of all other vertebrate

herbivores during population peaks(Krebs et al. 2001, Rexstad and Kielland 2006)

Krebs et al. 2013

White spruce (Picea glauca)

• Dominant conifer of the boreal forest• Reproduces via masting events

Ohse et al. 2009

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Observations at BNZ-LTER

BNZ LTER

Question 1:How does herbivory by snowshoe hares alter the survival and growth of white

spruce seedlings within the floodplain at BNZ-LTER?

Question 2:Does a relationship exist between white spruce age structure along the floodplain and the population cycle of snowshoe hares at BNZ-LTER?

Question 1:How does herbivory by snowshoe hares alter the survival and growth of white

spruce seedlings within the floodplain at BNZ-LTER?

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f(x) = 2.77396192581361 x + 3.10956538917369R² = 0.345214579288559

f(x) = 7.87403492138888 x − 15.7716936498736R² = 0.61753541661666

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Question 1: Preliminary Data

Planted spruce

All sites, 2010

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Outside

Question 1: Preliminary Data

Natural spruce

Hypothesis: The survival and growth rate of white spruce within floodplains is significantly reduced by snowshoe hare herbivory when mast seeding events coincide with a peak in the hare cycle.

Question 2:Does a relationship exist between white spruce age structure along the floodplain and the population cycle of snowshoe hares at BNZ-LTER?

Question 2: Theoretical Concept Hypothesis: Age structure of floodplain white spruce influenced by:• Mast event history• Snowshoe hare population abundance• Flood event history

𝑌 𝑡=𝑀 𝑡−𝐻𝑡−𝐻𝑡+1…−𝐻𝑡+𝑛−𝐹 𝑡…𝐹 𝑡+𝑚

: number trees established for year t: magnitude of cone production for year t: Abundance of snowshoe hares for year t: Flood level of Tanana River for year t n is the number of years it takes for a white spruce to outgrow risk of browsing by snowshoe haresm is the number of years it takes for a white spruce to outgrow risk of damage or burial by flooding

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Question 2: Preliminary Data White spruce seedfall data at BNZ-LTER

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Question 2: Preliminary Data

hare pellet data (from litter traps)

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Question 2: Hypothesis Age structure of floodplain white spruce influenced by:• Mast event history• Snowshoe hare population abundance• Flood event history

Significance

Projected climate change and polar amplification

• Increased drought stress reducing annual growth in white spruce (Barber et al. 2000)

• Reduced seed production and seed viability (Roland et al. 2013)

• Snowshoe hares may benefit from increased fire frequency(Bryant 1994)

No retrospective study on white spruce age structure has been conducted for floodplains of interior Alaska

No research has investigated the impact of snowshoe hare herbivory on conifers, like white spruce, in interior Alaska

Reduced presence of white spruce on the landscape

“Indeed, it is all too clear that every surviving oak is the product either of rabbit negligence or of rabbit scarcity. Some day some patient botanist will draw a frequency curve of oak birth years, and show that the curve humps every ten years, each hump originating from a low in the ten year rabbit cycle.”

- Aldo Leopold, A Sand County Almanac