Twenty year record of vegetation change from long-term plots in Alaskan tundra

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Presenter/Primary Author: 
William Gould
Joel A. Mercado Díaz
Jess K. Zimmerman

Twenty year record of vegetation change from long-term plots in Alaskan tundra
William A. Gould1, Joel A. Mercado Diaz1,2, Jess K. Zimmerman2
1. USDA Forest Service, International Institute of Tropical Forestry, Río Piedras PR,
2. University of Puerto Rico, Río Piedras, PR

Abstract. Emissions of greenhouse gases are expected to raise global mean temperature over the next century by 1.0-3.5 °C. Global warming trends are amplified at high latitudes because heating converts high-albedo (reflective) ice and snow surfaces to dark absorptive surfaces that absorb more solar energy and transfer it to the atmosphere. Scientists have argued that ecological responses to recent climate change in the Arctic will be complex and varied. Current models and experimental work have predicted an increase in shrub species and a decrease in graminoid species as a response to climatic warming. Warming of the Alaskan Arctic during the past 150 years has accelerated over the last three decades and is expected to increase vegetation productivity in tundra if shrubs become more abundant. This study investigates the changes in the vegetation structure and composition in a tundra landscape where changes may be both subtle and slow. Our approach involved resampling a series of permanent control plots over a multi-year interval (1989 to 2008), using the point- frame method. Our experiment was designed to look at changes in species abundance and vegetation structure over time and to understand the dynamics of the changes in dominance of growth forms in the Arctic tundra as a consequence of global climate change. Two sites were sampled, each with a 1 km2 grid of about 80 one meter sampling plots: Toolik Lake and Imnavait Creek. The Toolik Lake site is predominantly moist tundra and here shrub and graminoid species increased in abundance while bryophytes decreased in abundance over the last two decades. The Imnavait site is a mix of moist and dry tundra and showed an increase in shrub and graminoid species but no overall decrease in bryophyte species. The extent and complexity of the canopy has been increasing over time at both sites with the amount of surface having an understory (ground surface vegetation, typically bryophytes, lichens, and prostrate shrubs) and upperstory (typically erect shrubs, graminoids, or forbs) increasing from about 60% to 80%. The height of the canopy has also been increasing over time at both sites. The response at the more moist Toolik Lake site has been greater, with nearly a 3 fold increase in the difference between lower and upper canopy hits. The vegetation sampled is representative of vast areas of moist acidic tussock tundra in the more southern portions of the circumpolar arctic. The increase of canopy overstory represents a significant increase in above ground standing crop and a shift in carbon allocation to vascular plants vs. bryophytes. The increase in canopy height and complexity will likely affect snow redeposition, winter biological processes, and have feedbacks to nutrient allocation and cycling.