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r e s e a r c h.. f o c u s :
The
sustainability of grazing ecosystems
 The
overarching goal of this project is to understand the direct and
feedback effects of native grazing mammals that foster sustainability
of chronically and intensively grazed grassland. Our model system
of study is grassland in Yellowstone National Park, where we have
established exclosures to fence out elk, bison, and pronghorn,
at grassland sites that vary widely in elevation, topographic position,
soil properties, and species composition. At each of these sites
we are comparing plant production and nutrient cycling inside and
outside the exclosures to determine how site condition interacts
with grazers to control grassland processes. There are several components
of this project. Here I'll outline two areas.
Little
is known about the rate of belowground productivity in grasslands,
despite the qualitative understanding that in semi-arid grassland
most of the plant resources (carbon and nutrients) are allocated
below ground. A novel aspect of this study is that we are measuring
root production across a broad range of grasslands in Yellowstone
and determining how grazers influence that productivity. We measure
root production
with minirhizotrons, which allows us to track individual roots through
time to estimate root turnover. In contrast to a prevailing ecological
view that abovegound herbivory will reduce root productivity, we
have found that grazers in Yellowstone dramatically increase belowground
production (Frank et al. 2002). This result has exciting implications
for how grazers influence the belowground decomposer foodweb the
Yellowstone ecosystem.
A
second aspect of this work explores how grazing may alter microbial
processes in the rhizospheres of grazed plants. In a preliminary
laboratory experiment (Hamilton and Frank 2001), defoliating a common
Yellowstone grass stimulated the rate that roots exuded carbon to
the soil. In turn, this highly labile carbon was quickly assimilated
by a growing microbial biomass associated with roots of clipped
plants. The facilitating effects of defoliation on rhizospheric
C processes
positively fed back on soil inorganic N pools, plant N uptake, and
leaf N content and photosynthesis. These results are intriguing
because they suggest that  grazers can promote their forage production
by inducing plants to stimulate the activity of their rhizospheric
heterotrophic microbes and the availability of a limiting soil resource.
Next we need will determine if this phenomenon, observed in the
laboratory, is an important process occurring in the field.
Some
other key questions of the project are: Do Yellowstone ungulates
affect the structure of spatial variation in soil resources (Augustine
and Frank 2001) and plant species? What are the demographic properties
(e.g., longevity) of roots in Yellowstone grassland? How do grazers
influence grassland canopy architecture? How do grazer activity,
plant growth and turnover, and soil microclimatic factors interact
to determine soil N cycling? Does aboveground herbivory affect mycorrhizal
species composition? Could grazers indirectly facilitate ecosystem
energy and nutrient processes by indirectly altering the species
composition of the soil biota?
For
more details about my other research and related publications, please
select from the following:
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