Assessing grassland management impacts on soil C storage using remotely sensed data: Project overvie

1
Assessing grassland management impacts on soil C
storage using remotely sensed data Project
overview
Richard T. Conant and Moffatt K. Ngugi
Management and conservation in rangelands Past,
present impacts on NPP and C uptake
Intensive rotational grazing Effects on biomass,
NPP, C uptake in pasture land
Intensive rotational grazing is widely believed
to increase grassland forage production by
ensuring more uniform forage removal and allowing
a recovery period. A recent review found that
rotational grazing in dry rangelands does not
influence forage production, but in more humid
regions, forage production increased by 20-30. A
key component of the benefit of intensive
rotational grazing is more efficient use of
forage that is produced. Also, more frequent
forage removal keeps plants from reaching slower
growth phases associated with leaf maturity.
Therefore, while intensive rotational grazing
increases annual forage production, standing
aboveground biomass may actually be equal or
greater under traditional, non-rotational
grazing. Seasonally integrated or one-time
measurements of vegetation indices are unlikely
to be useful in identifying pastures under
intensive grazing management. However, more
frequent measurements throughout the growing
season would enable quantification of changes in
biomass over time and total biomass production.
Rangeland degradation through overgrazing is a
global problem, the extent of which has been
quantified only using large-scale survey data.
Rangeland degradation as a result of overgrazing
leads to many environmental problems including
soil erosion, changes in species composition,
and, perhaps most importantly for producers,
decreased production potential. Identifying
impacted rangelands, therefore, has important
ecological, economic, and policy
implications. Evidence of rangeland degradation
via remotely sensed parameters could include
decreased NDVI, increased soil reflectance,
decreased peak biomass, or altered seasonal
distribution of standing biomass. Determination
of rangeland condition, production, and/or
productivity using remote sensing have been
attempted using a variety of methods with varying
success. For example Landsat-TM measurements of
LAI agreed well with ground based measurements.
Others have had success quantifying impacts of
grazing in Australian rangelands with NDVI.
Multi-temporal NDVI can, therefore, be used to
identify grasslands that (1) have been adversely
impacted by grazing, (2) are progressing away
from a degraded stage, and (3) are degrading due
to poor management.
Rangeland grazing intensity Effects on NPP and C
uptake
Rangeland degradation through overgrazing is a
global problem, the extent of which has been
quantified only using large-scale survey data.
Rangeland degradation as a result of overgrazing
leads to many environmental problems including
soil erosion, changes in species composition,
and, perhaps most importantly for producers,
decreased production potential. Identifying
impacted rangelands, therefore, has important
ecological, economic, and policy
implications. Evidence of rangeland degradation
via remotely sensed parameters could include
decreased NDVI and SAVI, increased soil
reflectance, decreased peak biomass, or altered
seasonal distribution of standing crop.
Determination of rangeland condition, production,
and/or productivity using remote sensing have
been attempted using a variety of methods with
varying success. For example Landsat-TM
measurements of LAI agreed well with ground based
measurements. Others have had success
quantifying impacts of grazing in Australian
rangelands with NDVI or SAVI. Multi-temporal
NDVI or SAVI can, therefore, be used to identify
grasslands that (1) have been adversely impacted
by grazing, (2) are progressing away from a
degraded stage, and (3) are degrading due to poor
management.
In many cases grazing leads to decreased NPP,
but under certain conditions rangeland grazing of
moderate intensity (30-50 of NPP consumed) in
grasslands can increase NPP by as much as 10.
When overcompensation occurs, the magnitude of
plant NPP response is very likely to be less than
the proportion of NPP removed, seasonally
distributed, and interannually variable.
Grazing, therefore leads to decreased standing
biomass, LAI and APAR, even when grazing results
in increased NPP. I hypothesize that the
seasonal pattern of biomass production and
standing biomass follow seasonal patterns like
those illustrated below. Remote sensing of
rangeland grazing intensity has been compounded
by two main problems (1) sample frequencies
long enough to preclude detection of impacts of
grazing and (2) confounding variability due to
topography. Recent developments in remote
sensing technology (MODIS) that produce more
frequent moderate resolution measurements of
variables important in estimating NPP will
provide data frequently enough to overcome
problems associated with infrequent re-sampling.
Ground-based parameterization of spatial and
seasonal variation in LUE are necessary for
deriving relationships between spectral
characteristics and NPP.
Can pastures under intensive rotational grazing
be distinguished from pastures that are managed
less-intensively using remote sensing?
Q
Pastures under intensive rotational grazing are
(1) more productive (higher NPP) and (2) have
different seasonal distribution of aboveground
biomass and can, thus, be distinguished from from
extensively managed pastures remotely.
Can rangelands under long-term grazing exclosure
and different grazing histories be distinguished
using remote sensing?
Q
H
Long-term grazing treatments of varying intensity
differentially affect biomass seasonality and can
therefore be identified using remote sensing.
H
Can grazing intensity on extensively managed
rangeland be quantified using remote sensing?
Q
Rangeland sites with similar climate and grazing
history have similar production potentials that
are modified by current grazing intensity which
affects LAI and is, thus, amenable to detection
by remote sensing.
H
Standing biomass
Cumulative NPP
NPP/LUE
Standing biomass
Time
Time
Standing biomass
Time
Time
Cumulative NPP

• Pulaski, Virginia
• Rotational grazing
• Soil C root biomass (pasture project)
• Clipping experiment
• LAI, clipping after rest period
• Seasonal LAI, clipping, root biomass

CRP
Moderate
Time
Time
Comprehensive sites
Heavy

• Spartansburg, Tennessee
• Rotational grazing fertilization
• Soil C root biomass (pasture project)
• Clipping experiment
• LAI, clipping after rest period

Light
CRP

• CPER
• Three recent grazing treatments
• (light, moderate, heavy)
• Historical grazing treatments
• (various intensities including exclosure)
• Ground measurements
• LAI, NPPa 4x per season

Frequent measurements, like those of the MODIS
products, are required to resolve temporal
variation in standing biomass, but what about
spatial resolution?
MODIS NDVI (7/3/01)
MODIS NDVI (6/29/02)

• Crescent Lakes
• Rotational grazing
• Long history of stocking rates
• Ground measurements
• LAI, NPPa 4x per season

Paddock 3

• Grazing exclosures/treatments
• Grazing history
• Neighboring ranches w/ grazing
• Species composition data
• National grasslands
• National wildlife refuges
• ARS Experimental ranges
• LTER sites
• National parks
• National/state historic sites

Paddock 2
Short grass steppe LTER (USDA Central Plains
Experimental Range)
Crescent Lakes National Wildlife Refuge
Paddock 1
LANDSAT false-color composite (5/15/02)
MODIS NDVI (5/16/02)

• Questions to be addressed
• How does forage removal impact C uptake and C
  sequestration?
• Remote estimates of impacts on biomass (MODIS),
  remotely sensed NPPa to drive Century (model
  output C uptake and C sequestration), and new
  CASMGS-supported soil sampling for model
  evaluation.
• How has the Conservation Reserve Program impacted
  C uptake?
• Identification of CRP lands using seasonality of
  production (see next panel), ground-truthing on
  known CRP lands in short- and midgrass steppes
  (species composition, peak biomass, LAI, etc.),
  remotely sensed NPPa measurements to drive
  Century, comparison of C uptake on differently
  managed rangelands (CRP, light, moderate, heavily
  grazed sites).
• How did the severe 2002 drought impact production
  and C uptake in the short- and midgrass steppes?
  What is the magnitude of differences between
  differently managed rangeland?
• Like for pastureland, assessed using regional
  model outputs (above), interannual NPPa variation
  due to management practices.

• Questions to be addressed
• What are the farm-level impacts of rotational
  grazing on C uptake? C sequestration?
• Remote estimates of impacts on biomass (LANDSAT
  MODIS), space for time substitutions to test
  methods for estimating regrowth, ? biomass
  regrowth (to approximate NPPa) to drive Century
  (model output C uptake and C sequestration),
  and benchmark soil sampling (and re-sampling) for
  model evaluation.
• Implications for regional C uptake/sequestration?
• Characterization of grazing management practices
  by remotely measured seasonal patterns, grazing
  management practices drive Century (model output
  C uptake, C sequestration, spatial variation,
  changes over time). Must assess links between
  grazing management and other pasture management
  practices (i.e., are fertilizer and species
  managed better on rotationally grazed pasture
  land?).
• How did the 2001-2002 region-wide drought impact
  production? C uptake? Differences between
  differently managed pastures?
• Assessed using regional model outputs (above),
  interannual NPPa variation due to management
  practices.

• Questions to be addressed
• How has range management impacted species
  composition?
• Remote estimates of NPPa seasonality, assessment
  of functional group (i.e., C3/C4) composition,
  Range Resource Area assessment of increasers and
  decreasers (range condition assessment) for
  comprehensive sites.
• Impacts on regional C uptake/sequestration?
• Characterization of historical grazing management
  practices by remotely measured seasonal patterns,
  functional group composition, and current grazing
  management (panel 2) to drive Century (model
  output C uptake, C sequestration, spatial
  variation, changes over time).
• Are production and C uptake in well-managed
  grazing lands resistant/resilient to drought
  impacts?
• Assessed using regional model outputs (above),
  NPPa variation due to management practices.

This research project is funded by the NASA New
Investigator Program grant No. NAG5-10593 to
Conant.