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Mapping and Modeling the Biogeochemical Cycling of Turf Grasses in the United States

  • Institute of Public Health & Environment

Abstract and Figures

Turf grasses are ubiquitous in the urban landscape of the United States and are often associated with various types of environmental impacts, especially on water resources, yet there have been limited efforts to quantify their total surface and ecosystem functioning, such as their total impact on the continental water budget and potential net ecosystem exchange (NEE). In this study, relating turf grass area to an estimate of fractional impervious surface area, it was calculated that potentially 163,800 km2 (+/- 35,850 km2) of land are cultivated with turf grasses in the continental United States, an area three times larger than that of any irrigated crop. Using the Biome-BGC ecosystem process model, the growth of warm-season and cool-season turf grasses was modeled at a number of sites across the 48 conterminous states under different management scenarios, simulating potential carbon and water fluxes as if the entire turf surface was to be managed like a well-maintained lawn. The results indicate that well-watered and fertilized turf grasses act as a carbon sink. The potential NEE that could derive from the total surface potentially under turf (up to 17 Tg C/yr with the simulated scenarios) would require up to 695 to 900 liters of water per person per day, depending on the modeled water irrigation practices, suggesting that outdoor water conservation practices such as xeriscaping and irrigation with recycled waste-water may need to be extended as many municipalities continue to face increasing pressures on freshwater.
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Mapping and Modeling the Biogeochemical Cycling of
Turf Grasses in the United States
Numerical Terradynamic Simulation Group
College of Forestry and Conservation
University of Montana
Missoula, Montana 59812, USA
NOAA National Geophysical Data Center
325 Broadway, Boulder, Colorado 80303, USA
Cooperative Institute for Research on the Atmosphere (CIRA)
Colorado State University
Fort Collins, Colorado 80523-1375, USA
Cooperative Institute for Research in the Environmental
Sciences (CIRES)
216 University of Colorado
Boulder, Colorado 80309
NASA Ames Research Center
MS-242-4, Moffett Field, California 94035, USA
ABSTRACT / Turf grasses are ubiquitous in the urban
landscape of the United States and are often associated
with various types of environmental impacts, especially on
water resources, yet there have been limited efforts to
quantify their total surface and ecosystem functioning,
such as their total impact on the continental water budget
and potential net ecosystem exchange (NEE). In this
study, relating turf grass area to an estimate of fractional
impervious surface area, it was calculated that potentially
163,800 km
(€ 35,850 km
) of land are cultivated with turf
grasses in the continental United States, an area three
times larger than that of any irrigated crop. Using the
Biome-BGC ecosystem process model, the growth of
warm-season and cool-season turf grasses was modeled
at a number of sites across the 48 conterminous states
under different management scenarios, simulating poten-
tial carbon and water fluxes as if the entire turf surface
was to be managed like a well-maintained lawn. The re-
sults indicate that well-watered and fertilized turf grasses
act as a carbon sink. The potential NEE that could derive
from the total surface potentially under turf (up to 17 Tg C/
yr with the simulated scenarios) would require up to 695 to
900 liters of water per person per day, depending on the
modeled water irrigation practices, suggesting that out-
door water conservation practices such as xeriscaping
and irrigation with recycled waste-water may need to be
extended as many municipalities continue to face
increasing pressures on freshwater.
Turf grasses are ubiquitous in the American urban
landscape, in residential, commercial, and institutional
lawns, parks, most athletic fields and golf courses, often
as monocultures, independently of the local climate
(Jenkins 1994). Existing estimates indicate that in the
early 1990s, the surface cultivated with turf was up to
three times larger than that of irrigated corn, the
largest irrigated crop in the United States (DPRA,
Incorporated 1992). As the construction of new homes,
averaging 1.6 million per year in the late 1990s (U.S.
Bureau of the Census 1999), continues to expand the
American urban landscape, the total surface under turf
is expected to further increase.
Turf grasses contribute to soil carbon (C) sequestra-
tion (Bandaranayake and others 2003, Qian and Follett
2002, Van Dersal 1936) and, as a component of urban
vegetation, to the mitigation of the urban heat island ef-
fect (Spronken-Smith and others 2000) and to enhanced
water infiltration compared to bare soil or impervious
surfaces. However, turf has also been linked with a num-
ber of negative environmental impacts. Turf grasses often
pose a neglected environmental hazard through the use
of lawn chemicals and overfertilization (Robbins and
Birkenholtz 2003, Robbins and others 2001), and, where
used, irrigation of turf grasses sharply increases the sum-
mer water consumption for residential and commercial
use, especially if grown in arid and semiarid regions,
where it can account for 75% of the total household water
consumption (Mayer and others 1999).
KEY WORDS: Turf grasses; BIOME-BGC; Impervious surface area;
Carbon budget; Carbon sequestration potential; Wa-
ter use
Published online July 19, 2005.
*Author to whom correspondence should be addressed; email: milesi
Environmental Management Vol. 36, No. 3, pp. 426–438 ª 2005 Springer Science+Business Media, Inc.
DOI: 10.1007/s00267-004-0316-2
In spite of the pervading presence of turf grass sys-
tems in the urban and suburban landscape and their
considerable use of water resources, a national assess-
ment of the ecological functioning of these systems is
still missing. The fragmented distribution of residential
and commercial lawns and the large variability in
management practices adopted to grow the different
types of turf surfaces certainly challenges the task of
such an assessment.
In this study, we attempt a rst estimate of the po-
tential impact of turf grasses to the continental U.S.
carbon and water budgets by producing a spatially ex-
plicit estimate of their distribution within the contig-
uous 48 states and simulating their growth with an
ecosystem process model. Specic objectives of this
study are (1) to compare a remote sensing-based esti-
mate with other independent estimates of the total
surface under turf grasses, and (2) to evaluate the im-
pact of different turf management practices, such as
removal versus on-site decomposition of the grass
clippings, varying nitrogen fertilization regimes, and
alternative irrigation schedules, on the continental
carbon and water budgets.
Estimation of U.S. Turf Surface
A continental assessment of the carbon and water
balances of turf grasses requires their spatial distribu-
tion to be mapped. With the exception of some golf
courses, turf grasses are rarely cultivated on surfaces
large enough to be identiable with moderate resolu-
tion satellite data (1 km). Due to excessive costs and
time constraints, the use of high-resolution satellite
images or aerial photography has been limited. Past
efforts to estimate the continental surface of turf grasses
used indirect approaches that provided measures of the
total surface under lawn on a state-by-state basis,
therefore lacking the spatial detail required to calculate
spatially dependent biogeochemical cycles. Vinlove and
Torla (1995), for example, estimated the national total
home lawn area using methods based on adjusted
Federal Housing Authority (FHA) average and median
lot sizes by state, without accounting for the turf sur-
faces found in golf courses, parks, schools, roadsides,
etc. DPRA, Incorporated (1992), in a report commis-
sioned by the Environmental Protection Agency, esti-
mated the total area under turf on the basis of direct
surveys in 12 states, which were extrapolated to the
remaining states in proportion to their population.
In our study, also adopting an indirect approach,
we assumed the surface of turf grasses to be inversely
related to the amount of impervious surface associ-
ated with urban development (roads, roofs, parking
lots, sidewalks, etc.). We rst calculated a fractional
cover of Impervious Surface Area (ISA) for the 48
states at 1-km spatial resolution using 2001 radiance
calibrated nighttime lights, a 1-km grid of road den-
sity and Landsat-derived urban landcover classes (El-
vidge and others 2004). The road density was
calculated as the length of road per square kilometer
from 1998 TIGER (Topologically Integrated Geo-
graphic Encoding and Referencing System) road
vector data from the U.S. Census Bureau. The night-
time lights were produced using cloud-free portions
of DMSP/OLS (Defense Meteorological Satellite Pro-
gram/Operational Linear Scanner) data using meth-
ods described by Elvidge and others (1999). We also
used direct measurements of the proportion of con-
structed surface (roads, parking lots, buildings) versus
the proportion of vegetated (turf grasses and/or
trees) or other (undeveloped) surface calculated from
80 high-resolution aerial photographs collected along
development transect distributed across 13 major ur-
ban centers. The transects extended from the urban
cores out to the sparsely developed (or undeveloped)
fringes of the urban centers of Atlanta, Boston, Chi-
cago, Denver, Houston, Las Vegas, Miami, Minneap-
olis, New York, Phoenix, Portland, Sacramento, and
Seattle. The aerial photographs were from year
2000 ± 1 year (see Figures 1 and 2 for an example of
the aerial photographs over industrial and residential
areas of Chicago, respectively). The measurements
were done on square kilometer tiles extracted from
the aerial photographs to match the coverage of
specific cells in the satellite and road density grids,
and were used to develop an empirical relationship
between the fractional ISA and the radiance cali-
brated nighttime lights, road density, and Landsat-
derived urban land cover classes. The highly signifi-
cant regression model (P - value < 0.0001 and Root
Mean Square Error (RMSE) of 13.4, Figure 3) was
then applied to the conterminous United States to
produce a 1-km grid depicting the spatial distribution
of ISA in percentage terms. The total ISA for the
conterminous United States calculated with this
method was estimated to be 112,610 (±12,725) km
1.3% of the total area.
The proportions of impervious versus vegetated
surfaces derived from the high-resolution aerial pho-
tography tiles were then used to develop a predictive
relationship between the fractional ISA and the com-
bined fraction of turf and tree surface, given that turf
was present under the trees observed in the samples.
For this model, only samples over areas with more than
Biogeochemical Cycling of Turf Grasses in the US 427
10% fractional ISA were used, leaving out the sparsely
developed urban fringes, where the occurrence of very
low development density is often associated with for-
ested and other nonturf vegetated surfaces. The pre-
dictive model showed a moderately strong (R
= 0.69),
highly significant (P < .0001, RMSE = 11.2) relation-
ship between fractional ISA and fractional turf grass
area (Figure 4) and was subsequently applied to the
conterminous United States to produce a 1-km grid of
fractional turf area (Figure 5).
Modeling of Turf Grasses Growth
Management of turf grasses is highly variable, in
part because of the different uses for which these sur-
faces are dedicated. In order to withstand considerable
wear, golf courses and athletic elds usually receive
much higher doses of nitrogen (N) than residential
lawns (up to 490 kg/ha/yr; Sartain 1998). For resi-
dential lawns, the recommended rates range between
98 and 195 kg/ha/yr (Schultz 1999) and are lower
when the clippings are left to decompose on the turf
Figure 2. Detail of aerial photography used
to measure fractional impervious surface area
over Chicago (residential area).
Figure 1. Detail of aerial photography used to
measure fractional Impervious Surface Area
over Chicago (infrastructure and commercial
428 C. Milesi and others
surface rather than composted or bagged and sent to
the landll. Many residential lawns are managed by
homeowners who pay little attention to the amount of
resources invested for lawn maintenance and often
receive excess water and fertilizer. On the other hand,
there also are some areas cultivated with turf grasses
that are not adequately watered and fertilized, spend-
ing part of the growing season in a dormant stage.
In this study, the simulation of the impact of dif-
ferent turf grass management practices on the conti-
nental C and water budget was based on the
simplifying assumption that, under a given scenario,
the entire turf surface is managed homogeneously,
such as irrigated with the same criteria, fertilized with
the same amount of N, and mowed at the same
height, whether it would be part of a residential lawn
or a golf course. Although this assumption largely
simplies reality, it allows developing a rst estimate
of the potential national impacts of turf grasses on
ecosystem functioning by asking: how would the
continental C and water budgets be affected if all the
surface currently under turf was to be managed like a
well-maintained lawn?
We adapted the Biome-BGC ecosystems process
model to predict C and water uxes of turf ecosystems
at 865 sites distributed across the United States, cor-
responding to populated places that, according to the
2000 U.S. Census, had a population of at least 40,000
people (the list of populated places is available online
ces2k.html). Biome-BGC has been extensively docu-
mented and validated (Thornton and others 2002,
White and others 2000, Kimball and others 1997, Hunt
and others 1996, Running 1994, Running and Hunt
1993, Running and Gower 1991, Running and
Coughlan 1988). Biome-BGC uses prescribed site con-
ditions, meteorology, and parameter values to simulate
daily fluxes and states of C, water, and N for coarsely
defined biomes, at areas ranging from 1 m
to the
entire globe. Biome-BGC can be used to simulate these
fluxes for more specifically defined ecosystems when
appropriately parameterized. Adapting Biome-BGC for
simulating the ecosystem processes of turf grasses re-
quired modifying the default parameterization for C3
(cool season) and C4 (warm season) grasses to reflect
the higher specific leaf area as well as the lower C:N
ratio of leaf, litter, and fine roots of fertilized and wa-
tered turf grasses. Leaf C:N ratio was assigned to 20 and
litter C:N ratio was assigned to 40, as suggested by
Bandaranayake and others (2003). We also modified
the lignin, cellulose, and labile portions of fine roots
to, respectively, 12%,52%, and 36% (Bandaranayake
and others 2003), while the canopy average specific
leaf area (SLA) was increased to the upper range of
SLA values observed for grasses (White and others
2000) and set to 70 m
/kg C.
Mowing activities were simulated as mortality pro-
cesses that would take place every time the leaf area
index (LAI) reached a critical value of 1.5. The mor-
tality event was assumed to remove 20% of LAI and the
corresponding amount of ne roots. Removal of the
clippings was simulated by removing the portion of C
and N associated with the cut leaves from the ecosys-
Figure 3. Scatter diagram of the observed values of frac-
tional Impervious Surface Area (ISA) versus the values pre-
dicted from linear regression and equation for the predictive
regression model used to estimate the 1-km grid of fractional
ISA for the conterminous United States.
Figure 4. Scatter diagram of the direct measurements of
fractional turf grass area and fractional Impervious Surface
Area (ISA) and equation for the predictive regression model
used to estimate the total U.S. surface under turf from the 1-
km grid of fractional ISA.
Biogeochemical Cycling of Turf Grasses in the US 429
tem process. In the cycling scenario, the C and N
associated with the cut leaves were left on the site to
decompose as litter.
N was added to the system at a constant rate,
simulating a slow-release fertilizer. To evaluate the
effect of clipping cycling on grasses N availability, N
was applied at two different rates in contrasting sim-
ulation runs. Clippings were either removed or cycled
in scenarios simulating an application of 146 kg N/
ha/yr and cycled in scenarios with an application of
73 kg N/ha/yr.
Irrigation during the growing season was simulated
by adding water to the precipitation eld in the climate
parameterization. We assumed that the sprinkling
season of a certain location would start when the
minimum temperatures remained above 5C for 7
consecutive days in the spring, and end when mini-
mum temperatures decreased below 5C for 7 consec-
utive days in the fall. Although the start and end of the
sprinkling season are generally determined arbitrarily
and may incorporate other climatic factors, we found
the chosen temperature threshold to represent an
acceptable approximation of the growing season and,
consequently, of the evapotranspirational season of
turf grasses. The simulations assumed water to be
sprinkled following two different watering manage-
ment types. In one type of watering management, we
followed the common recommendation that during
the growing season, turf grasses require about 2.54 cm
(1 inch) of water per week (Schultz 1999). In the
simulations, in the case of rainfall, rain made up for
part of this amount. In the real world, it is common
that sprinklers, especially if automated, run also on
rainy days. The alternative watering management sce-
nario, rather than providing a xed weekly amount of
water, modulated the irrigation based on the potential
evapotranspiration (PET) and precipitation, the for-
mer calculated according to Priestly and Taylor (1972).
In this case, irrigation was simulated to be triggered
when the PET minus precipitation, accumulated since
the last watering event, exceeded 60% of the added
water. Irrigation then replaced 20% of the PET,
bringing the water availability to nearly 80% of PET.
The effect of the two different water management
practices on the C and water balance was evaluated
comparing scenarios in which N added through fertil-
ization was constant and irrigation was either xed at
2.54 cm of water/week or modulated according to
For the 865 selected populated places, soil texture
information was extracted from the STATSGO data-
base (Miller and White 1998) and 18 years of climate
data were obtained from the Daymet dataset (Thorn-
ton and others 1997). The simulation sites were as-
sumed to grow C3 (cool season) or C4 (warm season)
turf grasses based on adaptation zones (Beard 1973,
Time-Life Books 2000). In cities located in the C3-C4
transitional region, the grasses were assumed to be a
mixture of both photosynthetic models and the simu-
lation was run twice, once for each type of grass. The
resulting C and water uxes in the transitional region
were determined to be an average of the two runs,
assuming that half of the surface was growing C3
grasses and half C4 grasses. All the C and water uxes
are reported as an average of the 18-year model simu-
lation results.
Figure 5. Distribution of the
fractional turf grass area in the
conterminous United States.
430 C. Milesi and others
The growth of turf grasses at the 865 sites was sim-
ulated for the following ve different scenarios:
Control: Turf grasses growth was simulated with no
management (no irrigation and no N fertilization)
except for cycling of the clippings;
Removed-146N: The grass was irrigated during the
growing season so that a total of 2.54 cm of water per
week, rainfall included, was provided, fertilized with
146 kg N/ha/yr, and the clippings were removed
from the system after each mowing event;
Cycled-146N: same as Removed-146N, except for the
clippings, which were left on the site after each
mowing event;
Cycled-73N: same as Cycled-146N, except for the
amount of fertilizer, which was halved to 73 kg N/
Cycled-73N-PET: same as Cycled-73N, except for the
irrigation management, which was calculated based
on Priestly-Taylor PET.
Mann-Whitney U-test for differences was used to
evaluate whether model results under the five scenarios
differed significantly from each other.
The net accumulation of C in the ecosystem was
estimated by calculating the Net Ecosystem Exchange
(NEE), where NEE = Net Primary Productivity (NPP)
heterotrophic respiration –fluxes of C out of the
ecosystem. Fluxes of C out of the ecosystem refer here
to the C removed with the clippings.
The simulation results were extrapolated to the
continental surface, assuming that turf areas in the
vicinity of a simulation site displayed similar C and wa-
ter uxes. The continental United States was divided
into Thiessen polygons centered on the simulation sites
to identify individual ‘‘regions of inuence’’ around
each of the 865 simulation localities. The output results
at each simulation site were then multiplied by the total
turf area estimated within the respective polygon.
Model Validation
There are only a few studies on the effect of turf
grass management on the C budget. The adaptation of
Biome-BGC to simulate the growth of turf grasses was
validated by comparing the simulated clipping yield
with published clipping yield data (C was assumed to
represent 48% of the dry yield). Two studies (Kopp and
Guillard 2002, Heckman and others 2000) presented
clipping yields under different N fertilization rates for
C3 grasses. Kopp and Guillard (2002) present yields
both for removed clippings and for recycled clippings.
Only one value of clipping yield was available for C4
grasses (Harivandi and others 1996). The measured
versus modeled yield data showed a strong and highly
signicant correlation (r = 0.83, P < 0.0001) (Fig-
ure 6).
Results and Discussion
Estimation of Turf Grass Area
The total turf grass area estimated in this study
summed up to 163,800 km
35,850 km
for the
Figure 6. Scatter diagram of the modeled versus observed
grass clipping yields expressed in C biomass (kg/m
/yr). 1*
Observed and modeled data at Santa Clara, CA, clippings
removed, N rate 146.5 kg/ha/yr (Harivandi and others
1996); 2Observed at Spring Manor Farm (SM), Storrs, CT,
clippings removed (bagged), N rate 0 kg/ha/yr, modeled at
Hartford, CT (Kopp and Guillard 2002); 3Observed at SM,
Storrs, CT, clippings removed, N rate 98 kg/ha/yr, modeled
at Hartford, CT (Kopp and Guillard 2002); 4Observed at
SM, Storrs, CT, clippings removed, N rate 146.5 kg/ha/yr,
modeled at Hartford, CT (Kopp and Guillard 2002); 5Ob-
served at SM, Storrs, CT, clippings removed, N rate 196 kg/
ha/yr, modeled at Hartford, CT (Kopp and Guillard 2002);
6Observed at SM, Storrs, CT, clippings removed, N rate 392
kg/ha/yr, modeled at Hartford, CT (Kopp and Guillard
2002); 7Observed at SM, Storrs, CT, clippings cycled, N rate
0 kg/ha/yr, modeled at Hartford, CT (Kopp and Guillard
2002); 8Observed at SM, Storrs, CT, clippings cycled, N rate
98 kg/ha/yr, modeled at Hartford, CT (Kopp and Guillard
2002); 9Observed at SM, Storrs, CT, clippings cycled, N rate
196 kg/ha/yr, modeled at Hartford, CT (Kopp and Guillard
2002); 10Observed at SM, Storrs, CT, clippings cycled, N rate
392 kg/ha/yr, modeled at Hartford, CT (Kopp and Guillard
2002); 11Observed at Rutgers, NJ, clippings removed, N rate
97.6 kg/ha/yr, modeled at Edison, NJ (Heckman and others
2000); 12Observed at Rutgers, NJ, clippings removed, N rate
195.2 kg/ha/yr, modeled at Edison, NJ (Heckman and oth-
ers, 2000). *Only C4 grass site. Points 212 refer to sites
growing C3 grasses.
Biogeochemical Cycling of Turf Grasses in the US 431
upper and lower 95% confidence interval bounds)
(Table 1). This estimate, intended to include all resi-
dential, commercial, and institutional lawns, parks, golf
courses, and athletic fields, accounts for approximately
1.9% of the total continental U.S. area, which com-
pares with 3.54.9% of the total surface estimated to be
devoted to urban development (Nowak and others
2001, National Association of Realtors 2001). Although
it is difficult to validate the estimate of total turf grass
area derived from this analysis, it reasonably compares
to the estimates of the other studies, in particular when
considering the recent growth in population and ur-
ban areas in the United States (Fulton and others
2001). DPRA, Incorporated (1992), assuming turf sur-
face to be directly related to the population, estimated
a total surface of 188,180 km
, among which 94,090
were of home lawns (Grounds Maintenance 1996).
A 1987 study by Roberts and Roberts (1987) estimated
a total surface of 100,000120,000 km
. Another study,
focusing only on residential lawns, analyzing state-
based average lot sizes of single family homes, esti-
mated a total home lawn area ranging between 58,000
and 71,680 km
, considerably downsizing DPRAs
estimate of home lawns (Vinlove and Torla 1995). One
of the earliest estimates of total turf surface dates back
to the late 1960s, when it was reported that 67,000 km
of lawn existed nationally (Falk 1976).
Even when the estimate of total surface is consid-
ered to be closer to the lower bound of the 95% con-
dence interval (128,000 km
), it appears that turf
grasses would represent the single largest irrigated
‘‘crop’’ in the United States, occupying a total area
three times larger than the surface of irrigated corn
(43,000 km
according to the 1997 Census of Agricul-
ture, out of 202,000 km
of total irrigated cropland
Water Budget
The two alternate irrigation methods produced
watering requirements that varied widely across the
climatic regions of the 48 states, with the yearly total
amount of water that needed to be provided through
irrigation at each site depending both on the total
rainfall and its distribution during the growing season
and the length of the sprinkling season. In general, a
xed irrigation management based on turf require-
ments of 2.54 cm of water per week, including rain-
fall, resulted in a minimum of no irrigation in
Lincoln Park, Michigan (meaning that here rainfall
alone is able to satisfy the watering requirements of
the turf throughout the growing season) to a maxi-
mum of 125 cm of water per year to be added
through irrigation in Yuma, Arizona. In contrast, the
irrigation management based on PET tended to de-
crease the amount of water supplied through irriga-
tion in wet regions and increase it in arid and
semiarid regions of the United States, where it was by
far larger than 2.54 cm/week. Modulating irrigation
according to PET required a minimum of 17 cm/yr of
water to be added through irrigation in Pensacola,
Florida, to a maximum of 197 cm/yr in Yuma, Ari-
zona. The Mann-Whitney U-test for differences indi-
cated that the two irrigation methods would provide
significantly different annual amounts of water at 77%
of the 865 sites. The sites with no significant differ-
ence between the two irrigation methods were all but
three located just east of the Great Plains. The spa-
tially interpolated differences in irrigation water use
between the two irrigation managements (Figure 7)
indicates that adopting the PET-based method versus
applying constantly 2.54 cm/week would result in a
larger amount of water sprinkled in the West, with a
maximum difference of up to 72 cm/yr in the
Southwest, and less water in the southeastern United
States, with a reduction in water use of up to 38 cm/
yr in southern Florida, where the high relative
humidity reduces the evapotranspirational demand.
Extrapolating the water use for irrigation with the
two methods at each of the 865 sites to the surface of
turf grasses contained in the respective Thiessen poly-
gons yields an average total of 73,560 Mm
(Mega cubic
meters) of water with the constant 2.54 cm/week
method and 95,100 Mm
of water with the PET meth-
od, while rain contribution during the sprinkling sea-
son to the watering of the total estimated turf grass
area would amount to 99,130 Mm
(Figure 8).
These estimates indicate that, in the scenario that
the entire turf surface in the United States was to be
irrigated to satisfy the 2.54 cm/week water supply or at
80% of PET, domestic and commercial consumptive
water use would be, respectively, 695 to 900 liters of
water per person per day. Noteworthy is that in spite of
the elevated irrigation requirements, there appears to
be a considerable amount of water leaving the soil layer
as outow (water in excess of eld capacity) rather
than evapotranspiration (56,620 to 57,670 Mm
of wa-
ter, depending on the irrigation management scenar-
ios). Ninety percent of the estimated outflow takes
place in the eastern and southern United States, where
it is related to rainfall rather than sprinkling events. On
occasions of abundant rainfall, precipitation is larger
than the soil water-holding capacity and leaves the soil
before the grass can use it for evapotranspiration. In
spite of a surplus of available water during the rainy
periods, sprinkling is still required during the drier
432 C. Milesi and others
If irrigation could just replace actual evapotran-
spirational losses, the water to be added through
sprinkling would amount to 11,070 Mm
in the case
of the 2.54 cm/week method and 33,300 Mm
the PET-based method. The large increase in water
requirements with the PET-based method has to be
attributed to the arid western United States, where
grasses can evaporate much more than 2.54 cm of
water per week if more irrigation is supplied. Still,
part of the water reaching the surface during the
growing season, either from precipitation when
abundant rainfall occurs, or from the sprinkler, due
to Priestly-Taylor PET overestimating actual evapo-
transpiration, would not be used by the grass and
would leave the soil layer as outflow.
Carbon Budget
Table 2 reports the ranges in C fluxes and mowing
counts for the control and the four management
scenarios. In general, the simulation results indicate
that the C fluxes of a well-watered grass increase with
the amount of available N. For a certain amount of N
input through fertilization, the C fluxes were larger
when cycling of the grass clippings was simulated,
since the onsite decomposition of the mowed grass
clippings returned a consistent amount of N to the
soil. For each scenario, differences in the maximum
minimum ranges are related mainly to the growing
season length.
Unsurprisingly, the control scenario displays the
lowest range of C uxes, and the lowest range of
mowing counts, which are both signicantly different
from all the other scenarios at all 865 sites. The low
number of mowing counts simulated in the control
scenario let us infer the obvious: in the absence of
irrigation and fertilization, turf grasses would not be
able to grow in most of the United States. A general
guess of where turf grasses could grow with no added
resources of N and water can be inferred by calculating
the number of days between successive mowings (ratio
of yearly mowing counts to growing season length).
Assuming that turf grasses should grow back to an LAI
of 1.5 in at least 3035 days in order not to be out-
competed by weeds, Figure 9 shows that monocultures
of turf could probably grow without managed inputs of
water and fertilizer only in a few of the modeling sites,
all but one located in the northeastern portion of the
country (the site in the western United States corre-
sponds to Flagstaff, AZ). If turf grasses reach an LAI of
1.5 only six to seven times in areas where the growing
season is as long as 300360 days, then it is probable
that between subsequent cuts there are several
opportunities for nonturf species to invade the surface
and prosper over time. Because the LAI is reduced by
20% every time the LAI would reach the value of 1.5,
the NPP of an unmanaged turf would be considerably
lower than that of natural grasslands, which in tem-
Table 1. Estimates of turf grass area by state
Turf grass area (km
State Mean
95% C.I.
95% C.I.
Alabama 3130 3741 2520
Arizona 2559 3178 1941
Arkansas 2098 2519 1679
California 11159 13890 8434
Colorado 2478 3047 1910
Connecticut 2429 2946 1913
Delaware 533 644 422
District of Columbia 57 86 28
Florida 11570 14221 8925
Georgia 5688 6848 4530
Idaho 942 1133 751
Illinois 5729 7102 4359
Indiana 3843 4679 3008
Iowa 2227 2772 1822
Kansas 2004 2453 1555
Kentucky 2446 2935 1958
Louisiana 3377 4099 2656
Maine 975 1157 793
Maryland 2471 3013 1929
Massachusetts 4183 5054 3314
Michigan 4538 5598 3480
Minnesota 3176 3866 2487
Mississippi 1969 2362 1578
Missouri 3442 4217 2669
Montana 735 884 585
Nebraska 1149 1401 898
Nevada 928 1162 694
New Hampshire 1126 1339 913
New Jersey 3942 4885 3002
New Mexico 1545 1860 1231
New York 6320 7770 4873
North Carolina 8112 9715 6512
North Dakota 572 693 452
Ohio 6733 8213 5257
Oklahoma 2689 3294 2086
Oregon 1977 2406 1549
Pennsylvania 7293 8789 5799
Rhode Island 506 622 390
South Carolina 4034 4822 3248
South Dakota 692 829 555
Tennessee 4201 5064 3339
Texas 13187 16242 10138
Utah 1207 1493 922
Vermont 524 621 427
Virginia 4544 5510 3581
Washington 3579 4345 2814
West Virginia 1459 1731 1189
Wisconsin 3110 3764 2457
Wyoming 554 665 444
Total U.S. 163,812 199,679 128,016
Biogeochemical Cycling of Turf Grasses in the US 433
perate ecosystems ranges between 320 and 750 g/m
yr (Saugier and others 2001, Schlesinger 1997).
The largest C uxes are realized for scenario Cycled-
146N, in which Mann-Whitney U-test for differences in
C fluxes and mowing count indicates that this scenario
is significantly different from the other scenarios for all
the variables measured at the 865 sites (P < 0.01).
Abundant fertilization (146 kg N/ha/yr) and the re-
cycling of the N contained in the leaves left to de-
compose on the site boosts both the productivity as
well as the heterotrophic respiration. As a consequence
of the higher productivity of turf grasses in this sce-
nario, there is also an increase in mowing frequency,
which can reach 98 cuts per year in those sites where
climatic conditions favor year-round growing season,
resulting in about two cuts per week.
Scenario Removed-146N produces the second
highest NPP and clipped biomass ranges. Because the
clipped biomass is assumed to be removed from the
turf surface, very low on-site decomposition activity
results in the smallest C uxes from heterotrophic
In scenario Cycled-73N, the C uxes are signi-
cantly lower (P < 0.01) at all sites when compared to
those of Removed-146N and lower in 8892% of the
sites when compared to Cycled-146N.
Scenario Cycled-73N-PET, which differs from Cy-
cled-73N only for the type of water management,
modulating irrigation according to PET rather than
supplying a weekly xed amount of water, does not
produce signicantly different C uxes from Cycled-
73N at any of the 865 sites. Mann-Whitney U-test at 5%
Figure 7. Spatially interpolated differences in
irrigation water use between the two irrigation
Figure 8. Water budgets of the total
U.S. turf surface for the four
management scenarios. Error bars
indicate budget values calculated for
the 95% confidence interval lower and
upper bound estimate of total turf
434 C. Milesi and others
significance level indicates a water effect on NPP and
clipped C at 7% of the sites, and an effect on hetero-
trophic respiration at 5% of the sites.
Large differences in total C uxes can be realized
under the same irrigation management of 2.54 cm of
water per week, all resulting in very large losses of water
through outow (Figure 8). This result is most proba-
bly explained by the fact that in all the simulated
management scenarios water is not limiting growth,
which responds rather to increases in N availability.
The large increase in water application observed when
modulating irrigation according to PET, on the other
hand, results in an insignificant change in C fluxes,
indicating that the water is lost in luxury evapotrans-
The estimation of the total C budget for the conti-
nental U.S. turf surface under the ve scenarios
examined (Figure 10) indicates that the highest NEE is
recorded for the Cycled-146N scenario, amounting to
16.7 Tg C/year. The lowest NEE is recorded for the
Removed-146N scenario, for which the removal of the
clippings from onsite decomposition reduces the C
sink to just 5.9 Tg C/yr, in spite of the fact that the
same amount of N as in Cycled-146N is added through
fertilization (a total of 2.39 Tg N/yr for the total esti-
mated surface under turf). Offsite composting of the
clippings allows recuperating part of the C. On the
other hand, the practice, nowadays less common, of
sending the clippings in trash bags to the landfill leads,
through anaerobic decomposition, to the production
of methane, a powerful greenhouse gas. Reducing the
N fertilization by half in scenarios that recycle the
clippings (Cycled-73N and Cycled-73N-PET) lowers the
NPP by 3637% and the NEE by 45% compared to the
Cycled-146N scenario but also considerably lowers the
number of times the grass needs to be cut throughout
the growing season. The C budget for the control
scenario, on the other hand, results in a small source of
carbon, since the total heterotrophic respiration (15.5
Tg C/yr) slightly surpasses the total NPP (15.3 Tg C/
yr), bringing the NEE for the control scenario to 0.2
Tg C/yr.
Therefore, if the entire area was well watered and
fertilized, we would have a positive NEE, even when
assuming the bagging and removal of the grass clip-
pings. A positive NEE means that more carbon is
Table 2. Minimum and maximum values of the modeled C fluxes and mowing counts
Carbon fluxes (g C m
) Control Removed-146N Cycled-146N Cycled-73N Cycled-73N-PET
NPP 22121 257641 2811063 184604 195613
Clippings 034 79207 87348 55195 58198
Heterotrophic respiration 31121 138392 210922 140533 150542
Mowing counts (cuts yr
)071652 2298 1455 1656
Figure 9. Modeling sites where
growth of turf grasses appears to be
possible without irrigation or
Biogeochemical Cycling of Turf Grasses in the US 435
accumulated in the turf grass system than is released
through respiration processes and, eventually, re-
moved with the clippings, indicating that the soils of
lawns and golf courses that are left undisturbed for a
few decades have the potential to sequester a consistent
amount of C in their soils. The sink is generally
stronger when more N is available. N availability can be
increased both through increased fertilization or, more
efciently, by leaving the clippings to decompose on
the site after mowing. Merely applying a larger amount
of N through the use of synthetic fertilizer without
recycling the clippings would reduce the potential gain
in C sequestration because of increased discounts due
to the C costs of manufacturing, transporting, and
commercializing the fertilizer (Schlesinger 1999).
These C costs are in addition to the ones deriving from
the operation of lawn mower equipment, distributing
water for irrigation, and from transporting and
decomposing the clippings in the landll.
In addition to the associated C costs, the current
high-input choices made by consumers and profes-
sional turf managers for maintaining monocultures of
turf grasses typical of many lawns and play elds comes
at the risk, not analyzed here, of watershed pollution
due to improper fertilization and use of pesticides
(Petrovic 1990). The input levels of herbicide and
insecticide per unit area of turf are often several times
larger than in their agricultural counterparts, inde-
pendently from the consumers knowledge and
understanding of personal health risks and negative
environmental effects associated with the use of these
products (Robbins and Sharp 2003a, 2003b).
Benecial effects of turf grasses, such as a carbon
sequestration but also recreation, storm runoff reduc-
tion due to increased soil inltration in occasion of
intense rainfall, and removal of impurities and chem-
icals during percolation of the water through the root
zone, could be sought by minimizing the application of
fertilizers and pesticides, introduction of lower input
species mixes such as clover and other so-called weeds
(Bormann and others 1993), on-site decomposition of
the grass clippings, and extending the practice of irri-
gating with waste water rather than with drinking wa-
In this study, we mapped the total surface of turf
grasses in the continental United States and simulated
its potential C and water budgets. We also provided a
description of how the C and water budgets can be
affected by adopting different management practices
for irrigation, fertilization, and the fate of the clip-
pings. Rather than trying to accurately quantify the
existing uxes, we simulated scenarios in which the
entire surface was to be managed like a well-main-
tained lawn, a thick green carpet of turf grasses, wa-
tered, fertilized, and kept regularly mown. The
accuracy of the results is therefore limited by both the
uncertainty in the mapping of the total lawn area and
by the simplifying assumptions made in the modeling
of the growth of turf grasses.
The analysis indicates that turf grasses, occupying
1.9% of the surface of the continental United States,
would be the single largest irrigated crop in the
country. The scenarios described in this study also
indicate that a well-maintained lawn is a C sequestering
system, although the positive C balance has to be dis-
counted for a very large use of water and N and, not
quantied in this study, pesticides. The model simula-
Figure 10. Carbon budgets of the
total U.S. turf surface for the different
simulations. Error bars indicate budget
values calculated for the 95%
confidence interval lower and upper
bound estimate of total turf surface.
The carbon budget for the control
scenario is negligible and therefore not
displayed. NPP, Net Primary
Productivity; NEE, Net Ecosystem
436 C. Milesi and others
tions have assumed a conservative amount of fertiliza-
tion (a maximum of 146 kg N/ha/yr). In general, the
rates of N applications are similar to those used for row
crops, and N losses from turf surfaces can contribute to
non-point source pollution when fertilization takes
place improperly.
If the entire turf surface was well watered following
commonly recommended schedules, there would be
very large pressure on U.S. water resources, especially
when considering that drinking water is usually sprin-
kled. At the time of this writing, in most regions out-
door water use already reaches 5075% of the total
residential use. Because of demographic growth and
because more and more people are moving towards
the warmer regions of the country, the potential exists
for the amount of water used for turf grasses to in-
crease. Several counties in the arid and semiarid re-
gions of the United States have already implemented
lawn watering restrictions, the recycling of wastewater
to replace drinking water for outdoor sprinkling, and
incentives to increase the use of xeriscaping. Although
turf grasses also provide important ecological benets
such as slower storm runoff, improved water inltra-
tion, and holding soil in place on sloping terrains, to
protect our water resources as further urban growth
takes place other regions will probably need to extend
the practice of recycling wastewater for outdoor use
while continuing to educate the population on the
value of water resources.
This study was supported by the NASA Earth System
Science Fellowship program to the rst author and by
the NASA Land Cover Land Use Change research
program. We are grateful to Ronald Follett, Paul Rob-
bins, and Michael White, whose constructive comments
improved the quality of the manuscript. Many thanks
also to Faith Ann Heinsch, Carol Brewer, Eric Edlund,
Sarah Halvorson, David Jackson, and Stephen Siebert
at the University of Montana for interesting discussions
and insightful comments.
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438 C. Milesi and others
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With a range of programs focused on local plant and insect conservation, the Cincinnati Zoo & Botanical Garden works with partners and our communities to restore landscapes and thriving ecosystems for wildlife and people. We used organizational identity theory (OIT) to evaluate the current strategies and practices of five programs and determine opportunities for adaptation to better achieve our organizational conservation goals. Case studies ranged from habitat restoration of wetlands and gardens to community engagement to encourage individual gardeners and the reintroduction of endangered plants and insects. We present program characteristics and how collaborative partnerships facilitate opportunities for zoos to lead the conservation of local flora and fauna. The OIT framework reveals components critical to strategy implementation and best practices relevant to other zoos, aquariums, and botanical gardens when evaluating their collaborative conservation initiatives.
... The turfgrass industry in the United States continues to grow rapidly due to strong demand for residential, public, and commercial property development (Haydu et al. 2006). Valued at $40-60 billion, turfgrass is estimated to cover 10-20 million hectares (25-50 million acres) in the United States making it possibly the fourth largest crop in acreage, and the largest irrigated U.S. crop by three times (Milesi et al. 2005, Morris 2006, Robbins and Birkenholtz 2003. While turf is used for its recreational value and aesthetics, it also provides many environmental and ecosystem function benefits, including a reduction in soil erosion, improved water infiltration and reduced runoff, water purification, toxic remediation, carbon sequestration, and cooling to mitigate urban heat island effects (Beard 1996, Beard and Green 1994, Chang et al. 2021. ...
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This study investigated the effect of compost tea applications on turf quality and soil microbial activity. Evaluations of turfgrass quality were based on The National Turfgrass Evaluation Program's guidelines while soil samples were analyzed for chemical attributes and microbial activity. Four sites and treatments for the study included: 1) a soil drench compost tea application with irrigation, 2) a soil drench compost tea application with no irrigation, 3) no compost tea application with irrigation, and 4) no compost tea application nor irrigation. Fifteen soil samples and turf quality observations from each treatment were collected for pretest data. Then, post-test data were collected after each additional seasonal test period over the course of one year for each of the four plots. For the four plots, the site which received compost tea applications and regular irrigation received significantly higher turf quality ratings, and compost tea improved turf quality ratings beyond that of regular irrigation. No differences were found in microbial populations given the compost tea application. While the study results provided evidence of the value of compost tea to overall turf quality aesthetics, more research is recommended regarding compost tea applications and beneficial soil microbial populations in turf. Species used in this study: Bermudagrass (Cynodon dactylon) L. Pers.
... By infecting the root system, plant parasitic nematodes (PPN) influence the physiological processes of the entire plant, either directly or indirectly. Nematodes affect plant growth by disrupting cell structure, removing cell contents, altering metabolism and modifying the genetic expression of the host (Bongers and Bongers, 1998;Milesi et al., 2005;Cheng et al., 2008). As PPN feed on plant parts, subsequent damage is reduces the ability of root systems to uptake water and nutrients from the soil solution (Khan, 1993). ...
... Given the landowners' overwhelming appreciation of wildlife on their land and in their ponds, as well as the desire of many to make environmentally friendly management decisions, these areas may proffer valuable habitat to species and provide untapped opportunities to cultivate knowledge about the role individual management strategies could provide to local processes that are beneficial to the coupled human-natural system at large. However, because the exurbs tend to occur near natural features such as lakes and federal/state parks [10], exurban development could alter land cover by increasing lawn cover, which provides few ecosystem services and increased disservices [48], thereby negatively impacting natural areas. In an agricultural setting, however, the exurbs may restore less intensive and more natural systems to the landscape, increasing habitat heterogeneity, which would be beneficial to local biodiversity [49]. ...
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Exurban development is a prominent land use in the United States of America, particularly in the Midwest, where much of it occurs on farmland and remnant woodlands. While exurbanization may affect ecosystem services, its impact could be modulated by management decisions made by residents. We aimed to uncover how exurban residents in a midwestern county perceived ecosystem services and disservices provided by their property based on 26 semi-structured interviews of landowners on parcels between 1 and 20 acres with a pond in unincorporated areas. We found the ecosystem services people associated most with their land are classified as cultural services (dominated by recreation services), while the most common mentioned disservices were classified as regulating disservices. Many ecosystem services that would be categorized as supporting or regulating services were not mentioned by interviewees, including microclimate stabilization, carbon sequestration, disease regulation, and maintenance of genetic diversity. Residents spent an average of 1.4 h/acre each week managing their properties. However, as parcel size and forest cover increased, the residents reported managing less surface area. Our study suggested that residents cultivate landscape features that directly benefit them and view many of the services that benefit regional biodiversity and ecosystem processes as disservices, which, to rectify, may require coordinated landscape-level management or local policies/incentives.
Maintaining monoculture lawns is resource intensive and can have environmental impacts on arthropod and aquatic communities. Here we introduce the reader to some of the many native plants that establish in lawns and if left to grow can provide ecological benefits to urban landscapes. Most resources and marketing regarding these plants focus on killing them rather than on their ecological benefits. This publication introduces the audience to these species and provides information about the biology and benefits of these species, as well as resources to learn more about self-recruiting lawn species.
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Land-use change is highly dynamic globally and there is great uncertainty about the effects of land-use legacies on contemporary environmental performance. We used a chronosequence of urban grasslands (lawns) that were converted from agricultural and forested lands from 10 to over 130 years prior to determine if land-use legacy influences components of soil biodiversity and composition over time. We used historical aerial imagery to identify sites in Baltimore County, MD (USA) with agricultural versus forest land-use history. Soil samples were taken from these sites as well as from existing well-studied agricultural and forest sites used as historical references by the National Science Foundation Long-Term Ecological Research (NSF-LTER) Baltimore Ecosystem Study program. We found that the microbiomes in lawns of agricultural origin were similar to those in agricultural reference sites, which suggests that the ecological parameters on lawns and reference agricultural systems are similar in how they influence soil microbial community dynamics. In contrast, lawns that were previously forest showed distinct shifts in soil bacterial composition upon recent conversion but reverted back in composition similar to forest soils as the lawns aged over decades. Soil fungi did not follow similar trends as the bacteria in the previously forested lawns. Our results show that components of bacterial biodiversity and composition are resistant to change in previously forested lawns despite urbanization processes. Therefore land-use legacy, depending on the prior use, is an important factor to consider when examining urban ecological homogenization.
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ABSTRACT,ational facilities, and other greenbelts. Turfgrass ecosys- tems provide excellent soil erosion control, dust stabili- Soil organic C (SOC) directly affects soil quality by influencing zation, flood control, and urban heat dissipation. These aeration and water retention and serving as a major repository and reserve source of plant nutrients. Limited information is available grasslands may act as C sinks, absorbing more CO2 than concerning the long-term SOC changes in turfgrass systselection, mowing, and irrigation management. 0.83 for putting greens. Our results suggest that the CENTURY model Organic C change in soil is a slow process, and many can be used to simulate SOC changes in turfgrass systems and has the years and decades of measurements,are needed,to assess potential to compare C sequestration under various turf management C changes,as influenced,by,management,practices. conditions. Simulation results also suggest that warming temperatures Therefore, evaluation of different management options have greater degree of influence on SOC in turf systems compared with native grasslands.,relative to C sequestration is difficult to accomplish by sampling,and measuring,SOC content and C fractions over time. Simulation models,offer the opportunity,to predict long-term trends based on mathematical,repre- U nderstanding,long-term,SOC changes,in various sentations of nutrient-cycling processes in the soil–plant ecosystems,is of importance,because SOC directly systems. Predictive modeling,exercises allow significant affects soil quality by influencing the air-filled porosity insight into the ecosystem,dynamics. A number,of com- and water retention and serving as a major,repository puter models are available to evaluate SOC dynamics and reserve source of plant nutrients, especially N, P,
Terrestrial ecosystems significantly contribute to the global carbon cycle. The chapter discusses that stocks and fluxes are increasingly altered by human activities, through changes in land use, in atmospheric composition, and in climate. This carbon sink results from an increase in global terrestrial net primary productivity (NPP). Thus, it is essential to know what the present value of global NPP is, and whether it will continue to increase to sustain an important terrestrial carbon sink. This chapter uses recent information provided in each of the biome-updated estimates of global values for NPP and phytomass. Finally, the current and expected changes in global NPP and net ecosystem productivity are discussed. The main source of uncertainty on global NPP lies in the biome area, and especially in forest area. Significant progress on biome maps is expected in the near future from the use of satellite data, once the classification done using such data can be given biome names that can receive a large acceptance. There is also some uncertainty on the current and future rates of change in NPP. Research on this aspect, at all scales, is developing fast. The chapter emphasizes on predicting more precisely the impact of human activities on NPP and on the carbon cycle in general. This information is crucial to design and implement sustainable development of human societies.
Field measurements from the Oregon Transect Ecological Research project (OTTER) were used to validate selected process simulations in the FOREST-BGC ecosystem model. Certain hydrologic, carbon, and nitrogen cycle process simulations were tested in this validation, either comparatively across sites, or seasonally at single sites. The range of simulated ecosystem-process rates across the OTTER sites was large; annual evapotranspiration (ET) ranged from 15 to 82 cm, net photosynthesis (as carbon) from 2.2 to 22.8 Mg/ha, and decomposition (as carbon) from 1.0 to 7.2 Mg@?ha^-^1@?yr^-^1. High correlations between predicted and measured data were found for: aboveground net primary production, R^2 = 0.82; 100-yr stem biomass, R^2 = 0.79; and average leaf nitrogen concentration, R^2 = 0.88. However, correlations for pre-dawn leaf water potential and equilibrium leaf area index (LAI) were much lower, because successful simulation of these variables requires accurate data for soil water-holding capacity. Defining the water-holding capacity of the rooting zone and the maximum surface conductance for photosynthesis and transpiration rates proved to be critical system variables that defied routine field measurement. Although many other processes are simulated in FOREST-BGC, no other processes had repeated field data sets for validations. Problems in parameterizing the model from disparate data sets are also evaluated, with suggestions for using ecosystem modeling in future integrated research programs.
A study of the energetics of a sururban lawn was conducted in 1972-73 in Walnut Creek, California USA. Several major components of the annual primary and secondary production were measured, including man's role as manager and experimenter in the system. The system was extremely productive with net productivity of 1,020 g/m^2 per yr compared to cornfields with productivity of 1,066 g/m^2 per yr and exceeding tall grass prairie values of around 1,000 g/m^2 per yr. Homopterans, with maximal values of 19 mg/m^2 were plentiful; other typical grassland species, like Araneida, were scarce, representing only 1% by weight of the total invertebrate population. Food utilization per unit area by suburban birds considerably exceeded natural grassland bird utilization (46 kcal/m^2 per yr vs. 1.01-2.33 kcal/m^2 per yr); lawns are ideal foraging sites for open area adapted, flock-feeding species. Man was the dominant consumer in the community, accounting for 10% of the herbivory and nearly 100% of the scavenging. Energy inputs (labor, gasoline, fertilizer, etc.) amounted to 578 kcal/m"2 per yr, equalling or exceeding corn production for a comparable net productivity, but not necessarily utilitarian return.
BIOME-BGC is a general ecosystem model designed to simulate hydrologic and biogeochemical processes across multiple scales. The objectives of this investigation were to compare BIOME-BGC estimates of hydrologic processes with observed data for different boreal forest stands and investigate factors that control simulated water fluxes. Model results explained 62 and 98% of the respective variances in observed daily evapotranspiration and soil water; simulations of the onset of spring thaw and the dates of snowpack disappearance and accumulation also generally tracked observations. Differences between observed and simulated evapotranspiration were attributed to model assumptions of constant, growing season, overstory leaf areas that did not account for phenological changes and understory effects on stand daily water fluxes. Vapor pressure deficit and solar radiation accounted for 58–74% of the variances in simulated daily evapotranspiration during the growing season, though low air temperature and photosynthetic light levels were found to be the major limiting factors regulating simulated canopy conductances to water vapor. Humidity and soil moisture were generally not low enough to induce physiological water stress in black spruce stands, though low soil water potentials resulted in approximate 34% reductions in simulated mean daily canopy conductances for aspen and jack pine stands. The sensitivity of evapotranspiration simulations to leaf area (LAI) was less than expected because of opposing responses of transpiration and evaporation to LAI. The results of this investigation identify several components within boreal forest stands that are sensitive to climate change.
The effects of disturbance history, climate, and changes in atmospheric carbon dioxide (CO2) concentration and nitrogen deposition (Ndep) on carbon and water fluxes in seven North American evergreen forests are assessed using a coupled water–carbon–nitrogen model, canopy-scale flux observations, and descriptions of the vegetation type, management practices, and disturbance histories at each site. The effects of interannual climate variability, disturbance history, and vegetation ecophysiology on carbon and water fluxes and storage are integrated by the ecosystem process model Biome-BGC, with results compared to site biometric analyses and eddy covariance observations aggregated by month and year. Model results suggest that variation between sites in net ecosystem carbon exchange (NEE) is largely a function of disturbance history, with important secondary effects from site climate, vegetation ecophysiology, and changing atmospheric CO2 and Ndep. The timing and magnitude of fluxes following disturbance depend on disturbance type and intensity, and on post-harvest management treatments such as burning, fertilization and replanting. The modeled effects of increasing atmospheric CO2 on NEE are generally limited by N availability, but are greatly increased following disturbance due to increased N mineralization and reduced plant N demand. Modeled rates of carbon sequestration over the past 200 years are driven by the rate of change in CO2 concentration for old sites experiencing low rates of Ndep. The model produced good estimates of between-site variation in leaf area index, with mixed performance for between- and within-site variation in evapotranspiration. There is a model bias toward smaller annual carbon sinks at five sites, with a seasonal model bias toward smaller warm-season sink strength at all sites. Various lines of reasoning are explored to help to explain these differences.
Maintaining high quality surface and groundwater supplies is a national concern. Nitrate is a widespread contamination of groundwater. Nitrogenous fertilizer applied to turfgrass could pose a threat to groundwater quality. However, a review of the fate of N applied to turfgrass is lacking, but needed in developing management systems to minimize groundwater contamination. The discussion of the fate of N applied to turfgrass is developed around plant uptake, atmospheric loss, soil storage, leaching, and runoff. The proportion of the fertilizer N that is taken up by the turfgrass plant varied from 5 to 74% of applied N. Uptake was a function of N release rate, N rate and species of grass. Atmospheric loss, by either NH3 volatilization was generally <36% of applied N and can be reduced substantially by irrigation after application. Denitrification was only found to be significant (93% of applied N) of fine-textured, saturated, warm soils. The amount of fertilizer N foud in the soil plus thatch pool varied as a function of N source, release rate, age of site, and clipping management. With a soluble N source, fertilizer N found in the soil and thatch was 15 to 21% and 21 to 26% of applied N, respectively, with the higher values reflecting clippings being returned. Leaching losses for fertilizer N were highly influenced by fertilizer management practices (N rate, source, and timing), soil texture, and irrigation. Highest leaching losses were reported at 53% of applied N, but generally were far less than 10%. Runoff of N applied to turfgrass has been studied to a limited degree and has been found seldom to occur at concentrations above the federal drinking water standard for NO3-. Where turfgrass fertilization poses a threat to groundwater quality, management strategies can allow the turfgrass manager to minimize or eliminate NO3- leaching.
The construction and maintenance of impervious surfaces-buildings, roads, parking lots, roofs, etc.-constitutes a major human alteration of the land surface, changing the local hydrology, climate, and carbon cycling. Three types of national coverage data were used to model the spatial distribution and density of impervious surface area (ISA) for the conterminous U.S.A. The results (Figure 1) indicate that total ISA of the 48 states and Washington, D.C., is 112,610 km2 (+/- 12,725 km2), which is slightly smaller than the state of Ohio (116,534 km2) and slightly larger than the area of herbaceous wetlands (98,460 km2) of the conterminous United States. The same characteristics that make impervious surfaces ideal for use in construction produce a series of effects on the environment. Impervious surfaces alter sensible and latent heat fluxes, causing urban heat islands. In heavily vegetated areas, the proliferation of ISA reduces the sequestration of carbon from the atmosphere. ISA alters the character of watersheds by increasing the frequency and magnitude of surface runoff pulses. Watershed effects of ISA begin to be detectable once 10% of the surface is covered by impervious surfaces, altering the shape of stream channels, raising water temperatures, and sweeping urban debris and pollutants into aquatic environments. Consequences of ISA include reduced numbers and diversity of species in fish and aquatic insects, and degradation of wetlands and riparian zones.
Soil information is now widely required by many climate and hydrology models and soil-vegetation-atmosphere transfer schemes. This pa- per describes the development of a multilayer soil characteristics dataset for the conterminous United States (CONUS-SOIL) that specifically addresses the need for soil physical and hydraulic property information over large areas. The State Soil Geographic Database (STATSGO) developed by the U.S. De- partment of Agriculture-Natural Resources Conservation Service served as the starting point for CONUS-SOIL. Geographic information system and Perl computer programming language tools were used to create map coverages of soil properties including soil texture and rock fragment classes, depth-to-bed- rock, bulk density, porosity, rock fragment volume, particle-size (sand, silt, and clay) fractions, available water capacity, and hydrologic soil group. In- terpolation procedures for the continuous and categorical variables describing these soil properties were developed and applied to the original STATSGO data. In addition to any interpolation errors, the CONUS-SOIL dataset reflects the limitations of the procedures used to generate detailed county-level soil