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RESEARCH ARTICLE
Root Dynamics of Cultivar and Non-Cultivar
Population Sources of Two Dominant Grasses during
Initial Establishment of Tallgrass Prairie
Ryan P. Klopf1,2 and Sara G. Baer1
Abstract
Dominance of warm-season grasses modulates tallgrass
prairie ecosystem structure and function. Reintroduction
of these grasses is a widespread practice to conserve soil
and restore prairie ecosystems degraded from human land
use changes. Seed sources for reintroduction of dominant prairie grass species include local (non-cultivar) and
selected (cultivar) populations. The primary objective of
this study was to quantify whether intraspecific variation in developing root systems exists between population sources (non-cultivar and cultivar) of two dominant
grasses (Sorghastrum nutans and Schizachyrium scoparium) widely used in restoration. Non-cultivar and cultivar
grass seedlings of both species were isolated in an experimental prairie restoration at the Konza Prairie Biological
Station. We measured above- and belowground net primary production (ANPP and BNPP, respectively), root
architecture, and root tissue quality, as well as soil moisture
and plant available inorganic nitrogen (N) in soil associated
with each species and source at the end of the first growing season. Cultivars had greater root length, surface area,
and volume than non-cultivars. Available inorganic N and
soil moisture were present in lower amounts in soil proximal to roots of cultivars than non-cultivars. Additionally,
soil NO3 –N was negatively correlated with root volume
in S. nutans cultivars. While cultivars had greater BNPP
than non-cultivars, this was not reflected aboveground root
structure, as ANPP was similar between cultivars and
non-cultivars. Intraspecific variation in belowground root
structure and function exists between cultivar and noncultivar sources of the dominant prairie grasses during initial reestablishment of tallgrass prairie. Population source
selection should be considered in setting restoration goals
and objectives.
Introduction
2005), root systems of the dominant grasses are likely to influence trajectories and outcomes of community and ecosystem
reassembly. Previous research has demonstrated root morphology impacts functional properties of ecosystems such as the
distribution and availability of nitrogen, water, and carbon, as
well as competitive species interactions (Craine et al. 2002;
Schenk & Jackson 2002; Craine et al. 2003; Hui & Jackson
2005).
A variety of seed sources for grassland restoration exist,
including locally collected ecotypes (i.e., non-cultivars) and
genotypes selected for specific traits (i.e., cultivars). Use of
non-cultivars has been promoted in restoration because it has
been assumed that they contain genetic variability necessary
for the long-term success of the species (Lesica & Allendorf
1995; Hufford & Mazer 2003). Cultivars of the dominant
prairie grasses have been developed by the United States
Department of Agriculture (USDA) for high seed viability,
germination, and forage production (Fehr 1987), and as such
they may have been selected to grow faster than non-cultivar
sources. Cultivars of the dominant grasses may be genetically
distinct from remnant wild populations (Gustafson et al. 2004),
and higher net rates of photosynthesis have been observed
Conversion of the North American prairie to row-crop agriculture has reduced the extent of this grassland ecosystem
from 162 to 8.1 million hectares since the 1830s (Samson
& Knopf 1994; Manning 1995). Prairie restoration primarily relies on the sowing of native plant communities from
seed. Native plant species diversity in prairie restorations is
often lower than in remnant prairies due to the dominance
of a few C4 grass species (Baer et al. 2005; Polley et al.
2005; Taft et al. 2006), which can account for the majority
of aboveground biomass and cover in tallgrass prairie (Knapp
et al. 1998). These dominant grasses also drive recovery of
soil structure and function (e.g., carbon and nitrogen pools)
during grassland restoration (Baer et al. 2002; McLauchlan
et al. 2006). Because the majority of plant biomass in tallgrass
prairie occurs belowground (Gill et al. 2002; Milchunas et al.
1 Department of Plant Biology and Center for Ecology, Southern Illinois
University, Carbondale, IL, Carbondale, IL 62901, U.S.A.
2 Address correspondence to Ryan P. Klopf, email [email protected]
© 2009 Society for Ecological Restoration International
doi: 10.1111/j.1526-100X.2009.00539.x
112
Key words: cultivar, dominant grasses, local ecotype, roots,
tallgrass prairie restoration.
Restoration Ecology Vol. 19, No. 1, pp. 112–117
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Root Dynamics of Cultivar and Non-Cultivar Population Sources of Two Dominant Grasses
in cultivars of two widely seeded species, Panicum virgatum
L. and Andropogon gerardii Vitman, relative to non-cultivars
of these species (Skeel & Gibson 1996; Baer et al. 2005).
No studies have directly compared, in a common garden
experiment, belowground characteristics of cultivars and noncultivars of the dominant grasses that drive recovery of
ecosystem structure and function during prairie restoration
(Baer et al. 2002; McLauchlan et al. 2006).
The primary objective of this study was to determine
whether intraspecific variation exists in root structure (i.e.,
length, surface area, and volume) and function (i.e., soil inorganic N depletion) between cultivar and non-cultivar sources
of dominant grasses used for restoration, and whether this variation scales to affect ecosystem structure and function. We
hypothesized that cultivars would exhibit higher belowground
net primary production (BNPP) and have more extensive root
architecture than non-cultivars due to selection for greater
growth rates that would require larger fine root networks
to capture more resources and support higher aboveground
net primary productivity (ANPP). Specifically, we predicted
that larger fine root networks would increase the capacity
of cultivars to deplete soil water and available inorganic N.
In the short term, cultivar selection for faster growth may
result in greater root biomass and N uptake. Alternatively,
faster-growing cultivars may have been selected for aboveground production at the expense of allocation to belowground
biomass. While previous research has established the importance of the dominant grasses in determining the outcome of
a prairie restoration in terms of community composition (Baer
et al. 2003) and soil recovery (Knops & Tilman 2000; Baer
et al. 2002; McLauchlan et al. 2006), we aimed to elucidate
whether intraspecific variation in the belowground structure
and function of the dominant grasses might provide a mechanistic explanation for variation in the establishment success of
grasses during the initial stages of prairie restoration.
Methods
Experimental restoration plots were located in an agricultural
field at Konza Prairie Biological Station (KPBS), in the Flint
Hills of eastern Kansas (39◦ 05 N, 96◦ 35 W). Elevation at the
site was 340 m above sea level. Mean annual temperature is
13◦ C with a range of monthly mean temperatures of 6–19◦ C.
Mean annual precipitation is 835 mm, of which 75% occurs
during the growing season. During this study (May 1–August
1, 2006) precipitation was 204 mm, 42% (153 mm) below
the 1971–2000 average. During June and July of 2006, the
average maximum and minimum air temperatures were 32.5◦ C
and 18.9◦ C, respectively. The maximum and minimum 5 cm
soil temperatures for this period were 32.5◦ C and 23.6◦ C,
respectively (Kansas Weather Data Library 2007).
The experimental plots were established in a Reading silt
loam (fine-silt, mixed, superactive, mesic Pachic Arguidoll)
soil formed by colluvial and alluvial deposits. Historically,
the study site would have been a native prairie community,
dominated by A. gerardii (big bluestem), Sorghastrum nutans
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Restoration Ecology
(L.) Nash (Indian grass), and a suite of prairie forbs that
contribute most to diversity (Freeman et al. 1998). Before
establishment of the experimental restoration plots, the field
had been in cultivation since the early twentieth century. Since
1976, the field has been exclusively in Triticum aestivum
L. (winter wheat), Zea mays L. (maize), or Glycine max
(L.) Merr. (soybean) production. Liquid chemical fertilizers
(e.g., liquid ammonium phosphate 10-34-0) have been the
only supplemental source of nutrients applied since 1976 (S.
Runquist & T. Van Slyke 2006, Kansas State University,
personal communication).
In the winter of 2005, following harvest of T. aestivum and
subsequent mechanical disking, we delineated two blocks each
containing six 5 m × 25 m plots with 6 m buffers between
adjacent plots. Each plot was seeded by hand with a mixture
of three dominant warm-season grasses (A. gerardii, S. nutans,
and Schizachyrium scoparium [Michx.] Nash [little bluestem]),
two non-dominant grasses, and 13 forbs frequently used in
prairie restoration. Species richness and seeding rate (600 live
seeds/m2 ) were identical in all plots. The whole plot treatment
was dominant grass seed source. Seeds of the dominant grasses
were either non-cultivars hand collected from remnant prairie
at KPBS or cultivars purchased from the USDA that were
developed within the region. The three cultivars used in this
experiment were A. gerardii “Kaw,” S. nutans “Osage,” and
S. scoparium “Camper.” All seeds were hand broadcast and
raked into the soil in the winter of 2005 to ensure adequate
time for scarification and cold stratification.
In June 2005, shortly after germination and initial emergence of the grass seedlings in the restoration, we inserted
8 cm diameter by 20 cm depth PVC cores around one to two
individuals of each of the three dominant grass species in each
whole plot. Two holes were drilled into the aboveground rim of
the PVC to allow drainage of surface collected water. Isolated
seedlings developed in situ until peak biomass.
In the first week of September 2006, each PVC core containing soil and roots was removed and transported to the
laboratory. Plants were identified and mortality data were collected for cultivars and non-cultivars of each species. None
of the non-cultivar A. gerardii seedlings survived the first
growing season, whereas 7 out of 11 cultivars survived (36%
mortality). For S. nutans, 12 out of 15 non-cultivars survived
(20% mortality), while only 5 out of 13 cultivar plants (62%
mortality) survived. For S. scoparium, non-cultivar and cultivar plants experienced 50% (5 out of 10 survived) and 9% (10
out of 11 survived) mortality, respectively.
Aboveground biomass of each seedling was clipped, dried
(55◦ C) for 1 week, and weighed to calculate ANPP. The
belowground contents of each PVC core were removed and
divided into 0–10 cm and 10–20 cm depths. Roots from
each depth were manually separated from the soil, and then
washed clean with reagent grade water. Root networks were
scanned with WinRHIZO v. 2002c (Reagent Instruments, Inc.,
Quebec, Canada) on a flatbed scanner (Epson Expression 1640
XL) at a resolution of 800 dots per inch. Fragments of nonroot material present in the scanned image were manually
traced and excluded from analyses. WinRHIZO software,
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Root Dynamics of Cultivar and Non-Cultivar Population Sources of Two Dominant Grasses
using the automatic sensitivity setting, traces the roots in
an image and calculates length, surface area, and volume
for the prescribed diameter size classes. Because fine roots
function differently than coarse roots (Jastrow et al. 1998;
Craine et al. 2002), data were collected for five root diameter
classes: 0–0.5 mm, 0.5–1.0 mm, 1.0–1.5 mm, 1.5–2.0 mm,
and greater than 2.0 mm.
Following architectural analysis, roots were dried (55◦ C) for
1 week and weighed to calculate BNPP. Dried roots were then
ground and analyzed for percent C and N on a Flash 1112 CN
Analyzer (CE Elantech Corp., Lakewood, New Jersey, U.S.A.)
to determine root quality as indexed by C:N ratio.
After root removal, soil from each core was passed through
a 4-mm diameter sieve and a subsample was extracted for
plant available inorganic N. Inorganic nitrogen extractions
were performed by shaking 10–12 g of soil with 0.01 N
KCl for 1 hour. Solutions were decanted and filtered through
0.4 μm polycarbonate membranes. Extracts were analyzed for
NO3 –N and NH4 –N on a Flow IV Solution Autoanalyzer (OI
Analytical Corp., College Station, Texas, U.S.A.). Gravimetric
soil moisture was determined by drying approximately 20 g of
field moist soil at 105◦ C for 72 hours.
Because of the independent nature of plant growth among
the individual PVC cores, differences within and among
species and seed sources (i.e., cultivar or non-cultivar)
were analyzed according to a randomized complete block
design using the mixed model procedure in SAS (SAS Inc.
2003). Because one treatment combination (non-cultivars of
A. gerardii ) experienced 100% mortality, resulting in an
unbalanced treatment structure (Littell et al. 2006), we used
contrast and estimate statements to determine main effects
of seed source for each species (α = 0.05, p values < 0.10
reported). Root length, surface area, and volume data were log
transformed before analyses to improve normality. Pearson’s
correlation coefficients were used to examine relationships
between response variables.
Results
Approximately 80% of all roots occurred in the surface 10 cm
of each soil core. Across all species and sources, over 99%
of the roots were classified as fine, that is, less than 1 mm
in diameter. Of these fine roots, an average of 97% were
very fine, that is, less than 0.5 mm in diameter. In the surface 10 cm, across both S. nutans and S. scoparium, cultivars had greater total (F1,32 = 5.09, p = 0.031), fine (F1,32 =
5.06, p = 0.031), and coarse (F1,32 = 3.32, p = 0.078) root
length than non-cultivars (Table 1). Across S. nutans and
S. scoparium, root surface area (F1,32 = 4.43, p = 0.043) and
root volume (F1,33 = 4.97, p = 0.033) were also greater in
cultivars than non-cultivars (Table 1).
The only intraspecific difference in net primary production
(NPP) occurred belowground (Table 2). Across both S. nutans
and S. scoparium, BNPP in the 0–20 cm depth was greater for
cultivar than non-cultivar sources (F1,33 = 4.42, p = 0.043).
Root tissue quality (C:N) and N concentrations were similar between cultivars and non-cultivars (p > 0.10). The root
C:N ratios for S. nutans were 45.9 ± 6.4 and 44.1 ± 1.4 for
cultivars and non-cultivars, respectively; and for S. scoparium
were 48.5 ± 2.6 and 46.5 ± 3.6 for cultivars and non-cultivars,
Table 1. Average (±1 standard error) total, fine, and coarse root length, volume, surface area, and ratio of fine to coarse roots of cultivar and non-cultivar
sources of two dominant grasses in the surface 10 cm of soil. Means accompanied by two asterisks indicate the cultivar source was different than the
non-cultivar source (p < 0.05). Means accompanied by one asterisk indicate the cultivar source was different than the non-cultivar source (p < 0.10).
Species
Across Species
Sorghastrum nutans
Schizachyrium scoparium
Source
Source
Source
Cultivar
Root length (cm)
Total
Fine
Coarse
Fine: Coarse
Root volume (cm3 )
Total
Fine
Coarse
Fine: Coarse
Root surface area (cm2 )
Total
Fine
Coarse
Fine: Coarse
114
Non-cultivar
Cultivar
Non-cultivar
Cultivar
Non-cultivar
1117 ± 145∗∗
1104 ± 141∗∗
13.5 ± 14.2∗
221.2 ± 47.3
1017 ± 198
1005 ± 195
12.4 ± 3.8
240.3 ± 82.1
1752 ± 247∗
1721 ± 241∗
30.5 ± 6.9∗
62.0 ± 7.8
1221 ± 268
1205 ± 264
16.9 ± 5.0
143.0 ± 36.0
863 ± 97
857 ± 94
6.8 ± 3.3
285.0 ± 54.1
568 ± 41
566 ± 41
2.6 ± 0.8
455.0 ± 238.8
0.799 ± 0.17∗∗
0.516 ± 0.07∗∗
0.282 ± 0.10∗∗
7.52 ± 1.84
0.681 ± 0.153
0.454 ± 0.08
0.228 ± 0.08
5.41 ± 1.30
1.49 ± 0.29∗∗
0.79 ± 0.13∗
0.69 ± 0.17∗∗
1.22 ± 0.18
0.85 ± 0.20
0.53 ± 0.11
0.31 ± 0.10
4.27 ± 1.35
0.522 ± 0.12
0.406 ± 0.06
0.12 ± 0.06
10.04 ± 2.10
0.318 ± 0.05
0.280 ± 0.03
0.04 ± 0.02
8.56 ± 2.91
65.80 ± 9.48∗∗
59.43 ± 7.57∗∗
6.37 ± 2.03∗
28.77 ± 6.26
58.79 ± 10.69
53.15 ± 9.18
5.64 ± 1.81
30.78 ± 9.94
104.16 ± 16.7∗
89.35 ± 13.57∗
14.81 ± 3.48∗
6.53 ± 0.81
69.80 ± 14.46
62.08 ± 12.52
7.71 ± 2.39
17.57 ± 4.53
50.46 ± 7.29
47.47 ± 5.96
2.99 ± 1.53
37.67 ± 6.96
34.57 ± 2.87
33.49 ± 2.74
1.08 ± 0.35
59.86 ± 27.68
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Root Dynamics of Cultivar and Non-Cultivar Population Sources of Two Dominant Grasses
Table 2. Average (±1 standard error) below- and aboveground net
primary production (BNPP and ANPP, respectively) for individual plants
of cultivar and non-cultivar sources of each dominant grass species. Means
accompanied by an asterisk indicate the cultivar source was different than
the non-cultivar source (p < 0.05).
BNPP (g/plant/year)
A. gerardii
Cultivar
S. nutans
Cultivar
Non-cultivar
S. scoparium
Cultivar
Non-cultivar
Across all species
Cultivar
Non-cultivar
ANPP (g/plant/year)
0.380 ± 0.046
0.777 ± 0.122
0.565 ± 0.105∗
0.374 ± 0.061
0.856 ± 0.175
0.780 ± 0.125
0.355 ± 0.054
0.254 ± 0.038
0.782 ± 0.136
0.440 ± 0.092
0.415 ± 0.054∗
0.339 ± 0.046
0.806 ± 0.106
0.680 ± 0.098
25
25
Non-cultivar
Cultivar
B
20
NO3-N (μg g−1)
20
15
10
5
0
0.4
A. gerardii
S. nutans
NH4-N (μg g−1)
10
0
S. scoparium
14
C
0.3
0.2
0.1
0.0
15
5
Total Inorganic Nitrogen (μg g−1)
Total Inorganic nitrogen (μg g−1)
A
respectively. Variation in tissue quality is attributable to differences in root N concentration, which were similar in cultivars
(8.52 ± 0.78 mg·g−1 ) and non-cultivars (8.86 ± 0.26 mg·g−1 )
of S. nutans and in cultivars (7.95 ± 0.33 mg·g−1 ) and noncultivars (8.68 ± 0.77 mg·g−1 ) of S. scoparium.
Two important soil resources, available inorganic N and soil
moisture, were present in slightly lower amounts in soil proximal to roots of cultivars than non-cultivars. Averaged across
both S. nutans and S. scoparium, cultivar rhizosphere soil had
moderately lower levels of plant available NO3 –N (F1,34 =
3.33, p = 0.077) and total inorganic N (F1,34 = 3.41, p =
0.073) in the surface 10 cm than non-cultivar rhizosphere.
This variation in N availability between sources was driven
by strong differences in N availability between cultivars and
non-cultivars of S. scoparium. Soil associated with cultivars
of this species had lower plant available total inorganic N
(F1,34 = 5.53, p = 0.025) than non-cultivar sources of this
species due to less NO3 –N (F1,34 = 5.42, p = 0.026) (Fig. 1).
Averaged across species, gravimetric soil moisture was also
lower in soil from cultivar plants (11.92%) than soil from
A. gerardii
S. nutans S. scoparium
A. gerardii
S. nutans S. scoparium
D
12
10
8
6
4
2
0
Non-cultivar
Cultivar
Figure 1. Average (±1 standard error) plant available (A) total inorganic N, (B) NH4 –N, (C) NO3 –N in soil associated with cultivar and non-cultivar
sources of each species, and (D) total inorganic N across all species between cultivar and non-cultivar treatments in the 0–10 cm depth. Two asterisks
indicate a difference between cultivar and non-cultivar treatments (p < 0.05). A single asterisk indicates a difference between cultivar and non-cultivar
treatments (p < 0.10).
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115
Root Dynamics of Cultivar and Non-Cultivar Population Sources of Two Dominant Grasses
non-cultivar plants (12.99%) in the 0–10 cm depth (F1,33 =
4.16, p = 0.050).
BNPP was positively correlated with root surface area
(r = 0.86; p = 0.056) and volume (r = 0.85; p = 0.008) for
cultivars of both species. BNPP of the non-cultivar source of
S. nutans was also positively correlated with these architectural
response variables (r = 0.83; p = 0.020), but BNPP of the
non-cultivar source of S. scoparium was not correlated with
root length, surface area, or volume. Plant available soil
NO3 –N in the 0–10 cm depth was negatively correlated with
total root volume of S. nutans cultivars (r = −0.949; p =
0.051). No correlations between soil N and root architecture
were observed in non-cultivar sources of either species at either
depth.
Discussion
The overall importance of roots in grassland ecosystems is
well documented (Weaver 1961; Gill et al. 1999; Derner et al.
2001; Craine et al. 2002; Milchunas et al. 2005). No studies
have compared root architecture of dominant grasses during
restoration of tallgrass prairie. Greater BNPP, root length,
surface area, and volume in cultivars suggest that selection
for aboveground attributes during development of cultivars
(Fehr 1987; Falk 1990) also enhances morphological traits
belowground. Architectural differences were the strongest in
the surface 10 cm of soil most likely because fine fibrous
roots would have been concentrated where levels of available inorganic N were the greatest during the first growing
season of restoration following long-term cultivation (Weaver
1958; Ennos & Fitter 1992). In cultivars of both species, and
non-cultivars of S. nutans, greater BNPP was translated into
increased root length, surface area, and volume, indicating that
greater BNPP has increased the functional interface between
these plants and their environment. More extensive root architecture may enable cultivars to better exploit heterogeneous
soil resources to maximize growth with minimal energetic
costs (Craine et al. 2002; Hodge 2006). In addition to greater
root biomass and length, rhizosphere soil contained less available inorganic N (most strongly observed for S. scoparium),
suggesting that the cultivars more effectively monopolized
this limiting resource, or roots provided greater carbon inputs
that stimulated microbial activity and immobilization of inorganic N.
Intra- and/or interspecific variation in depletion of plant
available N and soil moisture at the onset of a restoration
may affect species richness and diversity in the developing
community (Baer et al. 2003; Hobbs & Norton 2004). In a
previous prairie reassembly experiment, inorganic N monopolization by S. scoparium precluded the establishment of species
with patterns of N uptake similar to S. scoparium (Fargione
& Tilman 2005). In our study, root volume was inversely
correlated with available soil N in cultivars of S. nutans,
suggesting cultivars of this species possess an architectural
advantage in the depletion of N available for plant uptake
over non-cultivars. Consequently, use of some grass cultivars
116
may increase productivity and reduce soil inorganic N, which
may have implications for plant diversity in restorations (Baer
et al. 2003).
Phenotypic variation can scale to affect ecosystem function
(LeRoy et al. 2006). While cultivars have been anthropogenically selected for enhanced aboveground performance, the
long-term implications for these phenotypic alterations are
unknown. This study demonstrates that cultivars of the dominant species used in prairie restoration also exhibit enhanced
belowground productivity and resource capture, which may
contribute to the higher forage production and reproductive
output that selection has aimed to enhance (Fehr 1987). During the second growing season of this restoration experiment (2007), cultivars of the dominant grasses had greater
ANPP (77 g m−2 yr−1 ) than non-cultivars (18 g m−2 yr−1 )
(F1,50 = 5.86, p = 0.019), suggesting that intraspecific variation can scale to affect ecosystem function in subsequent
years (R. Klopf, unpublished data). While this greater productivity of cultivar grasses increases their initial dominance
in the community, it is possible that there are fitness costs
associated with maintaining enhanced root and shoot production which will manifest themselves with stochastic events on
greater spatiotemporal scales.
Implications for Practice
• Different population sources of locally adapted species
used in restoration may exhibit variation in growth and
resource use.
• Cultivars of the dominant prairie grasses, selected for
enhanced aboveground traits, also have greater and more
extensive root production than non-cultivars from locally
collected seed.
• Phenotypic and functional variation associated with different population sources should be considered when
setting restoration goals, as variation in belowground
productivity and resource capture may affect species
composition and function in the restored community.
Acknowledgments
Funding for this research was provided by the National
Science Foundation (DEB-0516429), Konza Prairie LongTerm Ecological Research program, and the Southern Illinois
University Carbondale Faculty Seed Grant Program. We thank
David J. Gibson and Bryan G. Young for guidance with
this research. Field and laboratory assistance were provided
by Ryan Campbell, Dana Carpenter, David Dalzotto, Robin
Garcia, Luke Koett, Allison Lambert, Meredith Mendola,
Clinton K. Meyer, Lewis Reed, and Stephanie Welsh. We also
thank two anonymous reviewers and Dr Stuart Allison for
providing constructive suggestions toward this manuscript.
Restoration Ecology
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