Journal of Applied
Ecology 2006
Assessing grassland restoration success: relative roles of
seed additions and native ungulate activities
Blackwell Publishing Ltd
LEANNE M. MARTIN and BRIAN J. WILSEY
Iowa State University, Department of Ecology, Evolution and Organismal Biology, Ames, IA 50011, USA
Summary
1. Grassland restorations often lack rare forb and grass species that are found in intact
grasslands. The possible reasons for low diversity include seed limitation, microsite
limitation and a combination of both. Native ungulates may create microsites for
seedling establishment in tallgrass prairie restorations by grazing dominant species or
through trampling activities, but this has never been tested in developing prairies.
2. We experimentally tested for seed and microsite limitation in the largest tallgrass
prairie restoration in the USA by adding rare forb and grass seeds in two trials inside
and outside native ungulate exclosures. We measured seedling emergence because this
stage is crucial in recruiting species into a community. We also measured light, water
and standing crop biomass to test whether resource availability could help to explain
seedling emergence rates.
3. Ungulates increased light availability for each sampling time and also increased
above-ground net primary productivity (ANPP) during summer.
4. Seedling emergence of rare prairie forbs and grasses was consistently greater when
we added seeds.
5. Seedling emergence was conditionally greater with a combination of seed additions
and grazing, but grazing alone was unable to increase emergence.
6. When ungulates increased seedling enhancement, the mechanism was partially
associated with increased water and light availability.
7. Exotic and cosmopolitan weed seedling emergence was not affected by grazing.
8. Synthesis and applications. These results suggest that tallgrass prairie restorations
are primarily seed limited and that grazing alone may not be able to increase seedling
emergence of rare species without the addition of seeds. Therefore, adding seeds to
grassland restorations may increase seedling emergence of rare species, and mimicking
effects of grazing may increase emergence when seeds are added.
Key-words: Bos bison, Cervus elaphus, diversity, grazing, Iowa, net primary productivity,
seedling emergence, tallgrass prairie
Journal of Applied Ecology (2006)
doi: 10.1111/j.1365-2664.2006.01211.x
Introduction
Ecosystem restoration is becoming a more common
way to increase native species habitat. Typically, restorations are attempted by adding seeds from nearby
remnants to a previously converted ecosystem (Sluis
2002; Polley, Derner & Wilsey 2005). Seedlings of
multiple species are expected to emerge, survive and
© 2006 The Authors.
Journal compilation
© 2006 British
Ecological Society
Correspondence: Leanne M. Martin, University of
Nebraska at Omaha, Department of Biology, 6001 Dodge
St, Omaha, NE 68182, USA (fax 402 554 3532; e-mail
lmmartin@mail.unomaha.edu).
establish reproducing populations, and then populations are expected to assemble into a community
similar to the original system. The seedling emergence
stage is important in this process because it funnels
individuals into the system. Contrary to expectations,
restored ecosystems often have lower plant species
richness and diversity than their unaltered counterparts
(Galatowitsch & van der Valk 1996; Martin, Moloney
& Wilsey 2005; Polley, Derner & Wilsey 2005) and
species richness has been observed to decline over time
instead of increase as expected (Sluis 2002).
Typically, low species diversity is attributed to either
(i) seed limitation or (ii) seedling microsite limitation
2
L. M. Martin &
B. J. Wilsey
© 2006 The Authors.
Journal compilation
© 2006 British
Ecological Society,
Journal of Applied
Ecology
(Eriksson & Ehrlen 1992; Zobel et al. 2000; Foster
et al. 2004; Henry et al. 2004). The seed limitation
hypothesis suggests that plant community richness and
diversity are limited by the species pool (Gough, Grace
& Taylor 1994). Seed additions have increased species
richness and diversity of some native plant communities and agriculturally improved grasslands (Pywell
et al. 2002; Smith et al. 2002; but see Wilsey & Polley
2003). If restorations are seed limited, then adding
seeds of a large number of species should increase
diversity and recruitment of rare species even in systems with high dominance. Alternatively, the microsite
hypothesis suggests that one or a few strongly dominant
species suppress seedlings (Howe 2000; Sluis 2002;
Camill et al. 2004). In this scenario, seeds or propagules are not limiting but seedlings fail to establish
reproducing populations. Dominant grass patches are
usually larger in restorations than in intact grassland
(Derner et al. 2004). Dominance by C4 grasses, which
can occur as soon as 3 years after establishment, can be
especially high in the nutrient-rich environments that
characterize most restorations (Baer et al. 2002; Baer
et al. 2004; Camill et al. 2004). Large grass canopies
and abundant litter can reduce light and water availability, which are crucial to seedling survival (Fahnestock
& Knapp 1993; Haugland & Froud-Williams 1999;
Xiong & Nilsson 1999). Common management practices,
such as frequent spring burning and grazing exclusion,
could exacerbate this problem (Collins et al. 1998; Howe
2000). Thus, anything that reduces grass dominance
should alleviate competition with rare species and
should increase seedling establishment and diversity
(Foster & Gross 1997).
According to the intermediate disturbance and
grazing optimization hypotheses, intermediate levels
of grazing should produce the highest levels of species
diversity, but also the highest levels of net primary
productivity (NPP) in an ecosystem (Grime 1973;
Connell 1978). The question of whether native ungulate
grazing can increase seedling emergence and diversity
is becoming more relevant because grazers such as
bison Bos bison L. and wapiti Cervus elaphus L. are
increasingly being reintroduced (Knapp et al. 1999;
Larkin et al. 2004). Management strategies, such as
mowing, aimed at decreasing the biomass of dominant
species have shown increased seedling survival in
some experimental and pasture plantings (Burke &
Grime 1996; Hutchings & Booth 1996; Lawson, Ford &
Mitchley 2004). Moderate grazing by native ungulates,
a common grassland disturbance, could have positive
impacts on plant species diversity in intact grasslands
by reducing dominant grasses and increasing light
availability (Hartnett, Hickman & Walter 1996; Collins
et al. 1998; Knapp et al. 1999). Moderate grazing
would therefore be expected to produce non-linear
effects on diversity in restorations, with higher levels of
diversity at intermediate grazing intensities (Smith
et al. 2000). However, this may be restricted to intact
systems, where there is a propagule source available for
recruitment into the community (Hartnett, Hickman &
Walter 1996; Collins et al. 1998). Furthermore, because
intermediate levels of grazing can also produce the
highest levels of NPP (McNaughton 1979; Dyer,
Turner & Seastedt 1993), grazing may lead to increased
resource uptake by plants. Productivity is already
high in grassland restorations, often higher than in
comparable remnants (Baer et al. 2002; Camill et al.
2004; Martin, Moloney & Wilsey 2005). If intermediate grazing increases production of dominant species
above and beyond what is already high, then intermediate grazing in restorations, unlike in intact grasslands,
may actually lessen positive effects on seedling emergence
and diversity. Thus increased productivity in restorations
because of moderate grazing might nullify potentially
positive effects of grazing on microsite availability.
Finally, seed limitation and low seedling emergence
because of grass dominance may interact to limit diversity in grassland restorations. A combination of adding
seeds and increasing microsite availability may be
necessary to favour seedling emergence (Burke &
Grime 1996; Turnbull, Crawley & Rees 2000; Foster &
Dickson 2004).
Our objectives were to determine: (i) if native ungulates increase availability of resources crucial to seedling emergence and (ii) whether seed additions, native
ungulate grazing or a combination of both enhance
native prairie seedling emergence while having little or
no effect on non-native and cosmopolitan weeds in tallgrass prairie restorations. Our focus was on seedling
emergence, a key stage in the establishment of grassland plants. Whether seedlings can establish viable
populations is a longer term question that will not be
considered here.
Materials and methods
 
The objective of the Neal Smith National Wildlife
Refuge (NS) prairie project is to restore a large tallgrass
prairie ecosystem using locally collected seeds combined
with prescribed fire and grazing by native ungulates.
The restoration is located on the Walnut Creek watershed in Jasper County, Iowa, USA (41°33′N, 93°17′W).
The refuge currently spans 2104 ha, approximately
1200 ha of which have been seeded with tallgrass
prairie species, beginning in 1992 and continuing to the
present day. Grazing ungulates (B. bison and C. elaphus)
were introduced to a 303-ha enclosure in 1996 and
1998, respectively, which is where our study took place.
Approximately 35 B. bison and 15 C. elaphus occupied
the area during our study. Land use prior to prairie
seeding included corn Zea mays L. and soybean Glycine
max (L.) Merr rotations and a few scattered pastures.
There were 20 different plantings in this area (mean of
approximately 14 ha each) and each planting was seeded
with separate bulk seed mixes collected from local prairie
remnants. Management practices after planting included
3
Native ungulates
and seed additions
in restorations
yearly spring burning during the early years followed
by 2-year burn rotations, which is a common practice
for beginning restorations (Packard & Mutel 1997;
Copeland, Sluis & Howe 2002). Mowing was done
when necessary to control weedy and invasive species
(P. Drobney, personal communication). Our plots were
not burned or mowed in 2003 or 2004, the years of
sampling.
Historically, precipitation at the site has a unimodal
distribution and peaks in May and/or June, with an
average of approximately 880 mm year−1. Weather in 2003
was much warmer and drier than during 2004. Between
May and August, the peak growing months, temperatures and monthly precipitation averaged 21·7 °C and
13·3 mm in 2003 and 19·7 °C and 143·8 mm in 2004.
To standardize our sampling, we randomly selected
eight plantings within the enclosure that were seeded
between 1994 and 1996 on formerly cropped areas.
Four plantings north of a dirt road included 6·7 kg ha−1
of Elymus canadensis L. in the seed mixture as a putative cover crop, whereas four plantings south of the
road did not (for effects of the cover crop see Martin,
Moloney & Wilsey 2005).
 
A randomized complete block split-plot design with
unequal replication was used, with grazing or exclosures
applied to main plots and seed addition treatments
(described below) applied to subplots. Two 6 × 8-m
grazed plots were established 5 m away on either side of
a permanent 6 × 8-m permanent exclosure in June 2003
in each of the eight plantings (blocks). Two grazed
plots were sampled per planting because of increased
heterogeneity with grazing (Knapp et al. 1999). By
request of the refuge staff, exclosures were kept out of
view of visitors where possible and this precluded
completely random locations.
   
© 2006 The Authors.
Journal compilation
© 2006 British
Ecological Society,
Journal of Applied
Ecology
Biomass and above-ground net primary productivity
(ANPP) (general indicators of resource uptake and
competition intensity) were estimated to compare
grazed and exclosed plots (Baer et al. 2004). Aboveground biomass was clipped to 2 cm in a 40 × 100-cm
quadrat randomly placed in each exclosure (ng) and
grazed plot (gr), and surface litter was collected in June
and August 2003 and in March, June and August 2004.
Biomass was sorted into live and dead components,
and live material was sorted by species, dried for 48 h
at 65 °C, and weighed. Subsequently, estimates were
made for the following biomass components: proportion of exotic biomass (exotic/total), proportion of
total grass biomass (grass/total) and C4 grass biomass
(C4/total), and combined litter and standing dead biomass. Each of these different components of biomass
could suppress seedling emergence and species richness
in grassland restorations (Howe 2000; Camill et al.
2004). Plant species were designated as native or exotic
based on Eilers & Roosa (1994).
We estimated grazing intensity (GI) and used polynomial regressions to determine if GI was quadratically related to response variables (McNaughton 1979,
1985). Simply comparing grazed and ungrazed plots
can be misleading in cases where grazing is non-linearly
related to response variables (Grime 1973; Connell
1978; McNaughton 1979). Above-ground NPP and
GI were estimated for grazed plots (n = 16) using the
moveable exclosure approach (McNaughton 1985;
McNaughton, Milchunas & Frank 1996) during three
periods: June–August 2003 and March–June and
June–August 2004. One 3 × 4-m temporary exclosure
was established at each site in March 2004 and was
moved in June 2004 to measure consumption and GI
(McNaughton 1985). Above-ground biomass from the
centre of each temporary exclosure was collected using
the same quadrat size as explained above. Biomass from
the permanent exclosure was used to estimate consumption during June–August 2003 (i.e. the first growing
season). Consumption (C) was estimated as (ng – g)/
time, where ng was biomass inside and g was biomass
outside temporary exclosures at the end of the period,
and time was the number of days exclosures were in
place (McNaughton 1985; Wilsey et al. 2002). Aboveground NPP (g m−2 day−1) was calculated as a positive
biomass increment + consumption for each time period.
Grazing intensity was calculated as GI = C/NPP
(McNaughton 1985; Wilsey et al. 2002), with GI set to
0 if consumption estimates were negative.
Environmental variables were measured to determine
whether grazing was creating microsites favourable for
seedling emergence. Soil moisture and percentage light
at the soil surface were measured monthly from July to
September 2003 and from May to October 2004 (soil
moisture was not measured in July–August 2003 because
of equipment failure) in each plot using a Moisture
Point® MP-917 Time Domain Reflectometry system
(30 cm rods; Environmental Sensors, Victoria, Canada)
and a 1-m Decagon® AccuPar Ceptometer (Decagon
Devices Inc., Pullman, WA). Sampling points were
randomly located, and two measures of incident light
were taken during each sampling time.
Species diversity was calculated from biomass to
determine if grazing exclusion affected diversity.
Diversity was calculated at the quadrat scale for each
grazed and exclosed plot. Diversity was quantified
2
with Simpson’s diversity (1/D), where D = 1/ ∑ pi and
pi = relative biomass of each species i, and was then
decomposed into species richness (S) and evenness
(1/D/S) to determine if each component of diversity
differed (Buzas & Hayek 1996; Smith & Wilson 1996;
Martin, Moloney & Wilsey 2005).
  
Two seed additions of rare native prairie forbs and
grasses were made to separate, randomly located 1-m2
4
L. M. Martin &
B. J. Wilsey
© 2006 The Authors.
Journal compilation
© 2006 British
Ecological Society,
Journal of Applied
Ecology
subplots within each plot using two different methods.
These were compared to one control subplot (no seed
addition) within each main plot. Therefore, three
subplots were located in each main plot (exclosure or
grazed plot), with a total of nine subplots in each block.
The first seed addition treatment consisted of adding
seeds of 10 species collected by hand from local remnants in June 2003. The second treatment consisted of
adding seeds of 25 species from a local seed company
(Allendan Seed Co., Winterset, IA, USA) in April 2004
in a second set of subplots. More species were used in the
second trial than the first because seeds were more readily
available from the seed company, and we wanted to
mimic the high number of species found at the
neighbourhood scale in remnants (Martin, Moloney &
Wilsey 2005). Seeds were added with equal relative
abundances at a rate of 19 700 seeds m−2 for both trials
(1970 and 788 per species for addition experiments 1
and 2, respectively). Seed numbers were based on
number of seeds in a typical seed rain rate found by
Rabinowitz & Rapp (1980) in a Missouri tallgrass
prairie. Seed viability was not tested with seeds collected
from remnants, but all but one species readily germinated in greenhouse pots grown for seedling reference
samples. Mean seed viability for seeds obtained from
the seed company was 81% (range 49–96%). Seeds
were hand-scattered in each 1-m2 subplot and existing
vegetation and litter were shaken to aid seeds in
reaching the soil surface. Species added in the first
experiment were Bouteloua curtipendula (Michx.)
Torrey, Sporoblus asper (Michx.) Kunth, Solidago
speciosa Nutt., Pycnanthemum virginianum (L.) Dur. &
Jackson, Dalea purpurea Vent., Chamaecrista fasciculata
(Michx.) Greene, Amorpha canescens Pursh, Lespedeza
capitata Michx., Monarda fistulosa L. and Eryngium
yuccifolium (Michx.) (Eilers & Roosa 1994). Species
added in the second experiment included all those
added in the first experiment, plus Potentilla arguta
Pursh, Silphium laciniatum L., Echinacea pallida Nutt.,
Ratibida pinnata (Vent.) Barnh., Artemesia ludoviciana
Nutt., Liatris pycnostachya (Michx.), Verbena stricta
Vent., Helianthus rigidus (Cass.) Desf., Gentiana
andrewsii Griseb., Tradescantia bracteata Small, Viola
pedatifida G. Don, Anemone cylindrica Gray, Phlox
pilosa L., Schizachyrium scoparium (Michx.) Nash and
Solidago rigida L. (Eilers & Roosa 1994).
All forb seedlings, including species added from the
mix as well as volunteers, were identified to species and
counted in a randomly placed 20 × 50-cm quadrat
within each subplot, to estimate seedling emergence.
Volunteers were included because some added species
were already in the seed bank and therefore could not
be distinguished from experimentally sown seedlings.
Exotic and cosmopolitan weed seedlings were counted
to test concerns about disturbance from native ungulates increasing weeds in grasslands (Smith & Knapp
1999). Grass seedlings were only counted if the species
was added. Seedlings were counted if they were up
to 7·5 cm tall or up to any height if they were annuals,
and were counted once per month during the growing season, beginning the month after seeds were
added.
 
Randomized block split-plot s were used, with
planting as a random block term. All grazing effects were
tested with the main plot error term (planting × grazed),
and seed and seed–grazed interactions were tested with
the subplot error term. Repeated-measures 
was used to compare grazed (n = 16) and exclosed plots
(n = 8) for existing vegetation and resource variables,
with time 0 data (measurements taken before exclosures were constructed) as a covariate (except for NPP,
for which time 0 data could not be calculated). We
dropped the covariate from each model if it was not
significant (P > 0·05). Variables were logarithmically
transformed (biomass, standing dead and litter), squareroot transformed (proportion of exotic biomass) or
arcsin square-root transformed (proportion of C4 and
grass) to improve normality when necessary. All
analyses were done with   in SAS (Littell,
Stroup & Freund 2002).
The first and second seed additions were analysed
separately because they had different numbers of species, addition dates and weather conditions. A seedling
enhancement effect, ln[(added seedlings + 1)/(control
seedlings + 1)], was calculated to quantify the number
of seedlings that did not emerge from the existing seed
bank but emerged from added seeds. This derived variable eliminated non-normality in data as a result of
having many zeroes in control subplots. To test if seed
additions increased seedling numbers above those of
controls, seedling enhancement effects were tested against
0 with a t-test. Grazing effects on seedling enhancement
over time were analysed with repeated-measures 
of corresponding data. Exotic seedling and seedling
diversity variables were analysed with repeated-measures
(means for 2003 and 2004)  for the first seed addition, and with regular  for the second addition.
Data were averaged across months because raw data
had too many zeroes to analyse each sampling time.
Seedling Simpson’s diversity (1/D), species richness (S)
and evenness (1/D/S ) were calculated in each subplot
to determine if seed additions or grazing improved
seedling diversity.
We used polynomial regression to test for quadratic
and linear relationships between mean GI (grazed plots
only) and response variables. Mean GI was calculated
by averaging GI across time because of non-normally
distributed data. We used path analysis to test for direct
and indirect associations of grazing on seedling
enhancement. A direct pathway was tested of GI on
biomass, biomass on light and water availability, and
light and water on seedling enhancement. An indirect
path was tested of GI on NPP, and NPP on seedling
enhancement. This indirect pathway could be significant if increased NPP had additional effects on seedlings
Fig. 1. Grazing (n = 16) or exclosure from grazing (n = 8)
differences for (a) ANPP (P < 0·01) and (b) percentage light
availability at soil surface (P < 0·01). Vertical bars are ± 1 SE.
0·9
1·9
1·4
1·0
0·8
1·5
0·1
0·3
2·3
2·7†
2·4
0·7
5·6*
0·0
8·0*
1·9
6·7*
0·8
20·6**
2·7
1·4
6
132
0·4
0·1
0·7
0·8
1·6
7·2**
0·2
1·0
2·1
6·3**
0·9
4·3*
1·0
0·1
2
44
8·8**
1
1
14
2
2
2
28
NS
6·7*
1
15
6
NS
0·1
NS
2·4
NS
1·8
NS
0·6
Water
Alpha S
Proportion
grass
Proportion
exotic S
d.f.
Light
d.f.
Alpha E
Alpha
1/D
Proportion
C4 grass
Proportion
exotic biomass
Litter and
standing dead
NS
3·7†
NS
9·4**
NS
2·4
1
15
2
Time 0
Grazed
Error
Time
Time × time 0
Time × grazed
Error (time)
© 2006 The Authors.
Journal compilation
© 2006 British
Ecological Society,
Journal of Applied
Ecology
ANPP
Only two response variables differed significantly
between grazed and exclosed plots and another was
marginally significant (Tables 1 and 2). Above-ground
NPP m−2 was 1·2, 1·1 and 8·0 times as large in grazed
plots depending on time period, and the difference was
only significant for June–August 2004 (Table 1 and
Fig. 1a). Light availability at the soil surface was 1·7
times as great in grazed plots, and this was fairly
consistent across time periods (Table 1 and Fig. 1b).
Combined standing dead and litter biomass m−2 was
marginally significantly lower in grazed plots (Tables 1
and 2).
No response variables were quadratically related to
mean GI (grazing intensity) (F1,15 between 0·03 and
3·07, P between 0·10 and 0·88). Mean GI was highest in
June–August 2004, when 68% of NPP was consumed
(range 0–100%, SE 6), followed by 49% during June–
August 2003 (range 0–100%, SE 8). It was much
lower during spring 2004 (mean 14%, range 0–37%,
SE 3). Biomass m−2 was negatively related to mean GI
Biomass
   
d.f.
Results
Source
because of non-light and water effects, such as increased
nutrient uptake of vegetation.
Table 1.  results (F-values) for comparisons between grazed plots and exclosures in a tallgrass prairie restoration. Time 0 data were included as a covariate, but were removed if not significant (P > 0·05;
denoted by NS) Significance is indicated by †P between 0·06 and 0·1, *P between 0·02 and 0·05 and **P ≤ 0·01
5
Native ungulates
and seed additions
in restorations
28·23 (1·40)
34·69 (1·40)
35·75 (1·40)
26·66 (1·04)
35·10 (1·04)
34·84 (1·04)
37·23 (1·04)
38·24 (1·04)
Percentage water 38·92 (1·04)
July 2004
June 2004
September 2003 May 2004
Grazed (n = 16)
38·41 (1·40)
39·98 (1·40)
July 2004
June 2004
September 2003 May 2004
September 2004 October 2004
Exclosed (n = 8)
5·87 (0·14)
6·28 (0·14)
0·17 (0·06)
0·35 (0·03)
1·13 (0·06)
0·95 (0·08)
10·53 (0·82)
0·25 (0·02)
2·55 (0·33)
33·82 (1·40)
October 2004
September 2004
6·12 (0·17)
6·58 (0·14)
0·22 (0·07)
0·36 (0·04)
1·08 (0·08)
1·01 (0·11)
11·02 (1·10)
0·24 (0·03)
2·76 (0·44)
5·97 (0·17)
6·58 (0·14)
0·31 (0·07)
0·31 (0·04)
1·19 (0·08)
0·99 (0·11)
9·86 (1·10)
0·25 (0·03)
2·46 (0·44)
6·13 (0·17)
6·36 (0·14)
0·17 (0·07)
0·30 (0·04)
1·22 (0·08)
1·13 (0·11)
9·44 (1·10)
0·23 (0·03)
2·27 (0·44)
5·93 (0·14)
6·39 (0·14)
0·32 (0·06)
0·36 (0·03)
1·17 (0·06)
0·96 (0·08)
10·86 (0·82)
0·26 (0·02)
2·63 (0·33)
5·82 (0·14)
6·09 (0·14)
0·33 (0·06)
0·37 (0·03)
1·01 (0·06)
0·83 (0·08)
12·63 (0·82)
0·24 (0·02)
2·89 (0·33)
August 2003
August 2003
Grazed (n = 16)
June 2004
August 2004
Biomass (m−2)
Litter and standing dead (m−2)
Proportion exotic biomass (0·4 m−2)
Proportion exotic S (0·4 m−2)
Proportion grass (0·4 m−2)
Proportion C4 grass (0·4 m−2)
Alpha S (0·4 m−2)
Alpha E (0·4 m−2)
Alpha 1/D (0·4 m−2)
© 2006 The Authors.
Journal compilation
© 2006 British
Ecological Society,
Journal of Applied
Ecology
Table 2. Mean (SE) of response variables measured to test effects of native ungulate grazing in a tallgrass prairie restoration
Exclosed (n = 8)
June 2004
August 2004
6
L. M. Martin &
B. J. Wilsey
Fig. 2. Relationships between (a) biomass and (b) evenness
and grazing intensity (n = 16).
(F1,15 = 6·02, P = 0·03, r = −0·56, slope = −3·49; Fig. 2a),
as expected. Species evenness at the 0·4 m −2 scale
was positively related to mean GI (F1,15 = 5·61, P = 0·03,
r = 0·54, slope = 0·89; Fig. 2b).
- 
Native species seedling emergence
Seed additions increased the number of native species
seedlings 0·1 m−2 in both seed addition experiments
(Table 3 and Fig. 3). Adding seeds increased seedling
numbers by 2·5 times in 2003 (t = 1·97, P = 0·07) and
2·0 times in 2004 (t = 3·22, P < 0·01) in the first experiment (Fig. 3a). In the second experiment, the seedling
enhancement ratio as a result of adding seeds was
3·8 in May, 5·2 in June, 5·5 in July, 6·6 in August and
17·9 in October (t = 6·94, 6·48, 3·60, 4·43 and 2·14,
respectively; P-values < 0·01 in May–July, P = 0·05 in
October; Fig. 3a). Overall, seedling numbers decreased
significantly between June and October (Table 4 and
Fig. 3a).
Grazing alone, without seed additions, did not increase
number of seedlings 0·1 m−2 for either seed addition
experiment (Fig. 3a) but grazing conditionally affected
seedling enhancement. The seedling enhancement
effect did not differ between grazed and exclosed plots
in the first experiment but was on average 1·4 times as
large in grazed than exclosed plots in the second
(Table 4 and Fig. 3b). We did not find a significant
© 2006 The Authors.
Journal compilation
© 2006 British
Ecological Society,
Journal of Applied
Ecology
Seed addition 2
Aster pilosus
Chamaecrista fasciculata*
Conyza canadensis
Dalea purpurea*
Daucus carota
Lespedeza capitata*
Monarda fistulosa*
Taraxacum officinale
Other
Total numbers of seedlings
Seed addition 1
Ambrosia artemesiifolia
Aster pilosus
Chamaecrista fasciculata*
Conyza canadensis
Daucus carota
Lespedeza capitata*
Monarda fistulosa*
Taraxacum officinale
Other
Total number of seedlings
Scientific name
Hairy aster
Partridge pea
Horseweed
Purple prairie clover
Queen Anne’s lace
Round-headed bush clover
Wild bergamot
Dandelion
Common ragweed
Hairy aster
Partridge pea
Horseweed
Queen Anne’s lace
Round-headed bush clover
Wild bergamot
Dandelion
Common name
24·1 (20·1)
5·3 (2·9)
4·4 (1·6)
3·4 (1·8)
25·9 (15·0)
0·0 (0·0)
2·5 (1·8)
30·6 (11·1)
15·3 (7·3)
111·6 (35·6)
11·6 (5·2)
6·3 (3·7)
5·3 (1·7)
3·4 (1·3)
18·8 (6·5)
6·6 (2·5)
6·6 (2·2)
30·6 (7·3)
15·0 (35·0)
104·1 (14·8)
6·9 (4·5)
13·1 (11·1)
3·8 (2·6)
5·0 (2·7)
11·9 (5·5)
0·0 (0·0)
5·0 (3·3)
30·0 (7·8)
9·4 (3·9)
85·0 (15·9)
3·1 (2·5)
5·0 (3·4)
2·5 (1·3)
5·0 (2·5)
66·9 (46·2)
15·0 (6·9)
10·6 (7·2)
33·8 (11·3)
14·4 (5·7)
156·3 (50·7)
Exclosed, seed
(n = 8)
9·4 (5·5)
6·4 (2·9)
6·4 (5·1)
0·0 (0·0)
17·6 (10·1)
0·0 (0·0)
2·1 (1·1)
87·3 (26·5)
10·1 (2·6)
139·3 (34·5)
2·0 (0·9)
9·4 (5·5)
6·4 (2·9)
6·4 (2·9)
17·6 (10·1)
0·0 (0·0)
2·1 (1·1)
87·3 (26·5)
8·3 (2·6)
139·4 (34·5)
Grazed, no seed
(n = 16)
Exclosed, no seed
(n = 8)
Grazed, no seed
(n = 16)
Grazed, seed
(n = 16)
2004
2003
6·9 (3·0)
34·9 (7·6)
3·6 (1·8)
18·4 (4·2)
39·9 (14·6)
96·9 (9·6)
9·6 (2·5)
100·6 (26·5)
59·5 (9·7)
351·9 (50·5)
1·3 (0·9)
9·5 (7·3)
3·6 (1·3)
6·9 (4·8)
22·4 (9·7)
4·9 (1·8)
15·5 (4·7)
80·1 (19·2)
5·6 (1·8)
149·8 (35·7)
Grazed, seed
(n = 16)
14·0 (8·8)
3·3 (1·8)
6·0 (4·3)
0·0 (0·0)
14·3 (6·7)
1·5 (1·0)
7·8 (5·4)
59·8 (18·4)
8·8 (3·3)
115·3 (28·8)
0·5 (0·5)
13·0 (9·0)
3·3 (1·8)
6·0 (4·3)
14·3 (6·7)
1·5 (1·0)
7·8 (5·4)
59·8 (18·4)
9·3 (4·2)
115·3 (28·8)
Exclosed, no seed
(n = 8)
13·3 (5·2)
22·0 (4·9)
2·8 (0·9)
12·5 (4·0)
25·8 (8·6)
64·5 (16·8)
7·3 (3·1)
70·0 (35·9)
40·5 (10·3)
246·0 (67·3)
0·5 (0·5)
3·0 (1·7)
7·3 (2·4)
0·8 (0·5)
47·0 (27·1)
4·8 (2·1)
8·5 (3·2)
54·8 (17·0)
8·5 (3·6)
135·0 (39·3)
Exclosed, seed
(n = 8)
Table 3. Mean numbers of seedlings m−2 counted in the first and second seed addition experiments. Species listed comprised approximately 90% of those counted. Added species are denoted with an asterisk (*)
and exotic species are in bold (Eilers & Roosa 1994)
7
Native ungulates
and seed additions
in restorations
Exotic and cosmopolitan weed seedling emergence
were not clearly affected by treatments in either experiment (Table 4). Exotics such as Taraxacum officinale
2·1
0·2
0·4
1·1
6·5*
1·0
1·0
54·0**
1·8
88
22
0·5
0·6
15
1
1
15
4
4
7·3**
0·5
1/44
1/44
2/44
10·7**
0·1
1·2
9·3**
0·0
1·5
1·0
0·2
0·9
7·8**
0·2
0·4
0·0
60·2**
0·3
0·1
2·0
0·2
0·7
0·0
0·4
0·2
5·1*
0·2
0·3
4·0†
0·7
1/15
1/22
1/22
6·3*
1
0·02
Exotics
Evenness
Richness
Diversity
Diversity
Richness
Evenness
Exotics
Second experiment seedling numbers
First experiment seedling numbers
Num/Den d.f.
Seedling enhancement
d.f.
Seedling enhancement
d.f.
1
Grazed
Seed addition
Grazed × seed addition
Error
Time
Time × grazed
Time × grazed × seed addition
Error (time)
Exotic species seedling emergence
Source
© 2006 The Authors.
Journal compilation
© 2006 British
Ecological Society,
Journal of Applied
Ecology
Second experiment
relationship with GI in either experiment (first experiment, linear effects F1,13 = 0·01, P = 0·93, quadratic
effects F1,13 = 1·68, P = 0·22; second experiment, linear
effects F1,13 = 0·63, P = 0·44, quadratic effects F1,13 =
2·14, P = 0·17).
Path analysis from both experiments indicated that
biomass m−2 was negatively related to light and soil
water availability, and that water was positively related
to seedling enhancement more regularly and strongly
than light (Fig. 4 and Table 5). The indirect pathway
indicated that GI was positively related to NPP in the
first (significant for 2004 only) and second experiments,
but NPP never significantly explained seedling enhancement beyond effects of light and water (Fig. 4 and Table 5).
First experiment
Fig. 3. The (a) number of seedlings 0·1 m−2 in grazed and
exclosed plots when seeds were added or were not and (b)
effects of grazing on seedling enhancement for two experimental seed addition trials in a tallgrass prairie restoration
(n = 16 for grazed; n = 8 for ungrazed). The first seed addition
experiment is presented before the break in the x axis and the
second experiment is after the break. Vertical bars are ± 1 SE.
Table 4.  results (F-values) for seedling enhancement (increase in seedling numbers with seed additions) and seedling numbers between grazed plots and plots exclosed from grazing in a tallgrass prairie
restoration. Significance is indicated by †P between 0·06 and 0·1, *P between 0·02 and 0·05 and **P ≤ 0·01
8
L. M. Martin &
B. J. Wilsey
mean number of exotics was not related to mean GI
for either experiment (first, linear, exotics F1,13 = 0·07,
P = 0·79; quadratic, exotics F 1,13 = 2·08, P = 0·17;
second, linear, F1,13 = 0·59, P = 0·45, quadratic,
F1,13 = 0·23, P = 0·64).
9
Native ungulates
and seed additions
in restorations
  
Fig. 4. Path analysis diagram that tests direct effects of
grazing intensity on seedling enhancement through biomass,
light and water, and indirect effects of grazing intensity on
seedling enhancement through NPP effects in a tallgrass
prairie restoration. Ten and 25 rare prairie species were added
in plots inside and outside grazing exclosures in two separate
experiments.
and Daucus carota were among the most abundant
species in both experiments, and cosmopolitan weeds
Aster pilosus and Ambrosia artemisiifolia were also
abundant in the first experiment (Table 3). The mean
number of exotic seedlings did not significantly differ
between grazed and exclosed plots in either experiment
[mean (SE) number of exotics 0·1 m−2, n = 48; first
experiment 2003, grazed 1·51 (0·28), exclosed, 1·72
(0·31); 2004, grazed 2·03 (0·28), exclosed 1·91 (0·31);
second experiment, grazed 2·19 (0·29), exclosed 1·92
(0·31); Table 4]. Grazing effects did not interact with
seed additions in either experiment (Table 4). The
Seed additions increased seedling diversity and richness
in both experiments and slightly decreased seedling
evenness in the second experiment. Mean diversity in
the first experiment was 1·2 times as great with seed
additions in 2003 and 2004, but this difference was only
marginally significant [mean (SE) 1/D 0·1 m−2, n = 48;
2003, 2·76 (0·22), control 2·24 (0·22); 2004, 2·10 (0·22),
control 1·82 (0·22); Table 4]. Mean diversity in the
second experiment was 1·9 times as great in the seed
addition subplots than in controls [mean (SE) 1/D
0·1 m−2, n = 48; addition 3·37 (0·20), control 1·82 (0·20);
Table 4]. Mean richness in the first experiment was 1·3
and 1·2 times as great in seed addition than control subplots in 2003 and 2004, respectively, and was 2·3 times
as great in addition subplots in the second experiment
[mean (SE) S 0·1 m−2, n = 48; first experiment, 2003,
addition 3·81 (0·32), control 2·88 (0·32); 2004, addition
2·96 (0·32), control 2·53 (0·32); second experiment,
addition 5·82 (0·37), control 2·52 (0·37); Table 4]. Mean
evenness did not differ in the first experiment but was
slightly lower in seed addition subplots in the second
[mean (SE) E 0·1 m−2, n = 48; first 2003, addition 0·75
(0·03), control 0·77 (0·03); 2004, addition 0·74 (0·03),
control 0·72 (0·03); second, addition 0·65 (0·03), control 0·72 (0·03); Table 4].
Seedling species diversity did not differ between
grazed and exclosed plots for either experiment [mean
(SE) for grazed and exclosed, respectively, n = 48; first
experiment, diversity 2003 2·47 (0·19), 2·53 (0·25), 2004
Table 5. Path analysis results to determine direct effects of grazing intensity on biomass, biomass on light and water, and light and
water on seedling enhancement, and indirect effects of grazing intensity on NPP and NPP on seedling enhancement for the first
and second seed addition experiment. Ten (first) or 25 (second) rare prairie species were added to plots. Numbers represent
standardized regression coefficients. Significance is indicated by *P between 0·02 and 0·05 and **P ≤ 0·01
First experiment
© 2006 The Authors.
Journal compilation
© 2006 British
Ecological Society,
Journal of Applied
Ecology
Second experiment
Variable
Second half 2003
First half 2004
Second half 2004
First half 2004
Second half 2004
Direct effects
GI → biomass
Biomass → light
Biomass → water
Light → enhancement
Water → enhancement
0·00
− 0·73**
− 0·61**
0·30
− 0·23
0·05
0·04
− 0·24
0·15
0·51**
− 0·27
− 0·65**
− 0·65**
− 0·16
0·43*
0·05
0·04
− 0·24
0·16
0·56**
− 0·27
− 0·65**
− 0·65**
0·34*
0·53**
0·09
0·11
0·75**
− 0·19
0·55*
− 0·16
0·75**
− 0·06
0·55**
− 0·08
1·00**
1·00**
0·81**
0·97**
0·66**
0·96**
0·76**
0·68**
0·76**
0·83**
Indirect effects
GI → NPP
NPP → enhancement
Unobserved
o1 → biomass
o2 → light
o3 → enhancement
o4 → water
o5 → NPP
1·00**
0·68**
0·95**
0·79**
1·00**
1·00**
1·00**
0·82**
0·97**
0·66**
0·96**
0·76**
0·91**
0·76**
0·83**
10
L. M. Martin &
B. J. Wilsey
1·88 (0·19), 2·04 (0·25), richness 2003 3·28 (0·29), 3·41
(0·36), 2004 2·68 (0·29), 2·81 (0·36), evenness 2003 0·75
(0·03), 0·77 (0·04), 2004 0·71 (0·03), 0·75 (0·04); second
experiment diversity 2·58 (0·19), 2·61 (0·22), richness
4·39 (0·33), 3·95 (0·42), evenness 0·66 (0·02), 0·71
(0·04); all at the 0·1 m2 scale; Table 4]. Diversity
enhancement declined with GI in the first experiment
(linear effect F1,13 = 5·05, P = 0·04; quadratic effect
F1,13 = 2·29, P = 0·15; data not shown) and was quadratically related to GI in the second experiment (F1,13 =
6·02, P = 0·03; quadratic equation y = 2·4 ± 11·3x +
15·5x2) with an outlier included, and unrelated with
an outlier excluded (linear effect F1,13 = 3·22, P = 0·10,
r = 0·45, slope = 1·83; quadratic effect F1,13 = 0·53,
P = 0·48; data not shown). The evenness enhancement
effect was negatively related to GI in the first experiment (F1,13 = 4·8, P = 0·05, r = 0·53, slope = −0·53) only.
Discussion
Previously, we found that conventional prairie restoration at the study site was able to restore common native
species but not species diversity of nearby prairie
remnants (Martin, Moloney & Wilsey 2005). Here we
tested hypotheses regarding why diversity was lower.
Our results suggest that seedling emergence in low
diversity restorations is seed limited but that native
ungulates can sometimes increase emergence as well.
Seedling enhancement increased with water and light
availability, which suggests that, when grazing is enhancing emergence, the mechanism may be associated with
grazers having a direct effect on water and light. A
combination of seed additions and grazing led to the
highest amount of seedling emergence, but this result
was conditional, i.e. it was found only in the second
trial and year. However, grazing alone did not increase
seedling emergence in either trial.
Biomass and NPP, general indicators of resource
uptake in grasslands, are usually affected by grazing
(Semmartin & Oesterheld 1996). We found that total
biomass declined with grazing intensity, and water and
light availability were higher when biomass was lower,
which suggests that grazing decreased biomass enough
to increase resource availability. However, ANPP was
also higher with grazing, suggesting that defoliated plants
were readily recovering from defoliation and utilizing
available resources (Knapp et al. 1999; Wilsey et al.
2002). Nevertheless, increased levels of ANPP did
not appear to have effects above and beyond those
correlated with water and light availability in reducing
seedling enhancement.
© 2006 The Authors.
Journal compilation
© 2006 British
Ecological Society,
Journal of Applied
Ecology
tions are initiated by harvesting seeds from remnants
in the autumn (Polley, Derner & Wilsey 2005), when C4
grass seed is most abundant relative to other species.
Evidence from this study suggests that seedling
emergence of rare forbs is very low nearly 10 years after
initial seeding, and that adding seeds of rare forb and
grass species that are typically lacking in restorations
can increase seedling emergence, the first step in
recruiting species into the community.
Our finding that seed additions and grazing combined could increase seedling emergence suggests that
grazing mammals might increase seedling recruitment
in some situations. Turnbull, Crawley & Rees (2000)
found that, overall, a combination of adding seeds and
inducing disturbance to reduce dominant vegetation
was most important for recruitment. Rhinanthus minor,
a parasitic plant, also increased seedling recruitment
of added species by reducing competitive effects of
dominant vegetation (Pywell et al. 2004). However, the
effect of grazing on seedling emergence with seed additions in the restoration was conditional. This conditionality may have been the result of very different
weather between years, but our design could not determine this definitively. Although grazing combined with
seed additions conditionally improved seedling emergence, we did not find that emergence was quadratically
related to grazing intensity, as predicted by the intermediate disturbance hypothesis. Knapp et al. (1999)
proposed that target grazing intensity in intact tallgrass
prairies should be about 25% of annual above-ground
primary production, based on historic grazing intensities. We observed grazing intensities in the restoration
that were sometimes double that estimate. In our study,
it appears that grazing had an increasingly beneficial
(i.e. linear) effect on seedling emergence when seeds
were added.
Increases in exotic or cosmopolitan weed species may
negatively impact diversity and are a primary concern
in grazed grasslands (Smith & Knapp 1999; Hulme &
Bremner 2006). Grazing, which is utilized in management of both intact grasslands and restorations
(Collins et al. 1998; Knapp et al. 1999), could increase
weeds by the same mechanisms that increase native
plant recruitment (Smith & Knapp 1999). In contrast,
high dominance may enhance seedling emergence in
some grasslands (Wilsey & Polley 2002; Smith et al.
2004). We found no relationships with grazing on
exotics, suggesting that grazing may not be important
to exotic recruitment in these restored grassland
communities.
 
   

Seedling emergence was limited by seed availability in
these tallgrass prairie restorations, which suggests that
restorations are similar to many old fields in their lack
of propagule availability (Tilman 1997; Zobel et al.
2000; Pywell et al. 2002). Many tallgrass prairie restora-
Diversity components responded differently to seed
additions and grazing. With grazing, we found no
changes in species richness in the vegetation after
1.5 years of grazing, but evenness increased linearly
with grazing intensity. An increase in evenness can be
11
Native ungulates
and seed additions
in restorations
associated with a decrease in dominance from ungulate
activities (Hartnett, Hickman & Walter 1996). However, grazers may not be able to recruit new species and
enhance richness if rare plant species are not available
either in the seed bank or as vegetative propagules,
which appears to be the case in this restoration. Seed
additions, on the other hand, tended to increase richness but had a smaller negative effect on evenness.
It is important to point out that we only tested for
seed limitation with the emerging seedling community.
Although seedling emergence is a crucial step towards
recruitment into a community, seedlings must establish
viable populations before they will influence diversity
in the longer term. Wilsey & Polley (2003) found that,
even when seedling emergence was high, plant diversity
was unchanged because of low seedling survival in a
Texas grassland. It is too early to estimate establishment success, and longer term monitoring is needed to
test the hypothesis that seed additions and grazing will
increase diversity of vegetation in the long term.
  

Seedling numbers of rare species increased greatly when
seeds were added, suggesting that these restorations
were severely seed limited. Grazers increased water and
light availability by decreasing above-ground biomass,
and water increased seedling enhancement more regularly and strongly than did light. Grazing increased
ANPP, but ANPP did not explain seedling emergence
above and beyond water and light effects. Under
certain conditions, seedling emergence was positively
influenced by grazing when seeds were added. However,
grazing by itself did not increase seedling emergence,
probably because seeds of rare species were not
available to emerge. These results suggest that it may be
advantageous to mimic positive effects of native ungulates in restorations when seeds or propagules of rare
species are available to emerge. If propagule availability
is low, positive effects of grazing alone will not increase
seedling emergence. Improving seedling emergence in
restorations may therefore require adding seeds even
after dominant plants are established, while simultaneously mimicking grazing effects to increase light and
water availability. In contrast to management strategies
in intact grasslands (Collins et al. 1998), utilizing
grazing without propagules may not improve seedling
emergence in restorations. However, longer term monitoring is necessary to determine if seed additions and
grazing promote long-term plant diversity.
© 2006 The Authors.
Journal compilation
© 2006 British
Ecological Society,
Journal of Applied
Ecology
Acknowledgements
We thank Pauline Drobney at Neal Smith NWR,
Andrea Blong, Brennan Dolan, Dan Haug, and David
Losure for help in the field, and Tim Dickson and
two anonymous referees for comments on an earlier
version of this manuscript.
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Received 31 October 2005; final copy received 28 May 2006
Editor: Phil Hulme