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RESEARCH ARTICLE
Green hay transfer for grassland restoration: species
capture and establishment
Markus Wagner1,2 , Sarah Hulmes1, Lucy Hulmes1, John W. Redhead1, Marek Nowakowski3,
Richard F. Pywell1
Green hay transfer from species-rich donor sites is now commonly used in Europe to restore species-rich semi-natural grass-
land, both on ex-arable land and on former intensive grassland. However, species transfer rates are usually well below
100%, and due to lack of further colonization by additional target species after initial restoration, continued progress toward
the target plant community is often very slow. We used data from a restoration experiment aiming to reestablish species-rich
grazed meadows of the MG5 grassland type according to the British National Vegetation Classification to investigate relation-
ships between species abundance at a donor site, species capture by green hay and its seed content, and success of species estab-
lishment on experimental plots in formerly intensively managed species-poor grassland undergoing restoration. Our results
show that species with higher abundance at the donor site were more likely captured as seed in green hay, and were more likely
to establish after hay application at the recipient site. Species with low abundance at the donor site that also possessed specific
germination requirements that might prevent immediate establishment after green hay transfer were particularly unlikely to
get established after transfer. These findings can provide guidance for additional measures aimed at ensuring establishment
of a wider range of target species. Such measures could include targeted sowing of species in addition to green hay application,
and management of restored grassland swards to extend or reopen an initial window of opportunity for the establishment of
green hay species that might not be germinable immediately after hay transfer.
Key words: effective sowing rate, greenhouse emergence, hay seed content, MG5 grassland, seed dormancy, supplementary
sowing
Implications for Practice
•Failure to capture species as seeds in green hay for
meadow restoration and to establish them at a recipient
site is more likely for species less abundant at the donor
site, and establishment is also less likely for species with
germination requirements preventing efficient germina-
tion after green hay transfer.
•These obstacles can be addressed, for example, by sup-
plementing green hay transfer with targeted additional
sowing of species, or by managing the recipient site to
extend the initial window of opportunity for target species
establishment, or reopen it in the spring after hay transfer.
•Greenhouse emergence trials to determine seed content of
green hay must be carefully designed and interpreted, as
species differ from each other in their requirement for
cold stratification
Introduction
Species-rich European and U.K. lowland semi-natural grassland
has strongly declined in extent over the last 70 years (Isselstein
et al. 2005; Bullock et al. 2011). The principal driver of this
decline was agricultural intensification, mainly in the form of
conversion to intensively managed grassland (Walker
et al. 2004). For example, of grazed meadows of the MG5 type
according to the British National Vegetation Classification
(NVC; see Rodwell 1992), once the most widespread type of
lowland hay meadow in Britain (Rodwell et al. 2007), less than
10,000 ha are now left across England and Wales
(Maddock 2008). To restore a coherent and resilient network
of this and other types of semi-natural grassland, both the diver-
sification of extant species-poor grassland and the creation of
additional sites of high botanical quality are required (Lawton
Author contributions: RP conceived and designed the field experiment with the help of
MN, JR; SH, LH, MW carried out botanical monitoring; SH carried out greenhouse
emergence trials; MW analyzed the data and led and coordinated the writing of the
manuscript; SH, LH, MN, JR, RP edited the manuscript.
1
UK Centre for Ecology & Hydrology, Benson Lane, Wallingford, Oxfor dshire, OX10
8BB, U.K.
2
Address correspondence to M. Wagner, email mwagner@ceh.ac.uk
3
Wildlife Farming Company, Alchester Road, Bicester, Oxfordshire, OX26 1UN, U.K.
© 2020 The Authors. Restoration Ecology published by Wiley Periodicals LLC on
behalf of Society for Ecological Restoration.
This is an open access article under the terms of the Creative Commons Attribution
License, which permits use, distribution and reproduction in any medium, provided the
original work is properly cited.
doi: 10.1111/rec.13259
Supporting information at:
http://onlinelibrary.wiley.com/doi/10.1111/rec.13259/suppinfo
Restoration Ecology 1
et al. 2010). However, unassisted recovery of species-rich semi-
natural grassland often requires many decades, due to key con-
straints in the form of seed limitation and microsite limitation
(Bakker & Berendse 1999; Walker et al. 2004). Seed limitation
occurs due to the slow and limited dispersal of target species
propagules (Bakker & Berendse 1999; Walker et al. 2004).
Spontaneous colonization of sites undergoing reversion to
grassland by restoration target species is very limited even when
species-rich grassland that could act as a source of propagules is
present nearby (Stampfli & Zeiter 1999; Coulson et al. 2001;
Bischoff 2002). Microsite limitation, on the other hand, occurs
due to site-specific constraints preventing the establishment of
desired target species after their propagules have reached the site
(Bakker & Berendse 1999; Walker et al. 2004).
To overcome seed limitation, target species are actively intro-
duced during restoration (Bakker & Berendse 1999; Walker
et al. 2004). The two main methods for meadow creation on
ex-arable land and diversification of existing species-poor grass-
land are species-rich seed mixtures and transfer of green hay
from high-quality local grassland (Hedberg & Kotowski 2010;
Kiehl et al. 2010). The use of seed mixtures is generally straight-
forward as species composition can be adjusted as required.
With seed mixtures, the seeds of many species will have had
an opportunity to afterripen during dry storage, thus being capa-
ble of fast germination. Accordingly, on former arable land,
establishment of 75–100% of sown species is usually achieved
(Kiehl et al. 2010).
Several operational stages are involved during green hay
application (Trueman & Millett 2003), and there is less control
over species composition of seeds transferred with the hay,
which depends on choice of donor site and timing of the hay
cut. After the hay cut, transport of harvested material and its
spreading at the recipient site must take place within 24 hours.
In line with the fact that the set of species found as seeds in the
green hay is usually only a subset of the set of species found at
the donor site, two types of species transfer rate can be calcu-
lated. Calculation of the absolute transfer rate is based on the full
complement of species found at the donor site, calculation of the
relative transfer rate is based on the set of species that have actu-
ally been captured by the hay cut (Kiehl et al. 2010). When
restoring species-rich noncalcareous mesic to wet meadows
and floodplain grassland on former arable land, absolute transfer
rates using green hay are usually around 36–53%, with relative
transfer rates usually around 57–66% (Kiehl et al. 2010). Thus,
a significant proportion of species from green hay donor sites
regularly fail to establish at recipient sites, and for a sizable num-
ber of these species this is the case in spite of their seeds having
been transferred with the hay. Both kinds of transfer rate are
even lower when green hay is used to diversify species-poor
existing grassland (Kiehl et al. 2010). This is the case even
though in this situation, additional bare ground creation via
cultivation is usually carried out prior to hay spreading, to tem-
porarily produce a competition-free environment for establish-
ment of target species transferred with the hay (Trueman &
Millett 2003; Bischoff et al. 2018).
Irrespective of whether the recipient site is former arable land
or extant species-poor grassland, if absolute transfer rates are
low, this means that in the short term, only a core set of species
from the target community can be established. Furthermore,
given the already mentioned slow colonization, further progress
to a more complete target community is often slow or absent (e.
g. Sullivan et al. 2019), as those target species not already estab-
lished during initial restoration also fail to colonize subse-
quently. Thus, it might be better to combine green hay
application with seeding during initial restoration, to maximize
establishment of as wide a range of species as possible. This
could be done either via additional sowing of hand-collected
seed of species underrepresented or absent in the green hay
(Poschlod & Biewer 2005), or via additional sowing of commer-
cially supplied seed (Baasch et al. 2016). However, the cost of
purchased seed can be high, and about two thirds of European
grassland species are commercially unavailable (Ladouceur
et al. 2018). Hand collection on the other hand is very labor-
intensive (Stevenson et al. 1997). It would thus be advantageous
to have better knowledge of which species can be reliably trans-
ferred via green hay, and which species cannot. Such knowledge
requires both an understanding of the factors that determine
which species are captured as seed via green hay and which of
these captured species then successfully establish after hay
transfer. However, due to the rapid transfer requirement when
applying green hay, seed species composition of the applied
green hay remains unknown at the time of hay transfer. Species
capture can only subsequently be determined, either via green-
house emergence (Poschlod & Biewer 2005; Kiehl et al. 2006;
Kirmer & Tischew 2014) or via manual seed extraction (Scotton
et al. 2009; Albert et al. 2019). The results of such efforts show
that, as a rule, representation of different donor-site species in
green hay strongly varies, with seeds of a few species present
in large quantities, and those of many other species only present
at very low densities (Poschlod & Biewer 2005; Scotton 2016,
2018). However, among those studies relying on greenhouse
emergence, treatment of samples varies, with some authors
advocating a cold-moist stratification pretreatment of samples,
others allowing for such stratification halfway during emergence
trials (Kirmer & Tischew 2014), and yet others not making any
mention of cold stratification being applied. Hence, when using
greenhouse emergence to determine species capture, it appears
plausible that results might depend on the exact protocol used.
Species capture with green hay may also have consequences
for species establishment at the recipient site. One might expect
that species better represented in the hay are more likely to
establish than species whose seeds are only present at low den-
sities. Other factors including plant traits such as seed weight
or the presence of primary seed dormancy in many forb and
sedge species of grassland (Grime et al. 1981), which has also
been confirmed for freshly harvested seed in green hay for resto-
ration (Rasran et al. 2006), might also affect whether species
from green hay can successfully establish.
In this study, we investigated the factors determining seed
capture by green hay and the effects of green hay sample pre-
treatment on determination of seed content using greenhouse
emergence. In addition, we explored the factors determining
species establishment from green hay at the recipient site. In par-
ticular, we were interested in whether establishment success at
Restoration Ecology2
Species transfer with green hay
the recipient site could be predicted by a combination of species
composition and flowering phenology in the donor grassland,
and plant traits that might affect seed capture with green hay
and/or plant establishment at the recipient site. To this end, we
investigated four questions:
(1) Does greenhouse emergence from green hay depend in a
species-specific manner on length of prior cold-moist
stratification?
(2) Are species more likely to be captured as seed in green hay
if they have higher abundance at the donor site?
(3) Is species establishment at the recipient site influenced by
abundance at the donor site and by effective sowing rate via
green hay transfer?
(4) Can successful species establishment be predicted from a
combination of abundance and/or phenology at the donor site,
and species traits that might affect either seed capture with green
hay and/or plant establishment after hay transfer?
Methods
Field Site and Experimental Design
A 4-year meadow restoration experiment was set up in July
2013 in three species-poor grassland fields at the Hillesden
Estate, a c. 1,000 ha arable farm in Buckinghamshire, England
(515705800N0
5801500W), along the western bank of the small
river Padbury Brook, a tributary to the River Twins. These fields
were arable land until 2007, and were then sown with a mixture
of perennial ryegrass Lolium perenne and white clover Trifolium
repens. Subsequently, until the start of the experiment, they
were species-poor agriculturally improved grassland, corre-
sponding to the L. perenne–T. repens subcommunity (MG7a)
of L. perenne grassland in the U.K. NVC (Rodwell 1992). The
soil in all three fields is alluvial clay and clay loam, with a pH
of about 6.5. Mean annual temperature at Hillesden is 9.7C
and annual rainfall is 648 mm, of which 381 mm falls during
the months of April to October (based on data from 1981 to
2010; Met Office 2019).
The experimental design was a randomized complete block
design with four replicate blocks, two of which were located in
the largest of the three fields. Within each replicate block, three
restoration treatments were applied to experimental plots of
between 0.95 and 2.7 ha in size, depending on the overall size
and shape of the field in which the block was located. These
treatments were: (1) a green hay treatment to which target spe-
cies were introduced by application of freshly harvested hay
from a species-rich donor meadow; (2) a diverse seeding treat-
ment to which target species were introduced by sowing of a
specifically tailored diverse seed mixture containing a large
number of grasses and forbs; and (3) a control treatment as pro-
vided by the species-poor extant grassland. Treatments (1) and
(2) were designed to restore Cynosurus cristatus–Centaurea
nigra grassland, that is, MG5 grassland according to the NVC
(Rodwell 1992), which had been determined as a suitable resto-
ration target for the experimental area based on its location, soil,
hydrology, and proposed management as a grazed hay meadow.
A comparative analysis of vegetation development across all
treatments over a 4-year period is presented in a separate article
(Wagner et al. 2020). Here, we focus exclusively on the green
hay treatment, to investigate the factors that determine species
transfer by green hay application.
To prepare experimental plots for green hay application, a
silage cut took place in the w/c 17 June 2013. This was followed
by marking out experimental plots, and spraying with glypho-
sate followed by cultivation, to create a suitable shallow tilth
and bare ground for facilitating seedling establishment of resto-
ration target species. Glyphosate spraying was carried out in the
w/c 24 June 2013 using a self-propelled sprayer, and cultivation
was carried out in the w/c 8 July 2013. Cultivation consisted of a
series of operations including plowing, power harrowing, and
ring rolling, carried out in that order. Thus, we followed recom-
mendations by Natural England (2010) to combine creation of a
short sward with creation of bare ground prior to green hay
application, while exceeding their recommendation of single-
step cultivation by power harrowing. At the same time, as
suggested by Trueman and Millett (2003), our chosen site prep-
aration regime combined herbicide spraying and cultivation, thus
resulting in more lasting bare ground creation, and a widening of
the temporal window of opportunity for target species establish-
ment. The site preparation regime used in our experiment com-
bining all these elements resulted in close to 100% bare ground.
The green hay donor site was species-rich MG5 grassland,
3.68 ha in size and located within Rushbeds Wood SSSI, a
nature reserve c. 15 km south of the Hillesden experimental site.
The hay was cut on 24 July 2013 using a disk mower, and a for-
age harvester was used to load a farm tractor trailer for transport
of the freshly cut hay to the experimental site. On the same day,
using a muck spreader, the hay was evenly spread onto the green
hay treatment plots. After the green hay was spread, fields were
once more ring rolled. The total area of the four replicate green
hay restoration plots was 8.15 ha, resulting in a ratio of donor
to recipient area of 1:2.2, and green hay application at a higher
rate than with an area ratio for green hay application of 1:3 as
recommended by Natural England (2010). Post-establishment
management from 2014 onwards was similar to traditional man-
agement of MG5 grassland, involving a single cut in the sum-
mer, followed by aftermath grazing and winter grazing
(Rodwell 1992).
Vegetation Monitoring
Vegetation recording in the green hay donor meadow at Rush-
beds Wood SSSI was carried out on 3 July 2013, 3 weeks before
the green hay cut. Twenty-four quadrats of 1 m
2
were randomly
placed within the meadow, avoiding a margin of 2 m width
around the edge, and percentage cover was visually estimated
for all vascular plants, following the nomenclature of
Stace (2010). For each quadrat, we also recorded which species
flowered or were setting seed. The assessment also involved a
site-level assessment using the DAFOR scale (Kershaw & Loo-
ney 1985), to capture any additional species not picked up dur-
ing the quadrat-based assessment.
Vegetation sampling in the green hay experimental plots and
in the other two treatments was carried out annually in July
Restoration Ecology 3
Species transfer with green hay
between 2014 and 2017, when within each replicate plot, 14 ran-
domly placed quadrats of 1 m
2
were recorded. Again, care was
taken to avoid a margin of 2 m width around the edge of each
plot. For the analyses presented here on transfer and establish-
ment of donor-site species in the green hay treatment plots, veg-
etation data from the 56 quadrats recorded in the control
treatment plots in July 2014 were used to determine the extant
species pool at the recipient site. We took this approach as no
vegetation recording had been carried out at the recipient site
prior to the initiation of the experiment in 2013. We considered
this an adequate approximation of the extant species pool prior
to the onset of the experiment, as the experimental control plots
were very large, and as vegetation recording in 2014 took place
prior to the shift in management toward a regime more typically
associated with the restoration target vegetation.
Green Hay Seed Content and Species Composition
Green hay samples were collected during application at the
recipient sites to determine rate of application and seed content.
In each plot, prior to the hay being spread, 10 polythene bags of
0.45 m ×0.60 m size were placed on the ground and weighed
down with stones at equal distances along a transect crossing
the length of the plot from near the river channel to furthest away
from it. After the hay was spread, the hay on top of each bag was
bagged and its fresh weight determined in the field. Samples
were then transferred into the laboratory where they were stored
with bags opened to allow air-drying, and turned daily for
2 weeks to allow thorough drying and shedding of the seeds.
Once all seeds had been shed, each dried sample was again
weighed to determine dry weight, and then processed through
a 5-mm sieve several times until no further seeds appeared.
The resultant seed-containing chaff was homogenized, again
weighed, and then by weight split into two halves. Each half
was bagged separately, thus effectively creating two sets of sam-
ples. Prior to spreading in trays, each resulting sample was again
halved, and each quarter-sample was spread evenly in a plastic
tray of 15.5 cm ×21 cm, and pressed firmly onto a 3.5-cm layer
of a 3:1 mixture of peat-free multipurpose compost and sharp
sand. Then, all trays were moistened, placed into grip-seal bags,
and put into cold dark storage at 4C in a refrigerator. One set of
half-samples, with each half-sample consisting of two trays, was
retrieved for further processing after 5 weeks of cold-moist strat-
ification on 10 September 2013, and the other set was retrieved
after 5.5 months of cold-moist stratification on 20 January 2014.
The underlying rationale for this was that, as some species might
require a longer period of cold stratification, by doing so, we
would be more likely to detect species that might otherwise have
gone undetected if their requirements for germination would
have been insufficiently met. Also, this enabled us to investigate
any potential differences in seedling emergence that might arise
from using different pre-treatment protocols with respect to
length of stratification. Furthermore, assuming that such differ-
ences indeed existed, this would allow us to use the higher of
the two estimates of green hay seed content thus obtained as
an estimate for the effective sowing rate of individual species
with green hay application.
Upon removal from the refrigerator, each set of trays was
placed on growing benches in a heated glasshouse to allow ger-
mination. In each of the two trials we included five control trays
containing only the compost-sand mixture, to monitor for seed
contamination of the substrate and for airborne contaminants.
Trays were watered regularly from underneath via capillary mat-
ting to avoid seed loss. Starting 3 weeks after samples had been
spread into trays, seedlings were identified weekly, counted, and
carefully removed. Unidentifiable seedlings were potted on for
later identification. The soil in all trays was scarified once, after
the first flush of seedling emergence, to stimulate further emer-
gence. Each of the two trials was terminated after no more addi-
tional seedlings had emerged for at least 4 weeks.
Data Analysis
Green Hay Species Composition and Effective Sowing
Rates. For each species captured as seed with the green hay,
two estimates, one per stratification pre-treatment, were derived
of its effective sowing rate per m
2
, based on the area of ground
covered by the polythene bags used for sampling. To find out
whether length of stratification affected seedling emergence of
species during greenhouse emergence, we carried out two-tailed
Wilcoxon signed rank tests (at p< 0.05) for all species with a
total cumulative emergence of >20 seedlings across both trials,
comparing seedling emergence from paired half-samples
(n= 40). As a much larger number of species than expected by
chance were identified as being affected by length of stratifica-
tion (see “Results”section), the higher of two estimates was sub-
sequently assumed to represent the more realistic estimate of the
effective sowing rate for each species.
Species Capture With Green Hay and Establishment After
Hay Transfer. To test whether species captured as seed in
the green hay, as evidenced by greenhouse seedling emer-
gence, were characterized by higher abundance in the donor
site vegetation than species not detected in the greenhouse
emergence trials, we carried out a one-tailed Mann–Whitney
test (at p< 0.05). In addition, the range in donor site abun-
dance for both species groups was visualized using a box–
whisker plot.
With respect to analyzing species establishment after hay
transfer, additional consideration had to be given to the fact
that the experiment also included a control treatment of extant
grassland and another restoration treatment that involved the
sowing of a diverse seed mixture. For example, some of the
species transferred with green hay were also introduced with
the seed mixture used in adjacent plots assigned to the seeding
treatment, and even though plot sizes were very large com-
pared to those used in similar experiments, there was a possi-
bility of cross-colonization of green hay plots by these species
from seeded plots. In addition, some generalist grassland spe-
cies that might have been transferred with green hay were
already present in the extant species-poor grassland that
served as control treatment. To avoid “false positives”for
green hay transfer as a result of seed dispersal from outside
Restoration Ecology4
Species transfer with green hay
the green hay plots, for example from adjacent diverse seeding
plots that had received a seed mixture containing a number of
species also present in the sward of the green hay donor site
(Tables S1 & S2; Fig. S1), or from adjacent nonexperimental
areas, any species detected in only one or two quadrats in a
single year of the experiment was assumed to not have estab-
lished via green hay, but to have migrated into the green hay
plots during the experiment. The only exception was if a spe-
cies was recorded at such low frequency 1 year after green hay
application and was then not subsequently recorded, as in this
case the species would not yet have had the opportunity to set
seed after establishment in the diverse seeding treatment, and
to disperse into the green hay treatment. Furthermore, we con-
sidered any species found in ≥3 of the 56 quadrats recorded
within the control treatment in 2014, that is, 1 year after green
hay application, to be extant species that had already been pre-
sent at the experimental site. Such species were completely
excluded from analyses of species establishment. With these
adjustments made, we then carried out one-tailed Mann–
Whitney tests to test whether species successfully established
at the recipient site had higher abundance in the vegetation at
the donor site and/or higher abundance as seed in the trans-
ferred green hay, compared to species that failed to establish.
In the first analysis we included all species recorded at the
donor site but not initially present at the experimental site,
and in the second analysis we included all species whose seeds
were captured in the green hay but that were initially absent at
the experimental site. In addition, potential differences
between the groups of species successfully captured or estab-
lished and the groups of species to which this did not apply
were visualized, again using box–whisker plots.
To explore in more detail whether a priori predictions can be
made as to which species from donor sites establish after green
hay transfer, and which species do not, and hence might need
supplementing, for example via additional seed sowing, we con-
structed a classification tree model. We specified success of spe-
cies establishment from hay as the dependent binary factor.
Species abundance and phenological status 3 weeks prior to
the hay cut at the donor site, and several readily available plant
and seed traits with potential to affect either seed capture at the
hay donor site and/or initial establishment from seed at the recip-
ient site, served as candidate predictors (Table 1). Seed densities
of species in the green hay were not included as predictor, as the
specific intention of this analysis was to facilitate decision mak-
ing at the time of hay transfer, that is, at a time when such knowl-
edge is not available.
Species abundance at the donor site in the year of the hay cut
entered the analysis in the form of five-level DAFOR scale
values, recoded numerically (rare = 1; occasional = 2; fre-
quent = 3; abundant = 4; dominant = 5) to reflect this scale’s
ordinal nature. This abundance measure was used instead of
quadrat frequencies as several species found in the vegetation
at the donor site and as seed in the green hay had gone unrec-
orded in the 24 individual vegetation quadrats. Furthermore, a
site-level assessment of donor site vegetation in the form of a
DAFOR assessment or similar may be more within the means
of restoration practitioners than a detailed quadrat-based survey.
We also included two categorical variables based on the vegeta-
tion survey to characterize whether species were present only as
vegetative plants or only as plants that were seeding but no lon-
ger flowering. We did this to account for the possibility that
seeds of certain donor site species might not have been present
in the vegetation when the hay cut took place. Plant height as
specified in Hill et al. (2004) was included as potential explana-
tory variable to account for the possibility that some species at
the donor site may have been too small for their seeds to be cap-
tured by green hay. Capacity of fresh seeds for immediate germi-
nation, as listed in Grime et al. (2007), was included as potential
predictor. Many grassland species display primary physiologi-
cal seed dormancy (Grime et al. 1981), as also demonstrated in
at least one experiment of green hay sampling (Rasran
et al. 2006). In addition, grassland species from some plant fam-
ilies display other types of seed dormancy, such as, for example,
physical dormancy in the case of Fabaceae, and morphophysio-
logical dormancy in the case of Apiaceae (Baskin & Bas-
kin 2014). Such seed dormancy could put species at a
disadvantage during restoration, given that bare ground crea-
tion during grassland restoration only provides target species
with a relatively short-lived window of opportunity for initial
establishment (Wagner et al. 2011, 2016). Seed weight as listed
in the Seed Information Database (Royal Botanic Gardens
Kew 2008) was included as potential explanatory variable as
it is possible that large-seeded species are better able to estab-
lish. On the other hand, seed size may also be negatively corre-
lated with seed number in green hay, given that it might
negatively correlate with total seed output per plant at the spe-
cies level.
The classification tree was built using the “rpart”R package
vs. 4.1–9 (Therneau et al. 2015). The approach implemented
in this package starts with a full tree, which is then pruned back
with the aim of optimizing predictive accuracy of the final
“pruned”tree (Maindonald & Braun 2010). Splits were deter-
mined using the Gini criterion. To confirm optimal tree size
according to the 1-SE rule, we carried out 50 sets of 10-fold
cross-validation, and averaged estimated errors for each tree size
and their standard errors across all 50 sets (De’ath & Fabri-
cius 2000). We then calculated the absolute cross-validated per-
cent error rate (Maindonald & Braun 2010) as a measure of
predictive accuracy.
Results
Species Capture by Green Hay and Effective Sowing Rates
Sixty-two herbaceous plant species were recorded at the green
hay donor site (Table S1). Using greenhouse emergence, we
verified the presence of 47 of these species as seeds in the
green hay, with their individual contributions adding up to an
overall effective sowing rate of 5,119 viable propagules per
m
2
across all species (Table S3). As indicated by a Mann–
Whitney test, the 47 species captured by green hay occurred
on average at higher quadrat frequencies in the donor grass-
land than the 15 species not captured (W= 229.5, p= 0.021;
n= 62), thus indicating that the chances of individual species
Restoration Ecology 5
Species transfer with green hay
being captured depended to some extent on abundance at the
donor site (Fig. 1).
Estimation of effective sowing rates of species using the
higher of two values for emergence following different stratifi-
cation pre-treatments was justified, as Wilcoxon signed rank
tests were significant for 15 of 27 species with a cumulative total
emergence exceeding 20 seedlings, indicating a response to
length of stratification in more than half of the tested species
(Table S3). Ten species had higher emergence after extended
cold-stratification, and five had higher emergence after only
short stratification (Table S3).
Agrostis capillaris, by far the most abundant species in the
green hay, had an estimated effective sowing rate of 3,902 seeds
per m
2
(Table S3; Fig. S1). In contrast, 14 species were charac-
terized by estimated effective sowing rates of less than one seed
m
−2
(Table S3; Fig. S1).
Species Establishment
For the analysis of species establishment from green hay, we a
priori excluded six species that were already present at the
experimental site: Holcus lanatus,Lolium perenne,Cirsium
arvense,Ranunculus repens,Taraxacum officinale agg., and
Trifolium repens. Of the remaining 56 species, 29 successfully
established via green hay application, equivalent to an absolute
transfer rate of 52%. A one-tailed Mann–Whitney test compar-
ing median quadrat frequencies at the green hay donor site of
the 29 species established after hay transfer with those of the
27 species that failed to do so was highly significant
(W= 212.5, p= 0.002; n= 56), with frequencies of the former
over four times as high as those of the latter (Fig. 2A).
Of the 29 species successfully established after green hay
transfer, 21 were first recorded in 2014, with a further 6 species
firstrecordedin2015,and2speciesfirst recorded in 2016. Not
included in this count were four species only recorded in a sin-
gle year either 2 or 3 years after green hay application, and then
in only one or two quadrats. Transfer of these species with
green hay was instead considered to have failed. Three of these
species—Achillea millefolium,Galium verum,andVicia
cracca—were only recorded in 2016 and may have cross-colo-
nized from the diverse seeding treatment in which they were
also included as seed (Table S2). The fourth species, Senecio
jacobaea, which was recorded in a single quadrat in 2015,
may have spontaneously colonized from an adjacent nonex-
perimental area where it had also been observed. The interpre-
tation of these species having colonized from sources other
than green hay was further underlined by the fact that in the
greenhouse emergence trials, only two seedlings of G. verum
had emerged, and none of A. millefolium,S. jacobaea,andV.
cracca (Table S3).
The transfer of 29 of the 41 species found as seed in the green
hay and not already present at the recipient site is equivalent to a
relative transfer rate of 71%. Median effective sowing rate
across the 29 species successfully established was 9.4 seeds
m
−2
, whereas across the 12 species that were present as seed
in the green hay but failed to established after green hay applica-
tion it was only 1.6 seeds m
−2
(Fig. 2B). However, a one-tailed
Mann–Whitney comparing effective sowing rates of both
groups of species fell short of significance (W= 124,
p= 0.078; n= 41).
Table 1. Candidate predictors used in the construction of the classification tree model to predict whether species present at the green hay donor site will actually
establish at the recipient site. Candidate predictors have either been derived from the vegetation survey at the donor site or extracted from literature databases of
plant traits. Note that, as the vegetation survey took place 3 weeks prior to the hay cut, species phenology at the time of the cut was seasonally slightly more
advanced.
Predictor Data Type Description Source
DAFOR score Ordinal Species abundance at donor site Vegetation survey
Vegetative only Categorical Species only non-flowering at donor site Vegetation survey
Seeding only Categorical Species already past flowering at donor site Vegetation survey
Plant height Continuous Typical height of plants in the British flora Hill et al. (2004)
Seed weight Continuous Median seed weight in RBG Kew’s Seed Information Database Royal Botanic Gardens
Kew (2008)
Germination
requirement
Categorical Presence of specific requirements preventing efficient germination
of fresh seed
Grime et al. (2007)
Figure 1. Species capture in green hay in relation to abundance at the donor
site. Box–whisker plots illustrate the distribution of quadrat frequencies
(maximum = 24) at which a species was found at the donor site, both for
species captured as seed by green hay transfer and for species not captured,
as evidenced by greenhouse emergence trials.
Restoration Ecology6
Species transfer with green hay
The optimal classification tree for predicting species estab-
lishment from donor vegetation characteristics and plant traits
had two decision nodes. The final model used two predictors,
DAFOR abundance at the donor site and existence of specific
germination requirements (Fig. 3). The model predicts species
that are at least “Frequent”at the donor site according to the
DAFOR scale to establish successfully regardless of germina-
tion requirements, and species that are “Rare”or “Occasional”
to fail to establish if they have specific germination requirements
preventing them from quick establishment, but to establish suc-
cessfully in the absence of such requirements. Absolute cross-
validated error of the model was 21.9%.
Discussion
Effective Sowing Rates and Seed Capture
We found significant differences in seedling emergence between
stratification pre-treatments in 15 of 27 tested species. By
chance alone, only one or two species should have produced sig-
nificant results. Thus, our results confirm that greenhouse emer-
gence from green hay depends in a species-specific manner on
length of prior cold-moist stratification. This means that if ger-
mination requirements of a species are not successfully met dur-
ing greenhouse emergence, actual seed content of the hay for
this species could be underestimated by greenhouse emergence.
Moreover, 10 species behaved one way, and 5 species another
way, indicating that an optimal length of cold stratification
might have been somewhere between 5 weeks and 5.5 months.
Our results indicate that if the seed content of green hay is deter-
mined via greenhouse emergence, numbers of seedlings emerg-
ing of a given species might be affected by the chosen
stratification pre-treatment. Reassuringly, none of the species
reasonably common as seed in the green hay were found in only
one stratification treatment, underlining the fact that such spe-
cies should be reliably recorded regardless of the exact length
of cold stratification, even though numbers recorded might
somewhat vary.
We were also interested in the question of whether the prob-
ability of species being captured as seed in green hay would
depend on species abundance in the vegetation at the green
hay donor site. As indicated by nonparametric statistical testing,
Figure 2. Species establishment after green hay transfer in relation to (A)
quadrat frequencies (maximum = 24) at which a species was found at the
donor site (out of 24 quadrats) for species established after hay transfer and
for species that failed to establish, and (B) effective sowing rate as
determined by greenhouse emergence trials.
Figure 3. Classification tree model to predict which species from the green
hay donor site successfully establish after green hay application. Predictors
selected by the final model include species abundance at the donor site in the
form of ordinal DAFOR scale values and the presence of species-specific
germination requirements thought to prevent immediate germination after
hay transfer. Absolute cross-validated error rate of the model is 21.9%.
DAFOR abundance was coded as follows: rare = 1; occasional = 2;
frequent = 3; abundant = 4; dominant = 5. Proportions supplied with each
terminal node denote the proportion of species correctly classified using this
model.
Restoration Ecology 7
Species transfer with green hay
the 15 species that remained undetected during greenhouse
emergence were on average less abundant at the donor site than
the 47 species whose emergence was recorded, thus providing
an answer to our second question.
Species Establishment
We hypothesized that species successfully establishing after
green hay transfer would have been more abundant in the vege-
tation at the donor site and would have occurred at higher seed
densities in the green hay than the species failing to establish.
Nonparametric statistical testing confirmed the first relationship,
but fell short of significance for the second relationship. How-
ever, the control group of 27 species that failed to establish but
were present in the vegetation at the donor site was larger than
the group of 12 species that failed to establish in spite of having
been present as seed in the green hay, resulting in a less powerful
test for the second relationship. This means that the nonsignifi-
cance of the statistical test, while failing to establish such a rela-
tionship, does not preclude it either. Two studies by
Scotton (2016, 2018) also provide some indication for the exis-
tence of such a relationship, with species transferred with har-
vested material at densities of greater than 10 seeds m
−2
having mostly reliably established, and species transferred at
densities below that having more frequently failed to establish.
As indicated by our classification tree model, the capacity of
species’seeds for immediate germination appears to also play
a role in whether species get successfully established after trans-
fer. We found that if a species occurs at very low abundance at
the donor site, failure to be able to germinate quickly often
results in failure to establish at the recipient site. Previous work
led by one of us (Pywell et al. 2003) has shown that when seed
mixtures are used to restore grassland, high germinability of spe-
cies increases their chances for successful establishment. The
results of our tree model suggest that a similar relationship might
apply during green hay restoration. Green hay is spread at the
recipient site almost immediately after it has been cut at the
donor site, usually on the same day. As shown by Rasran
et al. (2006), at this point, the fresh seeds of many species trans-
ferred with green hay still exhibit primary seed dormancy. How-
ever, in species-poor grassland undergoing restoration, the
temporal window of opportunity for initial species establish-
ment that is created by preparatory soil cultivation often closes
rapidly (Wagner et al. 2016). Thus, when restoring temperate
semi-natural grassland using green hay, species whose freshly
produced seeds are capable of rapid germination might establish
more reliably than species lacking this capacity (Wagner
et al. 2011). In other situations and in ecosystems where win-
dows of opportunity for establishment are less predictable such
germination behavior can, however, be disadvantageous, as it
could result in loss of the whole seed population if establishment
fails (Cavers et al. 2000). To test the hypothesis of more easily
germinable species doing better during green hay restoration,
experiments like ours would have to be complemented by mea-
suring the germinability of freshly produced seed at the time of
hay transfer.
Delayed germination of species whose seeds are initially dor-
mant at the time of green hay application could also help explain
why during green hay restoration, often, a number of species
only establish in the second or third year after hay application
(see, e.g. Patzelt 1998; Mann & Tischew 2010). The same was
found in our study, in which only 21 of the 29 species success-
fully transferred with green hay were recorded in the first year
after hay application, with a further six and two species, respec-
tively, recorded for the first time in the second and third years
after hay application.
Overall, our findings suggest that species abundance at the
donor site, and the presence of specific germination require-
ments in species have the potential to affect whether a species
from a green hay donor site can successfully establish at the
recipient site. The practical implications of these findings for
restoration practice are discussed below. However, due to the
relatively small number of 56 species present at our hay donor
site that had also been initially absent from the recipient experi-
mental plots, we were likely only able to identify the most rele-
vant factors. This does, however, not mean that other factors
might not also have affected which species were successfully
transferred with green hay and which were not. One factor
known to play such a role is plant species’timing of seed-set.
The relevance of this factor for seed harvesting has for example
been demonstrated for continental European Arrhenatherum
elatius meadows that are traditionally cut twice during the grow-
ing season, and that contain late-flowering species that are evi-
dently specialized to produce flowering shoots and seeds only
after first hay cut in mid-June (Scotton & Ševcˇíková 2017). In
contrast, in our study, we focused on the restoration of a type
of grassland, MG5 grassland according to the British NVC, tra-
ditionally managed by a single hay cut, followed by aftermath
grazing. Moreover, at our green hay donor site, Rushbeds Wood
SSSI, hay usually gets cut between 15 July and the early part of
August, with the exact date depending on weather conditions
(M. Vallance, pers. comm.). Such a late cutting date likely limits
the possibility for specialized late-flowering species to complete
their life cycle, and may have thus selected against the presence
of such species at our donor site. However, some typical species
of MG5 grassland that were present at the site, notably Primula
veris and Luzula campestris, may have not been captured by
green hay due to their early phenology, with most of their seeds
having already potentially been dispersed by the time of the
hay cut.
Recommendations for Restoration
As indicated by our own results and those of other studies, even
though green hay application is a very useful method for restor-
ing a large number of species from target communities, nonethe-
less a sizeable proportion of species from green hay donor sites
usually fails to establish at green hay recipient sites. Our results
clearly indicate that this deficit is not just due to microsite limi-
tation at the recipient site (as, for example, demonstrated by
Pywell et al. 2003; Wagner et al. 2019; Löfgren et al. 2020),
but also due to limitations inherent to green hay method. Species
that are less abundant at a green hay donor site are less likely to
Restoration Ecology8
Species transfer with green hay
be captured as seeds, and in grassland types with a wide spread
in species phenology, using just a single cut may further limit the
range of species captured by the method (Scotton & Ševcˇí-
ková 2017). Moreover, even if species are represented as seed
in the green hay, some may not establish because their seeds lack
the capacity for quick and reliable germination while establish-
ment microsites are available at the recipient site during the win-
dow of opportunity created by site preparation in advance of hay
application.
However, our findings can also be used to provide some guid-
ance on how to reduce the deficit in species establishment from
green hay application, by highlighting which species are likely
underrepresented in the green hay, based on plant composition
at the donor site, and more generally by highlighting which spe-
cies might be least likely to establish from green hay, also due to
an inability to take advantage of the temporary window of
opportunity created during initial restoration. Being able to iden-
tify such critical species, their establishment can then be pro-
moted by targeted supplementation in the form of seed sowing
alongside green hay application. These additional seeds could
be either hand-collected (Poschlod & Biewer 2005), or where
this is not possible, purchased seeds could be used (Baasch
et al. 2016). Our results suggest that most of the species that
are common at a green hay donor site, and whose seeds can be
captured by a well-timed hay cut, should reliably establish after
transfer. Targeted supplementation in the form of additional
seed sowing should focus on target species absent from the
donor site, target species whose seed production does not coin-
cide with planned green hay cuts, and target species that are less
common or even rare in the vegetation of the donor site.
Among the latter group, those species least likely to establish
from green hay are those that are additionally characterized by
germination requirements that may prevent them from efficient
germination when spread as freshly produced seeds with green
hay. If such species are sown alongside an application of green
hay, the exact nature of their additional requirements for effi-
cient germination might warrant additional seed treatment prior
to sowing. For example, the seeds of many U.K. grassland
forbs and sedges display primary physiological seed dormancy
at the time of seed shedding, and require afterripening to
become germinable (Grime et al. 1981). For a subset of these
species that is characterized by nondeep physiological dor-
mancy, dry storage prior to sowing may provide sufficient
afterripening to allow germination when sown (Baskin & Bas-
kin 2020), and germination testing prior to sowing can help
confirm if this is the case. For other species, whose germination
requirements might be more complex, specific methodologies
for ex situ dormancy alleviation can and should be applied
(Kildisheva et al. 2020) prior to sowing, to ensure efficient
germination.
Another way to promote the establishment of species that
might not germinate efficiently soon after sowing, that would
not require additional sowing alongside the application of green
hay, would be to further extend the window of opportunity, for
example by heavy grazing soon after green hay transfer
(Garrouj et al. 2019). Alternatively, the window could be
reopened, for example by creating additional bare ground, pos-
sibly in the first spring after hay application, when transferred
seed characterized by primary seed dormancy has experienced
natural cold stratification.
The findings of our study provide some indication as to which
species are less likely to be established via standard green hay
application in the absence of additional measures. Such informa-
tion can feed into decisions regarding targeted additional mea-
sures specifically for these species. Ultimately, this could help
realize more ambitious grassland restoration projects that aim
to establish as many species as possible from a pre-defined ref-
erence plant community early on during restoration.
Acknowledgments
This project was funded by Defra and Natural England as part of
the Hillesden project BD5209. Greenhouse emergence trials
were supported by research programme NE/N018125/1
LTS-M ASSIST—Achieving Sustainable Agricultural Systems,
funded by NERC and BBSRC. Valuable advice was provided
by V. Robinson from Natural England. The experiment was
set up and managed in cooperation with Faccenda Farms Ltd
and farm manager Richard Franklin. We thank BBOWT who
own Rushbeds Wood SSSI, our green hay donor site, for permis-
sion to harvest green hay, and to carry out vegetation recording.
We thank Nadine Mitschunas, Pete Nuttall, and Jodey Peyton
for help with vegetation recording. We also thank two reviewers
whose insightful comments guided the revision of this article.
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Species transfer with green hay
Supporting Information
The following information may be found in the online version of this article:
Figure S1 Species composition and effective sowing rates of green hay and of the
diverse seed mixture that was used in an adjacent experimental treatment plots.
Table S1. Plant species composition of the green hay donor grassland at Rushbeds
Wood SSSI.
Table S2. Species composition of the seed mixture used in the diverse-seeding treat-
ment and estimated effective sowing rates for individual species.
Table S3. Seed species composition of the green hay based on greenhouse emerge nce
from hay samples collected after transfer to recipient plots.
Guest Coordinating Editor: Peter Török Received: 28 January, 2020; First decision: 6 April, 2020; Revised: 22 July,
2020; Accepted: 1 August, 2020
Restoration Ecology 11
Species transfer with green hay
