160 C.N.R. Critchley, J.A. Fowbert Agriculture, Ecosystems and Environment 79 2000 159–174
Sotherton, 1998, this was potentially significant for biodiversity on arable farmland.
The establishment of vegetation on abandoned arable land, and its early successional changes have
been comprehensively described from old-field sys- tems e.g. Bazzaz, 1975; Pickett, 1982; Lepš, 1987.
Similar processes in set-aside vegetation have been observed in studies of single sites and surveys within
restricted geographical areas Fisher et al., 1992; Wilson, 1992; Turley et al., 1994. The common se-
quence in naturally regenerated set-aside vegetation is the initial establishment of annual species along
with self-sown crops from the previous season’s shed seed volunteers, which decline rapidly in abundance
during the first 2–3 years, to be largely replaced by perennial grasses by the fifth year. Where vegetation
cover is sown, the sown species tend to dominate the vegetation during this early phase, suppressing other
species, and particularly the annuals that are other- wise characteristic of the first 2 years. Spatial pat-
terns of species richness within fields tend to reflect those observed for arable weeds and their seedbanks
within arable crops, with greatest concentrations be- ing within a few metres of the field edge Marshall,
1989; Rew et al., 1992; Wilson and Aebischer, 1995. Corbet 1995 noted that empirical studies of set-aside
have been short-term and influenced by site-specific factors. More recently, the effects of management
on set-aside vegetation have been reported from a 10-year experiment Hansson and Fogelfors, 1998.
Assessments of the value to biodiversity of set-aside vegetation have highlighted the need for appropriate
management if positive benefits are to be realised Firbank et al., 1993; Sotherton, 1998.
The first set-aside in the UK was introduced as a voluntary scheme in July 1988 Anon., 1988, and was
followed by a second voluntary scheme for 1 year in August 1991 Anon., 1991a. In May 1992, set-aside
became part of the Arable Areas Payment Scheme AAPS under the reform of the Common Agricul-
tural Policy of the European Union Anon., 1992. The original 1988 scheme in the UK required a minimum
of 20 of eligible land on a holding to be taken out of production in return for annual compensation pay-
ments, but this was subsequently reduced under the AAPS to 5 by 1996–1997. With these administra-
tive changes, also came modifications to the rules for set-aside management, but, in general, land entered
into set-aside could either be rotated annually around the holding rotational set-aside, or be kept in the
same place non-rotational set-aside. Set-aside land could either be in blocks usually whole fields or as
20 m wide strips that were usually sited along field boundaries. Vegetation on set-aside fallow had to be
established either by natural regeneration or by sown cover grasses and, in some cases, legumes. This veg-
etation had to be cut periodically at least once a year and cultivation was only allowed to control weeds or
before establishing new plant cover. Pesticides and fer- tilisers were prohibited except under certain exemp-
tions. From October 1991, some set-aside land could be grazed with livestock Anon., 1991b.
With some fallow set-aside sites now in existence since 1988, and set-aside widely adopted from 1992,
there was an opportunity to re-assess the development of its vegetation to include later years of succession,
and its variation at a national scale. The success of wildlife habitat recreation on arable land in the longer
term is largely dependent on the development of suit- able vegetation. The timescale over which this is likely
to occur, and its consistency in different geographical regions will have important implications for the future
management of set-aside land for biodiversity.
The aim of this study is to describe the variation in fallow set-aside vegetation from a national perspec-
tive, including the full age range of sites currently in existence. This was achieved by field survey of a sam-
ple of sites from England, UK.
2. Methods
2.1. Data collection A stratified sample of farms in England with
non-rotational set-aside was selected from the Inte- grated Arable Control System database of the UK
Ministry of Agriculture, Fisheries and Food MAFF. Fifty farms were randomly selected from each of two
geographical regions. These comprised counties clas- sified by MAFF as having predominantly arable agri-
culture, mainly in eastern England ‘arable’ region, and those with predominantly mixed agriculture,
mainly in western England ‘mixed’ region. One non-rotational set-aside site was randomly selected
from each farm, with first priority given to set-aside
C.N.R. Critchley, J.A. Fowbert Agriculture, Ecosystems and Environment 79 2000 159–174 161
in whole fields or blocks, as opposed to field margin strips.
Within each site, one field boundary was randomly selected and a sampling zone established. The sam-
pling zone had a minimum width of 20 m, and ex- tended 32 m into the field and to a minimum of 32 m
from each end of the chosen field boundary. Five tran- sects extending at right angles from the field bound-
ary were randomly located in the sampling zone. Each transect consisted of six 0.5 m×0.5 m quadrats i.e. 30
quadrats per site placed at distances of 1, 2, 4, 8, 16 and 32 m from the boundary, with individual quadrats
orientated diagonally at each position on the transect. To ensure that the sequence of quadrats in each tran-
sect represented a progression from the field bound- ary origin towards the field centre, quadrat number
six 32 m from the field boundary origin had to be at least 32 m from any other field boundary. Field mar-
gins that were less than 32 m in width had transects limited to 16 m in length, with five quadrats only i.e.
25 quadrats per site. Vegetation height was first mea- sured in the centre of each quadrat using a sward plate
meter comprising a plastic disk 10 cm diam mounted on a ruled fibreglass cane of 0.6 cm diameter through
the centre of the disk. The end of the cane was po- sitioned on the soil surface and the disk allowed to
settle on the vegetation to obtain the height measure- ment. To obtain estimates of top cover, a single pinhit
per quadrat was recorded. The pin was lowered ver- tically in one corner of the quadrat and the first hit
of plant species, bare ground or strawlitter noted. In each quadrat, the presence of plant species if rooted
in the quadrat, bare ground and strawlitter, was also recorded. For each site, cover of plant species, bare
ground and strawlitter was calculated as the percent- age out of 25 or 30 of pinhits per site, and species
frequencies as the percentage of quadrats in which they occurred. To maximise the chances of detect-
ing small changes between repeated surveys, species were recorded cumulatively from a series of smaller
nested cells within each quadrat Hodgson et al., 1994; Critchley and Poulton, 1998. However, for the pur-
pose of between-site comparisons reported here, the cover and frequency data were considered to be suffi-
ciently precise.
Plants were identified to species level apart from bryophytes and seedlings defined as plants with less
than two true leaves each of which was recorded as a single group. Occasionally, where species could not be
reliably separated in the field, they were recorded as amalgams e.g. Lolium multiflorumperenne. Nomen-
clature follows Stace 1991. Field surveys were carried out during 7 May–22 June 1996.
2.2. Data analysis Three sites were lost because of alterations to
farmers’ set-aside agreements, leaving 97 available for analysis. Sites were classified by the method used
to establish vegetation natural regeneration or sown cover and into two age classes according to the num-
ber of years of continuous management as set-aside 1–5 and 6–9 years. Five years was chosen as the
cut-off between the age classes because changes in set-aside vegetation had been previously described up
to that age. Some sites had been managed as set-aside since 1987 in anticipation of the forthcoming scheme
in 1988, so that the maximum age available was 9 years. Differences among region, establishment
method and age were analysed using a fixed-effects multivariate Analysis of Variance MANOVA. Four
separate analyses were carried out. These were, respectively, of
a the main cover components plant cover, bare ground and strawlitter,
b the cover of different species functional types annuals, perennials, monocotyledons, dicotyledons,
species with the canopy structure classes basal and leafy Hodgson et al., 1995 and species categorised
as ‘very characteristic of arable habitats’ and ‘mainly absent from arable habitats’ Grime et al., 1988;
Hodgson et al., 1995, c the relative frequency of the species functional
types outlined in b, and d the frequency of the nine most commonly occur-
ring species.
Relative frequencies for species functional types were the proportion of species records of that type
in each quadrat, expressed as site means. All cover and frequency data were transformed to arcsin
√ x
to meet the requirements of Analysis of Variance Zar,
1984. The effect of distance from the field boundary
on species richness within sites was analysed by a repeated-measures Analysis of Variance, where
162 C.N.R. Critchley, J.A. Fowbert Agriculture, Ecosystems and Environment 79 2000 159–174
establishment method, region, site age and distance from field boundary were fixed factors, and distance
was the repeated measure. Field margin sites 11 sites with shorter transects were omitted. Species
richness data were transformed to log
10
x+1 before analysis.
Relationships between vegetation and environmen- tal factors were analysed for 36 naturally regener-
ated sites for which management information could be obtained from farmers, using Canonical Correspon-
dence Analysis CCA ter Braak, 1987. Within-site species percentage frequencies were calculated, using
the 30 quadrats from set-aside blocks and 25 from field margin set-aside, respectively. All species and amal-
gams were included except bryophytes and unidenti- fied seedlings. Environmental data were region, soil
class, previous crop in the year before set-aside estab- lishment, management of set-aside vegetation in the
previous 7 months and age of each site in years. Soil data were derived from the Soil Survey and Land Re-
search Council dominant soils map of Great Britain at the 1 km scale, and re-classified as described by
Firbank et al. 1998. Management activities grazing, cutting and herbicide application were those carried
out during the 7-month period preceding the field sur- veys September 1995–May 1996. No sites had been
subjected to cultivation.
Stability of the analysis under re-ordering of data was checked as recommended by Oksanen and
Minchin 1997, and found to be satisfactory. En- vironmental variables were added individually by
forward selection, and significance testing done for each variable in turn, and for the overall trace, us-
ing 999 Monte Carlo permutations ter Braak, 1988, 1990. Only environmental variables that showed a
significant relationship with species were added to the model.
3. Results
