Botanzcal Journal
of the Linnean Sac@
(1998),126: 109-1 2 1. With 6 figures
Orchid population biology: conservation and challenges.
Edited by S. Waite
Population biology of the rare military orchid
(Orchis militaris L.) at an established site in
Suffolk, England
STEPHEN WAITE
Biology Division, Dtpartment of Phamay, UniuersiQ of Brighton, Cockcroft Building,
Moulsecoomb, Brghton BN2 4GJ
LYNNE FARRELL
Scottish Natural Heritage, 2 Anderson Place, Edinburgh EH6 5NP
In Britain, where it reaches the north-westerly limit of its European distribution, Orchis
militaris L. is extremely rare. Well-established and persistent populations of 0. militaris are
known to exist at only two sites. The largest extant population of 0. militaris occurs in a
disused chalk pit in Suffok. A preliminary demographic analysis of this population, covering
the period 1975 to 1991, along with estimates of key life stage transition probabilities are
presented here. From 1975 to 1986 the number of separate identifiable plants in the
population declined substantially. Until 1986 recruitment of rosettes was poor. The largest
cohort of new plants, recorded in 1976, was 35. Approximately 48% of new individuals
recruited between 1976 and 1985 failed to flower. Of those flowering, approximately 55%
flowered during their first year above ground. Of the original population recorded in 1975,
67.8% flowered at least once during the study. The reproductive performance, i.e. the
frequency of flowering and the period between episodes of flowering, varied considerably
between individuals. Some plants flowered every year while others only flowered once during
the study. Few plants remained below ground for more than one year, while several apparently
persisted below ground for more than 6 successive years. Although the number of plants
that can be identified as separate individuals has declined, the total number of rosettes in
the population has, from 1986, increased dramatically. Because of the dense clumping of
these recruits it is not possible to determine whether they are derived from seed or vegetative
propagation. When post 1986 recruitment is combined with the number of plants that
established before 1986 and survive, the apparent number of plants present at the site has
more than doubled between 1975 and 1991.
0 1998 The Linnean Society of London
ADDITIONAL KEY WORDS:-conservation
-
life-cycles - monitoring.
Correspondence to Dr S. Waite. Email [email protected]
0024-4074/98/0 10109 + 13 $25.00/0/bt970 148
109
0 1998 The Linnean Society of London
llO
S. WAITE Al'\lD L. FARRELL
CO:\'TE!\'TS
Introduction . . . . . . . .
Site history and management
Methods . . . . . . .
l\fonitoring of population . .
Results
. . . . . . . . .
Population structure
Recruitment and population trends .
Transition probabilities .
Discussion
Acknowledgements
References
110
110
Ill
111
112
112
113
114
118
120
120
Il\'TRODIJCTIOl\"
Orchis militaris L. is an extremely rare species in Britain, where it reaches the
north-westerly limit of its European distribution. Although it was thought to be
extinct in 1929, well-established and persistent populations of 0. militaris are now
known to exist at two sites. A small population was discovered, at a site at which
the species was previously unknown, in Buckinghamshire in 194 7 (see Hutchings,
Mendoza & Havers, this volume). A second, larger population was discovered in
1954 in a disused Suffolk chalk pit. Records of the size of the Suffolk population
have been kept since 1955. Since 197 5 plant coordinates have been recorded each
year during the flowering season, allowing the fate of plants to be determined. A
preliminary demographic analysis of the Suffolk population covering the period
197 5-91, along with estimates of key life stage transition probabilities, is presented
here.
On the European mainland the distribution of 0. militaris extends from southeastern Sweden and Russia, through central Europe to northern Spain, central Italy,
through Turkey and eastward to the Altai Mountains and Lake Baikal. Throughout
its range 0. militaris occurs in species-rich grasslands on calcareous soil, in full light
or partial shade. The species is reported to prefer warm spring and summer
conditions, to be tolerant of cold winters, but not of hot dry summers. The plants
of the Suffolk populations differ from the Buckinghamshire populations, being larger
and more closely resembling the plants of continental Europe and Asia. At the
Suffolk site plants emerge above ground in late December or earlyJanuary, increasing
rapidly in size during May, when the flower stalks elongate. Flowers are fully open
in early June, seed capsules being fully ripe in late August or early September
(Farrell, 1985, 1991).
Site history and management
The Suffolk population has been continuously monitored since its discovery in
1954. The original population of plants was largely confined to open areas of chalk.
It expanded rapidly in numbers from an estimated initial population of 500 plants
to a total of 2854 plants in 1958. The population remained at around this high
POPULATION BIOLOGY OF ORCHIS MZLJTARZS L.
111
level until the late 1960s, after which it declined rapidly to only 252 plants in 1971.
During this period the site had become overgrown and steps were subsequently
taken to remove invading scrub and tree species. The encroaching shrub corresponds
to the NVC Crataegus monogyna-Hedera helix W21 community (Rodwell, 1991). In
autumn 1972, Acer sp. trees were felled to reduce shading at the site. Subsequently,
in 1980 and 1982, all Betula spp. trees were removed from the main orchid area
(Farrell, 1985, 1991). During this period a 10 m belt of tall Corsican pine (I? niga
var. maritima) which had been planted in 1952, was removed from the perimeter of
the site. In some parts of the site areas of moss cover developed following tree
removal. Because of the concern that the moss may smother seedlings of 0. militaris,
and harbour small mammals that eat and damage orchid tubers, occasional hand
clearance of moss has also been conducted since 1980 (Farrell, 1991; M. Harding,
pers comm.). Since 1987, two annual work parties, in October and February, have
hand-cleared potentially competitive herbaceous species (e.g. liussilago fagara and
RubusJiuticosus agg.). This management appears to have been very successful, since
the decline of the population has been reversed. The total number of plants increased
substantially after 1986, although the high numbers recorded in the 1950s have not
been reached (Farrell, 1991).
METHODS
Monitoring of the population
Since 1975 the population has been recorded annually during the flowering
period, using the co-ordinate method (Farrell, 1985), which allows the fate of
individual plants to be followed (Wells, 1967). At each census date the positions of
individuals are recorded along with their status, i.e. whether they are flowering or
non-flowering (vegetative).Multi-leaved rosettes and plants with only a single leaf
were recorded as distinct classes. From 1986 onwards many small plants with only
a single leaf entered the population. The data set allows the fate of individual plants
to be easily determined from 1975 to 1987. After this date, although it is still possible
to follow the fate of many previously established plants, the identity and fate of
recruits cannot be followed. Recruits entering the population after 1986 were all
clustered closely around the co-ordinates of established plants, making it impossible
to identlf). individuals unambiguously using the co-ordinate method. Thus, some of
the population parameters presented here are based on the first 12 years of records
only. Records for some small sections of the population, which were incomplete or
inconsistent, have been excluded from the analysis. Because of this, figures presented
here underestimate the true size of the population at the site. The records of the
sections included in the present study contained approximately 80% of the all
rosettes present at the start of the study in 1975.
In common with other temperate orchid species (Wells, 1981; Mehrhoff, 1989;
Willems & Bik, 1991; Rasmussen, 1995),plants of 0. militaris appear to demonstrate
dormancy, failing to be recorded above ground during one or more consecutive
annual censuses, but re-emerging in subsequent years. Plants were classified as being
in one of three states: (1) above-ground flowering, (2) above-ground vegetative, or
(3) absent (i.e. below-ground dormant). Plants re-emerging after periods of apparent
1 I!:
S. LVhITE XKD L. FARRELL
dormancy at the position previously occupied by a plant were retrospectively classed
as dormant. In these cases it is possible that a small number of new recruits may
wrongly be classified as established plants.
It is impossible to distinguish unambiguously between vegetative and seedlingderkred recruits. Gi\Ten the rarity of the 0. militaris in the UK, excavation of
individuals is not possible. A pragmatic approach has been adopted. Following
FVillems (1989), an individual plant is considered to be either an isolated rosette or
a small group (2-4) of closely associated rosettes. Recruits (i.e. newly recorded
rosettes) that occupy locations clearly distinct from previously established plants are
treated as new plants, mostly probably derived from seed. The occurrence of more
than one multi-leaved rosette centred on the same co-ordinates previously occupied
by a single individual rosette has been treated as vegetative multiplication of
rosettes. While increasing the number of rosettes present in the population, rosette
multiplication is not considered to constitute recruitment of new plants.
Probabilities for all possible life-stage transitions (tn+tll+ J, were calculated from
the demographic data using the procedures outlined in van Groenendael, Kroon &
Caswell (1988) and detailed in Caswell (1989). The transition probabilities were
calculated independently, for each year of the study and for each class of aboveground plant, i.e. flowering or vegetative. N o distinction was made between plants
with differing past histories. Transition probabilities were tested for normality using
the Shapiro -Wilks test and found to be normally distributed. The Pearson product
moment correlation was used to test for relationships between transition probability
values and calendar year. None was found. To assess whether the dynamics of the
population had changed in response to site management transition probabilities
were averaged for the first 1 years of the study (1975-79) and for the period 1983-87,
the last 4 years for which reliable estimates of transition probabilities can be obtained.
Llveragevalues were compared using Student t-tests. Cohort and population depletion
curves were fitted by regression analysis, using the negative exponential model
(Harper, 1977). M statistical procedures follow Sokal & Rohlf (1987) and were
performed using MINITAB version 10.
RESULTS
Population structure
Of the 416 plants whose fate could be followed throughout the study period,
30.8O/o were absent from the above ground population for at least one census
between their first and last dates of recording. O n average in any year approximately
1 4 O O (range 9.55-22.360/0) are dormant. Mowing for the presence of dormant
plants, the estimated size of the initial 1975 population was 350 plants, made up of
304 above ground plants plus 46 plants below ground. Plants persist in the below
cground phase for differing lengths of time. The majority of plants remain below
,ground for one year only. Between 1975 and 1987, 95% of plants that entered and
returned from the below ground phases did so within 3 years, although some
indil-iduals appeared to persist below ground for up to 8 consecutive years (Fig. 1).
POPULATION BIOLOGY OF ORCHZS MZLKARZS L
113
70.0
1
2
3
4
5
6
7
Number of successive years
below ground
8
Figure I. Proportional distribution of records of dormancy between dormant periods of different
durations.
The majority (67.8%) of the 304 above-ground plants recorded in 1975 flowered
at least once during the study. Of the 108 plants entering the population between
1976 and 1986, 48.2% failed to flower between 1976 and 1991; 28.7% flowered in
the year of recruitment to the above ground population; 84% flowered within 3
years of first being recorded. Overall, flowering plants accounted for 28.7% of all
recruits recorded during the study (Fig. 2A). The frequency of flowering varied
between individuals. The majority of flowering plants flowered in each year for
which they were recorded above ground. However some individuals persisted as
non-flowering plants for up to 11 years between flowering events (Fig. 2B). The
proportion of the survivors of the 1975 cohort that flowered in any one year varied
considerably during the study (Fig. 3). It decreased from approximately 0.4 in 1975
to 0.1 7 in 1981, after which the proportion of surviving plants flowering increased,
so that in 1990 over 60% were recorded flowering.
Recruitment and population trends
Recruitment of individual plants which appeared not to be derived from vegetative
propagation was not sufficient to maintain the population during the study. Plant
losses far exceeded gains. The population in 1988, the last date for which a reliable
estimate of the number of plants in the below ground phase can be made, was
approximately half the size of the estimated 1975 population (Fig. 4). The original
population, and all cohorts, followed Deevey type I1 survivorship curves, implying
a constant risk of mortality. Cohort half-lives ranged from 7.8 years for the 1977
cohort to 2.2 years for the 1986 cohort. Approximately 15% of the 1975 population
survived until 1991 when they were at least 17 years old, and 18% of the 1976
cohort survived until 1991, when they were at least 16 years old. The progressive
decline in cohort sizes and half-lives suggests that conditions at the site have
S. WUTE AWD L. FARRELL
B
Irrym,,=,
0 1 2 3 4 5 6 1 8
Time to first recorded
flowering event
(years)
0 1 2 3 4 5 6 7 8 91011
Length of vegetative
period between flowering
events (years)
Figure 2. A, proportional distribution of the records for the length of time between a plant first being
recorded above ground and its first recorded flowering event, based on plants recruited between 1976
and 1986 (n= 108); 48.2% ofthese plants failed to flower during the study. B, proportional distribution
of the records of the length of time between successive flowering events, based on data for all flowering
plants and all flowering events.
deteriorated and are less favourable for the recruitment and/or survival of individuals
derived from seed (Table 1).
When the total numbers of rosettes in the population is considered, (i.e. no
attempt is made to distinguish between individual plants (possibly genets),vegetatively
derived rosettes (ramets) and plants whose original is uncertain) a different pattern
emerges (Fig. 5). Although the population decreased steadily in size from 1975 to
1986, this was followed by a rapid increase. In 1986, substantial numbers of singleleafed plants were recorded for the first time. These were all closely associated with
established plants. In 1987 they formed the largest single component of the population,
accounting for 56.6% of all plants. Although the increase in the total number of
rosettes present largely results from the recruitment of single-leafed individuals, it
also results partly from an increase in the number of rosettes produced by established
plants. The number of rosettes apparently associated with each of the surviving
plants first recorded in 1975 increased substantially from 1986 onwards (Fig. 6).
Transition probabilities
All transition probabilities were normally distributed and varied little during the
study. Between successive years the probability of either flowering or vegetative
plants entering the below ground phase was approximately 10%. This value did
not change significantly between the start and end of the study (Table 2). The risk
of mortality among flowering individuals (F-+D) was low (-3%). In contrast,
POPULATION BIOLOGY OF ORCHIS MIL!TARIS L.
115
0. 70 .-----------~------,
bll
·.:=
0.50
Q)
~
00
]
0.40
....-a
0
§ 0.30
t
8
il<
0.20
0.10
0. 00 L___l____L____L_L__j__L_J.._J_____L____L_L__j__L_L_j_____L_j_J
74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91
Year
Figure 3. Proportion of the survivors from the 19 75 cohorts for which plants flowered in each year
after their recruitment.
150.--------------------------------------------------.
100
00
·~
]
50
or---~~~~~~~~~~~~~~~.ahr~~~~~----------~
c:l
gj
-50
...s
-100
00
00
~
al -150
1-200
z
-250
_
79 80 81 82 83 84 85 86 87 88 89
-300L-_L_~-~~-_j______L
75
76
77
78
_j___L_L__L_L__L_~-~~-_j__~
90 91
Year
Figure 4. Cumulative changes in the population of plants after 1975. Net population change (.);
Cumulative losses from population (D); Cumulative number of recruits (Ill).
S. \ \ A l l E ,LUD L FARRELL
116
TABLE1. Cohort half-life values. Cohort size includes estimated numbrr of plants below ground.
Significance of fitted regression line *P<O.O5, **P<O.Ol, ***P<O.OOl
~~~~
Yrar of
~~
Half-life
cohort
cstahliahnieiit
~~
C:orrelation
coefficient of
fitted line ~ R I
6.+8
5.82
7.80
6.31
0.9705
0.933'.,
0.9798
0.9392
6.61
4.44
7.27
5.18
4.88
2.37
2.19
0.905
0.6708
0.9170
0.9813
0.9581
0.001 i
0.8283
3.81
0.9482
Significance
of fitted
linear
re<gression
n
(Initial size
of cohort)
***
***
***
**
350
44
16
8
0
***
10
13
1
*
***
***
***
9
9
-1
**
*
0
5
*
9
800-r
I
I
"
75 76 77 78 79 80 8 1 82 83 84 85 86 87 88 89 90 91
Year
Fi<gure5. Total number of status of rmettes present in the population 1975-91. Elowering rosettes
A):\.egetative rosettes
Estimated number of absent (dormant) individuals (0);
Single leafed
plants (+), total number of rosettes
i
(a);
(m).
POPULATION BIOLOGY OF ORCHIS MILLCARIS L.
117
-
5.0 -
5%
0
4.0 -
3.0
-
2.0
M
g
1.0
0.0
74 75 76 77 78 79808182 83 84858687 888990 9192
Year
Figure 6. The number of rosettes associated with surviving plants from the initial 1975 population.
TABLE
2. Mean (+SE) transition probabilities calculated before (1975-78) and following extensive
site management (1983-87). CV, percentage coefficient of variation; NS not significant, *P<0.05. F =
flowering plants, V =vegetative plants, A =plants absent from the above-ground population (i.e.
dormant), D =dead, FV =plants with two rosettes, one vegetative, one flowering
Transition
1975-78
Mean fSE
%CV
1983-87
Mean If: SE
%CV
t (df= 6)
V+V
V-F
V+A
69.05 f0.027
6.38+0.0073
9.29f0.0095
7.7
22.9
20.3
54.2 k 0.066
12.99 f0.024
10.94k 0.012
24.4
37.1
2 1.29
2.08 NS
2.63*
1.10 NS
F+F
F+V
F+A
39.6 f0.085
39.9f0.064
9.15k0.024
42.9
32.1
53.1
52.22 f0.059
19.0fO.Q56
9.53f0.017
22.6
59.0
35.3
1.22 NS
2.46*
0.13 NS
V+D
F-+D
10.90f0.018
2.75+0.027
32.2
199.6
13.20k0.022
17.9+0.0081
33.0
90.5
0.82 NS
0.33 NS
10.49+0.024
17.5f0.067
46.6
38.3
1.33 NS
2.76*
V+FV
F+FV
7.25f0.0019
8.44 0.0084
10.3
20.02
vegetative plants were substantially more likely to suffer mortality (V+D); approximately 10% were lost from the population each year. Between the start and
end of the study three transition probabilities changed significantly. The proportion
of flowering plants which survived as non-flowering the following year (F+V),
approximately halved from 40% to 20%. The proportion of vegetative plants
flowering the next year (V-tF) approximately doubled from around 6% to 13%,
and the probability of flowering plants flowering in successive years (F+F) also
increased, from around 40% to 52%, although this change was not significant (Table
2). The probability of flowering rosettes producing a second rosette (F+FV) increased
significantly. A similar, although not significant, increase also occurred in the
probability of vegetative rosettes giving rise to a second rosette (V-+FV). These
I la
S. IVAITE XYD L. FARRELL
changes all suggest that, for established plants at least, site conditions became more
favourable for growth and propagation.
DISCUSSION
Three main conclusions may be drawn from the results of this study: (i) the
beha\iour of individual plants is extremely variable, (ii) although the number of
indhidual plants has declined, total rosette numbers have increased substantially
during the study and (iii) the site has become more favourable for the persistence
and growth of established plants of 0. militaris during the study.
Although the majority of plants that enter the dormant phase do so for only one
)ear, some persist below ground for considerably longer (Fig. 1). O n average the
dormant fraction of the population accounted for 14% of all the plants present.
The length of apparent dormancy period. 1-8 years, and proportion of the population
in dormancy are similar to values recorded for other members of the Orchidaceae
(Lesica & Steele, 1994). For example, Tamm (1972) found that plants of 0. masculu
remain dormant for 1-12 years, and that the alrerage proportion of the population
dormant at any one time was 17Oh. For all species, the majority of plants recorded
in dormancy remained below ground for only one year (Lesica & Steele, 1994).
The frequency of flowering varied between individuals and from year to year.
IVhile the majority of flowering plants flowered each year, some plants persisted as
non-flowering plants for up to 11 years between flowering events (Fig. 2). The low
rate of mortaliq shown by flowering plants (Table 2), and the observation that
many plants flower in consecutive years suggests that reproduction does not impose
a heavy cost on flowering plants. There is no evidence of an increased postreproductive mortality risk. This may reflect the low levels of fruit set reported for
this species at this site. Reported levels of natural pollination of 0. militaris at the
site are low, ranging from 8.45 to 10.45% (Appleton, 1990; Hawke, 1989; Patmore
1988). An average value of 4.4% has been reported for fruit set (Farrell, 1985). The
occurrence of plants flowering in their first year above ground implies that during
the subterranean phase of seedling development, plants are able to accumulate
wfficient resources for flowering. The length of this phase is unknown. Farrell(l985)
reports values of 5, 4 and 3 years for the time between sowing seeds and the first
emergence of plants above ground. The shortest value reported, three years, was
for the appearance of flowering plants. The suggestion of Summerhayes (195 1) that
once seedlings emerged above ground they require a further 7-8 years of growth
before flowering is not supported by this study. It is possible that some plants that
appear to be dormant or which apparently flower in their first year above ground
have been missed during recording in previous years. This seems unlikely given the
carrful monitoring to which the population is subjected. It is also possible that a
small number of plants recorded as dormant may represent plants that emerge
above ground but lose their above ground parts before census (cf. Sanger & Waite,
this volume). Clearly, the behaviour of individual plants varies considerably. In
assessing the future development and management of the population it would be
extremely valuable to know how much of this variation may be attributed to
environmental heterogeneity, and how much to genetic factors.
Individuals of 0. militani can demonstrate considerable longevity with some
POPULATION BIOLOGY OF 0RCHZS.MZLITARZSL.
119
members of the initial 1975 population surviving for at least 17 years. If allowance
is made for the pre-emergence phase, these plants must be at least 20 years old.
The range of cohort half-lives (2.19-7.80 years, Table 1) is similar to the range of
values reported for other terrestrial orchid species (Wells, 1981). Cohort half-lives
reported in this study have taken account of the fate of dormant cohort members.
Since dormant plants have the highest probability of mortality, longer half-life
periods (greater than 10 years) can be obtained if calculations are based on the
performance of above-ground plants only. Survivors of the 1975 population accounted for approximately 60% of the ‘individual’ plants present in 1991. If
conditions remain the same, the population continues to decline in numbers at the
same rate recorded during the study, the half-life of 6.48 years implies that within
13 years only some 15 plants of this cohort will remain alive. The decline in halflives of consecutive cohorts recorded during the study (Table 1) may represent cause
for concern over the long-term viability of the population.
In contrast to the number of individual plants, total rosette numbers have increased
dramatically from 1986 onwards. This increase, although partially the product of
increased rosette production by established individual plants predominantly results
from the recruitment of single-leafed plants (Fig. 5). It is not possible to say whether
these single leafed plants are of sexual or vegetative (asexual)origin. The failure of
seed sown in 1979 to produce above ground plants, and the clustering of new plants
close to old established individuals all suggests that they are the result of vegetative
propagation. If so, and the number of individuals continues to decline, then the
genetic diversity of the population will decrease with the remaining population
consisting of fewer but larger individuals. Alternatively, these new rosettes could be
the product of sexual reproduction. Farrell(l985) reports that the number of flowers
per flower spike ranges from 7 to 42 and that each flower is likely to contain around
6200 seeds. Thus, even allowing for poor seed set, the total number of seed released
into the site each year might be substantial. The clustering of recruits around old
established plants may simply reflect predominantly local seed dispersal and variations
in habitat suitabilityfor seedling establishment and development, and the distribution
of the mycorrhizal fungi, which in Orchis milataris is known to continue to infect adult
plants (Farrell, 1985).
The prolonged period of population monitoring has allowed the effectiveness of
the site management to be assessed. Indications of increased plant vigour (e.g. Figs
3, 5, 6, Table 2) are most pronounced following management intervention during
the early 1980s. The beneficial effects of site management are reflected in the
differences between transition probabilities at the start of the study and after
substantial management intervention (Table 2). Although site management has
clearly been successful, it is also possible that populations of these species are prone
to natural cycles of ‘boom and bust’, with the Suffolk population currently entering
a boom period. Such population behaviour may result from the intrinsic characteristics of the species, for example, longevity combined with high reproductive
capacity, both asexual and sexual. Time lags or delays will also tend to destabilize
population size (Begon, Mortimer & Thompson, 1996). In the case of orchids, the
situation is complicated by the possibilitythat the fungal pattern may also demonstrate
cyclic or chaotic behaviour, in which case a stable population of 0. militak may
not be an achievable management goal even if favourable site conditions are
maintained.
The present high local density of rosettes around established individuals requires
!:20
S. WAITE
.~"iD
L. FARRELL
an alternative approach to coordinate mapping to be developed. One approach
that has been partly adopted at the site, is to map the position of clumps and record
the number and status of plants within a clump. However, in doing this much
valuable information is lost, not least the ability to follow individuals into and out
of the below-ground phase. The presence of this component of the population can
have important consequences for the survival and dynamics of a population. Waite
(1989) has shown how the size of, and recruitment from, the dormant phase can
affect the dynamics of a population, acting as a reservoir of potential recruits and
aiding the persistence of a population.
ACK.'\0\ H.EDGE:\lE::'-iTS
\Ve would like to thank English Nature and members of the Suffolk Wildlife
Trust for their help over the years with field recording, Terry Wells for his advice
on recording methods and Suffolk \Vildlife Trust voluntary wardens and the Forestry
Commission for help with site maintenance. \Ve also wish to thank Edgar MilneRedhead and John Trist for their early encouragement and continued interest in
the conservation of this species.
REFEREl'\CES
Appleton T. 1990. Report rifthe orchid warden for the military orchid site, Rex Graham, Mildenhall. Unpublished
Report, Ipswich, Suffolk Wildlife Trust.
Begon M, Mortimer M. Thompson DJ. 1996. Population ecology. A unified stuqy rif animals and plants,
3rd Edn. Oxford: Blackwell Science Ltd.
Caswell H. 1989. }vfatrix population models. Construction, anab•sis and interpretation. Sunderland, Massachusetts: Sinauer Associates.
Farrell L. 1985. Biological Flora of the British Isles: Orchis militaris L. Journal rif Ecology 73: I 041-1053.
Farrell L. 1991. Population changes and management of Orchis militaris at two sites in England. In:
Wells TCE, Willems JN, eds. Population ecology rif terrestrial orchids. The Hague: SPB Academic
Publishing, 63-68.
GroenendaelJM van, Kroon H de, Caswell H. 1988. Projection matrices in population biology.
Trends in Ecology and Emlution 3: 264-26.
Harper JL. 1977. Population biologr rif plants. London: Academic Press.
Hawke C. 1989. Rex Graham Resnve. Orrhid wardens report 1989. Unpublished report, Ipswich, Suffolk
Wildlife Trust.
Lesica P, Steele BM. 1994. Prolonged dormancy in vascular plants and implications for monitoring
studies. Natural Areas ]ouma/14: 209-212.
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