Botanical Journal of the Linnean Society, 73: 217-239. With 1 3 figures
July/September/October 1 9 7 6
The ecological status of bracken
A. S. WATT
Botany School, University of Cambridge
Information about the morphology of bracken (Pteridium aquilinum (L.) Kuhn var. aquilinum)
and the chief nutrients in the frond at different.times of the year introduce an account of litter
production and its accumulation in relation to the behaviour of frond, root and rhizome
systems.
Where litter gain exceeds loss there is a correlation between the thickness and/or kind of
litter and the level of the root and rhizome systems in relation to t h e mineral soil surface: with
increase of litter the bracken becomes progressively more dependent for its physical and
chemical soil environment on its own dkbris and less o n the underlying mineral soil. An
example of the limit of complete dependence has not been examined, but degeneration of the
community can take place before that stage is reached.
From a review of the chief factors affecting bracken the conclusion is reached that the
woodland habitat is both favourable and restrictive: in i t bracken is in equilibrium with its
environment, at a high social status. The relationship with other plant communities depends
largely on the degree of human interference t o which each is subject. Dominant bracken when
left alone, and where gain of litter exceeds loss, becomes the victim of its own success; local
degeneration opens t h e way for entry by other species.
CONTENTS
. . . . . . . . . . . . . . . . . . . . . . .
Introduction
Morphology and major elements in the frond
. . . . . . . . . . .
Litter and correlated changesin frond, rhizome and root
. . . . . .
Brief review of chief factors affecting bracken
. . . . . . . . .
Relation to other communities
. . . . . . . . . . . . .
Conclusions
. . . . . . . . . . . . . . . . . . . .
Acknowledgements
. . . . . . . . . . . . . . . . . . .
References
. . . . . . . . . . . . . . . . . . . .
. .
. . .
. . .
. . .
. . .
. .
. .
. .
21 7
217
221
229
232
237
238
238
INTRODUCTION
It is generally accepted by ecologists that bracken is primarily a woodland
plant and that its liberation on the destruction of forest has enabled it t o
spread. But, as far as I am aware its precise status in woodlands has never been
critically examined nor its relationship to those communities which replaced
woodland.
MORPHOLOGY AND MAJOR ELEMENTS IN THE FROND
The young sporophyte, established from a prothallus, spreads outwards
eventually to form a community in which a marginal zone of greater vigour
surrounds a central core of less. At the same time there are changes in
age-structure and dispersion : a developing even-aged community matures,
217
218
A. S. WATT
then becomes uneven-aged and patchy. The change may be likened t o the
development of an even-aged plantation of trees which, on maturity and the
death of one tree and then another, forms gaps which on regeneration change
the population in mean height, age-composition and dispersion.
Apart from the special problems which are the concern of the professional
morphologist, the form of the rhizome is basically simple: an axis, growing at
one end and dying at the other carries branches alternately right and left. These
are conveniently classified into two groups, short shoots which bear fronds
apparently directly on them and long shoots which do not. (As there are
intermediates and one kind of shoot may grow into the other, the validity of
the distinction, though convenient, may be questioned.) Generally too they
differ in behaviour: long shoots may grow in any direction in the soil, short
shoots generally grow upwards, and this pattern of behaviour is maintained
when one kind of shoot grows into another: short shoots passing into long
grow downwards, long shoots passing into short grow upwards.
At the base of the frond there is a bud (the basal bud), actual or potential,
which on Velenovsky’s (1905) hypothesis is the end of the suppressed
dichotomous arm. It commonly remains dormant and viable for an undetermined number of years but occasionally produces lateral branches under some
conditions including dying back of the main short shoot.
The frond makes its first visible appearance on the extension of the short
shoot after the emergence of the current frond. It grows slowly through the
summer and, when temperature permits, through the winter to emerge in
spring from April onwards. After emergence it grows fast and may reach its full
height in about two months, and about thirteen months after its first visible
appearance. The actual date of emergence and rate of growth vary with
temperature, and winter frosts may be lethal, spring frosts lethal or crippling.
Hence there is variation from year to year and, in uneven terrain, from place to
place. Also the duration of its photosynthetic activity may be shortened by the
incidence of frost and drought in spring and early summer and again in late
summer and autumn, and at all times by high evaporation rates due to intense
sunlight and/or exposure to wind (Bright, 1928; Tinklin & Bowling, 1969).
The young sporophyte produces more than one frond per annum
(Hofmeister, 1862; Braid & Conway, 1943; Schwabe, 1951), but in the mature
community, although there are exceptions to the generalization of one frond
per annum, the number of nodes on the short shoot may be used as an estimate
of relative age. Also since there is an inverse relationship between age and depth
of origin, the latter has been used (within certain limits which, as will emerge
later, are narrower than originally supposed) as an estimate of age.
Between fronds from young shoots and deep in origin, and fronds from old
shoots and shallow (called in this context ‘young’ and ‘old’ fronds respectively)
there are important behavioural and physical differences. ‘Young’ fronds grow
faster, emerge earlier, are taller and more robust. Figure 1 shows contrasting
situations where invasion is free, that is with little competition from heavily
grazed grassland: (a) in the invading front there is a change distally primarily
correlated with position on the plant and (b) beyond that, in the mature phase
where canopy is complete, there is a fall in height with age. In this mature stage
the differences are important in controlling the age composition as well as
affecting the kind of litter formed and very probably its rate of decay.
ECOLOGICAL STATUS OF BRACKEN
219
Contrasting situations are described but to these there are exceptions and
intermediates.
Where the first emerging fronds form a canopy and remain intact, the fate of
the older differentiated fronds, emerged or unemerged, depends on the weather.
If dry, the further growth of the frond is inhibited: they die and shrivel up. If
wet, they grow up under canopy and most become chlorotic and die. If this
situation is maintained for a number of years the older short shoots are starved
and die relatively young. Where, on the other hand, the fronds from younger
shoots are regularly killed (e .g. by spring frost) the differentiated fronds from
the older shoots are freed from inhibition and form the canopy, the short
shoots bearing them living to an old age.
Mean dimensions of fronds
in 10 ft Seclions from invoding margin
Mem dimensions of fronds in soil depth (cm
calagones ot 40-00 f l from invoding margin
Figure I . Transect across the marginal belt in bracken invading grazed grassland, Lakenheath
Warren. Mean dimensions of frond in (A, 10 f t ( 3 m) sections from 0-50 f t (0-15 m), and (B) in
depth of origin categories of the frond from 40-8- f t (12-24 rn).
The physical differences between fronds, whether related to position on the
plant or to age of shoot, are important in so far as they affect the thickness of
the litter formed and its probable rate of decay. The robust, sturdy, tall frond
breaks under pressure of wind, heavy rain or snow at a higher level forming a
canopy whose undulations reflect both internal differences in community
structure and external pressures, but in an atmosphere less favourable to decay.
The weaker fronds from older shoots break at a lower level and sooner enter an
atmosphere nearer the ground where conditions for decay are more favourable.
This difference is primarily applicable to the rachis framework: the laminar
portions become detached and fall off.
Between the two extremes of a young robust vigorous population of fronds
220
A. S. WATT
and one of old and weak, there are of course mixtures in various proportions
and the proportion of young to old and the way they are mixed will influence
the properties of the litter formed. Also the possibility of a differential reaction
of fronds of different ‘ages’ to herbicides cannot be ruled out and ‘age’ is
possibly a more informative criterion of results than number alone.
There are no data on the chemical composition of fronds from young shoots
as against fronds from old. But, at this point, we may remind ourselves of some
of the chemical changes in the frond and rhizome during the vegetative season.
From Huntei‘s (195 3) data showing the complementary relationship between
percentage dry matter in the rhizome and the actual dry weight of 100 fronds
(Fig. 2) the interpretation is generally accepted that the young developing
Figure 2. Weight (kg) of dry matter in 100 fronds from May t o October ( x ) and percentage dry
matter in rhizome during the year ( 0 ) . Based on data in Hunter (1953).
frond depends exclusively on the reserves in the rhizome until about the
unfolding of the second pair of pinnae: from then onwards the contribution
from the rhizome decreases, that from current photosynthesis increases,
completely replacing that from the rhizome when the frond has developed to
4-$ of its full height: current photosynthesis then completes the development of
the frond and begins restocking the reserves augmented later by withdrawal of
carbohydrates and nutrients during senescence.
Figure 3 shows the change in actual weight per 100 fronds of N , P, K and Ca
during the growing season, the parallelism between N , P and K (but particularly
between N and K) and the contrast between the high mid-summer values and
the greatly diminished values at the end of October. Thus fronds cut in summer
are richer in nutrients (including soluble carbohydrates) than those at death or
later and are thus more acceptable to organisms of decay. According to Berry
(1917) litter as a whole contains only about 2.5-3% crude protein, 0.83-1.3% K
and 0.065% P. As between ‘leaf’ and ‘stem’ in November, the ‘leaf’ contains
more N, Ca and P while the ‘stem’ has more K (at least later in the season)
ECOLOGICAL STATUS OF BRACKEN
22 1
(Ferguson & Armitage, 1944),most of which is water soluble and washed out
by the end of winter. Thus litter as a whole is a poor source of nutrients.
It may be added that the uniformity of chemical composition of the young
fronds from a variety of soil habitats contrasts with the diversity in older
fronds, a diversity which is related to differences in soil content (Berry,
Robinson & Russell, 1918). But once elements arrive in the plant, with its vast
reserves, it may take some time before the plant begins to react to a diminution
of the original source of supply.
Data from a series of soils in Breckland show that to each frond the length of
rhizome varied between 2.02 m and 3.4m and the dry weight of rhizome in
winter between 34.1g and 54.4g (Watt, 1964).
I
I-
M Y
'
Jun.
'
Jul.
'
Figure 3. Change in weight (g) of N (o), PXlO (x), K
vegetative season. Based on data in Hunter (1953).
d
I
I
Aup.
( 0 ) and
I
scp.
'
at.
-I
Ca (0)per 100 fronds, during the
LITTER AND CORRELATED CHANGES IN FROND, RHIZOME AND ROOT
These physical and chemical differences relate primarily to the individual
frond but the kind and quantity of litter formed on an areal basis depend on
the kind and quantity of fronds produced as well as on the conditions of the
habitat and their influence on the rate of decay.
The amount of litter, the balance between rate of gain and rate of loss, varies
much. Production in terms of dry weight of fronds in summer varies from the
negligible to communities in which bracken is virtually pure with 8000 to
14,000 kg/ha/ann (Pearsall & Gorham, 1956). In places exposed t o wind some
or all of the litter may be blown away and therefore exercises no influence on
bracken. Where it remains in place the rate of decay also varies from a loss
approximately equal to the annual production e.g. on the limestone of Hutton
Roof Crag (C. D. Pigott, pers. comm.), in birchwood in Skye on soil over
limestone (J. Birks, pers. comm.) and probably in most woods dominated by
broad-leaved trees. But in the open there are places in East Anglia, the Midlands
222
A. S. WATT
and in the west (Beeston Castle, Cheshire) where litter accumulates up to a
maximum of 34 in (86 cm). Between these extremes there are all degrees of
thickness of litter, much of it influenced by the direct or indirect interference
by man, now and in the past, by grazing, trampling, burning and cutting of the
live or dead frond.
During decay the litter layer (L s.s.) changes to the F layer (forna), the
partly decomposed material. In my own experience and in the literature (e.g. in
the Memoirs of the Soil Survey) there are many examples of an abrupt
transition from the F to the Ah soil horizon,.a result indicating the absence of
an active and effective agent mixing the organic material with the mineral soil
below (e.g. earthworms do not eat bracken and are rarely recorded below it).
There are records of a humifaction (H) layer but it is not clear whether this is a
later stage in the decomposition of bracken or a survival from the community
which bracken has replaced. Special studies of the effect, either physical or
chemical, of bracken on the soil do not appear to have been undertaken.
In the following account data are presented to show the correlation between
the accumulation of litter on the one hand, the vigour of the frond and the
position of the rhizome and root in relation to the mineral soil surface on the
other. The assumption of a causal connection is then the basis for the
formulation of the hypothesis of senescence-that, in short, bracken may be the
victim of its own success.
Lakenheath Warren
Data have already been presented (Watt, 1969, 1970, 1971a) to explain the
rise and fall in the height of the frond across the marginal belt on the acid (pH
3.6-4.0),
sandy (> 90% coarse and fine sand), nutrient-poor (exchangeable bases
in m.e./100 g of Ca 0.51, Mg 0.07, K 0.08, Na 0.04-total 0.70 in 0-5 cm
surface mineral soil), humus podsol with the surface of the B horizon at
16-20 in (40-50 cm) in an area on Lakenheath Wai-ren much liable to frost. The
change involved both the power of accumulating litter to protect underground
growing points as well as inducing the growth of the short shoot apices towards
the soil surface where, unless the rate of gain of litter is maintained against rate
of loss, the growing points become vulnerable to frost and drought. These
factors, it was decided, intensify degeneration but do not necessarily initiate it.
The rhizome system
In the present context the salient points in the change across the marginal
belt are depicted in Fig. 4,where bracken invades intensively grazed grass-heath
and passes through phases in linear sequence called pioneer, building, mature
and degenerate in which there is a change in the amount of continuity of the
litter, its differentiation into litter (L s.s.) and forna (F) layers, followed by the
loss of amount and continuity of L and the invasion of the exposed F by
grasses-the beginning of the return to grass-heath, until it is again invaded by
bracken.
There are correlated changes in the underground parts. The apices of the
long and the s h ~ r tshoots change in position in relation to the mineral soil
surface, both being more vulnerable to winter frost and drought in the pioneer
and degenerate phases than in the building and the mature. Also there is a
ECOLOGICAL STATUS OF BRACKEN
22 3
progressive rise in the level of the younger long shoots, as seen in Fig. 6A, of
which the data are based on the mature phase of bracken in the hinterland. The
rise in the general level of the rhizome system is accentuated by the death of
the rhizome at the lower levels, as seen in Fig. 12B from relict bracken on the
same podsol on Lakenheath Warren. Further detailed evidence of the rise in
level of rhizome in degenerate bracken is presented (Watt, 1971a) from
Blaxhall Heath, where Garrett-Jones (1969) first drew attention to the
phenomenon of retrogression.
20
-
-g
f
a
8
Grassheath and
pioneer phase
Building phase
Mature phase
Degenerate phase
Grassheath and
pioneer phrase
10
o
5
10
15
20
25
30
Figure 4. Diagrammatic representation of the changes in the litter, and in the behaviour of the
long and short shoots across the marginal belt on the podsol on Lakenheath Warren. Solid, live:
hollow. dead.
The root system
The rhizome invading the intensively grazed grass-heath lies at from 7-14 in
( 1 8 - 3 4 cm) from the soil surface. The roots (approximately one root per cm of
rhizome length) radiate in all directions (Fig. 5A) with about 3 from below the
position of the lateral lines and f above. The descending roots are fat (up t o
3 mm diameter), tapering slightly below (with short, stumpy lateral branches),
and reach but do not penetrate the B horizon, extending into pockets of the A
horizon to a length of 33 in (83 cm). Ascending and lateral roots begin fairly
fat but taper quickly, becoming thread-like (1 mm diameter) and branched,
attaining a length of 5 2 cm, the apices reaching to within an average of 8 cm of
the soil surface but varying from 2 to 13 cm. Thus the root system is
widespread but diffuse with an estimated length at its most concentrated near
the rhizome of less than 1 0 cm per 70 cm’ of soil.
As the short shoot grows towards the soil surface under cover of the
continuous litter, its upper part carries roots at from 4 to 5 per cm of length. A
few descend, some grow horizontally, but most grow upwards branching freely,
permeating the F(-H) layer and making it compact and tough. Fig. 5B shows
the steep gradient in length of root from bracken in the mature phase (Nos 1 ,
2, 3) on the podsol normal to the area; Nos 4 and 5 are from a profile with a
layer of blown sand over a buried A horizon. The short length of root in the
thin (1.3 cm: graph 4) F layer contrasts with the great length in the compact F
layer twice that thickness (graph 5). There are no adventitious roots and i t
should be stated that little is known about the duration of the functional life of
the roots.
With development of the bracken community there is thus a general rise in
level of both root and rhizome, the bracken plant becoming more and more
A. S. WATT
224
Short grazed grass
Soil,
horizon
50
cm
A
Length [cm) of roots per %cm3
100
I
n
~
p"
3
200
300
-_
5
400
500
600
,5
5olv
Surface of B horizon for I and 3
tI
'Ot
Surface of B horizon for 2
I
Buried A bixn at 76cm for
4 a n d Wcmfoc5
Figure 5 . The vertical distribution of roots (A) from the long shoot invading intensively grazed
grass heath, and (B) the length of roots over the B horizon at 40-50 ern (graphs 1 , 2, 3) and in
blown sand over a buried A horizon (nos 4 and 5 ) .
dependent on the upper soil layers and on its own dkbris and subject to the
hazards of such dependence. Part of the significance of this became evident
when during the dry summers of 1972 and 1973 the surface roots shrivelled;
also on 25/8/1935, when from 24/6 to 25/8 there was only 0.90 in (2.3 cm) of
rain and none between 11/7 and 8/8, the invading bracken t o a width of 2 m
was green, the rest behind was yellow.
In this transect the whole of the rhizome system lies in the mineral soil.
N o other area has been investigated in detail but data from a few other areas
form a series showing a progressive rise in mean level of the rhizome with the
accumulation of litter, not necessarily of bracken.
ECOLOGICAL STATUS OF BRACKEN
225
W eeting Heath
On the deep brown earth (sol lessivt) on the plateau of this heath whose
surface soil has a pH of 4.3, exchangeable bases in m.e./100g of Ca 1.2,
Mg 0.13, K 0.06, Na 0.04 (total 1.43), the bracken in the marginal belt is
vigorous (6ft, 1.8 m), with 3 3 fronds per 10 ft2 (0.93 m 2 ) in 1972. The L layer
varies in depth from 6-18 in ( 1 5 4 5 cm); the F, loose and open textured from
2-7 in (5-18 cm) contains abundant roots (402 cm per 70 cm3). The long
shoots carry 1.9 roots per cm length, the short shoots 6-7.
On the podsol on Lakenheath Warren the addition of fronds followed by
their decay and consolidation induced the short shoots to rise further towards
the mineral soil surface and several to emerge above it and travel along it (Watt,
1970). This happens naturally in the F layer on Weeting Heath.
Depth (in) of origin of short shoot
Figure 6. Relation between depth of origin of short shoot and number of nodes on the short
shoot in (A) the mature phase in the hinterland of Lakenheath Warren (area E) and (B)at 10 m
from the outpost fronds in the marginal belt o n Weeting Heath.
Again in the marginal belt at 35 ft (10.7 m) from an invading front
artificially checked by cutting, there is a significant, positive correlation
between the depth of the subtending long shoot and the number of nodes on
the short shoot (Fig. 6B) i.e. the older short shoots have a deeper origin. The
wide scatter of the points (as for the mature phase in the hinterland on
Lakenheath Warren) indicates that the general trend is subject to fluctuating
influences whether operating directly on the growing point or indirectly
through the plant. Similarly there are factors which cause the short shoot to
change direction (Fig. 7).
During the war the area was overrun by tanks and since the war the bracken
has been swiPed. It is now patchy with areas devoid of bracken covered by
grass-heath, but mainly by Chamaenerion angustifoliurn and mixtures of it and
short bracken. Areas with no live bracken have dead rotting rhizomes in the
A. S. WATT
226
Figure 7. Examples of short shoots changing direction of growth from the same year (1968).
soil. How far this patchiness is inherent or how far induced by the treatment is
now impossible to decide, but it is suggested that the present patchiness is in
part at least a reflection of inherent variation in the community organization.
West Tofts Plan tation
Pinus silvestris L. was planted in 1926 in dense vigorous bracken on deep
sandy podsolized soil which at some time in the past has been cultivated: the
surface 0-3 cm has a pH of 4.2 which increases with depth and the
exchangeable cations in m.e./100 g of Ca 0.68, Mg 0.07, K 0.03, Na 0.01 (total
only 0.79). There is an undergrowth of tall (7 ft: 2.1 m), sparse fronds (6.2 per
10 ft2 (0.93 m2 )) of shade form. A sparse upper L of bracken has below it a
lower L (3-8 cm thick) of pine needles, decaying branches, cones and some
bracken, and an F (0.3 cm) layer of partly decomposed brown loose remains.
10
5
0
61 cm
0 0 0
0
0 0 0
301
1
I
35
i
0
0
Figure 8. Profile in West Tofts plantation showing vertical distribution of live (e) and dead ( 0 )
rhizomes.
ECOLOGICAL STATUS OF BRACKEN
227
In the 2 x 2 f t (61 x 61 cm) pit dug down below the limit of dead rhizomes
the following features are noted. The total length of live rhizome is 55.5 f t
(17 m) of which 87% consisted of long shoots: the rest consists of bits but
mainly of short shoots (11.6%).Of the total length of rhizome 63%lies in the
litter and 37% only in the mineral soil below which most of it is superficial
(Fig. 8). Dead rhizomes are present throughout the profile (maximum depth in
the pit 15 in (38 cm)) and at deeper levels than the live. These dead bits in the
soil have a mean diameter of 1.22 cm compared with 0.85 cm for the dead bits
in the litter, the same as for the live rhizomes. The distribution of shoots with
1, 2, 3 etc nodes shows that of the 35 short shoots in the litter, 65.7%had only
one node, 14.3%had 3 , 5.7% had 7 (the maximum) and the rest had 5.7%or
less. Figure 9 shows the plan of the rhizomes in the litter.
24 in
24 in
Figure 9. West Tofts plantation. Plan of rhizomes in the litter, shown in two parts to avoid
overcrowding.
Charnwood Forest-Timberwood Hill
Originally forested, this area may have been grazed and at some time burned
(a small piece of charred materia1 found): it could never have been cultivated
because of the boulders. I t is now dominated by bracken 5.5 f t (1.7 m) tall
with 22.5 fronds per 10 ft2 (0.93 m2). The L layer has a mean thickness of
17 in (43 cm), and F of 14 in (36 cm) plus a possible H layer of 2 in (5 cm)
brown, powdery, open and friable but still containing incompletely
decomposed bracken.
In the pit, 2 x 2 ft, dug down to about 12 in (30 cm) some small part of the
rhizome may have been left descending below the large boulders. But of the
total length of live rhizome (108 ft, 3 3 m) 74.2% consisted of long shoots,
18.8%of short and 7.0% of unclassified bits. And of the total length 72%was
present in the F(-H) layer and only 28% in the mineral soil below.
That there is a rise in the general level of the younger rhizomes is indicated
by the much higher percentage of live rhizomes in the upper F (65%) as
compared with the lower F (only 19%): also by the maximum number of nodes
228
A. S. WATT
on the live and dead shoots respectively: in the upper and lower F respectively
there are, OR the live rhizomes 7 and 4, but on the dead 11 and 21 nodes.
On the distribution of shoots according to the number of nodes there are no
exact counts, but a near estimate, obtained from the distribution in 1 in length
categories, shows the rapid fall off in number with length (Table 1).
Table 1. Distribution of short shoots of different lengths in the forna
(F) layers in Charnwood Forest
0-1
Length (in) of live short shoot
1-2 2-3
3-4 4-5
5-6
~
6-7
Number of
liveshoots
~~
Percentage in upper F
Percentage in lower F
37.7 29.5 18.1
6.6
1.6
30.0 20.0 20.0 10.0 10,O
0.0
4.9
10.0
61
30
Percentageinupper andlower
35.2 26.4
1.0
6.6
91
18.7
7.7
4.4
1.6
Thus a high proportion of shoots produced either a small number of fronds or
none at all, that is, remained dormant. While the large number of shoot apices
(i.e. potential fronds) compared with the current number of emerged fronds is
a general feature of bracken, the ratio of 91 to 9 is high.
In the two samples collected for the measurement of root length, one from
the F(H) layer immediately above the mineral soil and the other in the top
7.5 cm of mineral soil, there is a sharp drop from 12.4 m to 4.2 m per
70 cm3 -the latter figure corresponding closely to the highest numbers from
Lakenheath Warren and Weeting Heath.
Discussion
Among the many issues raised by these observations, the major one for the
moment is that where the production of litter exceeds its rate of decay, i.e.
there is progressive accumulation, there is a rise in the general level of root and
rhizome so that the bracken becomes progressively more dependent on the
upper layers of soil and on its own dkbris-an effect brought about not only by
a rise in the level of the younger parts but also by the death from old age or
from unfavourable conditions which prevent re-entry of the younger rhizomes.
I t is not merely a question of the thickness of the litter (the F layer on
Lakenheath Warren is no more than 5 cm at most) but probably of its kind as
well and their effect on the physical and chemical conditions of the habitat.
There is also the progressive removal of the influence of the mineral soil below.
So far no example has been analysed where the whole of the root and
rhizome systems are restricted to bracken’s own dtbris. And this is perhaps not
surprising in view of the history of the past utilization of bracken and the
cutting, burning and trampling to which it has been subject. Nevertheless
examples of degeneration do occur and there are places where no live bracken
is present with evidence of its past existence in rhizome remains in the soil. In
the dry, acid, sandy soil in Breckland there is proof of copious remains 34 years
ECOLOGICAL STATUS OF BRACKEN
229
after death, and the persistent outer ‘skin’ of the rhizome may last much
longer.
Obviously in the extreme case of restriction to its own dtbris, bracken would
be called upon to grow on a wasting capital because some part of the nutrients,
returned in the litter are washed out by the end of winter; and this situation
may arise even before the extreme position is reached, especially in a nutrientpoor soil. The question of well-being and survival is then one of the rate of
production of litter and of its rate of decay and of factors affecting each. Also,
of course, the factors initiating retrogression may not be the same as those
affecting its progress later. The chief factors affecting the welfare of bracken
are now passed briefly in review.
BRIEF REVIEW OF CHIEF FACTORS AFFECTING BRACKEN
The greater abundance of bracken in the west compared with the east of the
country is probably as much a question of opportunity and of economics as of
suitability for the adult sporophyte, and the behaviour of bracken is probably
more closely linked with local variation in factors than with longitude as such.
For obvious reasons most experimental work on bracken has been done with
the young sporophyte (which is mesomorphic) under conditions of freedom to
expand and where the new fronds are not subject to the same controls and in
which the proportion of storage to photosynthesis is progressively increasing:
in short with a stage in life history when the relative significance of the various
factors and their amount may differ from those of the adult (Schwabe, 1953).
In the absence of dry weights per unit area in habitats where the influencing
factors are measured and their relative significance assessed, this account must
be largely qualitative.
Tempera ture
While the heat requirements of bracken for growth are moderate, its growing
points of frond and rhizome are sensitive to frost: severe winter frost kills
rhizome apices and apices of differentiated fronds before emergence; and spring
frosts kill or cripple the emerged frond while autumn frosts kill late emerging
fronds and shorten the life of the adult. Hence the duration of the growing
season may be shortened.
Bracken’s latitudinal limit in the Island of Foula (60” 8”) is set by factors
which include winter frost as a component (Messenger & Urquhart, 1959). So
too with its altitudinal limit in the Cairngorms. Both these areas are very
exposed to wind (Birse & Robertson, 1970), which adversely affects the
bracken both mechanicalIy and physiologically. Also in the Cairngorms the
duration of the growing season may be significant: at about 2000 f t (610 m)
the accumulated temperature over 5.6”C is only about 622 day C compared
with about 900 for Foula (Birse & Dry, 1970) and 1300 in the south of
England. Within these limits the effect of frost on the frond will vary with its
dates of emergence in relation to its incidence. Generally woodlands provide
natural protection.
16
2 30
A. S . WATT
Moisture
Nowhere in the country is the rainfall too low for bracken but its seasonal
incidence in low rainfall areas is important (Watt, 1964). There are no data on
loss of water per unit area of frond or soil surface, but the observations of
Bright (1928), confirmed and amplified by Tinklin & Bowling (1969), that the
stoma responds quickly at the higher evaporation rates, were extended by them
to show that at the higher rates not only are stomata closed but cuticular loss
from the saturated frond is nil or negligible. On the basis of Bright’s
observations this may be assumed t o follow either from exposure to wind or to
intense sunlight. Thus while bracken may survive in exposed places it is at the
expense of photosynthesis: not only is litter blown away but the fronds are
undernourished, few and short. Woods, hedges, hollows in uneven terrain etc.,
provide shelter. All the tallest fronds measured in the British Isles are in bushes
or woods where they are sheltered and supported mechanically by woody
branches dead or alive; 4.3 m (Braid, 1937), 4.2 m (Felshamhall Wood), 3.8 m
(Kenmare, S.W. Ireland), 3.8 m (West Tofts, Breckland), 3.5 m (Staverton
Thicks), 3.3 m (Cavenham Heath, Farrow, 1917).
Similarly bracken responds favourably to the provision of an adequate
supply of water, whether on soils with a high water table or in deep soils where
oxygen is not limiting: on shallow and dry soils fronds are few and short (Watt,
1964).
Soil aeration
The sensitiveness of bracken to the lack of oxygen is well established by the
experimental evidence of Poel (1961) and from field observations checked by
the measurement of oxygen diffusion rates (Poel, 1951, 1960; Watt, 1964).
Hence the absence of bracken from bogs, marsh, clay and waterlogged soils
may be readily explained on this basis. On drained peat bracken may dominate
(Berry, 1917; Avery, 1955). Similarly the vertical distribution of the rhizome
may be limited by the physiological barrier of oxygen deficiency, obviously in
soils with a high stagnant water table and possibly by the accumulation of litter
and the formation of a compact humus layer.
On the other hand Anderson (1961) attributes clumping of fronds to local
aeration through the hollow bases of dead fronds. The aggregation of roots in
hollow drainage channels formed of decayed rhizomes may be attributed to
improved aeration and/or water supply. Oxygen and/or water supply may have
a directional effect in the upward growth of roots from short shoots.
pH and nutrients
Although bracken is found mainly on acid, nutrient-poor soil it is not, as the
data from the literature (including many from the Memoirs of the Soil Survey)
in Fig. 10 show, confined to such soil (tor pH see also Salisbury, 1925). And its
absence or scarcity on nutrient-rich soil in present circumstances is primarily a
matter of history and of economics. I t is true that it is rare on calcareous soil
and chlorosis is common but the presence of calcium in larger or smaller
amounts is not regarded as the primary cause: it is not a calcifuge in the narrow
strict sense (Conway & Stephens, 1957).
ECOLOGICAL STATUS OF BRACKEN
231
According to Schwabe (1953) N, P and K are important nutrients for the
growth of the young sporophyte, and Conway & Stephens (1957) have shown a
favourable response of the young sporophyte in compost with plentiful
supplies of K and of N (particularly in the form of ammonium salts) and a less
but still favourable reaction to Ca and toP. Compared with the control plants,
which had low supplies of inorganic mineral nutrients, the initiation of foliar
organs at the apex of the stem was found to be one per apex: in treated plots
with plentiful supplies of N, P, K and Ca the number of organs initiated per
apex was markedly increased.
10
~
5
... .....
.
.. . .. . .
................. ..... ......
..................
.. ....... .
4
3
7
6
5
. ... .
9
8
pH
5
IJ)
cu
0
..
.::.
.....:.:.::.
......:........
2
3
Q.
E
5l
0
7 8 9 10 II 12
4
6
5
Co: exchangeable cations m.s/100 g
... . ..
5
d
z
0
2
4
4
6
8 10 12 14 16 18 20 22 24
K. exchangeable cations m.e/100 gxiO
t:. ............. •
0
..
8 10 12 14 16 18 20 22 24
6
Mg: exchangeable cations m.e/IOOg x 10
... . .
5
0
- . ..
13
•••
.. .
2
3
I
I
I
4
5
• ...
!
6 ..12.5
P2 0 5 acetic acid soluble mg /100 g
Figure 10, Chemical data (from various sources including many from Memoirs of the Soil
Survey) of the surface layer of mineral soils carrying bracken.
Hunter (19 53) compared the performance of bracken in several soil series in
the north-east of Scotland and concluded: "The composition of these samples
gives no indication of a relationship between the vigour of the plants and their
nutrient status: it is probable that the controlling factor in these cases was
water supply, or some other feature of the environment. Similarly the
differences between the three Tarves samples cannot be explained nutritionally
from the analyses." Again in his comparison between two sets of analyses of
100 fronds from two sites he states "The difference in vigour between the two
series of periodic samples is also inexplicable nutritionally though the higher
232
A. S. WATT
concentrations of nitrogen and potassium in the second series might have been
significant. ”
The basis of these comparisons is a unit number of fronds and not a number
per unit area, and as yet for the mature community there are no experimental
and convincing data on the relation between the nutrient status of the soil and
the response of the bracken per unit area, all other factors such as light
intensity and its duration, frost incidence, water availability, shelter,
age-composition of the population etc., being taken into account. This is not to
say that nutrient status is not important.
RELATION TO OTHER COMMUNITIES
Bracken in woodland
Bracken is well known as a plant anatomically and posturally as well adapted
to the shade of woodland as to full daylight (Boodle, 1904; Woodhead, 1906).
Similar responses to shelter are shown by Bright (1928). There are, however, no
data on the relationship between dry weight production and light-intensity over
the whole range from its compensation point in shade to full daylight: and the
few values for woodland-2000 to 6440 kg/ha from Roudsea Wood (Frankland,
1966) and 1470 kg/ha for Bogle Crag Wood (Carlisle, Brown & White,
1967)-are of litter collected in autumn. When seasonal change is allowed for
(on the basis of Hunter’s data (1953)) the August dry weight values vary
between 1740 and 10,540 kg/ha, the maximum failing short of 12,400 for
Grassland D (Watt, 1964) and much less than the maximum of 14,000 kg/ha
obtained by Pearsall & Gorham (1956), both these values being for dominant
bracken in the open. There seem to be no critical data bridging the gap between
moderate shade and full daylight (e.g. in a sheltered glade in a forest.)
Though protected from frost and high evaporation rates in woodland (from
exposure to wind and intense sunlight) bracken nevertheless would seem to
produce less dry weight than in the open. Also, if Schwabe’s (1953)
observations on the young sporophyte apply to the adult, then under shade the
proportion of frond weight to underground parts is increased as is also apical
dominance.
There are other sets of conditions which make a woodland habitat
favourable to bracken, namely, the nutrient supply and conditions for decay.
The detailed work on the source and amount of nutrients reaching the
undergrowth and the ground in Bogle Crag Wood shows that both the
dominant oak (Quercus petraea) and the bracken undergrowth absorb inorganic
N available in rainfall, a source therefore not exclusive to woodland. The
amounts of nutrients in kg/ha from all sources (litter, leachates, stem flow
rainfall) are given by Carlisle, Brown & White (1967) (Table 2).
While much variation in place, kind of woodland, density of trees, etc may
be expected, yet the contribution in nutrients, immediately available or on
slow release, is very considerable, and added to the soluble carbohydrates
(70.79 kg/ha) washed from vegetation (zero from rainfall and only 1.25 kg/ha
from fronds), is likely to have an effect both on the live bracken and on the
organisms of decay.
The physical conditions of higher humidity and moderated extremes of
ECOLOGICAL STATUS OF BRACKEN
233
temperature in woodland, as well as the chemical, would promote decay of the
dkbris, among the more resistant components of which are the petioles of
bracken (Frankland, 1966) surpassed only by the outer ‘skin’ of the rhizome.
The laminar portions of the fronds are both physically and chemically adapted
for a higher rate of decay but as to the rate and kind of transformation there
are no data. It may be suggested that herbaceous undergrowth to bracken,
either in or outside woodlands, would provide a more suitable base from whicb
fungi would attack bracken material unacceptable in its pure form.
Table 2. Net annual quantities of nutrients from all sources at Bogle
Crag Wood (kg/Ha)
Total
N
P
K
Ca
Mg
Na
Organic
matter
70.90
4.60
51.99
57.11
19.19
93.41
6942.99
8.71
0.28
2.84
6.72
6.10
50.76
76.56
Difference
62.19
4.32
49.15
50.39
13.09
42.65
6866.43
Contributed by
bracken fronds
-0.28
0.09
9.40
0.07
0.95
1.71
12.36
Difference
62.47
4.23
39.75
50.32
12.14
40.94
6854.07
Total reaching soil
from all sources
From rainfall
Thus the hypothesis is formulated that the woodland habitat is both
favourable and restrictive, preventing a rate of production of litter in excess of
the rate of decay. In the open, liberated from restriction and under favourable
temperature and moisture, the rate of production may exceed the rate of decay
to the point where bracken becomes the victim of its own success, leading to
local degeneration and to the opportunity of invasion by trees and other plants
excluded from the apparently impregnable fortress of a tall continuous canopy
of live bracken and its thick accumulated litter.
A special example of bracken in woodland is found in Flitwick Moor (Beds),
where under an open canopy of BetuZu pubescens there is a small patch
dominated by Sphagnum fimbriutum (kindly identified by Dr J. Birks) 15 cm
thick over a compact waterlogged sedge peat. About 2-5 cm above this lies the
horizontal rhizome of a complete bracken plant (a branch detached by decay
from its parent) 1.24 m long carrying 14 short shoots, 8 of which bear 1 node
and 6 bear 2 (the maximum) and 2 emerged fronds of markedly shade form (81
and 93 cm tall). The plant is an extension of a short shoot with increasing
internodal length, the terminal one ending in a swelling, a phenomenon
previously met with in Breckland of which the cause is unknown. The roots are
few but in proportions similar to those already reported: 0.5 per cm on the
long shoot, 1.92 cm on the short shoot and on the long shoot twice as many
below as above. A feature of special interest is that the roots radiate outwards
except those from below which begin vertical but bend outwards within the
S p hugnu m.
Protected by the tree canopy and the Sphagnum against frost and drought
and confined to the Sphagnum by the deficiency of oxygen below, the bracken
234
A. S. WATT
is probably kept alive by the leachates from the trees, absorbed and held by the
Sphagnum, and minerals in the water raised by capillary attraction.
Other communities besides bracken replace former woodland, notably
grassland of various kinds and moorland dominated especially by Calluna.
Relation to Calluna
The fluctuating equilibrium between bracken and heather has been described
in some detail from one area in Breckland (Watt, 1955, 1971b). The
relationship doubtless varies according to local conditions but the rise to
abundance and even dominance of bracken following the burning of heather
and its fall from that status on the return of heather is a familiar phenomenon
(Gimingham, 1972). Quite large areas of mature Culluna may have rare fronds
or none at all, but the previous presence and even abundance of bracken is
established by the remains under the Calluna mor. When the Calluna is burned,
surviving short shoots of bracken elongate, grow downwards in the soil and
start a colony: or it is possible, as Oinonen (1967) has affirmed on the basis of
the relationship between the size of patches of bracken and established dates of
burning, that the colonies have started from prothalli.
The disposition of the rhizome in relation to Calluna depends on the history
of their relationship. In the experimental area already alluded to on the podsol
in Lakenheath Warren, established patches of heather are spreading into the
surrounding Pteridietum with the following results, obtained from a transect
from outside the Cullunu patch (2.7 m in diameter) to its centre: there is a drop
in the length of long shoots from 7.6 m to 2.6 m, in short shoots from 4.5 m to
0.27 m, in the maximum number of nodes on live short shoots from 30 to 4, in
the number of terminal buds of short shoots from 3 1 to 5 . 5 and in the number
of differentiated live but unemerged fronds from 10 to 0-a11 calculated per
surface area of 61 x 30.5 cm. Although the maximum depth of live rhizome
from this set of observations was not recorded, in other similar circumstances
live rhizomes have been found at depth (e.g. 4 6 cm).
By contrast bracken invading mature Callunetum (c. 6 0 cm high) which has
grown up replacing grass heath on podsol since the incidence of myxomatosis
Figure 11. Bracken rhizome invading dense tall (60 cm) Calluna showing rise from a depth of
5 cm in the mineral soil into the mor and descent from it to its former level.
ECOLOGICAL STATUS OF BRACKEN
235
in 1954 shows the invading rhizome at shallow levels. There is a layer of
CaZZuna mor 3 cm thick. Of 11 invading rhizomes the mean depth (and range in
parenthesis) is shallow, 5.7 cm (-2.5 to 11.5 cm) below the mineral soil
surface. The behaviour of one which in the horizontal space of 23 cm emerged
from the mineral soil, formed a loop in the mor and returned to its previous
depth is shown in Fig. 11. Incidentally, the roots from the lower side on the
1972 rhizome extension grew horizontally. In these circumstances bracken
would be more susceptible to damage by fire, but where eliminated could
return by the establishment of prothalli.
A similar invasion of dominant Empetrum nigrum by bracken rhizomes at a
shallow level has already been reported from IIkley Moor (Watt, 1971a).
In both these examples bracken invades from a base of dominant bracken.
Relation to Deschampsia flexuosa
It is my impression, supported by remains of bracken in the soil (Fig. 12A),
that in certain communites with dominant Festuca ovina, Deschampsia
flexuosa or Carex arenaria fronds have decreased in number since myxomatosis.
There is no long-term record to prove this. But an experiment was started in
1966 to find out something about the relation between bracken and
Deschampsia.
Since 1954 (the first incidence of myxomatosis) Deschampsia flexuosa has
spread replacing dominant Festuca ovina over large areas on the podsol on
Lakenheath Warren. The cover is now (1974) tussocky with a mean thickness
of 24 cm and a range from 5 to 36 cm. The F layer is 7.5 cm thick with
remains of bracken below. From a 3 x 3 m plot (established 1966) the F layer
was removed and the Deschampsia cut to soil level where patches of denuded
soil were rapidly colonized by seedlings. The grass cover was cut periodically.
The results (Fig. 13) show a parallelism in number of fronds until 1970, after
which the numbers in the cut plot are fewer than in the control and in August
1974 there were only two dead fronds, one with symptoms of killing by frost
and the other by drought. The lamina in the cut plot is more uniform in length
throughout and is mainly shorter than in the control. The petiole shows a drop
in length in 1967, but this is greater in the cut plot: thereafter there is
increasing divergence, the petiole in the control returning to its initial length
and then remaining more or less uniform, that in the cut plot rising slightly
then gently falling.
A major change in the number of fronds before 1966 is not excluded, but
during the nine years of the experiment there is in the control no evidence of a
change in number or dimensions of the frond which cannot be explained in
terms of fluctuations in rainfall and spring frost. And from comparison with
the cut plot it could be inferred that the presence of the Deschammia cover
serves to maintain the number and dimensions of the frond, at least over this
period of time.
In the interpretation of the results, however, the shallow rhizome of the
relict bracken (Fig. 12A) in the immediate neighbourhood means that on the
removal of the Deschampsia the growing points are more exposed to winter
frost and summer drought: also the faster rise in surface soil temperature means
earlier emergence (up to 12 and 19 days are recorded) so that spring frost
236
/ I
/''
* '
'".
' / / '
/ /
I F layer of Deschampsia flexuoso
scanty bracken litter
4
/
\
0
5-.
0
0
0
8
n
0
0 0
f
0
0
0
20
0
0
25
=-
0
0 0
0
0
0
0
0
0
Figure 12. A. Shallow live ( 0 ) rhizome under a dense cover of Deschampsia flexuosa, with dead
( 0 ) rhizome at lower levels. B. Relict bracken on podsol (Lakenheath Warren) with shallow live
( 0 ) rhizome and dead ( 0 ) rhizome at lower levels.
damage is greater, and in the six years for which records were kept the
percentage mortality in the cut plot is twice that in the control (35.6%
(2.6-67.9%), and 18.9% (0.0-50.6%) respectively). Among the many spindly
fronds of the cut plot there are, however, some which are short-petioled and
stocky providing evidence of the descent of the rhizome and the beginning of
rejuvenation. I t is clear that, though the bracken is placed in a vulnerable
position by the removaI of the Deschampsia cover, adjustment to the new
situation is possible if weather conditions permit. This appeared to have been
happening in the wet springs of 1968 and 1969, but was checked by the
subsequent dry years.
It is interesting to note that though the number of fronds in the control
fluctuate, they are maintained over the nine years at an average of 2.9 per
10 ft2 (0.93 m2). There is further some evidence that there is phasic
interdigitation between the Deschampsia and the bracken; that is there are
short-petioled fronds in the degenerate and pioneer phases and long-petioled in
the taller tussocks of the mature phase (cf. the relations between bracken and
Calluna-Watt, 1955).
ECOLOGICAL STATUS OF BRACKEN
X-
237
--x-x---x---x\-
0
I!
1966 1967
1968 1969 1970 1971
1972 1973 1974
Figure 13. Change in number of fronds, their height and petiole length in the 3 m x 3 rn plot
with the Deschampsia flexuosa cover removed (x) compared with the control ( 0 ) .
CONCLUSIONS
There emerges from this study the need for further information on many
points including:
(1) factors affecting the production of fronds and the decay of litter,
(2) the mechanism(s) determining the level at which the rhizome runs,
( 3 ) the duration of the functional life of roots,
(4)factors affecting the dormancy of buds.
Viewed against the ecological and historical background bracken, in brief, is a
camp-follower of man. Liberated from forest by man it has flourished and
competes with other communities which have replaced forest and share similar
habitats with bracken, notably grassland and moorland, each of various kinds
and among the latter chiefly Callunetum. Among these three the competitive
relationships vary with the inorganic habitat; but outstanding also as a
differentiating factor is the direct and indirect influence of man, which has
varied much both in time and in place. And on the basis of a decreasing
interference one may formulate the following graded series, indicating in broad
outline the relationship between bracken and these other communities as well
as .itself.
(1) Where all three communities are managed intensively the relationship at
a given time remains virtually static.
A. S. WATT
238
(2) Where there is some let-up in the control of bracken, and grassland and
Calluna continue to be managed intensively, then bracken spreads at their
expense.
( 3 ) Where none is managed and the competitive power of each in relation to
the other is unrestricted there is evidence of a check to domination by bracken
even though it may remain a component with a subordinate status.
(4)Where bracken dominates and the gain of litter continues to exceed the
loss, then bracken through its reaction on itself degenerates, leaving gaps open
to colonization by woody and other plants.
( 5 ) Conditions in woodland enable bracken to maintain an equilibrium
between itself and its environment.
ACKNOWLEDGEMENTS
I am much indebted to Mr C. A. H. Hodge of the Soil Survey of England and
Wales for the data on the soil profiles in Breckland, to Mr E. Birse of the Soil
Survey of Scotland for soil data from communities with bracken in Scotland
and to Dr W. Block for telling me about the site in Charnwood Forest and for
help with the excavations: also to the Forestry Commission for access to their
plantations, to Miss S . Bishop for drawing the figures and to Mr. R. Worland for
help with the calculations.
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