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The composition and structure of deciduous woods in County Down, Northern Ireland
Forest Ecology and Management, 14 (1986) 219--234 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
219
THE C O M P O S I T I O N A N D S T R U C T U R E O F D E C I D U O U S W O O D S I N C O U N T Y DOWN, N O R T H E R N I R E L A N D
ALAN C O O P E R
Department of Environmental Studies, University of Ulster, Newtownabbey, Co. Antrim, BT37 OQB (Northern Ireland) (Accepted 17 October 1985)
ABSTRACT Cooper, A., 1986. The composition and structure of deciduous woods in County Down, Northern Ireland. For. Ecol. Manage., 14: 219--234. The results of an ecological survey of deciduous woods in County Down, Ireland, are presented. Variation in the composition of the ground flora, canopy composition and structure, and patterns of tree regeneration was related to environmental gradients of soil mineral nutrient status, the intensity of grazing and the degree of soil waterlogging. The effects of the dominant species of the tree canopy on the ground flora were shown to be secondary. Woodland clearance has occurred mainly at low elevation sites with changes in land use to either coniferous plantation or agricultural land. The woods are very different in character from native Irish Blechno--Quercetum and Corylo--Fraxinetum but are closely related to mixed deciduous woods which have previously been described in England and Scotland. The introduced trees, Fagus sylvatica and Acer pseudoplatanus, have naturalised and their regeneration patterns at different sites are related to edaphic factors. The regeneration status of the current canopy dominants and associated trees, however, suggests that other changes in canopy composition will take place if the woods are not managed further. Replacement of Pinus sylvestris and Fagus sylvatica by Fraxinus excelsior and Acer pseudoplatanus is predicted, particularly at higher elevation sites.
INTRODUCTION R e c e n t p h y t o s o c i o l o g i c a l studies o n s e m i n a t u r a l Irish w o o d s have c o n t r i b u t e d greatly t o o u r u n d e r s t a n d i n g o f t h e i r e c o l o g y (Kelly a n d M o o r e , 1 9 7 4 ; Kelly, 1 9 7 5 ; Kelly, 1 9 8 1 ; Kelly a n d Kirby, 1 9 8 2 ) . I n t e r e s t c e n t r e s a r o u n d these w o o d s m a i n l y b e c a u s e o f t h e i r age ( R a c k h a m , 1 9 8 1 ) a n d t h e i r uniquely oceanic character and botanical composition. They occupy, however, o n l y a very small f r a c t i o n o f t h e 5% or so o f t h e w o o d e d Irish landscape ( C o n v e r y , 1 9 7 9 ) a n d are usually c o n f i n e d t o sites t h a t are t o o steep, r o c k y , w e t or d i f f i c u l t o f access (Kelly a n d Kirby, 1 9 8 2 ) for a n y o t h e r land use. Most Irish w o o d l a n d is c o m p o s e d o f r e c e n t l y p l a n t e d c o n i f e r o u s species.
0378-1127/86/$03.50
© 1986 Elsevier Science Publishers B.V.
220
In Northern Ireland for example, 81% consists largely of conifer plantations. The remainder of either deciduous woodland, much of which was planted before the beginning of the 20th century, or seminatural scrub. Often the dominant trees have been introduced, either into former agricultural land or into felled seminatural woods (Neff, 1975; Kelly, 1981; Tomlinson, 1982). Few concurrent studies have been carried out on these plantation woods. Only Dierschke (1982) has made some initial observations at sites dominated by introduced European broadleaved trees. Little is known about their development, interspecific interactions, the species composition of the ground flora or the age-structure and regeneration status of the canopy trees. The present investigation was initiated, therefore, to investigate these aspects of deciduous woodland ecology in Ireland. North County Down was selected as a conveniently accessible area for study on the basis of a recent survey of private woodlands greater than 0.5 ha in Northern Ireland (Graham, 1981). This survey showed that in County Down, deciduous woodland predominated, covering 2000 ha compared with only 400 ha of conifer plantations. Almost 50% of all woods in the county were planted before the beginning of the present century. Moreover, the county itself contains 38% of all pre-1900 planted woods in the province. METHODS
Woodland sites in North Down (Fig. 1) covering more than a single hectare square of the Irish Grid and showing a cover of broadleaved trees greater than 70% were located on 1:10 000 Ordnance Survey maps. So as to reduce site heterogeneity, any hectare squares within the woods were
|Study iii) " E coO--n,Q Fig. 1. A m a p of County D o w n showing the study area (shaded).
221
excluded from the sampling programme if either paths, buildings, parkland, rough pasture or furze mapping symbols were present. A total of 26 woodland sites were listed during the map phase of the work. They contained 79 suitable hectare squares, of which a random sample of 37 (47%) were visited within the time available for the work. Of these, 13 (35%) had been clear-felled and were n o t considered further. The remaining 24 squares were subsequently sampled in July and August 1982 using the field survey technique described by Bunce and Shaw (1973). The percentage cover of the ground flora and tree density and girth measurements were recorded in 24 random plots, each of 200 m 2. Except where indicated, nomenclature follows Webb (1977) for flowering plants and Watson (1968} for bryophytes. Detrended correspondance analysis (DECORANA) ordination (Hill, 19qga) and two-way indicator species analysis (TWINSPAN) classification techniques (Hill, 1979b) were used to analyse the data. The default parameters given by Hill (1979a, b) were employed to specify the ordination and classification, except that for the classification, pseudospecies (sensu Hill, 1979a) were given values of 1%, 2--5%, 6--25% and > 25% cover. RESULTS
Of the clear-feUed squares, replanting with either sitka spruce (Picea sitehensis (Bong. Cart.)) or larch (Larix decidua Miller) had taken place in three (8%) of the 37 squares selected for field survey. The mean elevation of cleared squares was 71.6 m. The first axis of a DECORANA ordination of the plot ground floras (excluding tree seedlings) reflected a gradient of increasing acidity and decreasing soil mineral nutrient status (Fig. 2c). Species characteristic of more base-rich, ungrazed woods had a low loading on this axis, whereas species with a high loading were those which are generally found in acidic, grazed woods. The second axis could be interpreted as a gradient of soil water status, with species indicative of well drained soils having a low load. ing, and species tolerant of soils that can become waterlogged having a high loading. Two outliers, stands 18 and 20, were separated by the ordination (Fig. 2a). Their ground floras contained a larger proportion of marsh species and t h e y occupied the two wettest sites. The TWINSPAN classification segregated the base-rich woodland plots from the acidic grazed plots. The base-rich woods were floristically much more homogeneous than the acidic woods (Fig. 2a). Whilst the only indicator species (sensu Hill, 1979a) for the base-rich woods was Hedera helix, a species that is intolerant of grazing, other species with an indicator potential for nutrient-rich soils (Circaea lutetiana, Heracleum sphondylium and Urtica dioica) were frequent. The acidic woods were characterised mainly by the presence of calcifuge grasses (Tables 1 and 2). The soil water status
ell sl5
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] 800
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X 800
i 500
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't 000
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I 700
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;
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r 500
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6;0
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e Holcus mot/is
• Oryopteris di|st&ta
eOxalis acetosells
fl e xeu°sm Ag~o=tts c a ni n:,t Mnium ho:num
AXIS 1
2;Q
Oeschampstm • cseaDitoss oOryopier Is f l l i x - f e m i n a
• Hermcleum s p h o n d y l i u m
eRubus fruJt|cosus
Fig. 2. A DECORANA ordination o f (a) woodland plots using the ground flora; (b) woodland plots using the ground flora and presence o f canopy trees; (c) species in the ground flora. Plots delimited by a TWINSPAN classification based on the data used for each ordination have been delimited.
tOO
RO0
300
(c)
e Eury~chium praelongum 2 0 0 • Hederae * . hel|x Urtics d l o i c a
300,
12 • 13 ee24 |~
I 200
,21
010
400
(b)
i 100
+Z S~
n18
400 *
0
7 e13
(a)
500
o
100-
200-
400
SO0
b~
223 TABLE 1 P e r c e n t a g e f r e q u e n c y o f the g r o u n d flora in b a s e - r i c h a n d acidic w o o d s
Hedera helix Dryopteris dilatata Rubus fruticosus Dryopteris filix-mas Heracleum sphondylium Circaea lutetiana Urtica dioica Pteridium aquilinum Holcus mollis Oxalis acetosella Deschampsia flexuosa Agrostis capillaris Agrostis canina Mnium hornum Deschampsia caespitosa
Base-rich woods
Acidic woods
80 80 70 50 40 40 40 20 20
50 92 83 25 8 --42 83
10
75
10
58 58 50 50 42
----
--
The two main groups of stands were delimited by a T W I N S P A N classification of woodland plots. Only species with a frequency > 4 0 % in any one group are shown.
TABLE 2 M e a n p e r c e n t a g e cover o f t h e g r o u n d flora in b a s e - r i c h a n d acidic w o o d s
Hedera helix Eurynchium praelongum Oxalis acetosella Agrostis capillaris Holcus moUis Deschampsia caespitosa Pteridium aquilinum Mnium hornum Deschampsia flexuosa
Base-rich woods
Acidic woods
45.0 5.6 0.1 -0.6
5.0 0.2 14.8 12.0 7.8 6.5 6.2 5.3 5.2
-0.2 -0.1
The two main groups of stands were delimited by a TWINSPAN classification of woodl a n d plots. O n l y s p e c i e s w i t h a m e a n p e r c e n t a g e c o v e r > 5% in a n y o n e g r o u p are s h o w n .
3
Po ten tilla palus tris Geum urbanum D r y o p t e r t s pseudorrms A n t h r i s c u s sylvestris Hyacinthoides non-scriptus* Lonicera periclymenum Vaccinium myrtillus Rurnex sanqulneus H e d e r a helix V e r o n i c a officinalis Polytrichum forrnosum Poa trivialis Urttca dioica Circaea lutetiana Primula vulgaris* Eurynchlum praelongum Heracleum sphondylium Mnium longirostrum Cardamine f l e x u o s a Cerastium fontanum D r y o p t e r i s filix-mas Rubus fruticosus D r y p o t e r i s dilatata Viola rivintana Mnium hornum Dactylis glomerata H o l c u s mollis Pteridium aquilinum Chamaenerion angustifolium Oxalis acetosella * Cirsium palustre A g r o s t i s capillaris L u z u l a sylvatica * Teucrum scorodonia Agrostis stolonifera G a l i u m aporine
Plot number:
+
5 +
+
1
+
+
+
90
17
90
+
+
+
19
15 +
50
1"o
"5
13
Base-rich woods
"5
+
+
90
24
"1
+ 1
1"o
"5 50
+
1"0 + +
12
Percentage cover of gxound flora species in the woodland
TABLE
+
"1
",5
+
"5
70
16
+ + +
+
5"
11
plots
+ 5
10 5 5 + +
2
15
"5
,50 5
+ +
10
15
"5
+ 10 5 +
+
4
~'o
1"0 + +
+
+
+
14
~
+
+
6
Acidic woods
"5
+ 10
3
~o
~o
+
"6 15
+
+ 5
9
+
÷
÷
1
1 10 +
lo
21
+
"5
+ +
8
+
+ 5
7
+
+ +
+
5
1"o
"1 +
3"0
"5 7"o 3"o 5"0
70
+ 5
1
÷
+
+ +
+
22
"1 1
÷
+
+
23
+
+
10
8O +
5 +
+
5
+
+
18
Wet woods
+
20
t~ bo
5
1
1
2
9
0
3
2
2
1
0
5
3
2
0
4
1
2
0
10
2
0
0
6
1
2
0
5
2
3
0
7
3
0
1
8
2
2
1
7
1
1
5
14
1
0
0
4
3
1
3
10
2
1
3
8
3
1
1
13
1
2
0
7
2
0
3
12
10
3
2
3
8
5
1"o
1
0
2
11
15
10
2
2
5
13
+
25 + -1-
1
2
7
14
"1 ÷ 1 -t÷ 1
"5 1
1'0
1
2
8
16
5 ÷
+
÷ 5
io
+ 5
0
1
6
13
+
"5
0
1
5
7
15 15 10 25 5 10
Plots a n d species h a v e b e e n a r r a n g e d a c c o r d i n g t o t h e i r loadings o n a r e c i p r o c a l a v e r a g i n g o r d i n a t i o n . Also t a b u l a t e d are t h e t w o o u t l y i n g p l o t s ( w e t w o o d s ) d e l i m i t e d b y a D E C O R A N A o r d i n a t i o n . Species m a r k e d w i t h a n asterisk are p r i m a r y w o o d l a n d i n d i c a t o r species (sensu P e t e r k e n , 1 9 7 4 ) .
Species in t h e g r o u n d flora N o n - w o o d l a n d species in t h e g r o u n d flora N a t i v e tree species in t h e plot I n t r o d u c e d tree species in the plot
B l e c h n u m spicant Deschampsia flexuosa R u m e x acetosa D e s c h a m p s i a caespitosa Anthoxanthum odoratum H o l c u s lanatus Athyrium filix-femina R a n u n c u l u s repens Stellaria alsine E q u i s e t u m arvense P o t e n t i l l a erecta Calluna vulgaris GaUu m saxatile R u b u $ idaeus Ulex europaeus A g r o s t i s canina Juncos effusus E q u i s e t u m fluviatile Iris p s e u d o c o r u s Epilobium palustre G a l i u m palustre
7
i
i
5
17
4
i 6 1
19
2
i
13
24
3
11
12
7
16
8
II
2
1
2
"1 1
13
15
5
is
4
6
1
14
6
Acidic woods
i
1
5
9
2
3
4
21
1
1
8
i
2
7
Plots and species have been arranged according to their loadings on a reciprocal averaging ordination of the ground flora.
(B) A ve r pseudoplatanus Aeseulus hlppoeastanum Betula pendula Fagus sylvativa Picea sp. Pinus ntgra Pinus sy lvestris Rhododendron ponticum Tilta europaea
(A) A l n u s glutinosa Betula pubescens C o r y l u s avellana Crataegus m o no gyn a Fraxinus excelsior Populus tremula Quereus sp. 8ambucus nisra 8orbus aucuparia U l m u s glabra
Plot number:
Base-rich woods
T h e d e n s i t y o f ( A ) n a t i v e a n d (B) i n t r o d u c e d t r e e s in e a c h o f t h e w o o d l a n d P l o t s
TABLE 4
5
1
2
1
22
1
1
23
10
Wet
18
20
woods
bo t~ o~
227 of the acidic woods ranged from poorly drained to well drained. The acidic woodland plots had a mean elevation above mean sea level of 135.9 m compared with 36.6 m for those that were base-rich. The species diversity of the two ground floras was very low (Table 3). The mean number of species in each 200 m 2 plot in the base-rich and acidic woods, respectively, was 6.6 and 10.8. A total of 26 species were found in the base-rich woodlands compared with 42 in the acidic woods, but a higher number of the latter are typically found outside w o o d l a n d communities. Most of the species in the plots are characteristic of secondary woods (Webb, 1977) and all are c o m m o n plants usually disseminated by either wind or animals. Except for the two outlying plots, each wood contained introduced tree species (Table 4). Despite this, differences between the ground flora of the base-rich and acidic woods were mirrored in canopy composition and structure (Table 3, Fig. 3). Most of the introduced species, present usually as one or two individuals restricted to a single sampling site, were f o u n d on the better soils. The native species Sorbus aucuparia and Populus tremula, however, were restricted to acidic woods. Both Betula pubescens and Pinus sylvestris, also species of poorer soils, were more a b u n d a n t in the acidic woods. The most frequent trees in both types of wood were Fagus sylvatica and Acer pseudoplatanus, followed by Quercus sp. and Fraxinus excelsior, with Betula pubescens and Betula pendula both as c o m m o n canopy species. The oldest individuals in both communities were mainly Fagus sylvatica, Quercus sp. and Pinus sylvestris. With the exception of Pinus sylvestris, all of the c o m m o n trees were regenerating (Fig. 3). There were, however, essential differences in the size-structure of their populations, both between sites and species. Acer pseudoplatanus was regenerating in the base-rich woods but its performance in terms of numbers, regeneration and presence in the larger size-classes, was poorer in the acidic woods. In contrast, the size-structure of Fagus sylvatica indicated a population of largely mature individuals, and regeneration was not occurring in the acidic woods. The smaller numbers of Quercus sp. and Fraxinus excelsior makes comparison difficult, but in both species regeneration was more frequent in the base-rich woods. To examine whether observed differences between the woods were related to differences in c a n o p y composition rather than differences in their edaphic environments, the data were partitioned according to whether Quercus sp., Acer pseudoplatanus or Fagus sylvatica was a canopy d o m i n a n t (Tables 5, 6 and 7). Unlike the major differences in qualitative and quantitative species composition of the base-rich and acidic woods, differences between the ground floras of the three tree species were not so marked and were mainly quantitative. Plots dominated by Quercus sp. were more heavily grazed when judged by the lower frequency of Hedera helix and their greater mean percentage cover of grasses. Those dominated by Acer pseudo-
228
(a) 2O
111
(b)
]
B A n E - R I C H WOODS
]
ACIDIC WOOD3
II
10
i , n, nnn, . 1-10
11-20
21-10
31-40
41-80
:
>'80
1-10
(C)
~
11-20
21-30
31-40
41-80
>80
3t-40
41-80
>60
(d)
3
3
14
~ o'
~ , ~ n ,~ 1-10
11-20
21-30
31-40
,~ 41-30
~ ~ >80
4 'o
n,~ 1-10
8TI[M DIAMETER 8|ZE-CLA88
11-30
i 31-30
(cm)
Fig. 3. M e a n d e n s i t y 200 m - : o f (a) Acer pseudoplatanus; ( b ) Fagus sylvatica; (c) Quercus sp.; (d) Fraxinus excelsior > 1.3 m high, s h o w n b y s t e m d i a m e t e r size-class.
TABLE 5 P e r c e n t a g e f r e q u e n c y o f t h e g r o u n d flora in s t a n d s d o m i n a t e d b y Quercus sp. (Q), Acer
pseudoplatanus (A) a n d Fagus sylvatica ( F )
Dryopteris dilatata Oxalis acetosella Rubus fruticosus Holcus mollis Deschampsia flexuosa Descha mpsia caespitosa Hedera helix Dryopteris filix-mas Eurynchium praelongum Heracleum sphondylium Pteridium aquilinum Agrostis canina
Q
A
F
83 67 67 67 50 50
70 22 70 56 22 11
88 50 80 50 25 25
17
78
75
-17 33 33 17
68 44 45 22 22
25 -50 50
O n l y species w i t h a f r e q u e n c y > 40% in a n y o n e g r o u p h a v e b e e n s h o w n .
229 TABLE 6 Mean percentage cover of the ground flora in stands dominated by Quercus sp. (Q), A c e r pseudoplatanus (A) and Fagus sylvatica (F)
Agrostis capillaris Pteridium aquilinum Holcus mollis Deschampsia flexuosa Hedera helix Agrostis stolonifera E u r y n c h i u m praelongum Mnium hornum Oxalis acetosella Deschampsia caespitosa
Q
A
F
13.5 11.8 11.1 6.0 0.2 -0.2 0.2 2.0 1.2
5.8 0.2 2.9 0.7 24.8 8.9 6.3 5.7 1.8 0.1
1.4 0.5 1.5 3.3 29.1 --1.4 18.8 8.9
Only species with a mean percentage cover of 5% or more in any one group are shown.
TABLE 7 Density of trees greater than 1.3 m high, in stands dominated by Quercus sp. (Q), A c e r pseudoplatanus (A) and Fagus sylvatica (F)
Quercus sp. A c e r pseudoplatanus Fagus sylvatica Fraxinus excelsior S a m b u c u s nigra Tilia europaea Betula pubescens Pinus sylvestris Betula pendula Sorbus aucuparia Populus tremula A l n u s glutinosa Aesculus hippocastanum
Q
A
F
22 7 3
4 48 3
1 12 31
1 1 1 8 3
5 --4 1
5 --1 2
3
13
--
2 2 ---
--11 1
--6 --
had a species composition which suggested that they had a higher s o i l n u t r i e n t s t a t u s a n d less a c i d i c soils. An ordination and classification based on the ground flora together with tree species composition (Fig. 2b), delimited almost exactly the same two w o o d l a n d t y p e s as g i v e n b y t h e c l a s s i f i c a t i o n o f t h e g r o u n d f l o r a a l o n e {Fig. 2 a ) . C l e a r l y , s i t e d i f f e r e n c e s h a v e a m u c h g r e a t e r e f f e c t o n t h e c o m position of the woodland ground flora than the canopy dominants.
platanus
230 DISCUSSION Historically, the exploitation of indigenous woodland resources in County Down, proceeded more fully compared with neighbouring regions (Graham, 1981). McCracken (1971) states that by the 19th century, only small woods on demesnes and woods associated with the margins of interdrumlin lakes and bogs remained as remnants of the primary woodlands which were formerly extensive. It is within the context of a landscape that is almost entirely anthropogenic, therefore, that the ecology of present-day woods should be viewed. Land use in rural County Down is predominantly either arable or dairy farming (Boal, 1963). What deciduous woodland is present is generally associated with demesnes or former demesnes and was probably planted during the 18th century {Praeger, 1934; Harron, 1977) either for timber, amenity or game preserve. Often it may have been established around a nucleus of relict woodland possibly using seedlings propagated from local trees {Forbes, 1933). Species commonly used for planting were Fagus sylvatica, Pinus sylvestris and Quercus sp. Fraxinus excelsior and Acer pseudoplatanus were also planted along with a wide variety of exotic species usually propagated in smaller numbers (Forbes, 1933; Harron, 1977; Kelly, 1981). Fagus sylvatica, Pinus sylvestris and Acer pseudoplatanus are all species which have been introduced to Ireland (Webb, 1977). They can regenerate in Irish woods (Praeger, 1934; Neff, 1975; Graham, 1981; Kelly, 1981; Dierschke, 1982) and are generally considered to have a detrimental effect on the native ground flora. The results of the present investigation do not contradict the picture emerging from the literature review, of a set of woodland communities that owe their origin and tree species composition mainly to the activities of man. The regeneration of most of the canopy dominants bears out the hypothesis that, with the exception of Pinus sylvestris and the less common exotic species, introduced trees have become naturalised in this part of Ireland. The species composition of the ground flora is typical of secondary woods. Most of the woodland plants are characteristic of secondary woods in Ireland {Webb, 1977), with the exception of Hyacinthoides non-scriptus, Primula vulgaris, Cardamine flexuosa, Oxalis acetosella and Luzula sylvatica {Table 3). These are species which are given by Peterken (1974) as indicators of primary woods in Central Lincolnshire. In Ireland, however, they are not confined to woodland ecosystems and do not have the same indicator potential. Studies on native woodlands in Ireland (Kelly and Moore, 1974; Kelly and Kirby, 1982; Cooper, 1984) further testify to the secondary nature of the County Down woods. These woods have a rich ground flora of bryophytes, ferns and flowering plants and do not generally contain introduced trees.
231 In the Down woods there is a considerable influence of the edaphic environment on the composition of n o t only the w o o d l a n d tree canopy, but also the ground flora. This m a y be due in part to the planting of species such as Pinus sylvestris on soils on which sylviculturalists knew t h e y would do well, or the selective effects of the environment on establishment and survival after planting. Differences in the ground flora are more likely to be directly related to differences in the edaphic environment, either as the result of the inclusion of fragments of relict woodland in plantations or to the selective effect of the environment on plant propagules reaching woods by processes involving natural dispersal. The effects of canopy dominants on ground flora composition have been shown to be secondary to those of the environment. In mixed secondary woods such as those of the present study, extensive tracts of uniform flora associated for example with monocultures of Fagus sylvatica, did n o t occur. A further interaction affecting ground flora composition in the woods is between their use and their relative elevation. Many of the lower elevation base-rich woods are not grazed by farm stock. They are generally close to demesne houses and are often used to rear game birds. At higher elevations, grazing by sheep and cattle is more c o m m o n and the ground flora often consists predominantly of grasses. The poorer regeneration of Quercus sp., Acer pseudoplatanus, Fagus sylvatica and Fraxinus excelsior on the acidic sites is due in part to grazing animals, but it is difficult to separate this from the effects of the environment. The composition and diversity of the Down woods are similar to those given by Dierschke {1982) for seven relev~s taken in the Republic of Ireland. These were all chosen so as to contain Fagus sylvatica or Acer pseudoplatanus but even so, comparison can be drawn between the base-rich and acidic woods in Down and a similar division which can be detected in the relev~s presented by Dierschke (1982). This author tentatively compared the woods with the Atlantic beechwoods of Northwest France described by Durin et al. (1967) and pointed out a n u m e r of similarities. Further comparison of the floristic composition of the Down woods can be made with British woodland communities. When the samples were classified according to the scheme given by Bunce (1982), most of them keyed out to one of three main plot types. Eight woods belonged to plot type nine which is related to the U l m o - - F r a x i n e t u m / D r y o p t e r o - - F r a x i n e t u m (Klbtzli, 1970). Ten of the woods keyed out at the two closely related plot types 22 and 23, which correspond to the B l e c h n o - Q u e r c e t u m (BraunBlanquet and Tiixen, 1952), Fago--Quercetum (Tfixen, 1955) and Galio-saxatflis-Quercetum (Birse and Robertson, 1976). The latter association described lowland oakwoods on warm, mostly dry, acid brown earth soils in Scotland. The floristic similarities between this unit and the Down woods is high, and soils, climate and management practices are coincident (Boal, 1963; McConaghy and McAllister, 1963). All of the selective species (sensu Bunce, 1982) of plot type nine were
232 present in the Down w o o d s which keyed out at this plot type. Four from seven of the British constant species (sensu Bunce, 1982) were also constants in County Down and all of the dominants (sensu Bunce, 1982) listed were also dominants in Down. A similar level of convergence was also detected for plot types 22 and 23 when t h e y were compared with the County Down woods. The comparisons are crude but serve to highlight the similarities between the D o w n w o o d s and those o f the rest of the British Isles. Certainly the w o o d s bear little resemblance to the native oceanic B l e c h n o - Q u e r c e t u m and Corylo--Fraxinetum described by Kelly and Moore {1974) and Kelly and Kirby (1982). The only ecological study of w o o d l a n d in C o u n t y Down which I have been able to find is contained in a thesis b y Gilbertson {1969). Ballyhasson wood, on the river Quoile (outside the area covered by the present study), was found to contain b o t h native and introduced tree species and had a composition which Tomlinson (1982) described as being typical o f " m i x e d " deciduous woods in Ireland. Mature hardwood plantations have a considerable economic value. Since many individuals in the D o w n w o o d s are reaching maturity, their harvest as timber is probable unless they are specifically managed for their amenity value. Evidence of felling activity which the survey accumulated, shows that replacement with conifers or conversion to agricultural land are likely at many sites and that woodland clearance is concentrated at lower elevations. This conclusion was also reached by Graham {1981) for private woodlands in Northern Ireland. Whilst the ecological conservation value of the w o o d s is n o t high in terms of species rarity or c o m m u n i t y naturalness (Ratcliffe, 1977), felling will result in the loss of visual amenity and recreation potential. This will have, over the next two decades or so, a significant effect on the character of the landscape. Neff (1975) has outlined ideas for the economic management of deciduous w o o d s in Ireland, based on a programme of selective felling and natural regeneration. In areas such as County Down, serious scientific studies to maximise timber yields of hardwoods should be considered so as to ensure the continued efficient use of timber resources and the maintenance of landscape amenity and aesthetic quality. Relevant to this is the observation that tree regeneration patterns are n o t strongly related to canopy composition. Without further planting of Pinus sylvestris and Fagus sylvatica, for example, on the higher elevation, more acidic soils, species such as Acer pseudoplatanus and Fraxinus excelsior are likely to be much more c o m m o n as c a n o p y dominants than they are at present. ACKNOWLEDGEMENT The work of Jim McNair in carrying o u t the field survey for the project is gratefully acknowledged.
233 REFERENCES Brise, E.L. and Robertson, J.S., 1976. Plant Communities and Soils of the Lowland and Southern Upland Regions of Scotland. Macaulay Institute for Soil Research, Aberdeen, 226 pp. Boal, F.W., 1963. County Down. In: L. S y m o n s (Editor), Land Use in Northern Ireland. University of London Press, London, pp. 93--108. Braun-Blanquet and Tfixen, R., 1952. Irische Pflanzengesellschaften. VerSff. Geobot. Inst., Ziirich, 25: 224--415. Bunce, R.G.H., 1982. A Field Key F o r Classifying British Woodland Vegetation. Part 1. Institute of Terrestrial Ecology, Cambridge, 103 pp. Bunce, R.G.H. and Shaw, M.W., 1973. A standardised procedure for ecological survey. J. Environ. Manage., 1: 239--258. Comvery, F.J., 1979. Irish Forestry Policy. National Economic and Social Council. Stationary Office, Dublin, 225 pp. Cooper, A., 1984. Application of multivariate methods to the conservation management of hazel scrub in Northeast Ireland. Biol. Conserv., 30: 341--357. Dierschke, H., 1982. The significance of some introduced European broad-leaved trees for the present potential natural vegetation of Ireland. J. Life Sci. R. Dubl. Soc., 3: 199--207. Durin, L., G~hu, J.M., Noirfalise, A. and Sougnez, N., 1967. Les h~traies atlantiques et leur assaim climatique dans le nord-ouest et l'ouest de la France. Bull. Soc. Bot. N. Ft. No. special, 20: 59--89. Forbes, A.C., 1933. Tree planting in Ireland during four centuries. Proc. R. Ir. Acad., C41: 168--199. Gilbertson, M., 1969. The Ecology of the Quoile River. Unpublished M.Sc. thesis, Queens University Belfast, Belfast. Graham, T., 1981. Private Woodland Inventory of Northern Ireland, 1975--79. Forest Service, Department of Agriculture for Northern Ireland, Belfast, 76 pp. Harron, J., 1977. A Vegetation Survey of the Valley of the Linford Water near Glenarm, County Antrim. Landscape Section, Department of the Environment for Northern Ireland, Belfast, 23 pp. Hill, M.O., 1979a. DECORANA. Corne]l University, New York, NY, 52 pp. Hill, M.O., 1979b. TWINSPAN. Cornel] University, New York, NY, 90 pp. Kelly, D.L., 1975. Native Woodland in Western Ireland with especial Reference to the Region of Killarney. Unpublished Ph.D. thesis, University of Dublin, 337 pp. Kelly, D.L., 1981. The native forest vegetation of Killarney, south-west Ireland: an ecological account. J. Ecol., 69: 437--472. Kelly, D.L. and Kirby, E.N., 1982. Irish native woodlands over limestone. J. Life Sci. R. Dubl. Soc., 3: 181--198. Kelly, D.L. and Moore, J.J., 1974. A preliminary sketch of the Irish acidophilous oakwoods. Coll. Phytosoc., 3:375--387. KlStzli, F., 1970. Eichen-, Edellaub und Br~ichwa]der der Britischen Inseln. Schweiz. Z. Forstwes., 121: 329--366. McConaghy, S. and McAllister, J.S.V., 1963. Soils. In: L. Symons (Editor), Land Use in Northern Ireland. University of London, pp. 93--108. McCracken, E., 1971. The Irish Woods Since Tudor Times. David and Chambers, Newton Abbot, 184 pp. Neff, M.J., 1975. Woodland conservation in the Republic of Ireland. Coll. Phytosoc., 3: 273--285. Peterken, G.F., 1974. A method for assessing woodland flora for conservation using indicator species. Biol. Conserv., 6: 239--245. Praeger, R.L., 1934. The Botanist in Ireland. Hodges and Figgis, Dublin, 587 pp. Rackham, O., 1981. Ancient Woods. Arnold, London, 402 pp.
234 Ratcliffe, D.A., 1977. A Nature Conservation Review. Vol. 1. Cambridge University Press, 401 pp. Tomlinson, R.W., 1982. Vegetation. In: J.G. Cruickshank and D.N. Wilcock (Editors), Northern Ireland. Environment and Natural Resources. Queens University Belfast and New University of Ulster, Belfast, pp. 207--240. Tiixen, R., 1955. Das system der nordwestdeutschen Pflanzengesellschaften. Mitt. Florsoz. Arbeitsgem. NS, 5: 155--176. Watson, E.V., 1968. British Mosses and Liverworts. Cambridge University Press, Cambridge, 495 pp. Webb, D:A., 1977. An Irish Flora. Dundalgan Press, Dundalk, 277 pp.
219
THE C O M P O S I T I O N A N D S T R U C T U R E O F D E C I D U O U S W O O D S I N C O U N T Y DOWN, N O R T H E R N I R E L A N D
ALAN C O O P E R
Department of Environmental Studies, University of Ulster, Newtownabbey, Co. Antrim, BT37 OQB (Northern Ireland) (Accepted 17 October 1985)
ABSTRACT Cooper, A., 1986. The composition and structure of deciduous woods in County Down, Northern Ireland. For. Ecol. Manage., 14: 219--234. The results of an ecological survey of deciduous woods in County Down, Ireland, are presented. Variation in the composition of the ground flora, canopy composition and structure, and patterns of tree regeneration was related to environmental gradients of soil mineral nutrient status, the intensity of grazing and the degree of soil waterlogging. The effects of the dominant species of the tree canopy on the ground flora were shown to be secondary. Woodland clearance has occurred mainly at low elevation sites with changes in land use to either coniferous plantation or agricultural land. The woods are very different in character from native Irish Blechno--Quercetum and Corylo--Fraxinetum but are closely related to mixed deciduous woods which have previously been described in England and Scotland. The introduced trees, Fagus sylvatica and Acer pseudoplatanus, have naturalised and their regeneration patterns at different sites are related to edaphic factors. The regeneration status of the current canopy dominants and associated trees, however, suggests that other changes in canopy composition will take place if the woods are not managed further. Replacement of Pinus sylvestris and Fagus sylvatica by Fraxinus excelsior and Acer pseudoplatanus is predicted, particularly at higher elevation sites.
INTRODUCTION R e c e n t p h y t o s o c i o l o g i c a l studies o n s e m i n a t u r a l Irish w o o d s have c o n t r i b u t e d greatly t o o u r u n d e r s t a n d i n g o f t h e i r e c o l o g y (Kelly a n d M o o r e , 1 9 7 4 ; Kelly, 1 9 7 5 ; Kelly, 1 9 8 1 ; Kelly a n d Kirby, 1 9 8 2 ) . I n t e r e s t c e n t r e s a r o u n d these w o o d s m a i n l y b e c a u s e o f t h e i r age ( R a c k h a m , 1 9 8 1 ) a n d t h e i r uniquely oceanic character and botanical composition. They occupy, however, o n l y a very small f r a c t i o n o f t h e 5% or so o f t h e w o o d e d Irish landscape ( C o n v e r y , 1 9 7 9 ) a n d are usually c o n f i n e d t o sites t h a t are t o o steep, r o c k y , w e t or d i f f i c u l t o f access (Kelly a n d Kirby, 1 9 8 2 ) for a n y o t h e r land use. Most Irish w o o d l a n d is c o m p o s e d o f r e c e n t l y p l a n t e d c o n i f e r o u s species.
0378-1127/86/$03.50
© 1986 Elsevier Science Publishers B.V.
220
In Northern Ireland for example, 81% consists largely of conifer plantations. The remainder of either deciduous woodland, much of which was planted before the beginning of the 20th century, or seminatural scrub. Often the dominant trees have been introduced, either into former agricultural land or into felled seminatural woods (Neff, 1975; Kelly, 1981; Tomlinson, 1982). Few concurrent studies have been carried out on these plantation woods. Only Dierschke (1982) has made some initial observations at sites dominated by introduced European broadleaved trees. Little is known about their development, interspecific interactions, the species composition of the ground flora or the age-structure and regeneration status of the canopy trees. The present investigation was initiated, therefore, to investigate these aspects of deciduous woodland ecology in Ireland. North County Down was selected as a conveniently accessible area for study on the basis of a recent survey of private woodlands greater than 0.5 ha in Northern Ireland (Graham, 1981). This survey showed that in County Down, deciduous woodland predominated, covering 2000 ha compared with only 400 ha of conifer plantations. Almost 50% of all woods in the county were planted before the beginning of the present century. Moreover, the county itself contains 38% of all pre-1900 planted woods in the province. METHODS
Woodland sites in North Down (Fig. 1) covering more than a single hectare square of the Irish Grid and showing a cover of broadleaved trees greater than 70% were located on 1:10 000 Ordnance Survey maps. So as to reduce site heterogeneity, any hectare squares within the woods were
|Study iii) " E coO--n,Q Fig. 1. A m a p of County D o w n showing the study area (shaded).
221
excluded from the sampling programme if either paths, buildings, parkland, rough pasture or furze mapping symbols were present. A total of 26 woodland sites were listed during the map phase of the work. They contained 79 suitable hectare squares, of which a random sample of 37 (47%) were visited within the time available for the work. Of these, 13 (35%) had been clear-felled and were n o t considered further. The remaining 24 squares were subsequently sampled in July and August 1982 using the field survey technique described by Bunce and Shaw (1973). The percentage cover of the ground flora and tree density and girth measurements were recorded in 24 random plots, each of 200 m 2. Except where indicated, nomenclature follows Webb (1977) for flowering plants and Watson (1968} for bryophytes. Detrended correspondance analysis (DECORANA) ordination (Hill, 19qga) and two-way indicator species analysis (TWINSPAN) classification techniques (Hill, 1979b) were used to analyse the data. The default parameters given by Hill (1979a, b) were employed to specify the ordination and classification, except that for the classification, pseudospecies (sensu Hill, 1979a) were given values of 1%, 2--5%, 6--25% and > 25% cover. RESULTS
Of the clear-feUed squares, replanting with either sitka spruce (Picea sitehensis (Bong. Cart.)) or larch (Larix decidua Miller) had taken place in three (8%) of the 37 squares selected for field survey. The mean elevation of cleared squares was 71.6 m. The first axis of a DECORANA ordination of the plot ground floras (excluding tree seedlings) reflected a gradient of increasing acidity and decreasing soil mineral nutrient status (Fig. 2c). Species characteristic of more base-rich, ungrazed woods had a low loading on this axis, whereas species with a high loading were those which are generally found in acidic, grazed woods. The second axis could be interpreted as a gradient of soil water status, with species indicative of well drained soils having a low load. ing, and species tolerant of soils that can become waterlogged having a high loading. Two outliers, stands 18 and 20, were separated by the ordination (Fig. 2a). Their ground floras contained a larger proportion of marsh species and t h e y occupied the two wettest sites. The TWINSPAN classification segregated the base-rich woodland plots from the acidic grazed plots. The base-rich woods were floristically much more homogeneous than the acidic woods (Fig. 2a). Whilst the only indicator species (sensu Hill, 1979a) for the base-rich woods was Hedera helix, a species that is intolerant of grazing, other species with an indicator potential for nutrient-rich soils (Circaea lutetiana, Heracleum sphondylium and Urtica dioica) were frequent. The acidic woods were characterised mainly by the presence of calcifuge grasses (Tables 1 and 2). The soil water status
ell sl5
~ ~ ~ t
e6 e14
1'+8
*3 e9 i 400
elO
*S *? *8
*,23
e22
AXIS |
] 800
g
.14
X 800
i 500
"l ~00
't 000
'l 700
I 700
t~
_m
S
100
200
AXIS I
300
400
43 9a 8~e5
*7
e23
022
I
GIrcsea lutotiiiRm
-100 -100
O,
~00
;
1;0
Deschampsta
300'
4;0
r 500
e Pteridium aqu~ttnum
6;0
eA0rostis ¢:eDill l i r l s
e Holcus mot/is
• Oryopteris di|st&ta
eOxalis acetosells
fl e xeu°sm Ag~o=tts c a ni n:,t Mnium ho:num
AXIS 1
2;Q
Oeschampstm • cseaDitoss oOryopier Is f l l i x - f e m i n a
• Hermcleum s p h o n d y l i u m
eRubus fruJt|cosus
Fig. 2. A DECORANA ordination o f (a) woodland plots using the ground flora; (b) woodland plots using the ground flora and presence o f canopy trees; (c) species in the ground flora. Plots delimited by a TWINSPAN classification based on the data used for each ordination have been delimited.
tOO
RO0
300
(c)
e Eury~chium praelongum 2 0 0 • Hederae * . hel|x Urtics d l o i c a
300,
12 • 13 ee24 |~
I 200
,21
010
400
(b)
i 100
+Z S~
n18
400 *
0
7 e13
(a)
500
o
100-
200-
400
SO0
b~
223 TABLE 1 P e r c e n t a g e f r e q u e n c y o f the g r o u n d flora in b a s e - r i c h a n d acidic w o o d s
Hedera helix Dryopteris dilatata Rubus fruticosus Dryopteris filix-mas Heracleum sphondylium Circaea lutetiana Urtica dioica Pteridium aquilinum Holcus mollis Oxalis acetosella Deschampsia flexuosa Agrostis capillaris Agrostis canina Mnium hornum Deschampsia caespitosa
Base-rich woods
Acidic woods
80 80 70 50 40 40 40 20 20
50 92 83 25 8 --42 83
10
75
10
58 58 50 50 42
----
--
The two main groups of stands were delimited by a T W I N S P A N classification of woodland plots. Only species with a frequency > 4 0 % in any one group are shown.
TABLE 2 M e a n p e r c e n t a g e cover o f t h e g r o u n d flora in b a s e - r i c h a n d acidic w o o d s
Hedera helix Eurynchium praelongum Oxalis acetosella Agrostis capillaris Holcus moUis Deschampsia caespitosa Pteridium aquilinum Mnium hornum Deschampsia flexuosa
Base-rich woods
Acidic woods
45.0 5.6 0.1 -0.6
5.0 0.2 14.8 12.0 7.8 6.5 6.2 5.3 5.2
-0.2 -0.1
The two main groups of stands were delimited by a TWINSPAN classification of woodl a n d plots. O n l y s p e c i e s w i t h a m e a n p e r c e n t a g e c o v e r > 5% in a n y o n e g r o u p are s h o w n .
3
Po ten tilla palus tris Geum urbanum D r y o p t e r t s pseudorrms A n t h r i s c u s sylvestris Hyacinthoides non-scriptus* Lonicera periclymenum Vaccinium myrtillus Rurnex sanqulneus H e d e r a helix V e r o n i c a officinalis Polytrichum forrnosum Poa trivialis Urttca dioica Circaea lutetiana Primula vulgaris* Eurynchlum praelongum Heracleum sphondylium Mnium longirostrum Cardamine f l e x u o s a Cerastium fontanum D r y o p t e r i s filix-mas Rubus fruticosus D r y p o t e r i s dilatata Viola rivintana Mnium hornum Dactylis glomerata H o l c u s mollis Pteridium aquilinum Chamaenerion angustifolium Oxalis acetosella * Cirsium palustre A g r o s t i s capillaris L u z u l a sylvatica * Teucrum scorodonia Agrostis stolonifera G a l i u m aporine
Plot number:
+
5 +
+
1
+
+
+
90
17
90
+
+
+
19
15 +
50
1"o
"5
13
Base-rich woods
"5
+
+
90
24
"1
+ 1
1"o
"5 50
+
1"0 + +
12
Percentage cover of gxound flora species in the woodland
TABLE
+
"1
",5
+
"5
70
16
+ + +
+
5"
11
plots
+ 5
10 5 5 + +
2
15
"5
,50 5
+ +
10
15
"5
+ 10 5 +
+
4
~'o
1"0 + +
+
+
+
14
~
+
+
6
Acidic woods
"5
+ 10
3
~o
~o
+
"6 15
+
+ 5
9
+
÷
÷
1
1 10 +
lo
21
+
"5
+ +
8
+
+ 5
7
+
+ +
+
5
1"o
"1 +
3"0
"5 7"o 3"o 5"0
70
+ 5
1
÷
+
+ +
+
22
"1 1
÷
+
+
23
+
+
10
8O +
5 +
+
5
+
+
18
Wet woods
+
20
t~ bo
5
1
1
2
9
0
3
2
2
1
0
5
3
2
0
4
1
2
0
10
2
0
0
6
1
2
0
5
2
3
0
7
3
0
1
8
2
2
1
7
1
1
5
14
1
0
0
4
3
1
3
10
2
1
3
8
3
1
1
13
1
2
0
7
2
0
3
12
10
3
2
3
8
5
1"o
1
0
2
11
15
10
2
2
5
13
+
25 + -1-
1
2
7
14
"1 ÷ 1 -t÷ 1
"5 1
1'0
1
2
8
16
5 ÷
+
÷ 5
io
+ 5
0
1
6
13
+
"5
0
1
5
7
15 15 10 25 5 10
Plots a n d species h a v e b e e n a r r a n g e d a c c o r d i n g t o t h e i r loadings o n a r e c i p r o c a l a v e r a g i n g o r d i n a t i o n . Also t a b u l a t e d are t h e t w o o u t l y i n g p l o t s ( w e t w o o d s ) d e l i m i t e d b y a D E C O R A N A o r d i n a t i o n . Species m a r k e d w i t h a n asterisk are p r i m a r y w o o d l a n d i n d i c a t o r species (sensu P e t e r k e n , 1 9 7 4 ) .
Species in t h e g r o u n d flora N o n - w o o d l a n d species in t h e g r o u n d flora N a t i v e tree species in t h e plot I n t r o d u c e d tree species in the plot
B l e c h n u m spicant Deschampsia flexuosa R u m e x acetosa D e s c h a m p s i a caespitosa Anthoxanthum odoratum H o l c u s lanatus Athyrium filix-femina R a n u n c u l u s repens Stellaria alsine E q u i s e t u m arvense P o t e n t i l l a erecta Calluna vulgaris GaUu m saxatile R u b u $ idaeus Ulex europaeus A g r o s t i s canina Juncos effusus E q u i s e t u m fluviatile Iris p s e u d o c o r u s Epilobium palustre G a l i u m palustre
7
i
i
5
17
4
i 6 1
19
2
i
13
24
3
11
12
7
16
8
II
2
1
2
"1 1
13
15
5
is
4
6
1
14
6
Acidic woods
i
1
5
9
2
3
4
21
1
1
8
i
2
7
Plots and species have been arranged according to their loadings on a reciprocal averaging ordination of the ground flora.
(B) A ve r pseudoplatanus Aeseulus hlppoeastanum Betula pendula Fagus sylvativa Picea sp. Pinus ntgra Pinus sy lvestris Rhododendron ponticum Tilta europaea
(A) A l n u s glutinosa Betula pubescens C o r y l u s avellana Crataegus m o no gyn a Fraxinus excelsior Populus tremula Quereus sp. 8ambucus nisra 8orbus aucuparia U l m u s glabra
Plot number:
Base-rich woods
T h e d e n s i t y o f ( A ) n a t i v e a n d (B) i n t r o d u c e d t r e e s in e a c h o f t h e w o o d l a n d P l o t s
TABLE 4
5
1
2
1
22
1
1
23
10
Wet
18
20
woods
bo t~ o~
227 of the acidic woods ranged from poorly drained to well drained. The acidic woodland plots had a mean elevation above mean sea level of 135.9 m compared with 36.6 m for those that were base-rich. The species diversity of the two ground floras was very low (Table 3). The mean number of species in each 200 m 2 plot in the base-rich and acidic woods, respectively, was 6.6 and 10.8. A total of 26 species were found in the base-rich woodlands compared with 42 in the acidic woods, but a higher number of the latter are typically found outside w o o d l a n d communities. Most of the species in the plots are characteristic of secondary woods (Webb, 1977) and all are c o m m o n plants usually disseminated by either wind or animals. Except for the two outlying plots, each wood contained introduced tree species (Table 4). Despite this, differences between the ground flora of the base-rich and acidic woods were mirrored in canopy composition and structure (Table 3, Fig. 3). Most of the introduced species, present usually as one or two individuals restricted to a single sampling site, were f o u n d on the better soils. The native species Sorbus aucuparia and Populus tremula, however, were restricted to acidic woods. Both Betula pubescens and Pinus sylvestris, also species of poorer soils, were more a b u n d a n t in the acidic woods. The most frequent trees in both types of wood were Fagus sylvatica and Acer pseudoplatanus, followed by Quercus sp. and Fraxinus excelsior, with Betula pubescens and Betula pendula both as c o m m o n canopy species. The oldest individuals in both communities were mainly Fagus sylvatica, Quercus sp. and Pinus sylvestris. With the exception of Pinus sylvestris, all of the c o m m o n trees were regenerating (Fig. 3). There were, however, essential differences in the size-structure of their populations, both between sites and species. Acer pseudoplatanus was regenerating in the base-rich woods but its performance in terms of numbers, regeneration and presence in the larger size-classes, was poorer in the acidic woods. In contrast, the size-structure of Fagus sylvatica indicated a population of largely mature individuals, and regeneration was not occurring in the acidic woods. The smaller numbers of Quercus sp. and Fraxinus excelsior makes comparison difficult, but in both species regeneration was more frequent in the base-rich woods. To examine whether observed differences between the woods were related to differences in c a n o p y composition rather than differences in their edaphic environments, the data were partitioned according to whether Quercus sp., Acer pseudoplatanus or Fagus sylvatica was a canopy d o m i n a n t (Tables 5, 6 and 7). Unlike the major differences in qualitative and quantitative species composition of the base-rich and acidic woods, differences between the ground floras of the three tree species were not so marked and were mainly quantitative. Plots dominated by Quercus sp. were more heavily grazed when judged by the lower frequency of Hedera helix and their greater mean percentage cover of grasses. Those dominated by Acer pseudo-
228
(a) 2O
111
(b)
]
B A n E - R I C H WOODS
]
ACIDIC WOOD3
II
10
i , n, nnn, . 1-10
11-20
21-10
31-40
41-80
:
>'80
1-10
(C)
~
11-20
21-30
31-40
41-80
>80
3t-40
41-80
>60
(d)
3
3
14
~ o'
~ , ~ n ,~ 1-10
11-20
21-30
31-40
,~ 41-30
~ ~ >80
4 'o
n,~ 1-10
8TI[M DIAMETER 8|ZE-CLA88
11-30
i 31-30
(cm)
Fig. 3. M e a n d e n s i t y 200 m - : o f (a) Acer pseudoplatanus; ( b ) Fagus sylvatica; (c) Quercus sp.; (d) Fraxinus excelsior > 1.3 m high, s h o w n b y s t e m d i a m e t e r size-class.
TABLE 5 P e r c e n t a g e f r e q u e n c y o f t h e g r o u n d flora in s t a n d s d o m i n a t e d b y Quercus sp. (Q), Acer
pseudoplatanus (A) a n d Fagus sylvatica ( F )
Dryopteris dilatata Oxalis acetosella Rubus fruticosus Holcus mollis Deschampsia flexuosa Descha mpsia caespitosa Hedera helix Dryopteris filix-mas Eurynchium praelongum Heracleum sphondylium Pteridium aquilinum Agrostis canina
Q
A
F
83 67 67 67 50 50
70 22 70 56 22 11
88 50 80 50 25 25
17
78
75
-17 33 33 17
68 44 45 22 22
25 -50 50
O n l y species w i t h a f r e q u e n c y > 40% in a n y o n e g r o u p h a v e b e e n s h o w n .
229 TABLE 6 Mean percentage cover of the ground flora in stands dominated by Quercus sp. (Q), A c e r pseudoplatanus (A) and Fagus sylvatica (F)
Agrostis capillaris Pteridium aquilinum Holcus mollis Deschampsia flexuosa Hedera helix Agrostis stolonifera E u r y n c h i u m praelongum Mnium hornum Oxalis acetosella Deschampsia caespitosa
Q
A
F
13.5 11.8 11.1 6.0 0.2 -0.2 0.2 2.0 1.2
5.8 0.2 2.9 0.7 24.8 8.9 6.3 5.7 1.8 0.1
1.4 0.5 1.5 3.3 29.1 --1.4 18.8 8.9
Only species with a mean percentage cover of 5% or more in any one group are shown.
TABLE 7 Density of trees greater than 1.3 m high, in stands dominated by Quercus sp. (Q), A c e r pseudoplatanus (A) and Fagus sylvatica (F)
Quercus sp. A c e r pseudoplatanus Fagus sylvatica Fraxinus excelsior S a m b u c u s nigra Tilia europaea Betula pubescens Pinus sylvestris Betula pendula Sorbus aucuparia Populus tremula A l n u s glutinosa Aesculus hippocastanum
Q
A
F
22 7 3
4 48 3
1 12 31
1 1 1 8 3
5 --4 1
5 --1 2
3
13
--
2 2 ---
--11 1
--6 --
had a species composition which suggested that they had a higher s o i l n u t r i e n t s t a t u s a n d less a c i d i c soils. An ordination and classification based on the ground flora together with tree species composition (Fig. 2b), delimited almost exactly the same two w o o d l a n d t y p e s as g i v e n b y t h e c l a s s i f i c a t i o n o f t h e g r o u n d f l o r a a l o n e {Fig. 2 a ) . C l e a r l y , s i t e d i f f e r e n c e s h a v e a m u c h g r e a t e r e f f e c t o n t h e c o m position of the woodland ground flora than the canopy dominants.
platanus
230 DISCUSSION Historically, the exploitation of indigenous woodland resources in County Down, proceeded more fully compared with neighbouring regions (Graham, 1981). McCracken (1971) states that by the 19th century, only small woods on demesnes and woods associated with the margins of interdrumlin lakes and bogs remained as remnants of the primary woodlands which were formerly extensive. It is within the context of a landscape that is almost entirely anthropogenic, therefore, that the ecology of present-day woods should be viewed. Land use in rural County Down is predominantly either arable or dairy farming (Boal, 1963). What deciduous woodland is present is generally associated with demesnes or former demesnes and was probably planted during the 18th century {Praeger, 1934; Harron, 1977) either for timber, amenity or game preserve. Often it may have been established around a nucleus of relict woodland possibly using seedlings propagated from local trees {Forbes, 1933). Species commonly used for planting were Fagus sylvatica, Pinus sylvestris and Quercus sp. Fraxinus excelsior and Acer pseudoplatanus were also planted along with a wide variety of exotic species usually propagated in smaller numbers (Forbes, 1933; Harron, 1977; Kelly, 1981). Fagus sylvatica, Pinus sylvestris and Acer pseudoplatanus are all species which have been introduced to Ireland (Webb, 1977). They can regenerate in Irish woods (Praeger, 1934; Neff, 1975; Graham, 1981; Kelly, 1981; Dierschke, 1982) and are generally considered to have a detrimental effect on the native ground flora. The results of the present investigation do not contradict the picture emerging from the literature review, of a set of woodland communities that owe their origin and tree species composition mainly to the activities of man. The regeneration of most of the canopy dominants bears out the hypothesis that, with the exception of Pinus sylvestris and the less common exotic species, introduced trees have become naturalised in this part of Ireland. The species composition of the ground flora is typical of secondary woods. Most of the woodland plants are characteristic of secondary woods in Ireland {Webb, 1977), with the exception of Hyacinthoides non-scriptus, Primula vulgaris, Cardamine flexuosa, Oxalis acetosella and Luzula sylvatica {Table 3). These are species which are given by Peterken (1974) as indicators of primary woods in Central Lincolnshire. In Ireland, however, they are not confined to woodland ecosystems and do not have the same indicator potential. Studies on native woodlands in Ireland (Kelly and Moore, 1974; Kelly and Kirby, 1982; Cooper, 1984) further testify to the secondary nature of the County Down woods. These woods have a rich ground flora of bryophytes, ferns and flowering plants and do not generally contain introduced trees.
231 In the Down woods there is a considerable influence of the edaphic environment on the composition of n o t only the w o o d l a n d tree canopy, but also the ground flora. This m a y be due in part to the planting of species such as Pinus sylvestris on soils on which sylviculturalists knew t h e y would do well, or the selective effects of the environment on establishment and survival after planting. Differences in the ground flora are more likely to be directly related to differences in the edaphic environment, either as the result of the inclusion of fragments of relict woodland in plantations or to the selective effect of the environment on plant propagules reaching woods by processes involving natural dispersal. The effects of canopy dominants on ground flora composition have been shown to be secondary to those of the environment. In mixed secondary woods such as those of the present study, extensive tracts of uniform flora associated for example with monocultures of Fagus sylvatica, did n o t occur. A further interaction affecting ground flora composition in the woods is between their use and their relative elevation. Many of the lower elevation base-rich woods are not grazed by farm stock. They are generally close to demesne houses and are often used to rear game birds. At higher elevations, grazing by sheep and cattle is more c o m m o n and the ground flora often consists predominantly of grasses. The poorer regeneration of Quercus sp., Acer pseudoplatanus, Fagus sylvatica and Fraxinus excelsior on the acidic sites is due in part to grazing animals, but it is difficult to separate this from the effects of the environment. The composition and diversity of the Down woods are similar to those given by Dierschke {1982) for seven relev~s taken in the Republic of Ireland. These were all chosen so as to contain Fagus sylvatica or Acer pseudoplatanus but even so, comparison can be drawn between the base-rich and acidic woods in Down and a similar division which can be detected in the relev~s presented by Dierschke (1982). This author tentatively compared the woods with the Atlantic beechwoods of Northwest France described by Durin et al. (1967) and pointed out a n u m e r of similarities. Further comparison of the floristic composition of the Down woods can be made with British woodland communities. When the samples were classified according to the scheme given by Bunce (1982), most of them keyed out to one of three main plot types. Eight woods belonged to plot type nine which is related to the U l m o - - F r a x i n e t u m / D r y o p t e r o - - F r a x i n e t u m (Klbtzli, 1970). Ten of the woods keyed out at the two closely related plot types 22 and 23, which correspond to the B l e c h n o - Q u e r c e t u m (BraunBlanquet and Tiixen, 1952), Fago--Quercetum (Tfixen, 1955) and Galio-saxatflis-Quercetum (Birse and Robertson, 1976). The latter association described lowland oakwoods on warm, mostly dry, acid brown earth soils in Scotland. The floristic similarities between this unit and the Down woods is high, and soils, climate and management practices are coincident (Boal, 1963; McConaghy and McAllister, 1963). All of the selective species (sensu Bunce, 1982) of plot type nine were
232 present in the Down w o o d s which keyed out at this plot type. Four from seven of the British constant species (sensu Bunce, 1982) were also constants in County Down and all of the dominants (sensu Bunce, 1982) listed were also dominants in Down. A similar level of convergence was also detected for plot types 22 and 23 when t h e y were compared with the County Down woods. The comparisons are crude but serve to highlight the similarities between the D o w n w o o d s and those o f the rest of the British Isles. Certainly the w o o d s bear little resemblance to the native oceanic B l e c h n o - Q u e r c e t u m and Corylo--Fraxinetum described by Kelly and Moore {1974) and Kelly and Kirby (1982). The only ecological study of w o o d l a n d in C o u n t y Down which I have been able to find is contained in a thesis b y Gilbertson {1969). Ballyhasson wood, on the river Quoile (outside the area covered by the present study), was found to contain b o t h native and introduced tree species and had a composition which Tomlinson (1982) described as being typical o f " m i x e d " deciduous woods in Ireland. Mature hardwood plantations have a considerable economic value. Since many individuals in the D o w n w o o d s are reaching maturity, their harvest as timber is probable unless they are specifically managed for their amenity value. Evidence of felling activity which the survey accumulated, shows that replacement with conifers or conversion to agricultural land are likely at many sites and that woodland clearance is concentrated at lower elevations. This conclusion was also reached by Graham {1981) for private woodlands in Northern Ireland. Whilst the ecological conservation value of the w o o d s is n o t high in terms of species rarity or c o m m u n i t y naturalness (Ratcliffe, 1977), felling will result in the loss of visual amenity and recreation potential. This will have, over the next two decades or so, a significant effect on the character of the landscape. Neff (1975) has outlined ideas for the economic management of deciduous w o o d s in Ireland, based on a programme of selective felling and natural regeneration. In areas such as County Down, serious scientific studies to maximise timber yields of hardwoods should be considered so as to ensure the continued efficient use of timber resources and the maintenance of landscape amenity and aesthetic quality. Relevant to this is the observation that tree regeneration patterns are n o t strongly related to canopy composition. Without further planting of Pinus sylvestris and Fagus sylvatica, for example, on the higher elevation, more acidic soils, species such as Acer pseudoplatanus and Fraxinus excelsior are likely to be much more c o m m o n as c a n o p y dominants than they are at present. ACKNOWLEDGEMENT The work of Jim McNair in carrying o u t the field survey for the project is gratefully acknowledged.
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