REGULATED RIVERS: RESEARCH & MANAGEMENT, VOL. 5, 159-166 (1990)

THE RIPARIAN WOODY PLANT COMMUNITY OF REGULATED RIVERS IN EASTERN ENGLAND

C. F. MASON AND S . M. MACDONALD Department of Biology, University of Essex, Wivenhoe Park, Colchester, C04 3SQ, U.K.

ABSTRACT

The riparian woody vegetation (trees and shrubs) is described from a survey of fifty 1 km stretches of rivers in East Anglia, England, between 1985 and 1987. Tree loss along three rivers was estimated from 1879 maps and aerial photographs taken in 1960 and 1970. A total of 48 species of trees and shrubs were recorded, with Alnus glutinosa, Salix alba, and S. fragilis dominating the overall community. Overall tree density ranged from 0 to 185 km-l, average 73 km-'. The majority of stretches had less than 25 mature trees, with less than 10 individuals of any one species. Sites were classified, based on the frequency of occurrence of species, into four groups using TWINSPAN. Marked decreases in densities of riparian trees were apparent over the period 1879-1970, averaging 8 per cent per annum on the river with the greatest initial starting densities. Tree densities, counted from the aerial photographs of 1970, ranged from 25-35 km- '. The results from East Anglia are compared with a similar study from a less intensively managed region in western Britain.

KEY WORDS Riparian trees Management East Anglia

INTRODUCTION

Over most of the Northern Hemisphere rivers once flowed through extensive areas of swamp and swamp forest and the input of detritus from this terrestrial ecosystem formed the base of aquatic food chains, the stream fauna having evolved life cycles to take advantage of the large autumn input of tree leaves (Hynes, 1963, 1975; Ross, 1963; Cummins et al., 1984). Overhanging trees also provide a significant source of animal material, arboreal invertebrates falling from the canopy into the water in large numbers in spring and summer, forming an important component of the diet of fish at a time when within-stream biomass is reduced (Mason and Macdonald, 1982).

Great Britain has a long history of land drainage and river improvement (Purseglove, 1988) and trees and shrubs have been extensively removed from the banks of watercourses. However, there have been few estimates of either the rate of loss or the current resource of riparian trees. Changing landscapes in river valleys, including the loss of trees, have been considered in general terms by the Countryside Commission (1974, 1987), while Mason et al. (1984) provided a quantitative description of the riparian tree and shrub flora of West Midland England and Wales. This general lack of interest in riparian trees is surprising in view of their considerable landscape and wildlife value (Mason et al., 1984) and their importance in river management, especially as an aid to waterweed control (Dawson, 1989) and to agricultural pollution control (Lowrance et al., 1984; Peterjohn and Correll, 1984; Pinay and Decamps, 1988), because of the filtering role of riparian woods.

The present study provides a quantitative description of the vegetation of trees and woody shrubs (hereafter referred to as trees) along watercourses in East Anglia, an area which, overall, has received much greater management than that of the previous study of Mason et al. (1984). The loss of riparian trees over time is also described for selected rivers in the region.

0886-9375/90/020159-08$05.00 01990 by John Wiley & Sons, Ltd.

Received 31 August 1989 Accepted 24 November 1989

160 C. F. MASON AND S. M . MACDONALD

METHODS

Between September 1985 and September 1987,50 stretches of 1 km lengths of water course in the counties of Norfolk, Suffolk, and Essex were surveyed (Figure 1). The stretches were selected to represent a range of river discharges (with long-term average flows < 0.31 m3s-l to 40-80 m3 s - l , from maps in the Department of the Environment, 1978). Stretches consisted of stream order 1 to 3, and included both small brooks and major rivers, including upper, middle, and lower reaches of watercourses, but without prior knowledge of the tree flora. All trees on both banks, with their boles within 2 m of the channel, were identified, counted, and divided into categories of either mature trees or saplings (defined as less than 3 m tall with no evidence of having produced flowers or fruits). Stems arising in clusters in close proximity were considered to derive from one individual, whereas spaced stems were counted independently (see Mason et al., 1984, for further details of methods). A key to the genus Salix, based on the characteristics of twigs and mature leaves, was devised from species likely to occur in the region, using information in Meikle (1984) and Brendell (1985).

Figure 1. Study area in East Anglia showing rivers and sites

RIPARIAN TREE COMMUNITIES 161

To demonstrate the rate of tree loss from river banks, the rivers Colne, Blackwater, and Stour were considered. From the source to the tidal mouth, all riparian trees marked on the 1879 Ordnance Survey maps (scale 6 inches to 1 mile, equivalent to 9.5 cm km-') were counted. Similarly all trees observed on aerial photographs (scale 1: lO 000 at 6000 ft, approx. 1830 m) taken in 1960 and 1970 were counted. We have assumed that only mature trees were included on the 1879 map and that these were the only trees recognized as distinct on the aerial photographs.

RESULTS

The overall tree community of the 50 1-km stretches of watercourse is presented in Table I. A total of 48 species of trees were recorded. Two stretches (49, Wissey and 50, Blyth) had no trees, while the maximum density was 185 km-'. Alder (Alnus glutinosa) made up 19.4 per cent of the community, followed by white willow (Salix alba) (1 1.9 per cent), hawthorn (Crataegus monogyna) (10.2 per cent), and crack willow (Salix fragilis) (8.3 per cent). No other species made up more than 5 per cent of the overall community. Tree density averaged 73 km- '. Of mature trees alder, white willow, and crack willow made up 56.8 per cent of the total, which averaged 36 km-'. No one species dominated the sapling community, which averaged 37 trees km-'. Alder, white willow, and crack willow occurred most frequently as mature trees, while other potentially large species, such as oak (Quercus robur), ash (Fraxinus excelsior), sycamore (Acer pseudoplatanus), and elm (Ulmus carpinijiolia) were present predominantly as saplings, the latter mainly suckering regrowth from trees killed by Dutch elm disease.

The frequency distributions of mature trees and saplings are shown in Figure 2. Overall, 58 per cent and 46 per cent of stretches had less than 25 mature trees and saplings respectively. Only 10 per cent of stretches had more than 75 mature trees and 16 per cent more than 75 saplings. The frequency distributions of the commoner species are shown in Figure 3. The most frequent densities for all species were less than 10 trees km-'.

TWINSPAN (Hill, 1979) was used to classify sites and taxa using quantitative data on the trees (Hill et al., 1975). This method arranges sites into hierarchical groups on the basis of their taxonomic composition, while species are simultaneously classified on the basis of their occurrence in site groups. The technique also identifies indicator species, which show the greatest difference between site groups in their frequency of occurrence. Sites 49 and 50, with no trees, were omitted from the analysis. TWINSPAN was arrested at level 2 and the classification is shown in Figure 4. Table I1 gives the composition of the site groups and their indicator species.

Group A consists of all sites on the River Colne, together with one of the two sites on its tributary the Roman River. Its other site is from the River Gipping, thus the distribution is at the southeastern corner of the study area. Elm (Ulmus carpinifolia), blackthorn (Prunus spinosa), and maple (Acer campestre) were indicator species and formed a significant component of the tree flora. Group B includes eight sites on Breckland rivers, draining westwards into the Great Ouse. Both sites on the Ramsey River and three of six sites on the River Stour are also included. Salix species were well represented in this group. Group C contained six sites on Breckland rivers and four on rivers draining into the Broads, with a single site on the Stour. Sallow (Salix cinerea) and cultivars of black poplar (Populus x nigra) were characteristic of these sites. Group D is a geographically diverse group of sites, with a preponderance of white willow (Salix alba) in the tree community.

The flow class of the rivers had no influence on the groupings of rivers in terms of vegetation. For example Group B contains stretches with long-term average flows less than 0.3 m3 s- (31, 32,39) and a stretch with a long-term average flow greater than 20 m3 s- ' (26). Tree density was also highly variable within groups, ranging for example from 21 to 154 km-' in Group C. In the catchments of the Wissey, Little Ouse, Stour, and Colne, 7, 10,6, and 7 sites respectively were surveyed. The TWINSPAN groupings bore no relationship to the location within the catchment, for all four groupings had representatives of both the highest and the lowest survey sites within these catchments.

The tree densities on the Rivers Colne, Stour, and Blackwater in 1879, 1960, and 1970 are given in Table 111. It is assumed that these are equivalent to mature trees counted in the field. The Stour was navigable to

Tabl

e I.

The

over

all t

ree

com

mun

ity of

fift

y 1

km s

tret

ches

of w

ater

cour

se in

Eas

t Ang

lia

Mat

ure

Sapl

ings

T

otal

% o

f %

of

%

% o

f Sp

ecie

s nu

mbe

r to

tal

km-'

nu

mbe

r to

tal

km-'

Sa

plin

gs

num

ber

tota

l km-'

Salix

alb

a S.

frag

ilis

S. c

iner

ea

S, v

imin

alis

ot

her

Salix

spp

. Al

nus

glut

inos

a

Que

rcus

robu

r Fr

axin

us e

xcel

sior

A

cer p

seud

opla

tanu

s Sa

mbu

cus n

igra

C

rata

egus

mon

ogyn

a Pr

unus

spin

osa

UIm

us c

arpi

nifo

lia

othe

r sp

ecie

s

Tot

al tr

ees

Popu

lus s

pp.

315

17.5

6.3

23

3 12

.9 4.7

15

0.

8 0.3

0

0 0

18

1.0

0-4

477

26.4

9.5

96

5-3

1-9

40

2.2

08

81

4-

5 1.6

136

7.5

2.7

98

5.4

2.0

137

7.6

2.7

4 0.

2 0.1

11

9 6-

6 2.4

1804

10

0 36

35

1.9

a7

119

6.4

2.4

72

3.9

1.4

101

5-4

2.0

50

2,7

1.0

40

2.2

0.8

234

12.6

4.7

56

3.0

1.1

72

3.9

1 -4

88

4-7

1.8

109

5.9

2.2

230

124

4.6

277

14.9

5-

5 31

1 -

7 0.

6 14

1 7.6

2-

8 23

6 12

-7

4.7

1856

10

0 37

27.4

23.6

87.0

100.

0 69

.0

32.9

36

.8 64

.3 52

.1

76.7

62.8

73

.9

18.5

97

.2

66.5

51.6

434

11.9

8.7

305

8-3

6- 1

116

3.2

2.3

50

2.7

1 .o

58

1.6

1.2

711

19-4

32

.9 15

2 4.2

3-

0 11

2 3.1

2.2

16

9 4.6

3.4

14

4 3-

9 2.9

36

6 10

.0

7.3

375

10.2

7.5

168

4.6

3.4

145

4.0

2-9

355

9.7

7.1

3660

10

0 73

.2

0

7

RIPARIAN TREE COMMUNITIES

- 40-

20-

163

- Salix fragilis

mature

1 50 15 100 >loo 1 0

saplings

trees km-1

Figure 2. Frequency distribution of mature trees and saplings on 50 I-km stretches of watercourse in East Anglia

1 Fraxinus excelsior

L Salk alba 1 L

trees

M jl 0 1

km-1

Crataegus rnonogyna

-h 20 30 40 50 60>W

Figure 3. Frequency distribution of the commoner tree species in 50 I-km stretches of watercourse, East Anglia

164 C. F. MASON AND S. M. MACDONALD

Ulmus carpinifolia A c r r camp. s t re Prunus avium Taxus baccatr Prunui spinosa Crataegus monogyna Fraxinus excrlsior Acer pseudoplatanus Thtlycranir sanguine. Ligustrum vulgar. Syringa vulgrris fagu. sy1vatica Aesculus hippocastaneun

Salix tragtlis Alnus glutinosa Corylus avellana Tilia x europoa Rhrnnus catharticus Prunus Iaurocerasus Salix alba Salia vlninalim Salix caprra

Saabucus nigra Botula pendula

Laburnum magyroidrs Srltx alba coeruler Euonynus ruropaeus Srlix purpurra Salix pentandra Quercus robur Synphorrcrrpus rrvulare Salrx cinrrrr Salix triandra U l e x europaeus S l r x babylonrc. Populus x nlgra Viburnum opuius

I

k e y : 1-25.026-50;051-75..76-100 percent

Figure 4. Classification of tree species and site groups by TWINSPAN (species occurring at only a single site omitted). The percentage frequency of occurrence within sites in each group is indicated by the size of dot

Sudbury (Figure 1) during the last century, so that stretches of river above and below the town are treated separately. It is apparent that extensive tree removal had occurred on the rivers prior to 1879, especially on the navigable Stour. The rate of tree clearance along the River Colne, which still had a reasonable density in 1879, was 8 per cent per decade over the study period, though this does not allow for trees which grew and were felled between periods. All sites, except the navigable Stour, had a reduced density of trees in 1970 compared to 1879. The average density of trees, large enough to be counted on aerial photographs, which would appear to be acceptable to watercourse managers with current attitudes is 25-35 trees km- ' (Table HI), which is similar to the most frequent density of mature trees in the study area found during the field survey (Figure 2).

RIPARlAN TREE COMMUNITIES 165

Table 11. Composition of site groups and indicator species in a TWINSPAN analysis of riparian trees at 48 1 km stretches of watercourses in East Anglia

~~~~

Indicator species at the first division: Groups A and B, Fraxinus excelsior, Crataegus monogyna; Groups C and D, Salix cinerea.

Group A: sites 33,42,43,44,45 (Colne); 6 (Gipping); 38 (Roman).

Group B: sites 4,15,36 (Wissey); 13,20 (Little Ouse); 23 (Thet); 16,18 (Lark); 11 (Wensum); 26 (Yare); 46,47 (Blyth); 40, 41 (Gipping); 5,22, 34 (Stour); 31, 32 (Ramsey); 39 (Roman).

Group C: sites 14; 19; 24 (Little Ouse); 17 (Lark); 25 (Thet); 1 (Wissey) 10, 12 (Wensum); 8, 9 (Bure); 37 (Stour).

Group D: sites 28, 29 (Little Ouse); 2, 3 (Wissey); 7 (Bure); 27 (Chet); 30 (Waveney); 21, 35 (Stour); 48 (Minsmere).

indicator species, Acer campestre, Ulmus carpinifolia, Prunus spinosa.

indicator species, Salix cinerea, S. fragilis, S. alba.

indicator species Salix fragilis, Alnus glutinosa, Populus nigra, Sambucus nigra.

indicator species, Salix alba.

Table 111. Tree densities along three Essex rivers determined from maps (1879) and aerial photographs (1960, 1970)

River Length Trees per km (km) 1879 1960 1970

Colne 31 91 33 27 Blackwater 48 42 37 36 Stour, above Sudbury 31 46 24 25

below Sudbury 34 28 33 32

DISCUSSION

The only comparable study of riparian tree vegetation in Great Britain is that of Mason et al., (1984). Their study area in Wales and adjacent English counties, an area predominantly of livestock and mixed farming, had received generally less management that the present study area, where the agriculture is largely arable. In the Welsh study it was estimated that the maximum density of mature trees along river banks was 200 km- '. As early as 1879 the tree density along the River Colne, which had the greatest density of the three rivers studied over time, had less than 50 per cent of this potential maximum. Of the 50 stretches of river surveyed in the field, 68 per cent had a mature tree density less than 40 km-', compared with only 10 per cent of fifty 5-km stretches in Wales. In Wales 60 per cent of stretches had more than 80 trees km- ', compared with only 12 per cent in East Anglia. Some 12 per cent of stretches in Wales had more than 160 mature trees km-', i.e. approaching the maximum density, but no sites in East Anglia did. This emphasizes the much greater level of management applied to East Anglian rivers compared to those in the west of the country, with mature trees having been felled (Table 111) and saplings being cleared before they mature.

The tree communities of the two regions were also very different. Alder dominated both communities, but made up 64 per cent of the mature tree community in Wales, compared to 26 per cent in East Anglia. Conversely, combined species of willows made up only 6 per cent of the mature tree community in Wales, compared with 32 per cent in East Anglia. Mature specimens of oak, ash, and sycamore were also more significant in Wales, where they comprised 27 per cent of the total community of mature trees, compared to only 9 per cent in East Anglia. Densities of these species were also significantly greater in Wales, where they have considerable conservation value (Macdonald and Mason, 1983). The proportion of saplings in the two communities was similar, at about 50 per cent. Despite the low density of trees in East Anglia, the species richness was greater than in Wales, with 48 species recorded in 50 km of riverbank, compared with 43 species in 250 km.

166 C. F. MASON AND S. M . MACDONALD

Four groups of species were identified in the TWINSPAN analysis. Group A, with a high proportion of elm, maple, and blackthorn, characterized the sites in the River Colne, but the other three groupings of trees were geographically scattered. The species in Group A are characteristic of woodland, as opposed to riverside communities, but why they should occur in the Colne is unknown. There was no relationship of the groups with flow class, as observed also by Mason et al. (1984) in Wales, nor with distance from source or level of management. In a study of the entire length (118 km) of the River Teme (Wales/West Midlands), alders decreased in density downstream, while Salix spp. increased, but there were no consistent patterns with other species (Macdonald et al., 1978). All the rivers in the present study rise in the lowlands and are regulated from source. The tree community which develops at any particular site is probably dependent mainly on local and upstream sources of propagules. On unregulated rivers in Sweden, Nilsson et al., (1988, 1989) observed that species richness and the presence of rare species could not be predicted from downstream position or overall type of river habitat, each river having largely unique vegetation features.

With the current changing attitude to river management there is considerable scope to revegetating river banks in East Anglia with trees and shrubs. Thus far, any planting has been of cricket bat willow (Salix alba var coerulea) and poplars in regular rows. Natural regeneration could be encouraged by fencing off river banks, especially where arable fields abut the riparian zone. The resultant growth would benefit both landscape and wildlife and may go some way to suppressing the vigorous community of arable weeds which often dominate the narrow strip between crop and water, as well as significant weed growths within the river.

ACKNOWLEDGEMENTS

We would like to thank David Childs for help in computing and Nina Elston and Rita Bartlett for typing the manuscript.

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