Invasion of Alnus glutinosa (L.) Gaertn. in a former grazed meadow in relation to different grazing intensities

Biological Conservation 25 (1983) 75-89

Invasion of Alnus glutinosa (L.) Gaertn. in a Former Grazed Meadow in Relation to Different Grazing Intensities Erik Vinthert Botanical Institute, University of Arhus, Nordlandsvej 68, DK-8240 Risskov, Denmark ABSTRACT The invasion 6fAlnus glutinosa in a former grazed meadow in Horret Wood near .4thus (Eastern Jutland, Denmark) has been investigated. A dense Filipendula ulmaria vegetation, which has been developing since the grazing stopped, seems to prevent the establishment of seedlings of Alnus glutinosa, although the seeds are able to germinate in the spring, bejbre the growth of Filipendula ulmaria starts. By reconstructing the development of four Alnus glutinosa thickets, it was concluded that moderate grazing favours the spread of Alnus glutinosa, whereas intensive grazing or cessation of grazing inhibits further invasion. The optimal grazing intensity Jbr establishment of trees and bushes in meadows is discussed and related to the 'floristic optimum'. Two situations in the management of meadows with grazing animals are proposed.

INTRODUCTI ON All meadows are a product of culture, either by grazing or hay mowing, or by a combination of these two forms of agricultural use. The vegetation is low and often contains a considerable number of different plant species. T o d a y such marginal agricultural land is in many cases left idle, or utilisation is reduced, and this situation starts a vegetational succession t Present address: Fyns amtskommune, Fredningsafdelingen, Orbaekvej 100, DK-5220 Odense SO, Denmark. 75 Biol. Conserv. 0006-3207/83/0025-0075/$03.00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain

Erik Vinther

76

(Godwin, 1929, 1936; Regnell, 1975; Larsson, A., 1976; Larsson, B. M. P., 1976). The first step in this succession is typically a vegetation of tall herbs with a dense rhizomatous growth. This vegetation is generally poor in species, and in Denmark may consist of Filipendula ulmaria, Epilobium hirsutum,

Urtica dioica, Lysimachia vulgare, Phragmites australis, Carex disticha, Carex acutiformis, and in many cases also woody species such as Alnus glutinosa, Salix cinerea, Salix aurita, Salix pentandra, Betula spp., and Pinus mugo (Vinther, 1980a). In order to formulate management plans for such meadows and fens, it is very important to know the mechanisms of this succession. In the meadow in H~rret Wood the invasion of Alnus glutinosa was examined. The investigation was based upon experiments on seed germination and establishment of seedlings of A.glutinosa, and on a relation between the age structure in four A. glutinosa thickets and the different grazing intensities in the meadow.

THE E X P E R I M E N T A L SITE The experimental site is formerly grazed meadow, approx. 4 ha in extent, in Horret Wood near Arhus, Denmark (Fig. 1). The soil can be classified as a gley and is characterised by a high ground-water level, which fluctuates from the soil surface to a depth of about 60 cm. TABLE 1 The Former Utilisation of the Meadow

Period

Grazing pressure

c. 1925-48

c. 15 cows

c. 1950-60

c. 6 sheep

1961-66 1967-71

Intermittent grazing, including two years without grazing 4 ponies + 3 calves

1972

3 calves

1973-81

No grazing

Rejerence Mr E. Jensen, ranger, (pers. comm. 1979) Mr A. Nielsen, farmer, (pers. comm. 1980) Mr A. Nielsen, farmer, (pers. comm. 1980) Mr A. Nielsen, farmer, (pets. comm. 1980) Mr A. Nielsen, farmer, (pets. comm. 1980) Personal visits

Alnus glutinosa and grazing

Fig. 1.

Location

of the meadow in Hsrret Wood near Arhus, Denmark.

77

Eastern Jutland,

The former utilisation of the meadow is known from personal communications with people living nearby, and from personal visits to the site since 1971 (Table 1). In the period 1968 to 1972 the meadow was characterised by a low vegetation containing a large population of Dactylorhiza majalis. There is no vegetational analysis from this stage of succession, but the conditions are known from a series of excellent colour photographs taken by Mr K. Halberg. Today the vegetation consists of

Erik Vinther

78

N

195G

1968

A

•

OTHER WOODY PLANTS

1976

Fig. 2. The development of woody vegetation in the meadow as seen from air photographs. In 1976 there is a distinction between thickets of Alnus glutinosa and other trees. The position of thickets A, B, C and D is given.

tall perennial herbs, especially Filipendula ulmaria, and on the fringe of the area there are several thickets of A.glutinosa. Figure 2 shows the locality and the development of the thickets as seen from air photographs.

METHODS

Germination and establishment of Alnus glutinosa Two quadrats (2 x 2m) were laid out in the Filipendula ulmaria vegetation at the beginning of April 1979. One was cleared of litter and rhizomes of Filipendula ulrnaria, whereas the other was used as a control. In both plots five plastic pots filled with soil from the site were put down on a level with the soil surface. In filling the pots, the natural stratification in the soil was, as far as possible, kept intact. To allow free drainage the bottom of each pot was perforated. On 11 April 1979 50 seeds of A. glutinosa were sown in each pot, which were then covered with fine-meshed chicken wire to prevent vertebrate predators eating the seeds. The seeds used in the experiment were all collected from the same tree on 20 March 1979 and kept in a refrigerator.

Alnus glutinosa and grazing

79

The number of germinated seeds and established seedlings were counted on 15 May, 8 June, 14July, 10August and 14September 1979. Seeds were identified as germinated when root hairs were visible on the primary root, and seedlings were identified as established when the first leaf above the cotyledons had appeared. Colonisation Four thickets of A.glutinosa (A, B, C and D) were used in the investigation. A grid consisting of 10 x 10 m squares was set up in thickets B, C and D, and a grid with 2 x 2 m squares in A. In each square the positions of all trees were noted by measuring their distances from two of the corners in the square. The deviation was estimated to +2½cm. No cases of vegetative regeneration were found. The girth of the trees (total number of 658 trees) was measured at a height of 75 cm above ground level. 57 individuals of A. glutinosa were chosen at random in thickets A and D, and the trees were aged at the trunk base by increment boring and by counting growth rings. Linear, single logarithmic and double logarithmic regressions of age versus girth were calculated. The double logarithmic regression was used

Fig. 3. Double logarithmic regression of age versus girth at the height of 75 cm. The regression is based on 57 individuals of Alnus glutinosa. R 2 = 0.92. Slope = 1-87. Intersection with the ordinate: -0.55. Intersection with the abcissa: 0.2942, antilog 0.2942 = 1-97.

80

Erik Vinther

to predict the age of the rest of the trees because it had the best fit (R 2 = 0.92) of the three regressions. SE was calculated to 1.25 years. A girth of 80 cm was the maximum value used in the regression, and trees with a larger girth were aged directly. Figure 3 shows the regression. To check the fitness of the regression, 10 trees were aged at the trunk base and at a height of 75 cm. The difference in nine of these trees was two years and in the last tree one year. In theory this difference should be the same as the value at which the regression and the abcissa intersect. This value was calculated as 1.97 years. The trees in the four thickets were classified in age groups of four years, and their positions were pictured on charts on the scale of 1:100, which was later further reduced. From these charts the distribution of the thickets in 1965 and 1972 was reconstructed. The nomenclature of all plants follows Rostrup & J~rgensen (1977).

RESU LTS

Germination and establishment The results of the germination and establishment experiment (Table 2) show that on 15 May the germination conditions were about the same in the two plots. The Filipendula ulmaria vegetation gradually grew up, and on following inspections the cleared plot exceeded the control plot in number of established seedlings. Finally on 14 September there were no surviving seedlings in the control plot, and the soil was covered with litter from the F. ulmaria vegetation. Even in the cleared plot, there was a high rate of mortality in the period from germination to seedling establishment.

Colonisation The age structure of the four thickets (Fig. 4) indicates that the thickets were all established in 1959-60. In the period 1965-1970 the largest number of trees were established, but since 1976 no trees have been able to develop. In principle, then, the four thickets have the same age structure with very few old and young individuals, but many between 5 and 15 years old. Figures 5, 6 and 7 show the distribution of the trees in thickets C and D

Ainus glutinosa and grazing

81

TABLE 2 The Germination and Establishment Experiment with Seeds from Alnus glutinosa in a Filipendula ulmaria Vegetation a

Date 1979

15 May

8 June

14 July

10 August

14 September

Cleared plot Control plot Pot Number oJ Number oj no. Germinated Established Germinated Established seeds seedlings seeds seedlings 1 2 3 4 5 .~ 1 2 3 4 5 ~f 1 2 3 4 5 .f 1 2 3 4 5 ~? 1 2 3 4 5 ~?

12 19 5 17 22 15 2

1

1

12 12 2 6 3 7-0 + 4.3 12 13 2 7 6 8.0 ___4.0 13 14 2 6 6 8-2+4.6 12 13 2 6 3 7-2+4.5

20 23 8 22 21 19 3

Filipendula ulmaria

t'egetation

Sparse and about 10cm in height

7 1 0 0 0 1-6 + 2.7 7 0 0 0 0 1.4 + 2.8 2 0 0 0 0 0.4+0.8 0 0 0 0 0 0+0

Dense and about 70 cm in height

Very dense and about 120cm in height

Very dense and about 120cm in height

Dense and about 120cm in height. Dead leaves cover the soil surface

a On the five dates the seeds were classified as germinated from the moment when the root hairs were visible on the primary root. But when the first leaf above the cotyledons appeared, the seedlings were classified as established. The mean values ()~) are supplied with _+ SD (SD = x/E(x - ~)/n).

Erik Vinther

82 NUMBER OF TREES

NUMBER

OF TREES

A

B

16 12 8

0

0

~ 1975

12 ,16

1965

20 2~, 2B 32 36 &O ~.6 4B YEARS

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8

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D

2O

16 12 8

0 19~/5

12 16 20 26 28 32 36 60 ~ ~B YEARS 19~S 1965 1955 ' 19'35

0

0

~ B 12 16 20 26 28 32 36 60 ~ ~B YEARS 197S 19'bS 19'SS 19~5 193S

Fig. 4. The age structure of A lnus glutinosa in thickets A, B, C and D. (The dates refer to the moments when the trees in the different age classes have germinated.)

in 1965, 1972 and 1979, respectively, representing different grazing intensities (Table 1). In 1965 (Fig. 5) C consisted of only three trees in the age class 5 to 8 years, and these were in the eastern end of the thicket. Individuals in the age class 1 to 4 years extended the thicket towards the west, and a few newly germinated A. glutinosa enlarged the thicket in an easterly direction. D consisted of individuals in the age class 1 to 4 years and a few newly germinated plants. Two older trees standing between C and D were surveyed as belonging to D. The distribution of trees in C and D in 1972 (Fig. 6) indicated a large extension of the thickets. C had extended in the directions east, west and south, and at the same time, new individuals had grown up within the border of the thicket and filled up some of the spaces between the older trees. The extension towards the west followed a drainage channel. D had

1965

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(YEARS) -

GERMINATED

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Thickets C and D in 1965 reconstructed from their age structure in 1979. Open circles represent the positions of the trees and their ages.

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86

Erik Vinther

extended in all directions in two waves, which is clearly seen in the southwest direction. The extension of the thickets in the period from 1967 to 1972 appears on the 1979 chart (Fig. 7) as all individuals in the age classes 1 to 4, 5 to 8 and 9 to 12 years, apart from those trees marked with solid circles. Furthermore it can be seen that the thickets did not extend in the period from 1972 to 1979, the new individuals only filling up spaces between the older trees. The pattern of extension of the thickets A and B is in principle the same as the two just described above, and will not be shown. DISCUSSION The indirect method used in this paper to determine the age of the trees is a modification of a method of Leak (1974, 1975), who used a regression of age versus diameter at the trunk base. R/-values were between 0.5 and 0.92. In the present paper girth is preferred to diameter because of the irregularity of the stems, ensuring that the results can be reproduced. The measurement of the girth at a height of 75 cm is preferred to the trunk base, because many of the stems ramified below that height. The reconstruction of the development of the thickets only records trees that have survived until today. Thus the method cannot quantify the established trees, but it is possible to record historical events in the meadow. The results indicate that there is a correlation between the different grazing intensities and the establishment and development of the A.glutinosa thickets. Even if A.glutinosa is more or less avoided by grazing animals (13dum, 1969; Olsson, 1975), a grazing intensity of approximately 6 sheep or approximately 15 cows (Table 1) is able to prevent establishment of the trees (Fig. 4). After the sheep grazing stopped in 1960 and was replaced by intermittent grazing, the first A. glutinosa was able to establish. In the period from 1967 to 1972 the grazing intensity was too weak to prevent the growth of A. glutinosa, and the thickets extended (Fig. 7). But since the cessation of grazing in 1972, there has been no further extension of the thickets (Fig. 7), probably because the tall herb vegetation grew up, shading and smothering the tree seedlings either directly through growth or through litter fall (Table 2). In a Cladium mariscus and a Molinia coerulea vegetation, McVean (1956a) found the same effect on germinated seeds and seedlings of

A. glutinosa.

Alnus glutinosa and grazing

87

The invasion of other trees, e.g. Salix aurita and Crataegus sp., is also favoured by moderate grazing, but a sudden cessation of grazing prevents further establishment of the two species (Regnell, 1979). Results obtained by Haslam (1965) and Eriksson et al. (1976) also indicate the inability of tree seedlings to establish once grazing or mowing has stopped because of the developing herb vegetation. Only within the thickets do new individuals of A. glutmosa continue to establish in periods without grazing, but the filling up of spaces between the older trees stopped in 1976, probably because the canopies closed, and consequently the thickets have no internal regeneration. Such a lack of internal regeneration in A. glutinosa thickets has been proposed but not proved by Kujala (1924) and McVean (1953, 1956a, b). From the results discussed above, a generalised relationship between grazing intensity and tree establishment in meadow communities can be proposed (Fig. 8a). When there is no grazing, the herb vegetation inhibits the invasion of most trees, and only a few individuals are able to establish by chance. Moderate grazing favours establishment, probably by reducing the influence of the herb vegetation on the tree seedlings and by breaking up the surface turf (McVean, 1953). These events probably increase the numbers of 'safe sites' that contain the precise conditions for successful germination and establishment of seeds on a micro-scale (Harper, 1977). At a certain grazing intensity the food becomes scarce, and the grazing animals increasingly predate on the trees. This predation counteracts the advantages of establishment produced by the animals,

b

(2[ oJ

c

I I l

i

x2 grazing intensify

grazing intensity

Fig. 8, (a) The relationship between grazing intensity and establishment of trees for Alnus glutinosa but is proposed for all trees invading meadows, x2 represents the hypothetical grazing intensity that enables the largest number of trees to become established. (b) The relationship between grazing intensity and the number of plant species in meadows, x t represents the hypothetical grazing intensity that produces the 'floristic maximum' (maximum number of species). The figure is loosely outlined from results obtained by Steen (1956, 1957, 1958).

88

Erik Vinther

and at a certain intensity the number of newly established trees will fall. At a very high intensity only trees adapted to resist grazing may survive (Duffey et al., 1974). Steen (1956, 1957, 1958) has found that a certain grazing intensity in different kinds of meadows produces a 'floristic optimum' (maximum number of plant species). A reduction or increase in this intensity results in a reduction of the number of species. This relationship is loosely shown in Fig. 8b. Naturally the grazing intensity which produces the 'floristic optimum' differs in different meadow communities, depending on the physical, chemical and biological factors. In conservation practice the 'floristic optimum' is very often identical with the desired plant community. If in a plant community the grazing intensity which produces the 'floristic optimum' is called x l, and the intensity which allows the maximum number of trees to establish is called x 2 (Fig. 8a, b), it is very important to know the relationship x~ :x 2 in order to formulate a management plan for the area. If xa ~> x 2 or x~ ,~ x 2 management with the grazing intensity xa produces a relatively stable vegetation, rich in species, but if x 1 and x 2 are about the same, there will be a relatively quick invasion of trees. If this is the case, either mowing instead of grazing must be considered, or a removal of trees every 3 to 5 years must supplement grazing, as the trees change the flora and the soil conditions (Vinther, 1980b).

ACKN O W L E D G E M E N T S I am grateful to Mr K. Haiberg, Mr E. Jensen and Mr A. Nielsen for information on the area, to Mr O. F. Nielsen for help in the field, to Ms I. E. Vinther for help in the field and for preparing some of the figures, and to Mr V.J. Larsen, Mr S. Laegfird and Ms H. Pedersen for their comments on the manuscript.

REFERENCES Duffey, E., Morris, M. G., Sheail, J., Ward, L. K., Wells, D. A. & Wells, T. C. E. (1974). Grassland ecology and wildlije management. London, Chapman and Hall.

Alnus glutinosa and grazing

89

Eriksson, B., Kvist, K-A., Steen, E. & Wir~n, A. (1976). Ekologiska ~terverkningar av bete, slitter och fridlysning i tjugo~-riga f/iltf6rsSk. In Gjengroning av kulturmark. Internordisk symposium 27-28 Nov. 1975, 8 pp. As, Norges Landbrukshogskole. Godwin, H. (1929). The sedge and litter of Wicken Fen. J. Ecol., 17, 148-60. Godwin, H. (1936). Studies in the ecology of Wicken Fen, III. The establishment and development of fen scrub (carr). J. Ecol., 24, 82-116. Harper, J. E. (1977). Population biology of plants. London, Academic Press. Haslam, S. M. (1965). Ecological studies in the Breck Fens, I. Vegetation in relation to habitat. J. Ecol., 53, 599-619. Kujala, V. (1924). Das Vorkommen der Schwarzerle in Finland. Commun. Inst. ]br. Finl., 7. Larsson, A. (1976). Den sydsvenska fukt/ingen. Vegetation, dynamik och skStsel. Meddn. Avd. Ekol. Bot., Lund, 31. Larsson, B. M. P. (1976). Vegetation och markanv/inding i ett historisk perspektiv. In Gjengroning at, kulturmark, lnter-nordisk symposium 27-28 Not'. 1975. As. Norges Landbrukshogskole. Leak, W. B. (1974). Some effects of forest preservation. USDA For. Serf. Res. Pap. NE 186, 1-4. Leak, W. B. (1975). Age distribution in virgin red spruce and northern hardwoods. Ecology, 56, 1451-4. McVean, D. N. (1953). Biological flora of the British Isles. Alnus. J. Ecol., 41, 447-66. McVean, D. N. (1956a). Ecology of Alnus glutinosa (L.) Gaertn., III. J. Ecol., 44, 195-218. McVean, D. N. (1956b). Ecology ofAlnus glutinosa (L.) Gaertn., V. J. Ecol., 44, 321-30. IDdum, S. (1969). De vildtvoksende traeer og buske. Danmarks Natur(6), 143-99. Olsson, G. (1975). Inverkan av betning och annan skStsel p~ hagmarkers vegetation. Svensk bot. Tidsskr., 69, 393-404. Regnell, G. (1975). Dyngets Naturreservat. Meddn. Avd. Ekol. Bot., Lund, 8. Regnell, G. (1979). Vegetationsf6randringer vid upphSrande bete i en sk~nsk kalkfugtfing. Svensk bot. Tidsskr., 73, 139-59. Rostrup, E. & J~rgensen, C. A. (1977). Den danske flora. Copenhagen, Gyldendal. Steen, E. (1956). UndersSkningar 5ver betningens inflytande i tre naturbeten. Statens JordbruksJ~rs6k Medd., 74, Stockholm. Steen, E. (1957). Betningens inverkan p~ v/ixtlighet och mark i en malarstrand~ing. Statens JordbruksJ'6rs6k Medd., 83. Stockholm. Steen, E. (1958). Betesinflytelser i svensk vegetation. Statens Jordbruksf6rsSk Medd., 89, Uppsala. Vinther, E. (1980a). Enge og kaer--truede samfund. URT, 80, (4), 99-105. Vinther, E. (1980b). lndvandring afAlnus glutinosa (L.) Gaertn. pd en tidligere graesset eng i Horret Skov red Arhus reed efterfolgende aendringer i vegetation, grundvandstand og jordbund. Et successionsstudium. Thesis, Botanical Institute, University of Arhus.