Biological Conservation 19 (1980-81) 15-25

R E C O V E R Y O F P L A N T C O M M U N I T I E S O N C O A S T A L S A N D - D U N E S D I S T U R B E D B Y H U M A N T R A M P L I N G

TOVE HYLGAARD

Botanical Institute, University of Aarhus, 68, Nordlandsvej, DK-8240 Risskov, Denmark

ABSTRACT

The resilience of plant communities to disturbance can be characterised by the abilitv o/species and liJe-Jbrms to recover, provided that the desired degree of naturalness is present. Results of a case stud)' in two ecological types of Danish sand dunes, the grey and the white dune, show that dominant growth morphology and initial pattern ~?/ recovery are inter-related. The propagation of above-ground organs leads to progressive lateral recover),, while subterranean prol(feration by means of rhizomes leads to erratic: recover)'. Species behave in dij.]erent ways depending on the environment, and patches of bare ground generally grow over slowly, unlike well- established patches of vegetation where environmental conditions are more suitable. Because of this change of structure, vegetation recovery is not assessable hy traditional recording of individual factors such as number of species or percentage cover, and a list of the key factors involved is given. The erratic pattern of recovery o~ rare species stresses the importance of long-term recording of vegetation to assess recovery.

INTRODUCTION

The effect of outdoor recreation on natural and semi-natural habitats is a subject of current interest to research workers, conservationists and managers. Picnicking, horse riding, car driving and walking are types of activities, which have all been demonstrated to affect wildlife even at low intensities. The effects reported are numerous, the overall objects of concern being any easily recognisable response such as changes in the species composition, diversity, cover, biomass, primary and secondary production and soil compaction (Speight, 1973; Satchell, 1976). The extent of a response may vary from one habitat to another, indicating differences in

15 Biol. Conserv. 0006-3207/80/0019-0015/$02-25 © Applied Science Publishers Ltd, England, 1980 Printed in Great Britain

16 TOVE H Y L G A A R D

vulnerability between habitats. In relation to the impact of trampling Liddle (1973) has suggested that the vulnerability of different habitats may be compared by estimating the number of passages necessary to cause a 50 ~ reduction of vegetation cover or biomass. Besides direct responses of the biotic environment to physical impact, Bayfield (1979) has stressed the importance of an additional factor to characterise the resilience of an ecosystem, notably the need to assess the ability of species or communities to recover from disturbance.

The time needed for an area to recover depends on two main factors: (1) the severity of damage, which means the deterioration not only of the vegetation cover and animal populations but also of the physical and chemical environment; (2) the potential for recovery of the community. 'Recovery' is a multiple factor in time and space and it is no surprise that it has different meanings in different contexts. The same area may show a series of recoveries ranging from re-establishment of a grass cover as a result of seeding, addition of fertilizers and watering, to a complete return to the original state of the habitat. The first type of recovery may be called 'physical' and the second 'ecological'. Similar to 'carrying capacity', the implications of which have been discussed by Burden & Randerson (1972), the term 'recovery' is only valid in connection with a clear stipulation of the desired degree of naturalness and, in the discussion of recovery as an ecosystem property, this stipulation has not yet been given. It is the aim of this paper to discuss the term 'ecological recovery' in relation to time and to focus on the pattern or process of initial recovery in space. The results of a case study in two closely related habitats, the white and grey dunes of a Danish coastal ecosystem, are used to illustrate some of the ideas discussed.

Nomenclature of angiosperms follows Hansen (1973), of bryophytes Rosenvinge & Rostrup (1925) and lichens Dahl & Krogh (1973).

A CASE STUDY

Materials and methods Investigations were started in May 1976 on two old footpaths in the sand-dune

ecosystem of Skallingen, a peninsula on the southwest coast of Jutland, Denmark. One of these footpaths, stand I, was situated in the grey dunes dominated by Empetrum nigrum, while the other, stand II, was in the white dunes dominated by Ammophila arenaria and Festuca rubra. The widths of the both paths were approximately halfa metre and they ran in a N W S E direction, parallel to the dune ridges. Like the rest of the sand dunes of Skallingen the paths were only used for recreation during the summer but previous use of the two paths in terms of number of passes had not been recorded. Instead the paths were chosen on the criterion that they were typical of footpaths in the area. Both were very sparsely vegetated but only on stand II was the bare sand loose and easily moved by the wind. In the early spring of 1976 the two paths were fenced by a 1 m high wire fence to prevent further use.

The vegetation was sampled on four occasions--28 May 1976, 26 September

S A N D - D U N E P L A N T S A N D T R A M P L I N G EFFECTS 17

1976, 5 May 1977 and 20 June 1977. The vegetation was scored by the Lindquist Point Method (Lindquist, 1931), modified to the size of 0.25 m z to fit the narrow paths. With a pin distance of 10cm 25 points were made within each 0.25 m 2. On both stands 18 replicates were made at intervals of 0.5 m on the path as well as in a control plot which ran parallel to the path with a 1-m wide transition zone between path and control. All live angiosperms, bryophytes and lichens hitting the pointed pins were recorded. Pins that did not touch any live plants were scored as 'bare ground'. The degree of cover, the D-value, of each species was calculated as percentage of pin-to-plant contacts, and the accumulated cover by addition of the degree of cover of all species.

Results Data of accumulated cover, percentage of bare ground and number of species on

stands ! and II are given in Figs 1-3. In Figs 4 and 5 the degree of cover is plotted against time for species on the two stands with a D-value of 5 ~o or more on at least one of the sampling days. Species with a D-value below 5 ~ are listed as present or absent in Table 1.

DISCUSSION

The accumulated cover for both stands at Skallingen dunes indicates recovery after approximately one year (Fig. 1A & 1B). However, changes in percentage of bare ground show that recovery is incomplete (Fig. 2A & 2B). Although differences between the path and control sites in both cases has been reduced after one year, the percentage of bare ground is still distinctly higher on the paths than at the control

240

230

m 160

o

~. 120

d ~ 80

o 40

A

~- - - ~ ~ ~_

I I ~ I . I _ _ L

28-5 26 9 5 5 20 6 1976 1976 1977 1977

Fig. 1. T o t a l p l an t cover o f all species in re la t ion to t ime. Sites were fenced in Apr i l 1976 to prevent disturbance by trampling. A, stand I, the grey dune; B, stand II, the white dune. t i - -O, control plots:

O -- - O, footpaths. Vertical bars indicate standard error.

18 TOVE HYLGAARD

60

50

A

Fig. 2.

~40

# ~ 30

~2o [ 1o

I I I I [ [

26-5 26-9 5-5 20-6 1976 1976 1977 1977

Percentage of bare ground in relation to time. Sites were fenced in April 1976 to prevent disturbance by trampling. Symbols as in Fig. I.

sites. This indicates a changed structure of patches of vegetation on the paths, possibly towards an increased cover of individual species within the patches and/or an increased number of species present. In both stands the ratio between number of species on the path and at the control site had increased after one year and in stand II the number of species on the path exceeded that of the control site (Fig. 3A & 3B). Figures 4 and 5 show that by the end of the experiment more than half of the species have similar D-values on path and control sites. As, however, the vegetated areas are smaller on the paths, this indicates a higher percentage of total cover of all species

40

3O 03 Ld

U3 U - 2 0 O

z ~ io

Fig. 3.

A

2 8 - 5 1976

~ o

• I I I

2 6 - 9 5 - 5 20-6 1976 1977 1977

I & I

Number of species in relation to time. Sites were fenced in April 1976 to prevent disturbance by trampling• Symbols as in Fig. 1.

SAND-DUNE PLANTS AND TRAMPLING EFFECTS 19

't \ \

////i- Fo-I I ~ -

L _ _ I I

t i o::

z

c2. I ? 1 -

o2 <

L _ _

CF <~

J

2>

Z

. .J <~ Q)

< cr ~ r n

Q)

[

z

o 2 CL

w ~

1 n

<

u~

c n CK <

<

z 0 c ~ < J

L

t G I

:/ I l

/ /

/ /

/

\ \

\

I I

I I I

(°/o) EI3AO'D ~0 33W~30

L

3

< .E

z ~ '/ i r - . ~

2 ~

t ~

(3:

q c ~

~ 7 Z . - -

i 8

._~

2 0 TOVE HYLGAARD

0 Z

w

L J U')

b 3

I I I I

CO Z

(D

w I

i

c12 m

/

c~

z w mr

0

I I I L

0 0 0 0

n-" m

W LL

c r

7

C~

W Ck

~o-~

I I I

~ -

L I I

boric / \ ~ o ~

o o o o

(°/o) W3AO3 30 33W930

8

O

o ~

r ~

E -

r . r~

, r ~ t -

z w

0_ // ~

o o ~ i .

SAND-DUNE PLANTS AND TRAMPLING EFFECTS 2

TABLE 1 PRESENCE (-}-) OR ABSENCE ( - - ) OF SPECIES BELOW 5 o~ COVER ON STANDS 1 AND 11~ PATH/CONTROL

Species

,4 ira praecox Ammophila arenaria Anthyllis vulneraria Brachythecium albicans Ceratodon purpureus Cladonia Jurcata C. impexa Cor),nephorus canescens Dicranum scoparium Elymus arenarius Erophila verna Galium saxatile G. ITCFI, O~'I Hieracium umhellatum Holcus lanatus Hypnum cupressiforme Hypochoeris maculata Hypogymnia physodes Jasione montana Lotus corniculatus Luzula campestris Oenothera ammophila Phleum arenaria Poa pratensis Polygala vulgaris Polypodium vulgare Rhacomitrium canescens Rhinanthus minor Sedum acre Senecio vernalis S. vulgaris Silene otites Tortula subulata Trifolium arvense T. campestre Veronica officinalis Vicia cracca V. lathyroides Viola arvensis V. canina V. tricolor

Stand 1

28 26 5 20 May Sept. May June 1976 1976 1977 1977

+ i + + / + - i + i + + i + - i + - i - + ! + - I + - 1 + i - +,"+ - I - - i + - - I - - i -

+i+ +i+ +i+ +i+ - i + +i+ - i - - / + + i + + i + + i + +~+ - i + - / - - i - - i -

- / - + i' + - , -

- I + - i ' - - I ' - - ," -

+ 1 + +, '+ -- i '+ +, '+

- I + - , '+ i - - , ' -

- / - - , ' + - i / - / - / - - " + - / ' - -, ' -

+i'+ - i ' + - / - - , , ' -

/ + / + - / + - / + - / + - / + - / +

+ + + + - +

- - ] - -

- - +

- +

- +

- / - - / - - / + + i + + / + + I + - i + - / - - i - - / - - / - - ! + - / - - / - - i + - / + - / - - i - + i - + / + + i -

- ! - - / - - / - - ! + - ! + + / + - I - - / - - / - + / + - / - - i - - i + + ! + + / - - / + - / - - / - - i + - ! - - / + - / - - i - - / -

- / + - / - - / +

Stand II 28 26 5 20

May Sept. May June 1976 1976 1977 1977

+ i - - - i - + / -

Jr- ,' - - - - / - t - - - i ' - - ,< - -

--"+ - - i + - - i - - , ' - -

+,,+ --,,+ --, '+ + ' +

+ , + ,'+ --,'-- , ,-

- / - - ; - - i + i - + i - + ! - + i + ÷ i -

- i - + l - - i - - ; - + l + 1+ - i - - l +

- i - - i + - ! - + l -

i + - i - - - i - + i - - I + + i - - , ' - + ~ -

/-- --/-- + / + +,'-- + / + --/-- - - I + - - --,'+ - - / - - - - / ' - - - - " -

+i '+ + / + + / + +i '+

- / - + I - + / + + i ' -

+/+ +/+ --/-

+ i - - -- i - - --I-- , , + / _

+ / - + i' + - / -

+ +

+/+

-/+

22 TOVE HYLGAARD

per unit area. It seems that in the initial phases of recovery the patches of bare ground generally grow over slowly, unlike well-established patches of vegetation where environmental conditions are more suitable. Overall the results indicate a return to the original state after approximately one year, but individual sets of data such as the percent bare ground indicate a complicated pattern of recovery.

The planning and management of areas used for recreation necessitates classification systems but often these are different from those of experimental and descriptive ecology. One reason for the difference is that planning is mainly concerned with overall landscape qualities whereas ecology is concerned more with ecosystem function. Often classifications used in planning are less detailed than are ecological classifications. The areas investigated on Skallingen are a good example: although they are of two ecological types (white and grey dunes), they are often managed simply as vegetated dunes. Species composition on the two stands differs both in quality and quantity, On stand I only one species is really substantial in terms of percentage cover, Empetrum nigrum, ranging between 58 and 82~o depending on the season. The second most important species, Festuca rubra, has only 15 to 22 ~ cover. In stand I E. nigrum forms a rather loose tussock from which stolons are produced, increasing the size of the tussock. In stand II several species contribute substantially to the cover: Ammophila arenaria ranges between 8 and 60 %, F. rubra from 30 to 51 ~o, Cerastium semidecandrum from 0 to 34~o and Poa pratensis from 14 to 20 %. C. semidecandrum is a winter annual with a very quick reproduction cycle which is finished in the early spring, while the three other species are present throughout the year. These species have a tussock growth, proliferating by means of rhizomes which produce new aerial shoots. The two types of clonal growth dominant on stands I and II are shown in relation to recovery of vegetation of footpaths in Fig. 6A and B and a relationship between growth morphology and the pattern of recovery can be seen. Propagation of above-ground organs leads to progressive lateral recovery, a pattern which has also been demonstrated for alpine and tundra species by Willard & Marr ( 1971). Subterranean propagation leads to an erratic pattern of recovery, as shown most clearly by A. arenaria and F. rubra on stand II. Excellent recovery of species in which shoot initials are buried below the surface has been recorded by Bayfield (1979). This did not happen in stand II, indicating that similar life forms do not always respond in the same way; apparently the recovery capacity of plants is not a genetically fixed ability but is determined by the environment. As the capacity of vegetation to recover is dependent on the ability of the individual species to recover, habitat factors such as soil properties, microclimate and associate species composition will influence the time necessary for recovery as well as the pattern of recovery of vegetation.

So far only recovery of the more substantial species has been discussed. Species with D-values below 5 ~o are listed in Table 1 according to their presence or absence on footpaths and control sites. According to the table three types of behaviour are

SAND-DUNE PLANTS AND TRAMPLING EFFECTS 23

B

/

J

Fig. 6. Diagram showing relationship between growth morphology and the pattern of recovery of footpaths at Skallingen dunes. A, stand 1, Empetrum nigrum, progressive lateral recovery by means of above ground organs; B, stand I1, Ammophila arenaria and Festuca rubra, erratic recovery by

subterranean propagation.

exhibited. In the first type, illustrated by Aira praecox on stand I, the presence or absence on the path follows that on the control during the whole period after fencing, indicating that these species were not affected by trampling. The second type of response was shown by Anthyllis vulneraria on stand I; initially the species was absent from the footpath but increased in abundance during the year, indicating that it had recovered from disturbance. The third type is the most common and an example is Oenothera ammophila on stand II. Here the distribution in time was erratic on the path as well as on the control and the results gave no indication of the state of recovery of the species. The third type, however, stresses the importance of long-term recording of vegetation to assess recovery. Species with similar life-forms again behave in different ways depending on the environmental conditions. The two ecological types-- the white and grey dunes--behave differently, and ought to be considered separately in planning and management.

CONCLUSION

Results of the experiment in Skallingen dunes stress the importance of assessing recovery capacity when describing vulnerability of habitats to human impact. Published data on vegetation recovery indicate a complicated pattern concerning species and life-forms as these respond differently in space and time depending on environmental conditions. In many cases an erratic pattern of recovery in relation to

24 TOVE HYLGAARD

time can be observed, showing that long-term recording is a necessity. Ecological recovery should involve the following aspec ts - -not listed in order of importance but in the order in which they are most often recorded in the field:

The re-establishment of:

1. the original species composit ion; 2. the original frequencies of species present in the communities; 3. the original cover and height of species present; 4. the original production of species present; 5. the reproductive pattern of species present; 6. the original interrelations of plant and animal populations.

The term 'original' is used here in the sense 'compared with a corresponding natural, undamaged area or control plot', as distinct from 'compared with what the area was like before the impact' . This implies that secondary succession on disturbed areas may follow certain directions and still reach the desired 'final' stage. In relation to the fifth point a few explanatory remarks concerning primary production should be made. Here distinction should be made between production of vegetation on the sites and production of the whole ecosystem. In the first case gross production is a suitable measure but in the second case net production should also be considered. Even when the gross production has been re-established it will still take a further length of time before consumption of herbage by populations of herbivorous animals has reached the original level.

The six conditions mentioned above are all considered to be important and none of them should be left out of the theoretical discussion. This does not necessarily imply that investigations on ecological recovery must include all considerations. As several of the factors are interrelated, a combination of two or perhaps three of the factors will in most cases indicate the status of the others. One factor alone may, however, lead to misinterpretations, and a suitable method is still needed by which ecological recovery can be assessed in an adequate way.

ACKNOWLEDGEMENTS

I am grateful to A. Jensen for valuable discussions during planning of the case study and to S. P. Pinnerup for assistance with the field work. I also wish to thank M. J. Liddle, N. G. Bayfield and R. L. Jefferies for helpful criticism of the manuscript.

REFERENCES

BAYFIELD, N. (1979). Recovery of four montane heath communities on Cairngorm, Scotland, from disturbance by trampling. Biol. Conserv., 15, 165-79.

BURDEN, R. F. 8z RANDERSON, P. F. (1972). Quantitative studies of the effects of human trampling on vegetation as an aid to management of semi-natural areas. J. appl. Ecol., 9, 439-57.

SAND-DUNE PLANTS AND TRAMPLING EFFECTS 25

DAHL, E. & KROGH, H. (1973). Macrolichens of Denmark, Finland, Norway and Sweden, I st edn. Oslo, Universitetsforlaget.

HANSEN, A. (1973). Den danske Flora, 20th edn. Kopenhagen, Gyldendahl. LIDDLE, M. J. (1973). The effects of trampling and vehicles on natural vegetation. PhD thesis, University

College of North Wales. LINDQUIST, B. (1931). Den skandinaviska bokskogens biologi. (The ecology of Scandinavian beech-

woods.)!Svenska SkogsvF6r. Tidsk., 3, 179 532. ROSENV1NGE, L. K. & ROSTRUP, O. (1925). Den danske Flora. 2. Blomsterlose planter, 2rid edn.

Kopenhagen, Gyldendahl. SATCHELL, J. E. (1976). The effects of recreation on the ecology of natural landscapes. Nature am1

Ent~ironment Series, 1 i. Strasbourg. SPEIGHT, C, D. (1973). Outdoor recreation and its ecological effects. Unit, ersit)" College, London,

Discussion Papers in Conservation, 4. WILLARD, B. E. & MARR, J. W. (1971). Recovery of alpine tundra under protection after damage by

human activities in the Rocky Mountains of Colorado. Biol. Conserl:., 3, 181 90.