Densities and biomass of invertebrates in stands of rotationally managed coppice woodland

Biological Conservation 51 (1990) 167 176

Densities and Biomass of Invertebrates in Stands of Rotationally Managed Coppice Woodland David Hill,* Peter Roberts Royal Society for the Protection of Birds, Sandy, Bedfordshire SG19 2DL, UK

& Nigel Stork Department of Entomology, British Museum (Natural History), South Kensington, London SW7 5BD, UK (Received 3 February 1989: revised version received 13 April 1989: accepted 17 April 1989)

A BSTRA CT This paper investigates the effect on the three major invertebrate groups--Diptera, Hemiptera and Arachnida--together with total invertebrates and total biomass of (a) coppice species (sweet chestnut Castanea sativa and birch Betula pendula), (b) coppice age, (c) mature coppice understorey species (hazel Corylus avellana, hornbeam Carpinus betulus and sweet chestnut), living under oak Quercus standards at the Blean Woods complex in Kent. Generally birch coppice, irrespective of age, had the highest densities and biomass of the invertebrate groups studied. Middle-aged (6 7 year old) sweet chestnut coppice had the lowest density c~["invertebrates, although between-age class differences were significant in only a few cases. The age o[ birch coppice had no clear effect on invertebrate abundance and sample biomass. The species of coppice understorey in mature stands under oak standards had a significant effect on invertebrate abundance and sample biomass in the understorey. Generally densities and biomass values were highest in hornbeam and lowest in sweet chestnut. Total invertebrate densities and total biomass values were between 5-8 times higher and between 3-12 * Present address: British Trust for Ornithology, Beech Grove, Tring, Hertfordshire HP 23 5NR, UK 167 Biol. Conserv. 0006-3207/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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David Hill, Peter Roberts, Nigel Stork times higher, respectively, in hornbeam than in sweet chestnut understorey. The implications of management for mixed broadleaved woodland bird communities are discussed.

INTRODUCTION The ecology of invertebrate communities of rotationally managed coppice woodland in Britain has been little studied, although some studies have investigated soil invertebrates or phenology (Welch, 1969; Russel-Smith & Swann, 1972). Other groups such as birds have received more attention, largely for the benefits such understanding would give to their management. It can be assumed that birds exploit principally two broad resources during the spring and summer within coppice woodland--invertebrate food and structural features relating to breeding sites, etc. The effects of structural diversity in broadleaved woodland, particularly coppice, on the bird community have been studied in some detail (Fuller, 1982; Fuller & Moreton, 1987; Smith, 1988). Coppice woodland of 7 years of age, particularly of hazel Corylus avellana (or a species mixture) under oak Quercus standards, has been shown to be favoured by nightingales Luscinia megarhynchos (Bayes & Henderson, 1988). Further, in a study of bird distributions in ash Fraxinus-lime Tilia woodlands in Lincolnshire, Fuller & Whittington (1987) found that edge effects, often created between coppice blocks of different ages, resulted in some bird species being recorded at higher densities than in areas with few or no edges. Fuller & Moreton (1987) also showed for sweet chestnut Castanea sativa coppice, that the ratio of breeding migrant birds to resident birds was highest for young coppice, and declined (as did bird diversity) as the coppice aged and reached the time for further cutting. This present study resulted from the need to identify differences in biomass of major invertebrate groups between different coppice rotations. A high density of certain invertebrate groups is thought to be important as food of birds during the breeding season. At Blean Woods in Kent the large areas of sweet chestnut were thought to be important historically and for the specific invertebrate communities living in them. The Royal Society for the Protection of Birds has a reserve within the Blean complex and wished to remove areas of sweet chestnut in order to manage the wood for its botanical interest and to develop a mixed broadleaved woodland bird community. Until recently it has been difficult to assess the biomass of invertebrates in trees. However, many biologists are now using non-residual knockdown insecticides to obtain samples for studies of arthropod communities in trees (Moran & Southwood, 1982; Southwood et al., 1982a, b; Stork, 1987a, b, 1988; Morse et al., 1988). Such techniques have been employed in the present

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study to investigate the invertebrate load of coppiced woodland of varying age structure and tree species composition. This paper aims (1) to compare densities and biomass of major invertebrate groups between two dominant coppice species sweet chestnut and birch Betulapendula; (2) to identify any effects associated with the age of coppice; and (3) to identify any effects associated with different understorey species (chestnut, hazel and hornbeam Carpinus betulus) all growing under oak Quercus robur. STUDY A R E A Church Wood, in the Blean Woods complex to the north and west of Canterbury, Kent (TRl15600), is managed by the Royal Society for the Protection of Birds. It is 180ha in extent, comprising 120ha of mature, predominantly oak, woodland, 40 ha of mixed coppice, and 20 ha of other habitats including open heathland. The mature oak is in larger blocks than the various-aged coppice sweet chestnut and birch, which occurs in both mixed and single species stands. METHODS

Invertebrate sampling A Fontan back-pack sprayer was used to spray whole sample trees with Reslin 50E, a synthetic pyrethroid diluted with water at a ratio of 1:20. Trees were sprayed at a constant emission rate for a standard length of time in calm dry weather conditions during the first 2 h after dawn. A total of 5 catching trays measuring 1 m 2, placed beneath sample trees on the previous day, were emptied 2 h after spraying. The invertebrates collected were placed in alcohol for later analysis. Sample trees were selected at random within study plots chosen on the basis of being a large enough area of a single tree species thereby eliminating edge effects. For each sample category (e.g. sweet chestnut, birch, etc.), five replicate trees were sampled in each of five sampling periods in 1988: (1) 20-28 May, (2) 7-9 June, (3) 22-24 June, (4) 11-18 July, (5) 28-29 July. Additional samples from outside the study plots were taken from which dry weight measurements (biomass) for each of five size classes in each order were derived. Size classes were based on a log (base 10) scale, 0-1 mm, 1 2-5 mm, 2"5-5 mm, 5 10 mm, and 10-20 mm. These values were used to convert numbers of sorted size classes from the samples to total biomass. Invertebrates were sorted to order, each replicate treated separately. Larval and adult stages were also separated.

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Analysis of data For the three invertebrate groups chosen to represent (1) tourists--Diptera, (2) sap-feeding--Hemiptera, (3) predators--Arachnida, numbers sampled m -2 were transformed by log~o(X+ 1) in order to normalise the distributions. The same transformation was applied to total invertebrates (all groups) and total biomass. Three sets of analysis were conducted using number sampled m-2 for periods 1, 3 and 5 only in order to span the season but to reduce the volume of data: (1) the effects on the above of coppice species and age (and the interaction) were analysed using two-way analysis of variance (ANOVA) for sweet chestnut and birch of three age categories (young, 1-3 years; middle, 6-7 years; and late, > 7 years); (2) the effects of coppice age on invertebrate densities independent of coppice species were analysed by one-way ANOVA for chestnut and birch; (3) the effects of different understorey species (hazel, hornbeam and chestnut) under oak standards were analysed by one-way ANOVA.

RESULTS

Effect of coppice species on invertebrate density Generally, birch contained higher numbers of total invertebrates (FI,24 = 91, 71, 5) and total biomass ( F 1 , 2 4 -~ 67, 63, 21) than sweet chestnut, for the three sampling periods, respectively. Hemiptera were consistently more abundant in birch than in chestnut ( F 1 , 2 4 = 147, 131, 46) for the three sampling periods, respectively. Arachnida were significantly more abundant in birch only in the earliest sample (F1,24 = 34). Mean values for each age category for chestnut and birch are given in Tables 1 and 2, respectively. Age was not a consistently compounding variable when data for the two coppice species were analysed together. However, age did have an effect for the final sample, with significant differences occurring in Hemiptera ( F l , 2 4 = 7), Arachnida ( F 1 . 2 4 = 7), and total invertebrates ( F 1 . 2 4 = 10). In some cases (Diptera, total invertebrates and total biomass in period 1, total biomass in period 3, and Diptera and total invertebrates in period 5), coppice species and age gave significant interaction effects.

Effect of coppice age on invertebrate density In order to investigate more closely the interaction effects identified in the above two-way ANOVAs, the effect of age was independently analysed for

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each c o p p i c e species. G e n e r a l l y , for sweet c h e s t n u t the m i d d l e - a g e d c a t e g o r y h a d the lowest densities o f those i n v e r t e b r a t e g r o u p s studied. In s a m p l i n g p e r i o d 1 the oldest c o p p i c e age class h a d the highest densities o f all i n v e r t e b r a t e groups, total i n v e r t e b r a t e s a n d total biomass, a l t h o u g h the differences b e t w e e n age classes were o n l y significant for D i p t e r a (Table 1 ). In sampling p e r i o d 3 densities were generally h i g h e r in the y o u n g e s t age class, b u t differences b e t w e e n age classes were significant o n l y for total biomass (Table 1). D i p t e r a densities were significantly higher in the oldest age class. In p e r i o d 5 n o clear effect o f age was a p p a r e n t a l t h o u g h densities o f A r a c h n i d a a n d total i n v e r t e b r a t e s were highest in the y o u n g e s t and oldest age class, respectively (Table 1). T h e r e was n o clear effect o f age o f birch coppice o n i n v e r t e b r a t e a b u n d a n c e a n d sample b i o m a s s (Table 2). T h e densities o f H e m i p t e r a a n d total i n v e r t e b r a t e s were highest in the y o u n g e s t age class, a l t h o u g h there were n o significant differences b e t w e e n age class, i n v e r t e b r a t e g r o u p and sample period, e x c e p t for D i p t e r a in sampling p e r i o d 5 (Table 2). TABLE 1

The Effect of Age of Sweet Chestnut Coppice on Invertebrate Densities (no. m 2) and Biomass (mgm-2), (ant±log log~o (x + 1) ± SE) for Three Sampling Periods Age (yearst

Signi[icance"

Number

O/ 3

8

12

sumple.~

2(~28 May Diptera Hemiptera Arachnida Total invertebrates Total biomass

15"8+04 1.1 +_ 1.8 1-1 4- 1' 1 25"9 _+0"4 32-7 _+0-9

7-1 +_0.4 0.9_+0.4 1"1 4 1"1 18"3 ± 07 14.5 _+0"4

235__+0'2 3-6+__29 14 ± 0-4 48.4 + 0.4 40"1 ± 0"9

* ns ns ns ns

5 5 5 5 5

22 24 June Diptera Hemiptera Arachnida Total invertebrates Total biomass

75"3 +_0.7 42'3±1-1 6"8 ± 1'3 162"8 ± 0.7 73.2_+0.7

29-94-0.4 11-6+_0.2 2'8 ± 0.4 80-3 _+0.4 24.7+-0-2

86.1 +0.4 16-7±1.1 3.2 y 0.9 150.4± 0.2 65.1 ±0-2

* ns ns ns **

5 5 5 5 5

28 29 July Diptera Hemiptera Arachnida Total invertebrates Total biomass

31"8+-0'7 27.2+-0-4 362+_0-7 44.6_+0-7 24.1 _+0-4 17-2± 1-6 38.2_+0.9 6.4 ± 1.1 4.8+0.4 289-1 ± 0"4 106"2± 0"2 536"0+- 0-7 71'1 +0"4 32"1 -+0"4 43-7+0.7

ns ns ** ** ns

5 5 5 5 5

a One-way ANOVA, * p <0.01; ** p < 0.001; ns, not significant.

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David Hill, Peter Roberts, Nigel Stork

TABLE 2 The Effect of Age of Birch Coppice on Invertebrate Densities (no. m - 2 ) and Biomass (mg m-2), (antilog log10 (x + 1) + SE) for Three Sampling Periods

Age (years)

Significance"

Number

of 3

8

14.9 + 1-6 203.9 + 0-2 6.8 + 0.4 246.8 + 0.2 134.7+0.2

25"9 _+ 0"4 101"3 + 0"2 6.2 + 0.7 155.9 ___0.2 150.3 + 0 . 2

67"1 -+0"9 360.3-+ 0.9 4-2 -+ 0.9 469.9_+ 0'7 119.2-+ 1-6 32'8-+0'7 246.1 -+ 0.4 16"9-+1'6 530.7-+ 0"2 122.7+_0.4

12

samples

20-28 May

Diptera Hemiptera Arachnida Total invertebrates Total biomass

9.7 60-7 4-8 91.3 85.1

+ 1-9 + 1-8 + 4.9 + 1.3 -+0.9

ns ns ns ns ns

5 5 5 5 5

74"9-+0"2 280.8-+ 2.5 4.1 -+ 0.7 445"7-+ 0'9 133.9-+0.4

130"8-+0-2 230.1 -+ 0.7 9.9 -+ 0-9 451'1 -+ 0-4 192-9_+0-2

ns ns ns ns ns

5 5 5 5 5

106.1 -+0.7 106.2 -+ 1.1 21'9-+0.9 330.1 _+ 0.9 113"8-+0"7

25.9-+0.7 73.1 -+ 0.7 6'8-+2"5 287.4-+ 0"7 72-4+_0.9

* ns ns ns ns

5 5 5 5 5

22 24 June

Diptera Hemiptera Arachnida Total invertebrates Totalbiomass 28 29 July

Diptera Hemiptera Arachnida Total invertebrates Total biomass

" One-way A N O V A , * p < 0.01: ** p < 0.001: ns, not significant.

Age therefore appears to have a minimal effect on invertebrate densities and biomass per unit area sampled, for those age classes of coppice studied. Effect of coppice understorey species Loglo(X+ 1)-transformed densities from hazel, hornbeam and sweet chestnut were compared using a one-way ANOVA, with species as the treatment, for three sampling periods. There were significant differences in densities and biomass between the understorey species (Table 3 expressed as antilog of transformed densities), except for Diptera and Arachnida in sampling period 1, and Diptera in sampling period 3. Generally densities and biomass were highest in hornbeam and lowest in sweet chestnut (Table 3). Hemiptera showed the greatest differences when comparing hazel and hornbeam. Total invertebrate densities and total biomass values were between 5 and 8 times higher and between 3 and 12 times higher, respectively, in hornbeam than in sweet chestnut understorey.

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173

TABLE 3 Mean Densities (no. m 2, antilog log10 (x + 1)) of Invertebrates and Biomass img m 2) Sampled from 3 Understorey Coppice Species for 3 Sampling Periods Understorey coppice species

Signoqcance ~

Number

Hazel

Hornbeam

Chestnut

sample,~

65"1 + 0 ' 8 58"3 _+ 0"4 1 2 . 5 + 1.0 189"5 _+ 0.7 161-2_+0-9

73.1 _+0'2 222"8 _+ 0'4 9-5_+0.4 345.7 _+ 0.4 165-0_+0"4

35"3 + 0 . 7 5"5 _+ 1'8 6-6_+0.7 71.4 _+ 0.4 50"3_+0'7

ns ** ns ** *

5 5 5 5 5

64-3 + 0"4 66.6 + 0.7 17"6+0"7 233-4_+ 1-1 127.8_+0.7

80"3 -+- 0"4 466.7 -+- 1.3 15"6_+0-7 690.8 _+ 1.3 256.0_+0.9

41-2 _+ 0 3 9.4 _+ 0 9 3-6_+0.7 86.1 _+ 0.4 36-2_+0.5

ns ** * ** **

5 5 5 5 5

69.7 65.1 22.4 268"2 130"8

108.6 + 0.2 373-5 _+ 0-4 36.2 _+ 0.4 720"4_+ 0-2 359-1 _+ 0-7

23.5 16-4 8-5 90-2 28-5

** ** * ** **

5 5 5 5 5

20-28 May

Diptera Hemiptera Arachnida Total invertebrates Total biomass 22 24 June

Diptera Hemiptera Arachnida Total invertebrates Total biomass 28-29 July

Diptera Hemiptera Arachnida

Total invertebrates Total biomass

_+ 0-4 _+ 0"9 _+ 0.7 _+ 1"3 _+ 0"7

_+ 0.5 _+ 0-9 _+ 0.6 _+ 0-5 _+ 0-6

One-way A N O V A , * p < 0-01; ** p < 0,001; ns, not significant.

DISCUSSION This paper has three main findings: (1) birch has significantly higher invertebrate densities and biomass than sweet chestnut; (2) age of coppice has little effect on invertebrate densities and biomass; (3) different species of coppice managed as mature understorey under oak have significantly different densities and biomass of invertebrates, with, in this study, hornbeam supporting higher invertebrate densities than both hazel and sweet chestnut. Age apparently had little effect although previous work has suggested that leaf-mining microlepidoptera and spiders are more abundant in old and neglected coppice than in that which has been newly cut (Sterling & Hambler, 1988). The wood ant Formica rufa L. is very common at Blean Woods and this species, unlike many of the British ants, commonly forages in trees and low shrubs. As a result the samples of insects were often dominated by this species. A number of studies (Janzen, 1967; Bentley,

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David Hill, Peter Roberts, Nigel Stork

1977a, b; Lawton & Heads, 1984; Heads & Lawton, 1985) have shown that the presence of ants as voracious predators and tenders of Homoptera can influence the community structure of the arthropod populations on those trees. We do not know if our conclusions from the present study would be different in areas not populated by F. rufa, but this is part of a further study. Some trees are more invertebrate-species rich than others. In a preliminary analysis Southwood (1961) identified a positive correlation between the number of invertebrate species associating with the tree species and tree history, as evidenced by records of Quaternary remains, thus supporting the hypothesis that dominant native trees will have the most insect species. This was further supported in a later paper (Southwood et al., 1982a) in which tree abundance and insect species richness were found to be strongly positively correlated. In a further paper 82% of the variation in insect species richness in British trees was explained by five variables: time present in Britain, whether evergreen or coniferous, taxonomic relatedness, tree height and leaf length (Kennedy & Southwood, 1984). For those tree species studied at Blean the descending order of insect species richness according to Southwood is as follows: birch, hazel, hornbeam, sweet chestnut. In this paper species richness has not been studied because the primary aim has been to identify broad differences in resources to breeding birds. Sweet chestnut is an introduced species and consequently supports fewer invertebrates than birch. The fact that age of coppice had little effect on invertebrate density and biomass suggests that coppice species and structure might be more important to birds than purely food supplies. Southwood et al. (1982a) also found that biomass and numbers of individual arthropod groups were not clearly related to structural features of the tree species. Birch corresponded to expected density and biomass importance but the invertebrate samples from the hornbeam understorey contained much higher densities and total biomass than expected based on the ranking of hornbeam in the literature. It is not known whether this is a local effect, although further identification of samples may indicate the scale of the difference at Blean Woods. Surrounding vegetation types may also have important implications for samples taken from hornbeam, although the sampling design was rigorous enough to overcome these external effects. The management implications of these findings for birds are relatively simple. Birch should be an important component of the Blean Woods complex, whereas areas of sweet chestnut could be reduced on avian grounds (breeding passerines) and substituted with species which support more invertebrates. Hornbeam should also be encouraged. Not only would it appear that hornbeam at Blean is rich in total invertebrates but it has other features which make it important for other key woodland bird species, for

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example woodpeckers Picidae (Smith, 1987) and hawfinch Coccothraustes coccothraustes (Newton, 1972). It is planned to identify invertebrates further in sweet chestnut samples in order to assess the conservation importance of this tree to specific invertebrates.

ACKNOWLEDGEMENTS We wish to thank Dr P. Skidmore of the Wellcome Research Laboratories for supplying the insecticide; Dr K. Brown of Shell Research, Sittingbourne for help and advice on sampling techniques; M. Walter at the RSPB reserve for logistical help; R. Plowright and Miss H o u g h t o n of the University of Kent, and T. H a r m a n of the Canterbury Field Studies Centre for the kind provision of facilities. Part of the work was assisted by a grant to P.J.R. from the British Ecological Society.

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