Clonal variation in monoterpene concentrations in Sitka spruce (Picea sitchensis) saplings and its effect on their susceptibility to browsing damage by red deer (Cervus elaphus)

Forest Ecology and Management 148 (2001) 259±269

Clonal variation in monoterpene concentrations in Sitka spruce (Picea sitchensis) saplings and its effect on their susceptibility to browsing damage by red deer (Cervus elaphus) Alan J. Duncana,*, Susan E. Hartleyb, Maddie Thurlowb, Sheila Younga, Brian W. Stainesb b

a Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, UK Institute of Terrestrial Ecology, Banchory Research Station, Banchory, Kincardineshire AB31 4BY, UK

Received 9 March 2000; received in revised form 14 June 2000; accepted 14 June 2000

Abstract Damage to young trees as a result of browsing by deer, can lead to signi®cant loss of revenue in commercial forestry plantations. This study investigated the chemical and morphological characteristics of Sitka spruce saplings which in¯uenced their susceptibility to browsing. Speci®cally, we tested whether genetic variability in monoterpene content of saplings affected deer feeding behaviour. Clonal cuttings of Sitka spruce were grown for 5 years following propagation from 15 parent trees growing on the West Coast of Scotland. Monoterpene concentrations and morphological characteristics of individual saplings were recorded prior to offering them to a group of tame red deer hinds in a series of preference trials, conducted within an experimental enclosure on a moorland site in northeast Scotland. Browsing intensity was assessed after each trial by recording the number of distinct signs of damage on each tree. Results showed that a high proportion of variation in browsing damage could be attributed to inter-trial variability, possibly due to increasing familiarity with the Sitka saplings as the experiment progressed. However, individual saplings were found to differ in their susceptibility to browsing according to predictable criteria: height had a positive in¯uence on browsing damage while monoterpene concentration had a negative in¯uence. The different clones also varied in the amount of damage they received, and between-clone variation in monoterpene concentration was an important factor in explaining variation in preference. These results demonstrate the important genetic in¯uence on susceptibility of Sitka spruce to browsing damage by red deer and suggest that foresters may be able to exploit genetic variation in monoterpene concentrations to improve the resistance of Sitka spruce to deer browsing. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Red deer; Sitka spruce; Browsing damage; Monoterpenes; Clone

1. Introduction *

Corresponding author. Tel.: ‡44-1224-318611; fax: ‡44-1224-311556. E-mail address: [email protected] (A.J. Duncan).

Large herbivores such as red deer (Cervus elaphus) can have a major impact on the early growth of conifer trees in many forest ecosystems (Motta, 1996). Young

0378-1127/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 5 4 0 - 5

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conifers can represent a major food source for deer in northern Europe (Staines and Welch, 1984), particularly during the winter months when food resources are scarce. Trees are also particularly susceptible to browsing during bud-burst in the early spring. The rising populations and increasing range-sizes of several deer species in UK, particularly roe deer (Capreolus capreolus) and red deer (Staines and Ratcliffe, 1987), have implications for the extent of deer damage to forests. In commercial forestry plantations, browsing by red deer can impede the establishment of the forest and may lead to economic losses: browsed trees take longer to reach maturity and, if the leading shoot is damaged, they may develop into multi-trunked trees which produce low-quality timber at harvest (Welch et al., 1992; Welch et al., 1995). Herbivory by deer is, therefore, particularly signi®cant for juvenile trees (less than 80 cm tall), where the entire plant including the leading shoot is within the browsing horizon (Welch et al., 1991). A number of factors may in¯uence the likelihood of a particular sapling being browsed by deer (Gill, 1992; Hanley, 1997; Bergstrom and Bergqvist, 1997). For example, previous studies have highlighted the importance of site-speci®c factors, such as altitude (Miller et al., 1982), stand size (Andren and Anglestam, 1993), the nutritional quality of under-storey vegetation and the openness of the canopy (Welch et al., 1991). Characteristics of the trees themselves, however, also have an important in¯uence on susceptibility to browsing. Such characteristics include the nutritive quality of the foliage (Edenius, 1993; Tixier et al., 1997), the gross morphology of the tree (Niemela and Danell, 1988; Hartley et al., 1997), and the concentrations of secondary compounds. Monoterpenes are an important group of secondary compounds in conifers (Gershenzon and Croteau, 1991). They are found in resin canals in both foliar and cortical tissues and in some studies their concentrations have been shown to be inversely related to degree of browsing damage to trees by mammalian herbivores (Farentinos et al., 1981; Duncan et al., 1994). Many studies of the factors that in¯uence the browsing preferences of deer have involved growing trees under different environmental conditions to alter their growth and chemical composition, and then offering them to deer in experimental feeding trials. Although environmental factors such as soil nutrient

status and ambient light levels are known to produce changes in the levels of secondary compounds such as phenolics (Bryant et al., 1983; Jones and Hartley, 1999), any changes in browsing susceptibility detected in these experiments have usually been found to be primarily mediated through changes in the morphology of the trees, rather than their chemical composition (Iason et al., 1996; Hartley et al., 1997). It is possible that previous experiments which have used environmental manipulation of saplings have over-emphasised the in¯uence of tree morphology on browsing preference for two reasons. Firstly, in contrast to many types of secondary compounds (Bryant et al., 1991), both the composition (Hanover, 1966) and absolute concentration (Tomlin et al., 1997) of foliar monoterpenes in conifers are under strong genetic control. Environmental manipulation results in relatively small changes to monoterpene concentrations (Heyworth et al., 1998; Koricheva et al., 1998), which may still result in changes in browsing preference, but the considerable genetic contribution to variation in monoterpene concentration will tend to be masked in such experiments. The strong genetic control of monoterpene production, however, means that the role of these compounds in determining susceptibility to browsing can be investigated separately from other factors by the use of vegetatively propagated trees. A second limitation of many previous trials investigating the role of terpenes in browsing preference is that they have been conducted under relatively con®ned conditions, with individual animals foraging in small arti®cial enclosures under close observation in the absence of a matrix of background vegetation. Thus, there is a risk that the deer preferences observed are not representative of the preferences which would be found if the deer were exposed to the same trees under more natural conditions. The experiment described here advances research on the link between the chemical composition of conifer saplings and the feeding preferences of deer by: (1) directly investigating the in¯uence of genetic variation on browsing susceptibility by the use of cloned material differing in monoterpene content; and (2) conducting browsing trials in as natural a physical context as possible, to reduce the possibility that observed deer feeding behaviour is an artefact of experimental conditions. The experiment tests the hypothesis that genetic variation in monoterpene

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concentrations in Sitka spruce in¯uences the susceptibility of saplings to browsing damage by red deer. 2. Materials and methods 2.1. Growth of clonal cuttings Clonal cuttings were taken from Sitka spruce (Picea sitchensis) trees growing in Glen Branter, Argyll, on the West Coast of Scotland. The trees formed part of a commercial forestry plantation and were approximately 15 years of age. Cuttings were taken in February 1989, by removing the leading shoot and several leading lateral branches from selected parent trees. Cuttings were stored in labelled polythene bags before being transported within 24 h in insulated boxes to the Institute of Terrestrial Ecology, Edinburgh, UK. The contents of each bag were soaked in a fungicidal dip (mildothane) as a precaution against pathogens. The cuttings were made by cutting off the previous year's growth with bud from the branches. Cuttings were generally at least 10 cm in length. The basal end of the cutting was cut with a scalpel to give a clean cut and the bottom 2 cm was dipped in a commercially available hormone rooting powder (`Strike'; mixture of NAA and IBA in a fungicide). The cuttings were placed on an unheated mist bench (25% peat, 75% quartz grit) surrounded with a polythene tent to reduce water loss. Temperature was maintained between 16 and 258C. After 12 weeks, cuttings showing signs of rooting were potted up in 7.5 cm pots using compost containing peat, loam and quartz grit (4:1:1) and fertiliser (Vitax Q4) and placed on a hard standing. Remaining cuttings were potted up at 2-week intervals as they showed signs of rooting. After 1 year, surviving cuttings were re-potted into 12.5 cm square pots using the same compost and after a further year they were transferred to 25 cm pots again using the same compost. Whilst on the hard standing, cuttings were watered using overhead irrigation equipment. The cuttings were top dressed with peat and grit containing OSMOCOTE slow-release fertiliser (6±9 month) during the early summer of 1994. 2.2. Tree morphology and chemical composition One month before the start of the experimental feeding trials, each sapling was sampled for monoterpene

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analysis by removing two randomly selected lateral branches from each tree. Samples were stored at 208C pending monoterpene analysis. Monoterpenes were analysed by gas chromatography following extraction of macerated foliage in pentane (Duncan et al., 1994). Immediately prior to being offered to the deer in the trials, all saplings were subjected to a series of morphological measurements. The morphological characteristics measured were as follows: Height Span Number of laterals Leader length Leader twig width Leader leaf width

Distance from soil surface to tip of leader branch (cm) Horizontal span of sapling at its widest point (cm) Number of individual lateral branches leaving main stem Length of leader branch from terminal whorl to tip of leader branch (cm) Diameter of leader branch immediately above terminal whorl (mm) Maximum span of foliage on leader branch (mm)

2.3. Preference experiment The preference experiment consisted of a series of 10 feeding trials, during which eight red deer hinds were allowed to browse 30 saplings at a time, over sequential 24 h periods. The experiment was conducted at the end of November 1995. The experimental area was a fenced enclosure (1 ha) at the Macaulay Land Use Research Institute's Glensaugh Research Station. The enclosure was at an altitude of 300 m a.s.l. and the predominant vegetation consisted of a 15year old stand of Calluna vulgaris with small amounts of underlying Vaccinium myrtillus. Eight red deer hinds were allowed to graze this enclosure for 2 weeks prior to the trials to become familiar with the experimental habitat. At the start of the week leading up to the trials, 30 spare Sitka spruce saplings were placed in a series of pre-dug holes at the centre of the enclosure, to accustom the animals to the experimental material. At the start of the experimental trials, the 30 spare trees were removed. Each of the experimental trials

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began at 0900 h, when 30 experimental saplings were arranged at 5 m intervals in a 56 array in pre-dug holes in the centre of the enclosure. Two trees from each of 15 clones were used for each trial. The array was divided into two adjacent blocks of 15 saplings each (35 saplings) and in each of the two blocks one sapling from each of the 15 clones was placed randomly in each of the pre-dug holes. Following introduction of saplings to the experimental enclosure, deer were left to feed undisturbed for 24 h. At the end of this time all 30 trees were replaced with a fresh batch for the next trial adopting the same tree placement procedure. The position of clones within blocks was re-randomized for each trial. This procedure was repeated for 10 trials in total. Browsing damage to Sitka spruce saplings was recorded immediately after each trial as follows. The number of `bites' taken from each tree was recorded by counting the number of distinct damaged loci on foliage or branches. This may have underestimated the actual number of bites but provided an index of browsing damage. 2.4. Statistical analysis Relationships between all combinations of morphological parameters and individual monoterpene concentrations were investigated using simple correlation. The in¯uence of sapling variables on the degree of browsing damage observed was examined by stepwise multiple regression. In each case, the response variable was the logarithm of the number of damaged loci on individual trees. The response variable was log transformed to improve the homogeneity of the variance. Terms available for inclusion in the regression model were trial, clone, all six morphological variables, total terpene concentration and the interaction between trial and clone. Terms were included in the regression model in the order in which they improved the model with the proviso that terms were only included in the model if the change improved the model at the P<0.05 level. To examine which features of individual clones had most in¯uence on browsing damage, a further step-wise regression was performed using within- and between-clone variation in monoterpene concentrations and height (the morphological term with most in¯uence on browsing damage) as explanatory variables. To this end, clone was excluded

from the list of available terms, and variation in monoterpene concentrations and height were separated into within- and between-clone components. For between-clone variation in monoterpene concentrations, each sapling was assigned the average monoterpene concentration for its clone. For within-clone variation, monoterpene concentrations of individual trees minus average values for their respective clones were used in the regression analysis. A similar calculation was performed using height to arrive at withinand between-clone height parameters. 3. Results 3.1. Tree morphology and chemical composition Total monoterpene concentrations varied markedly between different clones (range 1.60±22.14 mg/g dry herbage) but were relatively consistent within individual clones (Fig. 1). Concentrations of individual monoterpenes covaried within tree: correlation coef®cients between different individual monoterpenes exceeded 0.5 in all cases and considerably higher values were common (Table 1). For this reason, total monoterpene concentrations were used as the response variable in subsequent regression analyses. There was no obvious relationship between any of the morphological characteristics measured and total monoterpene concentrations (Fig. 1) with correlation coef®cients being less than 0.2 in all cases (Table 1). 3.2. Deer feeding behaviour Much of the observed variation in browsing damage examined by the regression analysis was found to be due to between-trial variation (Tables 2 and 3), re¯ecting the large increase in the amount of browsing damage suffered by the trees as the experiment progressed (Fig. 2). Thus, trial was the term which explained the largest proportion of the observed variation in browsing damage between trees. Subsequent terms which improved the ®t of the model were thus explaining variation within individual trials. In the ®rst regression model, clone was available for inclusion in the model as a categorical variable (Table 2). In this analysis, clone was the term which gave the greatest improvement to the ®t of the regression

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Fig. 1. Inter-clonal variation in morphological parameters and monoterpene concentrations in Sitka spruce saplings. Height denotes distance from soil surface to tip of leader branch (cm); Span denotes horizontal span of sapling at its widest point (cm); Lats denotes number of individual lateral branches leaving main stem; Llen denotes length of leader branch from terminal whorl to tip of leader branch (cm); Twid denotes diameter of leader branch immediately above terminal whorl (mm); Lwid denotes maximum span of foliage on leader branch (mm); Terp denotes total monoterpene concentration in foliage in mg/g DM. Variance ratios (F statistics) derived from analyses of variance with clone as the sole treatment are shown as a comparative measure of the variability of each parameter at the level of clone.

model, after inclusion of trial, indicating that a signi®cant amount of variation in browsing damage by deer could be attributed to genetic provenance. Sapling height was also a signi®cant factor in the model, with a positive regression coef®cient indicating that taller trees were subject to a greater degree of browsing damage per tree. No other morphological characteristic of the trees signi®cantly affected deer preference in this model. The interaction between clone and trial was available for inclusion in the model

but did not signi®cantly improve it, indicating little inter-trial variability in clone preference. Total monoterpene concentration was the last term to signi®cantly improve the model, the negative coef®cient demonstrating a negative relationship between monoterpene concentration and the degree of browsing damage on individual trees (Table 2). Because of the strong link between monoterpene concentrations and clone, a second regression analysis was performed to examine in more detail which

264

Height Height Span Laterals Leader length Twig diameter Needle span a-pinene Camphene b-pinene Myrcine Car-limb Camphor Total terpenes a

1 0.407 0.266 0.329 0.305 0.014 0.022 0.115 0.088 0.013 0.078 0.104 0.014

Span

1 0.287 0.023 0.065 0.012 0.166 0.207 0.053 0.093 0.004 0.226 0.106

Laterals

1 0.186 0.059 0.089 0.023 0.037 0.015 0.083 0.054 0.020 0.068

Leader length

Twig diameter

Needle span

1 0.463 0.112 0.118 0.048 0.134 0.120 0.154 0.091 0.126

1 0.024 0.011 0.041 0.066 0.074 0.078 0.019 0.065

1 0.078 0.042 0.076 0.049 0.048 0.070 0.055

b-pinene

Camphene a-pinene

Myrcine

Car-lim

Camphor

1 0.921 0.874 0.771 0.697 0.935 0.843

1 0.750 0.630 0.511 0.927 0.714

1 0.904 0.643 0.990

1 0.532 0.915

1 0.728

1 0.777 0.770 0.757 0.829

Values greater than 0.113 (or less than 0.113) are significant at P<0.05. Values greater than 0.148 (or less than b `Car-lim' denotes a composite peak consisting of 3-carene and limonene.

0.148) are significant at P<0.01.

Total terpenes

1

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Table 1 Correlation matrix of sapling morphological variables of the trees and their individual monoterpene concentrationsa

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Table 2 Summary of the initial forward step-wise regression analysis carried out to assess which tree characteristics significantly affected deer feeding behavioura Variable Constant Trial (10 levels)b Clone (15 levels)c Heightd Total monoterpenee

Estimate

S.E.

0.689 ± ± 0.01068 0.01725

0.346 0.00301 0.00732

F

P

Cumulative (%) variance accounted for

68.47 3.64 13.23 5.55

P<0.001 P<0.001 P<0.001 P<0.05

63.1 67.0 68.3 68.9

a Response variable: logarithm of number of browsing damage loci. Explanatory variables available: trial, clone, height, span, number of lateral branches, leader length, leader needle span, leader twig diameter, total monoterpene concentration, interaction between trial and clone. b Trial: number of trial (factor with 10 levels). c Clone: identity of clone to which tree belongs. d Height: distance from soil surface to tip of leader branch (cm). e Total monoterpene: sum of all measured individual terpene concentrations (mg/g dry herbage).

characteristics of clones had most in¯uence on browsing damage. In the second model (Table 3), clone was, therefore, excluded from the explanatory variables available for the model, to give an indication of which characteristics of clones were responsible for the observed effects on the levels of browsing damage. Trial was again the dominant term in explaining observed variation in browsing damage. In this model, between-clone variation in monoterpene concentra-

tions was the next term to signi®cantly improve the model with a negative coef®cient showing that the clones with the highest average monoterpene concentrations were avoided and received the least browsing damage. Between- and within-clone components of variation in height also led to improvements to the model, the positive coef®cients indicating that taller trees received more browsing damage. Within-clone variation in monoterpene concentrations also led to a

Table 3 Summary of multiple regression analysis examining the relative influence of between-clone and within-clone variation in monoterpene concentrations and height on browsing damagea Variable Constant Trial (10 levels)b Between-clonemonoterpenec Between-clone heightd,h Within-clone heighte Within-clone monoterpenef Number of lateral branchesg

Estimate

S.E.

1.057

0.370

0.0321 0.0158 0.0090 0.0155 0.0119

0.00731 0.00412 0.00313 0.00739 0.00599

F

68.20 19.02 15.66 13.18 5.53 3.97

P

Cumulative (%) variance accounted for

P<0.001 P<0.001 P<0.001 P<0.001 P<0.05 P<0.05

63.1 65.0 66.6 67.9 68.4 68.7

a Response variable: logarithm of number of browsing damage loci. Explanatory variables available: trial, span, number of lateral branches, leader length, leader needle span, leader twig diameter, between clone variation in monoterpene concentrations, within-clone variation in monoterpene concentrations, between clone variation in height, within-clone variation in height. b Trial: number of trial (factor with 10 levels). c Between-clone monoterpene: total monoterpene concentrations (mg/g dry herbage) averaged by clone. d Between-clone height: total height (cm) averaged by clone. e Within-clone height: total height (cm) of individual trees minus mean value for their respective clones. f Within-clone monoterpene: total monoterpene concentrations (mg/g dry herbage) of individual trees minus mean value for their respective clones. g Laterals: number of lateral branches leaving main stem. h Height: distance from soil surface to tip of leader branch (cm).

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Fig. 2. Average number of browsing damage loci per tree during 10 preference trials carried out on successive days. Error bars denote standard errors of means.

slight, but signi®cant, improvement to the ®nal model, suggesting that even the relatively small within-clone differences in monoterpene concentration were suf®cient to affect deer preferences. The number of lateral branches was the ®nal term which signi®cantly improved this model. 4. Discussion 4.1. The chemical composition of the clones The results from our study reinforce previous work, which found a strong genetic in¯uence on the monoterpene content of conifers (Radwan, 1972; Radwan and Ellis, 1975), including Sitka spruce (Forrest and Samuel, 1988). Clone-to-clone differences in chemical composition in deciduous trees have been demonstrated for nutrient as well as for secondary compound concentrations (Jelinski and Fisher, 1991) and the in¯uence of such variation on browsing preference has been demonstrated for moose browsing birch saplings (Jia et al., 1997). In our experiment, although between-clone variation in monoterpene-mediated browsing preference was certainly much greater than within-clone variation, trees from the same clone did differ suf®ciently to signi®cantly affect deer behaviour (see below), suggesting that there is at least some environmental in¯uence on monoterpene concentrations in this species. Light and soil nutrient levels have been found to have a rather small in¯uence on

monoterpene content of Sitka (Duncan et al., 1994; Hartley et al., 1997), but geographic and seasonal factors seem more important (Forrest, 1980a; Forrest, 1980b). The correlation between the foliar concentrations of monoterpenes in a given clone and its morphological characteristics tended to be relatively small and generally non-signi®cant. The clones were exactly the same age and grown under the same conditions so it is perhaps unsurprising that they displayed a similar morphology. Clones with similar morphology had very different monoterpene concentrations which suggests little relationship between the partitioning of resources for monoterpene synthesis and growth, in agreement with recent studies (Koricheva et al., 1998). 4.2. Deer feeding behaviour This experiment showed that tree genotype had a highly signi®cant in¯uence on the degree of browsing damage in¯icted on trees by red deer. In our analysis, variation in monoterpene concentrations could be divided into that which occurred between clones and that which occurred within clones in the experimental population of saplings. This demonstrated that for variation in browsing damage attributable to clone, the factor with most in¯uence on browsing damage was variation in monoterpene concentrations occurring in different clones. This analysis showed that trees suffered reduced levels of browsing damage when their foliage contained high concentrations of monoterpenes, and that this reduction was closely related to genotype. This work extends previous work on factors in¯uencing browsing susceptibility of conifers to large herbivores (Niemela and Danell, 1988; Danell et al., 1992; Iason et al., 1996), much of which has focused on manipulating environmental conditions to alter the morphology and chemical composition of conifer foliage. Such experiments have often found that the morphological characteristics of the trees were the dominant mediator of changes in susceptibility to browsing, masking any effects due to alterations in chemical composition (Hartley et al., 1997). In this study we used genetic differences rather than environmental manipulation to alter chemical composition, which allowed us to more clearly distinguish between the effects of morphology and chemical composition. Using this approach, we also found that aspects of tree

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morphology, namely height and number of lateral branches, affected deer feeding behaviour. Our use of clonal cuttings showed that genetic origin can also have an important in¯uence on susceptibility to browsing, notably through its in¯uence on secondary compound concentrations which are under strong genetic control in conifers. Although we could detect an effect of within-clone variation in monoterpene content on browsing damage, this was much smaller than the effect of the between-clone variation which had a highly signi®cant effect on the degree of browsing damage to Sitka saplings. Several previous studies have also found that conifer genotypes differed in their susceptibility to deer browsing. Different genotypes of Douglas ®r (Pseudotsuga menziesii) vary widely in their palatability to black-tailed deer, despite relatively minor genotypic differences in morphology (Radwan, 1972; Dimock et al., 1976), suggesting chemical factors were responsible. Clones resistant to deer browsing were found to contain essential oils with a greater inhibitory action on rumen microbial activity (Radwan, 1972), but further experiments failed to ®nd a consistent relationship between deer browsing and yield or composition of essential oils; the level of chlorogenic acid within the clones was a better predictor of preference (Radwan and Crouch, 1978). It may be that different compounds are responsible for resistance in different conifer species and that particular compounds are not equally effective against all potential herbivores. One potential problem, which complicates the interpretation of feeding trials, is the arti®cial conditions under which they are often conducted. This study was carried out on an open hill with groups of red deer hinds in an attempt to mimic the natural conditions of exposure of tree saplings to deer as closely as possible. However, our ®ndings were broadly similar to those of a previous study conducted under con®ned conditions with individual animals, which also reported an important role for monoterpenes in in¯uencing browsing susceptibility (Duncan et al., 1994). This lends con®dence to the results from feeding trials conducted in experimental arenas which have the advantage of easier observation and attribution of observed damage to particular individuals. No information on interindividual variability in deer browsing behaviour was collected in the current experiment. This was because one of the purposes of the experiment was

267

to assess browsing damage in as natural a context as possible without disturbance from human observers. It was, therefore, not possible to determine the extent to which observed damage was caused by a small number of animals. Previous work in more con®ned conditions has shown that hinds do differ in their rate of browsing on Sitka spruce but that their diet selection decisions are broadly similar (Duncan et al., 1994; Hartley et al., 1997). Although the current experiment was conducted on eight captive deer, we have no reason to suppose that the behaviour exhibited in the current experiment was unrepresentative of the wild deer population. Our ®ndings on the effects of chemical and morphological variation on browsing susceptibility of Sitka spruce were set against a background of increasing damage to saplings as the trials progressed. This indicated some degree of learning by deer. Initial caution with novel food items is a well recognised phenomenon among herbivores and probably represents an adaptation to reduce the chances of consuming excessive amounts of toxic plants (Provenza, 1996). The increasing amounts of damage observed as the experiment progressed may also have been caused by increasing hunger of the hinds and lower overall food availability. 4.3. Implications for foresters Damage to commercial Sitka saplings by red deer is a signi®cant problem in UK (Gill, 1992). In ®nancial terms it has been estimated that up to 7% of revenue from timber in the UK public sector is lost to deer (Gill, personal communication). Current protection measures include deer fencing and deer culling. There are good reasons for seeking alternative means of protecting against deer damage bearing in mind the impact of deer fences on other wildlife species, such as red grouse and capercaillie (Baines and Summers, 1997). Deer fencing also represents a considerable cost to the UK commercial forestry sector. In commercial Sitka plantations, anecdotal evidence indicates that certain trees are particularly prone to browsing damage. The results from our experiment suggest that at least a component of this inter-tree variation in browsing susceptibility is genetically mediated and that high inter-tree variation in monoterpene concentrations are partially responsible for this phenomenon.

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Bearing in mind the strong heritability of monoterpene concentrations, their ease of measurement and their link with browsing susceptibility, there is a case for including terpene concentrations as a selection trait for commercial Sitka spruce trees for use in areas where browsing damage is likely to be a signi®cant problem. To date, breeding programmes in UK have tended to focus on growth and conformation characteristics (Lee, 1999). There is now a need to validate the ®ndings of this study, by planting saplings from browse-resistant clones at a number of ®eld locations and assessing their resistance to browsing under the conditions found in commercial plantations. 5. Conclusion The results of the experiment demonstrate that different genotypes of Sitka spruce are browsed to differing extents by red deer. Foliar monoterpenes are a strong mediator of this phenomenon, suggesting that there may be merit in their inclusion as a selection trait in future breeding programmes for Sitka spruce. Acknowledgements Thanks are due to Ian Paterson, Caroline Young, Louise Amos and Suzi MacIntosh for assistance with the conduct of the experiment. Thanks also due to Glenn Iason for helpful comments on an earlier draft. Early phases of this work were conducted under a research grant awarded jointly by the Natural Environment Research Council and the Agricultural and Food Research Council under the Joint Agriculture and Environment Programme. The principal investigators were Brian Staines, Martyn Gorman and Glenn Iason. The experiment described here was funded by the Scottish Executive Rural Affairs Department.

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