Nursery conditions affect seedling chemistry, morphology and herbivore preferences for Eucalyptus nitens
Introduction
Eucalyptus nitens is the major plantation eucalypt species in Tasmania at altitudes above ca. 400 m (Eldridge et al., 1993). It is currently being grown primarily as a pulpwood crop (Wardlaw and de Little, 2000). Browsing damage to newly planted E. nitens seedlings by mammalian herbivores is a widespread problem and a significant economic cost to the forestry industry (Bulinski, 1999, Bulinski and McArthur, 1999, Coleman et al., 1997, Wilkinson and Neilsen, 1995). The most common method currently used for reducing browsing damage is lethal control of herbivore populations by poisoning or shooting. Non-lethal methods for reducing this damage are being sought due to public concern over the killing of native animals. One approach involves reducing palatability of seedlings, in essence increasing resistance to herbivores. Intraspecific variation in resistance of plants to herbivory can have a genetic (Dimock et al., 1976, Marquis, 1990, Mutikainen et al., 2000) and/or an environmental basis, both of which are potentially exploitable.
A genetic basis to variation in resistance to browsing mammals has recently been demonstrated in E. globulus, its hydrids with E. gunnii (O’Reilly-Wapstra et al., 2002, Scott et al., 2002) and in E. nitens (Dungey, 1996). Variation in damage to E. globulus seedlings by swamp wallabies (Wallabia bicolor) and European rabbits (Oryctolagus cuniculus) has been associated with different nursery practices (Marks and Moore, 1998), although the mechanisms behind this variation were unknown. Shade and nutrient levels are two environmental variables that can influence resistance of plants to herbivores through their effects on leaf chemistry (Hartley et al., 1997, Lawler et al., 1997). An explanation for these effects has been formalised in the carbon-nutrient balance (CNB) hypothesis (Bryant et al., 1983, Bryant et al., 1987, Bryant et al., 1991). This hypothesis proposes that when nutrients are limiting or light intensity is high, excess carbon is produced that cannot be used for growth, but can be allocated to carbon-based secondary metabolites such as phenolics and essential oils. Although this hypothesis has been questioned recently (Hamilton et al., 2001), the fact remains that intraspecific differences in plant chemistry are often observed in conjunction with differences in the abiotic environment.
Both genetic and environmental factors affecting herbivory are worth incorporating into a strategy for managing browsing damage in forestry. Browsing often occurs soon after seedlings are planted (Bulinski, 1999). Manipulation of seedling chemistry in the nursery may, therefore, provide one step in reducing this early browsing. In this study, our first aim was to test whether three nursery regimes, which incorporated three different slow-release fertilisers and levels of shading, affected the morphology, primary and secondary leaf chemistry of E. nitens seedlings grown for commercial use. Our second aim was to determine whether red-bellied pademelons, Thylogale billardierii, and common brushtail possums, Trichosurus vulpecula, demonstrated any preferences (differences in intake) for seedlings grown under these three treatments. These are two of the three native herbivore species that cause browsing damage in forestry plantations in Tasmania (Coleman et al., 1997, Statham, 1983). Our third aim was to determine whether any preferences could be explained by observed differences in seedling characteristics, including leaf chemistry.
Section snippets
Animal husbandry
We used captive animals in a paired-treatment feeding trial: six pademelons (three males and three females, mean body weight 7.0 kg, S.D. 1.9) and six brushtail possums (three males and three females, mean body weight 3.5 kg, S.D. 0.5). Pademelons were from a captive population maintained in an animal enclosure at the School of Zoology, University of Tasmania, Hobart. Possums were caught in mascot traps in the bushland surrounding the University of Tasmania. Animals were housed in individual pens
Seedling characteristics
Details of morphological and dry matter characteristics of seedlings grown using the three nursery treatments are shown in Table 1. Treatment B seedlings were significantly taller than the other seedlings (treatment effect ), while Treatment A seedlings had significantly less biomass than the other seedlings (treatment effect ). Despite these differences, all three treatments produced seedlings with a similar proportion of leaf to stem (73–74% leaf)
Seedling responses to nursery treatments
Nursery treatments induced morphological and chemical differences between seedlings. Relatively low height, biomass and SLA in Treatments A and C compared with B is consistent with light limitation under 50% shadecloth. Fifty percent shadecloth, other factors being equal, has been demonstrated using growth analysis to cause light limitation to E. nitens seedlings (Close, 2001). Further, given the similar light environment, the fertiliser used in Treatment A resulted in nitrogen limitation
Conclusions
Nursery conditions induced different chemical and structural characteristics between seedling treatments. Differences were not extreme, but were sufficient to be detected by the herbivores, as shown by their distinct preferences. These preferences were evident despite the fact that animals did not need to eat any foliage for nutritional or energetic reasons, because they were provided with alternative food. This suggests that whatever seedling characteristics affected their choice, animals did
Acknowledgements
Captive animal feeding trials were performed under Parks and Wildlife Permit #FA99053, and University of Tasmania Animal Ethics Permit #99038. We thank the staff at Forestry Tasmania Nursery for supplying seedlings, Bart Dryden and Julianne O’Reilly for assistance with animal husbandry, Dr. Karen Barry for the use of tetra-galloylglucose standard data and Drs. Bart Eschler and William Foley for the sideroxylonal standard. This research was funded by the CRC-SPF, the Browsing Damage Management
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