We investigated effects of light and nutrients on whole plant growth, ecophysiology, and leaf chemistry of Eucalyptus nitens seedlings, and the effect of these characteristics on herbivory by common brushtail possums (Trichosurus vulpecula) and red‐bellied pademelons (Thylogale billardierii). We compared results of the response of plant secondary chemistry to changes in environmental conditions against two contrasting hypotheses: (1) the carbon:nutrient balance hypothesis (CNBH) and (2) the photoinhibition hypothesis. Nutrient and light levels affected relative growth rate and leaf area ratio of seedlings. Electron transport rates and optimum photochemical efficiencies indicated different rates of carbon fixation between treatments, consistent with levels of chlorophyll and xanthophyll pigments. Nonphotochemical quenching and xanthophylls per unit chlorophyll were negatively correlated with total chlorophyll content. Consistent with the photoinhibition hypothesis, levels of leaf phenolics (flavonols, hydrolyzable tannins, and sideroxylonals) per unit chlorophyll were also negatively correlated with total chlorophyll content. In contrast, phenolic levels on a dry‐mass basis were not consistent with predictions of the CNBH; they were better explained as a response to severity of photoinhibition than by reallocation of resources from growth to defense. Total essential oil and cineole levels were positively correlated with nitrogen levels, but there was an interaction in their response to nutrients and light. Thus, levels of essential oils were not well predicted by the CNBH. Sideroxylonal and cineole levels were not correlated, and we hypothesize that different processes and requirements drive their production. Intake of seedlings by possums and pademelons was not directly negatively correlated with levels of hydrolyzable tannins, sideroxylonals, or essential oils. An interaction of the costs of these compounds against benefits of nitrogen levels may explain patterns in intake. We suggest that, while environmentally induced levels of carbon‐based plant secondary metabolites have consequences for herbivory, photoinhibition may drive the adjustment of levels of leaf phenolics in response to variation in light and nutrients. This contrasts with the view that these adjustments represent a trade‐off in resource allocation between growth and defense.
Summary 1.In south-eastern Australia, browsing of commercial tree seedlings by mammalian herbivores in plantation forestry incurs a large economic cost to the industry, through its effect on tree survival, growth and form. In the absence of lethal methods to control herbivore populations, appropriate manipulation of vegetation through silvicultural management may reduce browsing but it relies on understanding the feeding responses of the herbivores to such changes. 2. We evaluated the extent to which vegetation patch characteristics affected browsing of pine Pinus radiata seedlings by generalist mammalian herbivores, including redbellied pademelons Thylogale billardierii , Bennett's wallaby Macropus rufogriseus rufogriseus and the common brushtail possum Trichosurus vulpecula . 3. In field trials, browsing of pine seedlings was greatest when they were located in patches of high quality, i.e. palatable short vegetation (grass), intermediate in high-quality tall vegetation ( Lomandra longifolia ) and least in low-quality tall vegetation (bracken and shrub). Patch structure also affected browsing: in low-quality patches pine seedlings were browsed more in short vegetation patches (12-24 cm shorter than the seedling) than in tall vegetation patches (at least as tall as the seedling). This difference was at least partly because herbivores took, on average, 2 weeks longer to locate seedlings in tall than in short vegetation. 4. In trials with captive red-bellied pademelons and brushtail possums, pine seedlings were browsed similarly in small short grass and soil patches. However, when these patches were large and spatially separated, they were browsed more in short grass than soil patches. Selection at the patch or individual plant level therefore depended on spatial scale and proximity of the patches. 5. Synthesis and applications . The influence of patch characteristics on browsing has the potential to be exploited for the management of browsing damage to commercially important tree seedlings in forestry plantations. Retention of relatively tall vegetation, particularly if it is unpalatable, in areas at risk of heavy browsing, for example by delaying the use of herbicide, may be a useful management strategy for reducing browsing. This needs to be weighed against the potential detrimental competitive effects of neighbouring vegetation on the seedlings.
Terpenes are a diverse group of plant secondary metabolites that mediate a plethora of ecological interactions in many plant species. Despite increasing research into the genetic control of important adaptive traits in some plant species, the genetic control of terpenes in forest tree species is still relatively poorly studied. In this study, we use quantitative genetic and quantitative trait loci (QTL) analysis to investigate the genetic control of foliar terpenes in an ecologically and commercially important eucalypt species, Eucalyptus globulus. We show a moderate to high within-family broad-sense heritability and significant genetic basis to the variation in 14 of the 16 terpenes assayed. This is the first report of QTL for terpenes in this species. Eleven QTL influenced the terpenes overall. One QTL on linkage group 6 affected six of the seven different sesquiterpenes assayed (plus one monoterpene), which, in combination with highly significant correlations between these compounds, argues that their variation is influenced by a QTL with pleiotropic effect early in the biosynthetic pathway. We examine the homology of these QTL to those found in a closely related eucalypt, Eucalyptus nitens, and provide evidence that both common and unique QTL influence terpene levels.
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