Phytochemistry, allelopathy and the capability attributes of camphor laurel (Cinnamomum camphora (L.) Ness & Eberm.)

The camphor laurel tree (Cinnamomum camphora (L.) Nees & Eberm.) was tested for allelopathic influence in studies of seed germination, seedling growth and soil algal populations of species identified in the regeneration assemblages of the Big Scrub region in north-eastern New South Wales. This included an allelopathy glasshouse trial on germinating seed and soil algae, the development of a technique for field identification of the camphor laurel chemo-types, an application of the chemo-type differentiation technique in a field assessment of allelopathic influence on seedling growth, and a review of the tree’s capability attributes providing a greater understanding of its role in plant succession. The allelopathy glasshouse trial was performed on germinating seed from 52 vascular plant species and 27 soil algal species associated with the camphor laurel assemblages. Aqueous solutions obtained from macerated camphor laurel leaves of the ‘cineole’ and ‘camphor’ chemo-types were prepared including nutrient and acidity adjustments. These treatments were applied to the germinating seed until germination was complete. Measurements of germination number and percentage over time and growth measurements at completion of germination for radicle length, shoot length, leaf number and leaf area were performed. Germination over time was modeled for each species with a non-linear regression using the Richards function which provided an asymptote, mid-inflection point, days to mid-inflection and germination rate. A deviance test and residual analysis were undertaken for each of these modeled variables which identified significant effects due to the extracts. In the statistical assessment of growth measurements this used t-tests for each species and growth attribute including a summary effect table which also identified significant effect. For the field identification of the chemo-types several methods were used to investigate possible differences including sectioning of the lamina and use of 11 histological stains, leaf chlorophyll extraction and precipitate comparison, leaf venation and sclerophylly comparison and olfactory recognition of the chemo-types using gas chromatography of the leaves compared with a blind olfactory test. In the field recognition of allelopathy, quadrating of seedlings found across various sites where both chemo-types of camphor laurel and other vegetation types were located produced measurements for shoot length, leaf number and leaf area. Measurements were compared for each species and growth variable across these sites using t-tests which provided a measure of significant influence due to the camphor laurel canopy types on seedling growth. The glasshouse trial identified that direct allelopathy originating from camphor laurel leaves influences vascular plant seedlings through a significant delay to germination and the significant reduction of radicle length, shoot length, leaf area and leaf number in most native and exotic vascular species associated with camphor laurel plant communities. No significant difference between the effects of the two chemo-types was found. Alpha selection of plant species was indicated to be occurring through this process of inhibition with potential to alter plant successional sequence in the field. Soil algal population and growth in the glasshouse were also significantly influenced by the allelochemics with many species either disappearing from the soil or possessing reduced vigour. This was identified as being important, as soil algae assist in many ecological functions such as soil wet-ability, moisture retention, humification processes, nutrient fixation and release, seed germination enhancement and as a food source for invertebrates. This indicates that allelopathy may also be indirect in its operation on vascular plants through influencing soil algal succession which in turn influences seed germination and plant growth, suggesting that Beta selection is also occurring in the plant succession. The testing for the field identification of the chemo-types resulted in olfactory recognition being the most reliable, repeatable and inexpensive technique to assess large numbers of camphor laurel trees, and was further applied in the field assessment of allelopathy. During the process of gas chromatography 19 new oils were identified in the leaves of camphor laurel. The field assessment for allelopathy of seedling shoot length, leaf numbers and leaf areas across the sampled sites found that significant reductions in all these growth attributes occurs in many of the species including camphor laurel itself. This was further verified against the glasshouse growth trial for most species indicating the allelopathic effect is active but also appeared to be more obvious than in the glasshouse trial. The comparison of effect between the chemo-types on field seedling growth was only possible for camphor laurel seedlings, as the ‘cineole’ chemo-type was found to be confined to sporadic locations along creek lines which prevented adequate sampling. It was found that growth in camphor laurel seedlings is significantly reduced below a camphor laurel canopy of both chemo-types indicating that greater competitive advantage for the species occurs outside the area of allelopathic influence. The factor of rarity of ‘cineole’ also suggested that the ‘camphor’ chemo-type was more invasive, being in greater abundance in the regeneration assemblage and able to exist in drier ridgeline environments. An extensive number of capability attributes was identified for camphor laurel thereby indicating that the tree is both highly stress tolerant and highly competitive, contributing to the influence of allelopathy and invasiveness of the tree. The process of allelopathy was seen to provide ecological advantage to the camphor laurel tree by reducing the competitiveness of the surrounding vegetation which provides a facilitated growth of the species, as well as a factor contributing to the trees dominance and persistence. Ultimately, the findings indicate that camphor laurel does have a significant influence in altering plant succession. Therefore as the species expands its presence in the region, habitat change will also increase, placing further pressure or threat on both plant and animal communities. The work also suggests that there is a need to seek a biological control, assess the trees’ impact on leaf litter invertebrates and water quality, and to improve management of camphor laurel recruitment in forested areas.