Intraspecific variation in defence and recovery of Eucalyptus globulus from mammalian herbivory
Creators: name: Borzak, CL
Publisher: name: University Of Tasmania
Year: 2023

description: The interaction between plants and their herbivores has played an important role in driving the form of many plant traits, including defensive chemistry and plant recovery following loss of biomass. Identifying intraspecific variation in such traits is not only crucial for our ecological and evolutionary understanding of plant-herbivore associations, but also for how they can shape the structure of natural ecosystems. In a series of glasshouse, nursery and field trials I explored plant-mammal associations by investigating intraspecific variation in foliar chemical defence and plant recovery after damage (including physiological and morphological traits) in early life stage Eucalyptus globulus. The key research questions were 1). Is there ontogenetic variation in foliar terpenes during early stage development of E. globulus seedlings, and does this vary at the population and family level? 2). Is there intraspecific genetic variation in photosynthetic rate, chlorophyll content, defensive chemistry and growth response to partial artificial browsing of juvenile E. globulus. 3). Is there intraspecific genetic variation in E. globulus seedling survival and growth from lignotubers after severe artificial browsing. 4). What is the pattern (spatial and genetic) and direct impact of natural mammalian browsing on E. globulus seedlings in the field, and subsequent indirect effects on a dependent foliar organism. A comparison of different populations of E. globulus showed significant genetic and ontogenetic variation in foliar terpenes throughout seedling development with a series of both dynamic and stable responses. Ontogenetic trajectories differed among populations suggesting that evolution may exploit the ontogenetic patterns of change in these compounds to create a diverse chemical mosaic with which to defend the plant. A key finding was the different behaviour of the two major terpene groups, which reflects their biosynthetic origins. Sesquiterpenes changed more rapidly throughout early ontogeny compared to monoterpenes, with sesquiterpenes expressing opposite trajectories between compounds. Despite the large constitutive differences that exist in foliar chemistry and storage allocation between E. globulus populations, defoliation elicited conservative recovery responses in physiological, growth and foliar chemical traits. Populations subject to a partial defoliation treatment (50% leaf removal) initiated regrowth from axillary buds. The remaining leaves on treatment plants showed dramatic increases in photosynthetic rates and chlorophyll content, whilst there was increased allocation to regrowth biomass as well as a change in leaf structure to increase light capture. A number of foliar chemical compounds showed elevated levels in the regrowth arising from axillary buds compared with new growth derived from apical meristems. In another trial, severe defoliation (decapitation) had clear effects on plant morphology, but after 20 months of growth in a field trial, the treatment effects on above-ground growth were overridden by population differences. There was, however, evidence of genetic divergence among populations in survival after severe defoliation. Survival among populations was different after a decapitation treatment, whereby the population with increased survival had relatively lower concentrations of foliar chemistry and better developed lignotubers (dormant buds). This result suggests that resource allocation to storage structures has been responsive to selection, and affects the ability of plants to recover from severe defoliation. The survival and growth consequence of browsing by free ranging marsupials was assessed in a large scale genetic pedigreed field trial. While no genetic differences between trees in browsing damage were detected, damaged trees suffered increased mortality and reduced growth (assessed up to 4 years). Herbivore-induced changes in plant morphology also affected subsequent tree use by an invertebrate herbivore. This demonstrates the potential role that mammals could play in driving ecological communities, and provides new evidence of plant mediated indirect effects structuring dependent ecological communities in a eucalypt system. The work presented in this thesis demonstrates the complexity and dynamics of the different strategies eucalypts use to deal with herbivore interactions. Mammal browsing damage had important direct effects, but there was genetic stability in many chemical and plant recovery responses, with changes in some traits potentially having flow-on consequences on other herbivores. In addition to ecological and evolutionary relevance, these results have important applied implications for the development of management strategies by enabling prediction of tree productivity after a browsing event, and the implementation of herbivore strategies in managed systems, such as the use of genotypes with improved recovery.