Foliar nutrition, site quality, and temperature influence foliar chemistry of Tallowwood ( Eucalyptus microcorys )

Abstract

The distribution of marsupial folivores in Australian eucalypt forests is extremely patchy and, although distribution patterns are known to be partly determined by nutritional factors, the role of plant secondary metabolites (PSMs) has been little studied. We investigated variation in the foliar concentrations of five constituents: total phenolics (TP); the foliar component bound by polyethylene glycol (PEGBC); the formylphloroglucinol compound, sideroxylonal (a strong deterrent of mammalian herbivory); the terpene, cineole; and nitrogen (N). These constituents were analyzed using near infrared spectroscopy in 501 individual tallowwood (Eucalyptus microcorys) trees at 42 sites across a broad geographic range in eastern Australia. We tested three hypotheses: (1) trees with lower foliar N produce higher PSM concentrations; (2) PSM concentrations are greater at colder sites; and (3) PSM concentrations are higher at sites with low resource availability. Hypothesis 1 was true for all PSMs considered and was the result of variation that occurred within sites. This is consistent with the carbon/nutrient balance hypothesis and with the hypothesis that PSMs protect leaves from photodamage. Hypothesis 2 was upheld for all constituents. This was consistent with both the growth-differentiation balance hypothesis, and with the hypothesis that PSMs represent an evolutionary adaptation to greater risks of photodamage and frost damage in cold environments. Hypothesis 3 was rejected, with one PSM, sideroxylonal, being affected by resource availability in a manner opposite to that predicted. The optimal defense hypothesis offers an explanation for this result (as sideroxylonal concentrations increased along with the apparent nutritional quality of the foliage, possibly to provide defense against higher rates of herbivory). We found that trees associated with koala fecal pellets had lower concentrations of sideroxylonal and cineole. We discuss this finding and further implications for the herbivores of Eucalyptus of the patterns of plant defense that we detected.