Interactive effects of elevated carbon dioxide concentration, temperature and soil P on growth and physiology of eucalypts

This PhD research investigated the impact of variable soil [P] on the growth and photosynthetic responses of three eucalypt species (E. tereticornis, E. saligna and E. sideroxylon) of varying intrinsic growth rate, and the interactive effects of soil [P], CE and TE on the response of E. tereticornis. This study found that increasing soil P availability increased DM production of the three Eucalyptus species and that the magnitude of that response scaled positively with intrinsic growth rate (i.e. fastest growing E. tereticornis showed the largest response in growth to increasing soil [P]). The impact on the rate of photosynthesis per unit area was negligible. Mass production of seedlings of E. tereticornis was positively responsive to CE, but only if soil [P] > 9 mg kg-1, indicating that low soil [P] may limit the positive response to CE. TE had a marginal impact on growth. Soil [P] did not modify the photosynthetic responses of seedlings to growth in CE or TE. CE increased Asat by an average of 41% at all [P] conditions, while TE did not affect photosynthesis. There were no interactive effects of CO2 and temperature on these processes. The wood density of E. tereticornis seedlings increased with increasing soil [P], and CE did not further modify that effect. Higher nocturnal gs by E. tereticornis seedlings growing at low soil [P], in part due to changes in wood density, increased P uptake at night, but did not substantially affect daily P uptake. In conclusion, these results suggest that under well-watered conditions, E. tereticornis will exhibit positive growth and photosynthetic responses to CE when not limited by soil [P], and that TE will not alter this response. However, in native soils with low soil [P], stimulation of growth under future climate conditions of CE and TE is not expected to occur. Further study is needed to understand the impact of soil [P] on the metabolic and storage pools of leaf Pi, and the subsequent effect on plant physiology and DM production. Adaptive strategies of roots to low soil [P] under different climate change scenarios require clarification, including the effect on root exudates and the quantity and composition of mycorrhizal populations. Though my study showed that soil P deficiency resulted in lower wood density and larger stem xylem vessels, the hydraulic conductivity of these vessels was not directly measured. Knowledge of stem hydraulic conductivity under the interactive effects of soil [P], CE and TE may provide further insight into the linkages between water and carbon transport