Effects of elevated carbon dioxide and elevated temperature on morphological, physiological and anatomical responses of Eucalyptus tereticornis along a soil phosphorus gradient.

Eucalypts are likely to play a critical role in the response of Australian forests to rising atmospheric CO2 concentration ([CO2]) and temperature. Although eucalypts are frequently phosphorus (P) limited in native soils, few studies have examined the main and interactive effects of P availability, [CO2] and temperature on eucalypt morphology, physiology and anatomy. To address this issue, we grew seedlings of Eucalyptus tereticornis Smith across its P-responsive range (6-500 mg kg-1) for 120 days under two [CO2] (ambient: 400 µmol mol-1 (Ca) and elevated: 640 µmol mol-1 (Ce)) and two temperature (ambient: 24/16 °C (Ta) and elevated: 28/20 °C (Te) day/night) treatments in a sunlit glasshouse. Seedlings were well-watered and supplied with otherwise non-limiting macro- and micro-nutrients. Increasing soil P supply increased growth responses to Ce and Te. At the highest P supplies, Ce increased total dry mass, leaf number and total leaf area by ~50%, and Te increased leaf number by ~40%. By contrast, Ce and Te had limited effects on seedling growth at the lowest P supply. Soil P supply did not consistently modify photosynthetic responses to Ce or Te. Overall, effects of Ce and Te on growth, physiological and anatomical responses of E. tereticornis seedlings were generally neutral or negative at low soil P supply, suggesting that native tree responses to future climates may be relatively small in native low-P soils in Australian forests.

Soil phosphorous and endogenous rhythms exert a larger impact than CO2 or temperature on nocturnal stomatal conductance in Eucalyptus tereticornis.

High nocturnal transpiration rates (5-15% of total water loss in terrestrial plants) may be adaptive under limited fertility, by increasing nutrient uptake or transport via transpiration-induced mass flow, but the response of stomata in the dark to environmental variables is poorly understood. Here we tested the impact of soil phosphorous (P) concentration, atmospheric CO2 concentration and air temperature on stomatal conductance (gs) during early and late periods in the night, as well as at midday in naturally, sun-lit glasshouse-grown Eucalyptus tereticornis Sm. seedlings. Soil P was the main driver of nocturnal gs, which was consistently higher in low soil P (37.3-79.9 mmol m(-2) s(-1)) than in high soil P (17.7-49.3 mmol m(-2)(-1)). Elevated temperature had only a marginal (P = 0.07) effect on gs early in the night (gs decreased from 34.7 to 25.8 mmol m(-2) s(-1) with an increase in temperature of 4 °C). The effect of CO2 depended on its interaction with temperature. Stomatal conductance responses to soil P were apparently driven by indirect effects of soil P on plant anatomy, since gs was significantly and negatively correlated with wood density. However, the relationship of gs with environmental factors became weaker late in the night, relative to early in the night, likely due to apparent endogenous processes; gs late in the night was two times larger than gs observed early in the night. Time-dependent controls over nocturnal gs suggest that daytime stomatal models may not apply during the night, and that different types of regulation may occur even within a single night. We conclude that the enhancement of nocturnal gs under low soil P availability is unlikely to be adaptive in our species because of the relatively small amount of transpiration-induced mass flow that can be achieved through rates of nocturnal water loss (3-6% of daytime mass flow).