Effects of elevated CO2 on the foliar chemistry of seedlings of two rainforest trees from north-east Australia:: Implications for folivorous marsupials

The present study examined the effects of elevated levels of atmospheric CO2 on foliar concentrations of nitrogen, mineral nutrients and phenolics, and leaf toughness, in seedlings of two common rainforest trees from north-east Queensland, Australia. The trees were the pioneer species Alphitonia petriei Braid and C. White and the mid-successional species Flindersia brayleyana F. Muell. Both species are important in the diets of folivorous marsupials endemic to the region. Seedlings were grown in native rainforest soils (nutrient-rich basalt and nutrient-poor rhyolite) and exposed to unreplicated treatments of ambient (350 p.p.m.) and elevated (790 p.p.m.) CO2 for 60 days in a glasshouse. The foliage of seedlings exposed to elevated CO2 had lower concentrations of nitrogen and sodium than did seedlings exposed to ambient conditions. Nitrogen levels declined by 4.5 mg g(-1) in Alphitonia and 5.9 mg/g in Flindersia, or 25 and 29% of ambient levels, respectively. Sodium levels declined by 44% in both species. In Flindersia, concentrations of phosphorus, potassium and calcium were also reduced in elevated CO2 by 19-28% of ambient levels, but these minerals did not vary with CO2 treatment in Alphitonia. In elevated CO2, levels of condensed tannins were higher in Flindersia, but not Alphitonia. Levels of total phenolics did not vary significantly with CO2 in Flindersia; whereas in Alphitonia, total phenolics were lower in elevated CO2, but only on basalt soils. Leaves were thicker in both species in elevated CO2. Leaves were tougher in both species in elevated CO2, but only on rhyolite soils. If the results of the present study can be extrapolated to mature trees exposed to elevated CO2 over the long-term, folivores would be expected to become less abundant under elevated CO2 conditions, as foliar chemistry is a good predictor of folivore abundance in the higher elevation rainforests of north-east Queensland.