Changes in concentrations of chemical components in green leaves, litters, twigs and fine roots usually take place when the plants grow under elevated atmospheric CO2. Variations in phytochemistry may influence species interactions and ecosystem processes such as competition, consumption, decomposition, and biogeochemical cycling. Increase in atmospheric CO2 results in increase in foliar starch concentrations, carbon-based secondary metabolites, particularly phenolics, while reduction in foliar nitrogen concentrations. CO₂-induced changes in foliar chemistry tend to reduce leaf quality and may further affect insect herbivores. Increasing atmospheric CO₂ also has potential influence on decomposition as the chemical components of all plant tissues are changed.
To examine the likely changes in the nutritional quality of tree tissues and the performance of leaf-feeding forest insect under increased atmospheric CO₂, samples of poplar (Populus pseudo-simonii ［Kitag.］) grown in Open Top Chambers at ambient and elevated (650μl L^－1) CO₂ were collected for measuring nitrogen concentration, C/N ratio, soluble sugar and starch contents in leaves, barks, coarse roots (>2mm in diameter), and fine roots (<2mm in diameter). Gypsy moth (Lymantria dispar) larvae were reared on a single branch of experimental trees in a nylon bag with 1 mm×1 mm grid. The response of larval growth was observed in situ.
The results indicated that increase in CO₂ significantly reduced nitrogen concentration in poplar leaves (-8.94% in average within growth season), bark (-9.13%), fine roots (-5.35%), and coarse roots (-10.53%). C/N ratios of green leaves (+9.98% in average within growth season), barks (+9.54%), and coarse roots (+10.27%) increased significantly in response to increased CO₂. C/N ratio in fine roots increased only 4.12% (not significantly). Total carbon contents in all tissues were unaffected by CO₂ treatment. Elevated CO₂ significantly increased soluble sugar (+13.89% in average within growth season) and non-structural carbohydrate (+12.67% in average within growth season) in poplar leaves, but starch concentration increased 24.67% only on 20 July, no significant changes in leaf starch concentration were found on the other sampling dates. Carbohydrate concentrations in poplar roots and barks were insensitive to CO₂ treatment, but soluble sugar contents in fine roots decreased by 10.54% in response to elevated CO₂. When second-stadium gypsy moth larvae consuming poplars grew under increased CO₂ for the first thirteen days, their body weight was 30.95% lower than the larvae at ambient CO₂, but when larvae fed in the same treatment for the next eleven days, no significant difference was found.
Increase in atmospheric CO₂ had adverse effects on nutritional quality of Populus pseudo-simonii ［Kitag.］tissues and the resultant variations in chemical components of green leaves had a significant but negative impact on the growth of early instar gypsy moth larvae.