Chemical responses of two deciduous trees after 10-years of exposure to Free Air CO2 enrichment

Globally, the mean atmospheric carbon dioxide level has risen steadily since pre-industrial times, which is largely attributable to human activities such as increased emissions from fossil fuel burning, and clearing of forests. Numerous research efforts have been made to understand responses of trees in higher atmospheric carbon dioxide levels in short term, small-scale experiments. To overcome the limitations of these experiments and understand the response of trees in a natural environment, the Free Air Carbon dioxide Enrichment (FACE) project was proposed. FACE is a global project that involves experimentally enriching carbon dioxide in the atmosphere of terrestrial ecosystems. The FACE project was designed to address long-term responses of ecosystems to climate-related changes that will occur in the future. The objective of this research was to assess the chemical composition of two deciduous tree species, sweetgum (Liquidambar styraciflua) and aspen (Populus tremuloides) which underwent long term (ten years) exposure to elevated carbon dioxide in two FACE sites. Changes due to carbon dioxide enrichment in the structural chemical components such as cellulose, hemicellulose, lignin, and the non-structural components including nutrients, ash and extractives were analyzed.

The results indicated that the structural components of sweetgum and aspen were not significantly affected by long term exposure to elevated carbon dioxide. However, statistical differences were found in non-structural components. In the case of sweetgum, total phenolics, ash content - specifically calcium, and magnesium - were increased in trees exposed to carbon dioxide enrichment. In aspen trees exposed to elevated carbon dioxide, ash (potassium and phosphorus), were increased. By principal component analysis of all the measured responses, a separation between trees exposed to elevated and ambient carbon dioxide was obtained. In addition to the separation, the multivariate approach revealed that some of the structural components of wood may also be affected at a lesser degree by elevated carbon dioxide.