Effects of long-term elevated CO2 treatment on the inner and outer bark chemistry of sweetgum (Liquidambar styraciflua L.) trees

Abstract

Long-term exposure of sweetgum trees to elevated atmospheric CO 2 concentrations significantly shifted inner bark (phloem) and outer bark (rhytidome) chemical compositions, having implications for both defense and nutrient cycling. Changes in plant tissue chemistry due to increasing atmospheric carbon dioxide (CO2) concentrations have direct implications for tissue resistance to abiotic and biotic stress while living, and soil nutrient cycling when senesced as litter. Although the effects of elevated CO2 concentrations on tree foliar chemistry are well documented, the effects on tree bark chemistry are largely unknown. The objective of this study was to determine the effects of a long-term elevated CO2 treatment on the contents of individual elements, extractives, ash, lignin, and polysaccharide sugars of sweetgum (Liquidambar styraciflua L.) bark. Trees were harvested from sweetgum plots equipped with the Free-Air CO2 Enrichment (FACE) apparatus, receiving either elevated or ambient CO2 treatments over a 12-year period. Whole bark sections were partitioned into inner bark (phloem) and outer bark (rhytidome) samples before analysis. Principal component analysis, coupled with either Fourier transform infrared spectroscopy or pyrolysis–gas chromatography–mass spectrometry data, was also used to screen for differences. Elevated CO2 reduced the N content (0.42 vs. 0.35 %) and increased the C:N ratio (109 vs. 136 %) of the outer bark. For the inner bark, elevated CO2 increased the Mn content (470 vs. 815 mg kg−1), total extractives (13.0 vs. 15.6 %), and residual ash content (8.1 vs. 10.8 %) as compared to ambient CO2; differences were also observed for some hemicellulosic sugars, but not lignin. Shifts in bark chemistry can affect the success of herbivores and pathogens in living trees, and as litter, bark can affect the biogeochemical cycling of nutrients within the forest floor. Results demonstrate that increasing atmospheric CO2 concentrations have the potential to impact the chemistry of temperate, deciduous tree bark such as sweetgum.