Temporal and spatial variations of stem CO2 efflux of three species in subtropical China

Although stem CO2 efflux is critical to ecosystem carbon and energy balance and its feedback to future climate change, little information is available on stem CO2 efflux and its responses to temperature, especially in subtropical China. This study aims to (i) evaluate the temporal and spatial variations of stem CO2 efflux of three species, including oak (Quercus acutissima Carr.), masson pine (Pinus massoniana Lamb.) and loblolly pine (Pinus taeda Linn.) in subtropical China and (ii) analyze the temperature sensitivity of stem CO2 efflux in the three species based on 2-year field measurements. We measured stem CO2 efflux and stem temperature (at 3 cm depth) of the three species using the horizontally oriented soil chamber technique from September 2008 to August 2010. We also conducted a 24-h measurement to examine the diurnal variation of stem CO2 efflux in three consecutive days in April 2009. The temporal dynamics of stem CO2 efflux followed the change of the stem temperature in a 3-cm depth with a bell-shaped curve in the three species. Stem temperature explained 77-85% of the seasonal variations of stem CO2 efflux over the entire study period in the three species. The temperature sensitivity (Q(10)) of stem CO2 efflux was obviously different among the three species with higher Q(10) value found in oak (2.24) and lower values in the coniferous species (1.76 and 1.63). Our results also showed that the Q(10) values of stem CO2 efflux in all the three species were lower in the growing season than that in the non-growing season, indicating that the growth and maintenance respiration had different temperature responses. Moreover, we found that the temperature-normalized stem CO2 efflux (R-10) changed greatly between the growing and non-growing seasons in oak and masson pine, but not in loblolly pine. Additionally, we also found that in the non-growing season, the principal factor responsible for the spatial variation of stem CO2 efflux among the 15 sampling trees was sapwood volume, whereas in the growing season, stem CO2 efflux was closely related to annual dry-matter production in the three subtropical species.