Soil moisture overshadows temperature control over soil CO_2 efflux in a Pinus canariensis forest at treeline in Tenerife, Canary Islands

摘要

Soil CO_2 efflux (E_S) of a 50 year-old Mediterranean Pinus canariensis Chr. Sm. Ex DC. at treeline in Tenerife (Canary Islands), an ecotone with strong seasonal changes in soil temperature and soil water availability. E_S was measured in weekly to monthly intervals between October 2008 and September 2009. Although E_S varied markedly throughout the year, the temperature sensitivity of soil CO_2 efflux (Q_S) and E_S normalized to 10 °C temperature (E_(S10)) did not vary considerably with respect to season. The Q_(10) values were 1.92 for the cold and wet season and 1.98 for the warm and dry season, respectively. The corresponding E_(S10) values were 0.61 and 0.50 μmol m~(-2) s~(-1), respectively. E_S was largely controlled by soil temperature (T_s) and 76% of the annual variation in E_S was explained by this factor. However, during the warm and dry period when soil water potential in 25-30 cm depth (Ψ) was lower than -0.04 MPa and during the cold and wet period when Ψ was higher than -0.02 MPa, T_s control over E_S was limited. A model combining soil temperature and soil water availability explained 75% of the annual variation in E_S. When integrated over an entire year, E_S was 314 g carbon m~(-2) and the contribution of the cold and wet season (October-May; 214 days) and of the warm and dry season (June-September; 154 days) to annual E_S was 48 and 52%, respectively. Our results reveal that soil water availability limits temperature control over E_S during the wet and cold season and during the warm and dry period in Mediterranean forest ecosystems at their upper distribution limit.