Soil moisture overshadows temperature control over soil CO2 efflux in a Pinus canariensis forest at treeline in Tenerife, Canary Islands

Abstract

Soil CO2 efflux (ES) 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. ES was measured in weekly to monthly intervals between October 2008 and September 2009. Although ES varied markedly throughout the year, the temperature sensitivity of soil CO2 efflux (QS) and ES normalized to 10 °C temperature (ES10) did not vary considerably with respect to season. The Q10 values were 1.92 for the cold and wet season and 1.98 for the warm and dry season, respectively. The corresponding ES10 values were 0.61 and 0.50 μmol m−2 s−1, respectively. ES was largely controlled by soil temperature (Ts) and 76% of the annual variation in ES 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, Ts control over ES was limited. A model combining soil temperature and soil water availability explained 75% of the annual variation in ES. When integrated over an entire year, ES 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 ES was 48 and 52%, respectively.Our results reveal that soil water availability limits temperature control over ES during the wet and cold season and during the warm and dry period in Mediterranean forest ecosystems at their upper distribution limit.