Abstract. Soil respiration (SR) is a major component of ecosystem's carbon cycle and represents the second largest CO2 flux of the terrestrial biosphere. Soil temperature is considered to be the primary control on SR whereas soil moisture as the secondary control factor. However, soil moisture can become the dominant control on SR in very wet or dry conditions. Determining the trigger that switches-on soil moisture as the primary control factor of SR will provide a deeper understanding on how SR changes under projected future increased droughts. Specific objectives of this study were (1) to investigate the seasonal variations and the relationship between SR and both soil temperature and moisture in a Mediterranean riparian forest along a groundwater level gradient; (2) to determine soil moisture thresholds at which SR is rather controlled by soil moisture than by temperature; (3) to compare SR responses under different tree species present in a Mediterranean riparian forest (Alnus, glutinosa, Populus nigra and Fraxinus excelsior). Results showed that the heterotrophic soil respiration rate, groundwater level and 30 cm integral soil moisture (SM30) decreased significantly from riverside to uphill and showed a pronounced seasonality. SR rates showed significant differences among tree species, with higher SR for P. nigra and lower SR for A. glutinosa. The lower threshold of soil moisture was 20 and 17% for heterotrophic and total SR respectively. Daily mean SR rate was positively correlated with soil temperature when soil moisture exceeded the threshold, with Q10 values ranging from 1.19 to 2.14; nevertheless, SR became decoupled from soil temperature when soil moisture dropped below these thresholds.
Coexistence Mechanism of Trees in a Sedimentation-dominated Riparian Forest in a Warm-temperate Mountainous Region in Southern Japan with Special Focus on Suppression
