Warm and Fertile Sub-Humid Conditions Enhance Litterfall to Sustain High Soil Respiration Fluxes in a Mediterranean Cork Oak Forest

Abstract

Soil respiration is a major component of the global carbon budget and Mediterranean ecosystems have usually been studied in locations with shallow soils, mild temperatures, and a prolonged dry season. This study investigates seasonal soil respiration rates and underlying mechanisms under wetter, warmer, and more fertile conditions in a Mediterranean cork oak forest of Northern Tunisia (Africa), acknowledged as one of the most productive forests in the Mediterranean basin. We applied a soil respiration model based on soil temperature and relative water content and investigated how ecosystem functioning under these favorable conditions affected soil carbon storage through carbon inputs to the soil litter. Annual soil respiration rates varied between 1774 gC m −2 year −1 and 2227 gC m −2 year −1 , which is on the highest range of observations under Mediterranean climate conditions. We attributed this high soil carbon flux as a response to favorable temperatures and soil water content, but this could be sustained only by a small carbon allocation to roots (root/shoot ratio = 0.31-0.41) leading to a large allocation to leaves OPEN ACCESSForests 2015, 6 2919 with a multiannual leaf production, enhanced annual twig elongation (11.5-28.5 cm) with a reduced leaf life span (<1 year) maintaining a low LAI (1.68-1.88) and generating a high litterfall (386-636 gC m −2 year −1 ). Thus, the favorable climatic and edaphic conditions experienced by these Mediterranean cork oak forests drove high soil respiration fluxes which balanced the high carbon assimilation leading to a relatively small overall contribution (10.96-14.79 kgC m −2 ) to soil carbon storage.

Deep Soil Conditions Make Mediterranean Cork Oak Stem Growth Vulnerable to Autumnal Rainfall Decline in Tunisia

“…This relationship was enhanced when additionally considering a secondary phenological phase in the fall, suggesting a significant contribution of the secondary growth in the annual tree growth in our site. This result then extends early results found for Quercus ilex [8,79] to Quercus suber, and highlights the potential role of the secondary growth under a mild Mediterranean climate where secondary growth is common, as previously observed on this site for leaf production and twig elongation [20].…”

“…Precipitations mainly occur from September to March and a drought period extended from May to August. The average annual precipitation is 1113 mm and the average annual temperature 19.3 °C, on the combined wettest and warmest side of the Mediterranean climate leading to high forest productivity [20]. Maximum and minimum air temperatures were 47.2 °C and 0.1 °C, respectively.…”

“…t1 was predicted by the DOY when the simulated plant water potential reached a threshold of −1 MPa, a generic critical threshold for cambial elongation for this species [8]. t2 corresponds to the DOY when predawn water potential reaches −1 MPa after the dry period, and allows for secondary growth as previously observed on twig elongation in the study site [20].…”

Estimate of biomass and carbon pools in disturbed and undisturbed oak forests in Tunisia

“…The soil is the major reservoir of carbon in such ecosystem with 79.52% and 76.26% of the organic carbon was found in the YS and the AS, respectively within the 1.50 m soil depth. These stocks, which are relatively high, can be explained by a deep and fertile soil, a high soil water content, and a high mean annual precipitation associated with hot and dry summers, overall leading to extremely favorable conditions in the range of Mediterranean climates (Zribi et al, 2015).…”

Soil Respiration Behaviour in a Mediterranean Aleppo Pine Forest in North Tunisia

“…Soil temperature was simulated using simple regression: We estimated α and β to be 0.91 and 2.58, respectively. These models have been validated in the same period by Zribi et al (2015) in a cork oak forest in Tunisia. The Daily meteorological data were provided from the weather station at the Oued Elkbir (Elfahs, Zaghouan).…”