Ecosystem respiration in two Mediterranean evergreen Holm Oak forests: drought effects and decomposition dynamics

1. We present ecosystem respiration data from two Mediterranean forest sites in central Italy (Castelporziano) and southern France (Puechabon) in order to analyse the role of soil drought and decomposition dynamics using different models. 2. Ecosystem respiration was derived from continuous eddy covariance measurements. The entire data set was separated into 5-day periods. For each period a function depending on three parameters was fitted to the scatter of eddy CO2 flux versus photosynthetic photon flux density. The y intercept of each curve was taken as an estimate of the average night-time ecosystem respiration during the period. The ecosystem respiration was analysed with different regression models as a function of soil water content and temperature. 3. Ecosystem respiration ranged from 1 to 7 mumol m(-2) s(-1) and showed a clear seasonality, with low rates during drought periods and in winter. The regression model analysis revealed that in drier soil, ecosystem respiration was more sensitive to soil moisture than is expressed by the often used hyperbolic model. 4. In contradiction to a simple multiplicative model, the Q(10) of ecosystem respiration was not independent of moisture, but increased from nearly 1.0 at low moisture to above 2.0 at field capacity. Several explanations are discussed. 5. Of the variance in ecosystem respiration, 70-80% was explained with a model where Q(10) of ecosystem respiration is a function of soil water content. 6. For the Puechabon site, a soil carbon-balance model predicted only small-changes in litter pool size (max. 7%), which caused only minor changes in soil microbial respiration (0.1 mumol m(-2) s(-1)). In contrast, the contribution of microbial regrowth dynamics to ecosystem respiration is estimated to be substantial (approximate to1.6 mumol m(-2) s(-1)). The model predicted that soil microbial respiration probably provides the largest contribution to ecosystem respiration (approximate to50%). The importance of below-ground processes for ecosystem C balances is thus emphasized.