Modeling daily gas exchange of a Douglas-fir forest: comparison of three stomatal conductance models with and without a soil water stress function.

Modeling stomatal conductance is a key element in predicting tree growth and water use at the stand scale. We compared three commonly used models of stomatal conductance, the Jarvis-Loustau, Ball-Berry and Leuning models, for their suitability for incorporating soil water stress into their formulation, and for their performance in modeling forest ecosystem fluxes. We optimized the parameters of each of the three models with sap flow and soil water content data. The optimized Ball-Berry model showed clear relationships with air temperature and soil water content, whereas the optimized Leuning and Jarvis-Loustau models only showed a relationship with soil water content. We conclude that use of relative humidity instead of vapor pressure deficit, as in the Ball-Berry model, is not suitable for modeling daily gas exchange in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in the Speulderbos forest near the village of Garderen, The Netherlands. Based on the calculated responses to soil water content, we linked a model of forest growth, FORGRO, with a model of soil water, SWIF, to obtain a forest water-balance model that satisfactorily simulated carbon and water (transpiration) fluxes and soil water contents in the Douglas-fir forest for 1995.

Pyrgeometer formula.

In a recent article [Appl. Opt. 34 , 1598 (1995) a new pyrgeometer formula was proposed. This formula leads to a] bias at isothermal conditions. The origin of the bias is explored by a more rigorous treatment of the radiation processes involved in the energy balance of a pyrgeometer. As a result it is found that, for an accuracy of a few watts per square meter, the spectral transmission characteristics of the dome have to be taken into account.

Observation of the structure parameters C(2)(T), C(TQ), and C(2)(Q) in the mixed layer over land.

We obtained simultaneous measurements of the structure parameters C(2)(T), C(TQ), and C(2)(Q) from the KNMI meteorological tower at Cabauw and investigated the behavior of these structure parameters in the mixed layer. In the lower part of the mixed layer, the structure parameters were found to follow the free-convection prediction (z/z(i))(-4/3). In the upper part of the mixed layer a frequency-dependent phase shift between temperature and humidity fluctuations was found.