Impacts of soil moisture on de novo monoterpene emissions from European beech, Holm oak, Scots pine, and Norway spruce

Impacts of soil moisture on de novo monoterpene (MT) emissions from Holm oak, European beech, Scots pine, and Norway spruce were studied in laboratory experiments. The volumetric water content of the soil, Theta, was used as the reference quantity to parameterize the dependency of MT emissions on soil moisture and to characterize the severity of the drought. When Theta dropped from 0.4 m(3) x m(-3) to similar to 0.2 m(3) x m(-3) slight increases of de novo MT emissions were observed but with further progressing drought the emissions decreased to almost zero. In most cases the increases of MT emissions observed under conditions of mild drought were explainable by increases of leaf temperature due to lowered transpirational cooling. When Theta fell below certain thresholds, MT emissions decreased simultaneously with Theta and the relationship between Theta and MT emissions was approximately linear. The thresholds of Theta (0.044-0.19 m(3) x m(-3)) were determined, as well as other parameters required to describe the soil moisture dependence of de novo MT emissions for application in the Model of Emissions of Gases and Aerosols from Nature, MEGAN. A factorial approach was found appropriate to describe the impacts of Theta, temperature, and light. Temperature and Theta influenced the emissions largely independently from each other, and, in a similar manner, light intensity and Theta acted independently on de novo MT emissions. The use of Theta as the reference quantity in a factorial approach was tenable in predicting constitutive de novo MT emissions when Theta changed on a time scale of days. Empirical parameterization with Theta as a reference was only unsuccessful when soil moisture changed rapidly