Indirect determination of rooting depth and permanent wilting point

Abstract

The relationship between production, rooting depth, permanent wilting point, soil water content and soil temperature was modeled. We hypothesized that shoot production would not be significantly correlated with soil water contents below permanent wilting or with soil water at depths below the rooting zone. Exposure, the accumulation of simultaneously suitable soil water and temperature conditions, was modeled as the product of a function of soil water and a function of soil temperature summed to a postulated rooting depth over the growing season. The two soil water functions included postulated values of permanent wilting point and rooting depth, while the four soil temperature functions considered the effects of soil temperature on production near the freezing point. The most probable rooting depth, permanent wilting point, and soil water and temperature functions used to model the exposure were those best correlated with production. These hypotheses were tested with data from two annual and one perennial communities in a semi-arid shrub steppe in Washington state, U.S.A. Results showed that the permanent wilting point was between −2.0 and −5.0 mPa, but the rooting depth was not well defined, possibly due to lack of soil water data below the rooting zone. The best combination of soil water and soil temperature functions was respectively soil water potential and frozen or unfrozen soil.