Use of power index and two-phase density approach to study fine root dynamics

Dynamics of fine roots are analyzed in terms of variation in functional soil volume, i.e., the volume of soil occupied by active fine roots. Functional soil volume decreases with drier climatic conditions while the substantiated rooting density, i.e., rooting density within the functional soil volume, remains constant. Substantiated rooting density differs from mean root density, which is defined as root biomass averaged over the entire rooting volume. This approach reflects the biological reality that, as a fraction of fine roots cease to function, the soil volume they inhabit can be considered to be non-functional. Thus functional soil volume becomes increasingly porous, and this porosity can be represented in terms of a volumetric power index. Hydrological equilibrium theory (Grier and Running, 1977; Eagleson, 1982; Nemani and Running, 1989; Pierce et al., 1993) implies that climate, functional soil volume, and total leaf area for a community of plants are in equilibrium. By expressing the dynamic characteristic of functional soil volume in terms of changing leaf area measurements, i.e., that soil rooting volume expressed as a function of leaf area by a scaling exponent, the relationship between transpiration and leaf area based on hydrologic equilibrium theory is established (Hatton and Wu, 1995). Using measurements of plant transpiration rate, parameter values relating these variables can be obtained from nonlinear fitting procedures. Four data sets from Wycanna, Qld are use to illustrate the procedure of describing fine root dynamics