Biotic Stress and Yield Loss

where c iis a proportionality factor (0.9 and 0.7 for C 3and C 4species, respectively), 38L wis the latent heat of vaporization of water (J kg 1 ), and 10 is required to obtainappropriate units for T p,i(cm).The ratio of actual (T a,i) to potential (T p,i) transpiration has been shown todecrease linearly with soil water: 43T a, i a wp,i where Tp,i cr,i wp,i a cr,i[13.9]wp,iwhere ais actual volumetric soil water content (obtained from R wand soil bulk density)and the subscripts wp and cr refer to species-specific water content at permanentwilting point, and the critical soil water content (below which T a,i/T p,ibegins todecline), respectively. 38Equation 13.8 estimates the potential demand for soil water by species i. Notethat the only way leaf area influences potential transpiration is through its effect onlight interception (F a,i). To my knowledge, this relationship has not been tested.Equation 13.9 estimates the effects of actual soil water content on the quantity ofwater transpired through the canopy of a given species. Ray and Sinclair 44 reporteddata useful for obtaining an empirical estimate of crand wpfor maize, and Moreshetet al. 45 provided the only example of experiments designed to obtain side-by-sidecomparisons of this relationship for crop and weed species.Equations 13.6 to 13.9 can be used to simulate indirect competition forsoil water. In other words, during periods when water is not limiting, plants utilizeas much water as is required using Equation 13.9, which subsequently reducesthe pool of available soil water. As the growing season progresses, if precipitationdoes not replenish the pool of available soil water, eventually the water contentwill fall below the cr,iand growth will be reduced. Direct competition isnot accounted for using this approach because when soil water becomes limiting,there is no accounting for how much soil water is available and whether that quantitymeets the transpiration demand of all species in the mixed canopy. In otherwords, Equation 13.8 simply indicates that if the volumetric soil water falls belowa threshold ( cr,i), transpiration is reduced. Under conditions of limiting soil water,it is assumed that the quantity of available water is not great enough to meetthe demands of both (or all) species (e.g., (T a,c T a,w) R w). Under such conditions,how should T a,ibe modified? Actual water uptake by a species will dependin part upon root length density and the efficiency with which the roots activelytake up water. Should the quantity of available water be divided among speciesbased on the relative quantity of roots in the soil profile? What if the species differ intheir uptake efficiency? What if the depth of the actual root zone of the twospecies differs? These are factors that substantially complicate our ability to simulatedirect competition for soil water. Moreover, because root physiology research isdifficult and costly, there are few useful examples reported in the literature.Therefore, algorithms for simulating interplant competition for soil water are not asrefined as those for simulating competition for light. Further research is clearlyneeded.