Competition for Water Among Plants: Resolving the Root System.

Competition for water and nutrients by plants plays a major role in controlling growth and the level of reactive forms of nutrients and their availability for off-site leakage.  The processes controlling competition are, however, poorly understood and quantified.  The goal of this research is to develop a mechanistic model of plant competition applicable to multi-species pastures.  To this end, an existing, physically-based algorithm for modeling water uptake is being tested by comparing model outputs with measurement in the field and in root boxes (0.02 x 0.5 x 1.2 m) having plants of contrasting root patterns.  The species that we propose to use are orchardgrass (Dactylis glomerata L.) and white clover (Trifolium repens L.) and the model-plants maize (Zea mays L.) and sunflower (Helianthus annuus).  In root boxes and after a drying phase, the soil immediately below the plant crown had the highest root density and the lowest water potential, in the vicinity of wilting point.  In the field and after a severe drying phase, plants of maize and sunflower growing side by side and without light competition have similar pre-dawn crown water potential (-600 J kg^-1, equivalent to approximately 15 to 20% plant available water) yet by mid-morning maize plants closed the stomata while sunflower stomata remained open (> 0.5 mol H₂O m² s).  This was associated to the deeper rooting pattern of sunflower.  However, both maize and sunflower stopped leaf expansion, indicating that in sunflower leaf expansion had higher sensitivity than transpiration in a drying soil.  Similar measurements with a perennial grass and clover are methodologically challenging but necessary to test the model in pastures.  These measurements in roots boxes will allow calculating root and shoot resistances to water flow, the role of root density, and provide information to preserve or modify the competition algorithm for water uptake.