Gas exchange in arid-adapted shrubs: when is efficient water use a disadvantage?

Potted individuals of two perennial shrub species of South Africa’s arid region were subjected to a soil-drying cycle. Plant stomatal conductances and water-use efficiencies were related to soil water content. High, opportunistic stomatal conductance and low water-use efficiency occurred at high soil water content, when the lower stomatal conductance required for optimum water-use efficiency may represent a competitive disadvantage. Water-use efficiency increased as soil dried to intermediate soil water contents, but stomatal conductance finally became conservative and water-use efficiency decreased again at very low soil water content. This response may lengthen growing season and defer lethal water stress. This combination of opportunistic and parsimonious water use may facilitate the competitive success and resilience of these species under semi-arid and arid conditions