Gender-specific responses to winter defoliation of greenhouse-grown Atriplex canescens clones under contrasting soil moisture regimes

A greenhouse experiment with female, male and monecious clones (36 of each, 108 total) of tetraploid Atriplex canescens (Pursh) Nutt. (fourwing saltbush) was conducted to determine gender-specific responses to simulated herbivory under contrasting conditions of soil moisture. We imposed three levels of winter defoliation (control, 40%, and 70% stem removal) and two soil moisture treatments (control and water stress). Gender-related responses to defoliation were context-specific. Defoliated female clones produced significantly less shoot biomass than male or monecious clones when soil moisture conditions were not limiting. All genders produced similar amounts of shoot biomass under conditions of soil moisture stress. Root biomass responses exhibited the opposite trend. With adequate soil moisture all defoliated clones produced similar amounts of root biomass, but water-stressed defoliated females usually produced less root biomass than their male and monecious counterparts. Leaf gas-exchange measurements indicated that females were less efficient than males in the use of water. Monecious clones outperformed both females and males when conditions of stress were greatest. Defoliated female shrubs may be at a competitive disadvantage regardless of the environmental context, and could therefore be expected to be the shrub gender most negatively affected by herbivory.