Drought Tolerance of Artemisia Tridentata in Response to Herbivory and Mycorrhizal Colonization

Interactions with other organisms can affect a plant’s ability to cope with drought. The re-establishment of Artemisia tridentata , a keystone species of the sagebrush steppe, is often limited by summer drought. This study investigated the effect of two biotic factors, herbivory and symbiosis with arbuscular mycorrhizal fungi (AMF), on the drought tolerance of Artemisia tridentata ssp. wyomingensis (Wyoming big sagebrush) seedlings. For this purpose, I conducted two separate but concurrent field experiments. The herbivory experiment had three treatments: seedlings without tree protectors and seedlings within Vexar or metal-mesh tree protectors. In the mycorrhizal experiment, all seedlings were within metal-mesh tree protectors, and the experiment had two treatments: without and with the addition of trap culture inoculum. The effects of the treatments were evaluated on seedling survival, leaf water potential, stomatal conductance, and inflorescence development. To better assess the impact of AMF colonization on drought tolerance, I also conducted greenhouse experiments. These experiments investigated whether AMF colonization affected the decline in stomatal conductance and photosynthesis induced by drought. Herbivory damage, presumably caused by ground squirrels, mainly occurred in early spring, about five months after outplanting. Most seedlings recovered from this damage, but herbivory was associated with higher mortality during the subsequent summer, fall, and winter. Eighteen months after outplanting, the survival in plants within metal protectors was 12 and 39% higher than in those within plastic and no protectors, respectively. In addition, the percentage of lived plants that developed inflorescences was approximately threefold higher in plants with protectors than those without them. The results of this experiment indicate that most plants that suffered herbivory did not die directly from this disturbance but from increased susceptibility to abiotic stresses, including drought. The addition of trap culture inoculum increased AMF colonization by 23%, which represented a more than 100% rise over the levels measured in non-inoculated seedlings. However, this did not affect survival, which was above 90% for both treatments. The seedlings experienced water stress, as evidenced by water potentials ranging from -2 to -4 MPa and a decrease in stomatal conductance. Yet, despite the continued drought, the water potentials did not reach lethal levels and remained rather constant after midsummer. This response was partially mediated by reductions in stomatal conductance and plant hydraulic conductivity. The only response variable affected by inoculation was the percentage of plants developing inflorescence. These percentages were 45.4 and 59.0% for non-inoculated and inoculated seedlings, respectively ( p = 0.028). The greenhouse experiment showed that AMF colonization did not affect photosynthesis under well-watered conditions or instantaneous water use efficiency. In contrast, AMF colonization delayed the drought-induced decline in stomatal conductance and photosynthesis, or this decline occurred at a lower soil water content. These effects were not related to a lower water potential threshold for stomatal closure but rather to an increased ability to extract water from the potting substrate. The results are consistent with the notion that AMF colonization increases the drought tolerance of A. tridentata seedlings. However, the significance of the observed effect on increasing survival in natural habitats remains to be tested under more extreme water stress conditions than those experienced by the plants in my field experiment.