An assessment of the differential sensitivity of four summer-deciduous chamaephytes to grazing and plant interactions using translational asymmetry

The effect of grazing on plant species can vary depending on plant strategy and available resources. We investigated the effect of grazing (sheep and goats) on four summer-deciduous perennial chamaephytes living in a semi-arid matorral of SE Spain. We used translational asymmetry to measure developmental instability, which is a proposed estimator of plant condition and, consequently, of environmental stress. In addition, we examined growth rhythm and floral production. To understand better plant interactions under grazing pressure, we used information on translational asymmetry and growth in the presence of co-occurring species. Sideritis osteoxylla was the most sensitive species to grazing, declining translational symmetry and floral production with grazing pressure. As in other palatable species, early growth in Ballota hirsuta accelerated in response to grazing by taking advantage of herbivores that are concentrated on annual plants. Grazing did not have a negative effect on Thymus hyemalis, but favored growth in Teucrium lusitanicum. Both of those species are characteristic species of the arid garrigue formation. In addition, we observed a positive association among target species and co-occurring perennial species. The strength and direction of the effect of the association with grazing was more evident in the most palatable species, B. hirsuta, which associated with unpalatable species as grazing pressure increased.