Temporal dynamics of herbivory and water availability interactively modulate the outcome of a grass-shrub interaction in a semi-arid ecosystem

The study of plant-plant interactions along grazing and abiotic stress gradients is a major research topic in plant ecology, but the joint effects of both stressors on the outcome of plant-plant interactions remains poorly understood. We used two different factorial experiments conducted in a semi-arid Mediterranean steppe to assess: 1) the role of the perennial grass Stipa tenacissima, a low-palatability species, providing protection from rabbit herbivory to the shrub Retama sphaerocarpa (experiment 1), and 2) the effects of environmental amelioration provided by Stipa on the recovery of Retama after rabbit damage under two contrasted levels of water availability (experiment 2). In the experiment 1, water stress worked as an indirect modulator of herbivore protection by Stipa. This species protected Retama seedlings from rabbit herbivory during the wetter conditions of spring and winter, but this effect dissapeared when rabbit pressure on Retama increased during summer drought due to the decrease in alternative food resources. In the experiment 2, Stipa exerted a negative effect on the survival of Retama seedlings during the three years of the experiment, regardless of inter-annual differences in rainfall or the watering level applied. This negative effect was mainly due to excessive shading. However, Stipa increased Retama recovery after initial rabbit impact, overriding in part this negative shade effect. Conversely, Stipa impact on the Fv/Fm of Retama seedlings depended on the intra-annual water dynamics and its experimental manipulation, overall contradicting predictions from the stress-gradient hypothesis. The complex interactions found between herbivory, microclimatic amelioration from Stipa, and water availability as drivers of Retama performance illustrate the importance of considering the temporal dynamics of both biotic and abiotic stressors to fully understand the outcome of plant-plant interactions.