Reshaping phenology: grazing has stronger effects than climate on flowering and fruiting phenology in desert plants

Abstract Resources and climatic conditions impose bottom-up selective forces on flowering phenology, enabling flowering to occur when climatic conditions are the most suitable for reproduction. However, phenological rhythms also suffer from top-down forces imposed by herbivory, which may alter plant resources intake and allocation. I analysed the net effect of grazing and climate on phenological variables in an arid environment in northern Patagonian, using a hypothetical d-separation hierarchical path model, postulating two main pathways. One path analysing the effect of large exotic herbivores on flowering phenology and plant fitness, through plant damage. A second path, estimating the effect of two main climatic variables (temperature and precipitation) on flowering phenology and plant fitness. Therefore, if climate has a stronger influence on flowering phenology than grazing, precipitation and/or temperature should show a stronger association with phenological variables than large herbivores’ density. I selected eight of the most common native plant species in seven independent rangelands located under the same environmental conditions but with increasing grazing intensity to study flowering phenology over four spring-summer seasons. I found that herbivore density had a stronger (negative) effect than climatic variables on flowering phenology impacting on plant fitness. As grazing intensity increased, blooming started earlier, decreased in intensity, lasted less and fruiting occurred earlier. Temperature and precipitation had both a positive effect on flowering phenology and plant fitness, being temperature stronger than precipitation. My results suggest that grazing can imposed top-down effects on phenological variables and fitness on desert vegetation that are not countered by the bottom-up effects inflicted by climate. This illustrates a novel way through which exotic animals can affect ecosystem dynamic: by reshaping the patterns of flowering phenology.