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

摘要

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 Patagonia, using a hypothetical d-separation hierarchical path model, postulating two main pathways. One path analysed the effect of large exotic herbivores on flowering phenology and plant fitness, through plant damage. A second path estimated the effects 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 from seven independent rangelands located under the same environmental conditions but which differed in 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 affecting plant fitness. As grazing intensity increased, blooming started earlier, decreased in intensity, lasted for a shorter period of time leading to earlier fruiting. Temperature and precipitation had a positive effect on both flowering phenology and plant fitness, temperature having more influence than precipitation. My results suggest that grazing can impose top-down effects on the phenological variables and fitness of desert vegetation and that these are not counteracted by the bottom-up effects produced by climate. This illustrates a novel way through which exotic animals can affect ecosystem dynamics: by reshaping patterns of flowering phenology.