The effect of local perennial plants on the occurrence and traits of the Brachypodium distachyon complex along an aridity gradient

Plant-plant interactions are well-known drivers of plant community dynamics and plant traits. Less well known is to what extent these interactions and their effects change along stress gradients. To study this, we assessed the local spatial association of the annual grass species Brachypodium hybridum and Brachypodium distachyon to perennial plants in ten sites along an aridity gradient in southeastern Spain and compared ten phenotypic traits of Brachypodium occurring underneath and outside the perennial plant canopy. We found that Brachypodium was less associated with perennials with increasing aridity but with a significant site effect. This site effect seemed to be due to one particular site, the removal of which from our analysis resulted in an even stronger negative correlation and uncovered a significant influence of the perennial plant. We also found that while overall three phenotypic traits (plant height, spike length, and seed number) were different between plants underneath perennial plants and plants outside perennials, only one of these differences (spike length) changed significantly along the gradient. Further, in controlled conditions, we found that the offspring of annual plants collected from outside perennial plant canopies in the least arid site flowered more rapidly than those from underneath perennial plants, but this difference was not observed for the most arid site. These results indicate that in patchy arid landscapes dominated by perennial plants, Brachypodium’s response to the microhabitat provided by neighboring perennial plants changes along the aridity gradient, but other factors such as site and perennial plant may counteract or strengthen the effects of aridity.