Variation of morphological and chemical traits of perennial grasses in arid ecosystems. Are these patterns influenced by the relative abundance of shrubs?

We asked whether morphological and chemical traits of perennial grasses in semiarid-arid ecosystems are influenced by the abundance of coexisting shrubs, in northern-central Patagonia, Chubut Province, Argentina. We selected populations of two perennial grass species, highly preferred by herbivores (4 for Festuca pallescens and 8 for Poa ligularis) at 10 sites distributed across a wide range of aridity. We assessed the relative shrub cover (shrub cover as percent of total cover) of each site and randomly collected 5 to 10 bunches of each grass species per site in December 2007 (late vegetative-early reproductive growth period). We measured the height of vegetative tillers, and morphological (length, width, area, dry mass, and specific area) and chemical (N, C, soluble phenolics and lignin concentration) attributes of full expanded green blades in the collected bunches. Further, we computed the aridity index of each site using temperature and precipitation data. The aridity index ranged from 1.07 to 3.9 at F. pallescens sites and from 3.6 to 7.3 at P. ligularis sites. The relative shrub cover increased significantly with aridity varying from 0.01 to 99%. Tiller height, blade length, and blade area of P. ligularis increased significantly with increasing relative shrub cover and aridity. Concentration of soluble phenolics in blades of both species decreased with increasing relative shrub cover and aridity. N concentration in blades of P. ligularis and specific blade area in F. pallescens decreased with increasing relative shrub cover and aridity. We conclude that some traits of perennial grasses (phenolics concentration in green blades in both species, and tiller height and some blade attributes in P. ligularis) were influenced by shrub cover itself. The variation in these traits was opposite to that expected by the effect of aridity and could be associated with escape from herbivores and/or drought, enhanced mesophytism, and reduced chemical defenses.