Pushing the limits of C3 intrinsic water use efficiency in Mediterranean semiarid steppes: responses of a drought-avoider perennial grass to climate aridification

Intrinsic water use efficiency (WUEi) reflects the trade-off between photosynthetic carbon gain and water loss through stomatal conductance and is key for understanding dryland plant responses to climate change. Stipa tenacissima is a perennial tussock C3 grass with an opportunistic, drought-avoiding water use strategy that dominates arid and semiarid steppes across the western Mediterranean region. However, its ecophysiological responses to aridification and woody shrub encroachment, a major land-use change in drylands worldwide, are not well understood.
We investigated the variations in leaf stable isotopes (δ18O, δ13C, δ15N), nutrient concentrations (N, P, K), and culm water content and isotopic composition (δ18O, δ2H) of paired pure-grass and shrub-encroached S. tenacissima steppes along a 350 km aridity gradient in Spain (10 sites, 160 individuals). 
Culm water isotopes revealed that S. tenacissima is a shallow-rooted grass that depends heavily on recent rainwater for water uptake..., We selected 10 locations along a ~350 km transect from Central to Southeastern Spain (Fig. S1) representing an increasing climatic aridity gradient encompassing much of the geographical distribution of S. tenacissima in the Iberian Peninsula, including areas with MAP ranging from 346 to 464 mm and mean annual temperature (MAT) from 12.6 to 16.3℃ (Supporting Information Table S1, S2). The climate is semiarid Mediterranean, with asynchronous wet and warm seasons, as maximum and minimum precipitation fall in winter and summer, respectively. The soils are classified as Calcisols and are weakly developed and thin, with sandy loam texture and high pH values (>7–8) (Maestre et al. 2009). Transpiration fluxes and primary productivity in semiarid S. tenacissima steppes are rather low, with peak plant ecophysiological activity occurring shortly after the irregular rainfall pulses (Domingo et al. 2011).
Using the geographical coordinates of each location, we obtained the elevation and available..., , # Pushing the limits of C3 intrinsic water use efficiency in Mediterranean semiarid steppes: responses of a drought-avoider perennial grass to climate aridification

## Description of the data and file structure

We investigated the variations in multiple plant traits in the perennial tussock grass *Stipa tenacissima* along an aridity gradient in Spain. Traits measured include leaf stable isotopes (δ18O, δ13C, δ15N), nutrient concentrations (N, P, K), and culm water content and isotopic composition (δ18O, δ2H) of paired pure-grass and shrub-encroached *S. tenacissima* steppes along a 350 km aridity gradient in Spain (10 sites, 160 individuals). Here, we provide a summary of mean (± SE), minimum and maximum values of *S. tenacissima* traits for pure grass and shrub-encroached steppes at ten sampling locations. The blank cells for the variable \"Plant cover\" indicate that there are no maximum or minimum values per study plot, just a single overall value for each study plot.