Changes in traits of shrub canopies across an aridity gradient in northern Patagonia, Argentina

Low and highly variable precipitation pulses exert a strong selective pressure on plant traits and this might provide axes of ecological differentiation among plant species in arid ecosystems. We asked whether aridity contributes to maintain high diversity of species and morphotypes in shrub canopies. We selected eleven study sites evenly distributed across a 400-km transect in northern Patagonia, Argentina. Precipitation is low and highly variable within and between years but almost homogeneous across the transect (125–150 mm). Mean annual temperature varied, however, ranging from 8 °C (west) to 13.5 °C (east) creating a west–east gradient of aridity (aridity index from 3.7 to 7.3, respectively). Sheep grazing commenced in the early 1900s at a similar intensity across the transect. We recorded the richness and cover of shrubs by species and by morphotypes (drought deciduous tall shrubs, evergreen tall shrubs, medium shrubs, and dwarf shrubs), and further calculated the species and morphotype Shannon diversity index at each site. We assessed the presence of spiny leaves, leaf pubescence, thorny stems, and photosynthetic stems in shrub species of all morphotypes and collected green leaves of the dominant shrub species (more than 80% of the total shrub cover) to assess the leaf area, leaf mass per unit area, N-, lignin- and soluble phenolic-concentrations per species at each site. Richness and diversity of shrub species and morphotypes were positively associated with aridity. The richness and diversity of shrub species with pubescent leaves and thorny stems, and nitrogen concentration in green leaves of dominant shrubs increased with increasing aridity. We conclude that our findings on increased diversification in life history traits, species and morhotypes in shrub canopies with increasing aridity support the hypothesis that variability in aridity provides axes of ecological differentiation among shrub species facilitating their coexistence.