Intra- and interspecific performance in growth and reproduction increase with altitude: A case study with two Saxifraga species from northern Spain

1. Improving our knowledge on plant functions underlying shifts in species ranges along altitudinal and latitudinal gradients is a research priority to understand biotic responses to climate change. Although much is known about biological responses to elevation, coupled observations of ecophysiological and reproductive traits with whole-plant performance metrics are rare, and thus our understanding disjunct rather than holistic. 2. We expected growth and reproductive performance to decrease with elevation, irrespective of adjustments in traits, modifications in allocation, or congeneric shifts in populations. To test this hypothesis we assessed growth, reproductive investments and traits related to resource use efficiency and reproductive success in populations of two vicariant species growing along an elevation gradient: Saxifraga trifurcata (lowland species) and S. canaliculata (highland species). 3. For most metrics, a relationship with elevation arose, mostly related to the replacement of species, but also to within-species variation in S. trifurcata. Leaves of S. canaliculata, and of the higher populations of S. trifurcata, were denser, had lower specific leaf area (SLA) and lower N concentration but higher N resorption efficiency. Collectively, these trends indicate a more conservative resource use pattern at the highlands as compared to the lowlands. Despite this, vegetative growth per unit canopy area was higher in S. canaliculata than in S. trifurcata, and in the highland than lowland populations of S. trifurcata. Reproductive investments, measured as the reproductive mass per unit canopy area, were lower, and reproductive maturity was reached later, in S. canaliculata, but reproductive success (fruit set and seed germination) increased at the high altitude limit of each species' range. 4. Performance did not uniformly decrease with altitude. Growth was higher in the uplands possibly because preferential biomass and meristem allocation to the vegetative function compensated for the decrease in growth season length. Similarly, the decrease in reproductive investments in the highland populations was partially compensated for, since fruit set was higher and more viable seeds (per gram invested in reproduction) were produced in the highland limit of each species. 5. This suggests that populations growing in increasingly limiting environments can partially compensate by modifications in allocation and morpho-functional traits.