There is mounting observational evidence of species’ ranges shifting poleward, concurrent with recent climate change. However, such observational evidence does not decouple climate change from other potential environmental drivers of range shifts. Thus, manipulative experiments across natural climate gradients with range-restricted species are necessary. To that end, we asked: how will future climatic conditions affect plant species’ demographic responses within and beyond their current ranges? Additionally, how will biotic interactions influence responses under various climate treatments, and will this be dependent on species’ current range distributions?
At three prairies across a 520 km latitudinal Mediterranean climate gradient within the Pacific Northwest, we embedded four climate treatments (control, warming, drought, and warming with increased precipitation) and two biotic environments (with natural or reduced aboveground vegetation) to determine the demographic vital rates of 14 native prairie grasses and forbs. We increased canopy temperatures in the warming treatments by 2.5°C year-round, decreased annual precipitation by 40% using rainout shelters in the drought treatment, matched ambient soil moisture in the warming with increased precipitation treatment, and reduced the presence of non-focal species via aboveground clipping in the reduced-vegetation treatments. Species were seeded in the fall of 2015 and 2016, and we measured germination, survival, growth and reproduction.
Site had a strong effect on recruitment (the product of germination and survival), but the strength of this effect has varied between years (2016 – 2017). Treatment effects thus far vary between species and have been mixed between years, but drought has consistently increased recruitment for one perennial forb in particular, Sidalcea malviflora, when planted both within and north of its current geographic range. Warming decreases recruitment for species within their current ranges, but increases recruitment when seeded north of those ranges. In all cases, germination generally appears to be a greater barrier than seedling survival to recruitment. Natural vegetation environments reduce recruitment across sites relative to weeded environments, but the strength of this effect varies with site and treatments. While species’ responses to climate treatments depend on whether they are within or beyond their current geographic ranges, our results suggest that life history characteristics, along with interannual variability, are important drivers of demographic responses across a latitudinal gradient. Furthermore, climate may influence the strength of biotic interactions, adding a layer of complexity to species’ demographic responses. Our results demonstrate the importance of using regional-scale climate manipulations on the demographic responses that control species’ distributions.