Data from: Reciprocal transplants reveal asymmetric local adaptation of Himalayan Rhododendron approaching elevational range limit

As plant species expand their upper limit of distribution under current warming, some retain both traditional climate space and biotic environment while others encounter novel conditions.  The latter is the case for Rhododendron campanulatum,a woody shrub that grows both above and below treeline at our study site in the Eastern Himalaya where a very conspicuous, stable treeline was defined by a nearly contiguous canopy of tall Abies spectabilis trees, many of them over a century old.  Prior work showed that treeline had remained static in this region while R. campanulatum expanded its elevational range limit. We tested local adaptation of R. campanulatum by performing reciprocal transplants between the species' current elevational range limit (4023masl) and just above treeline (3876 masl). Contrary to expectation, the coldest temperatures of late winter and early-mid spring were experienced by plants at the lower elevation: R. campanulatum at species' limit (upper site) were covered by snow for a longer period (40 more days) and escaped the coldest temperatures suffered by conspecifics at treeline (lower site). The harsher spring conditions at treeline likely explains why leaves were smaller at treeline (15.3 cm²) than at species-limit (21.3 cm²). Contrary to results from equivalent studies in other regions, survival was reduced more by downslope than by upslope movement, again potentially due to extreme cold temperatures observed at treeline in spring.  Upslope transplantation had no effect on mortality, but mortality of species-limit saplings transplanted downslope was three times higher than that of residents at both sites.  A general expectation is that locals should survive better than foreign transplants, but survival of locals and immigrants at our species-limit site was identical. However, those species-limit saplings that survive the transplant to treeline grew faster than both locals at treeline and the transplants at species-limit. Overall, we found asymmetric adaptation: compared to treeline saplings, those at species-limit (147 m above treeline) were more tolerant of extremes in the growing season but less tolerant of extremes in winter and early-mid spring, displaying local adaptation in a more complex manner than simply home advantage, and complicating predictions about impacts of future regional climate change.