Local adaptation constrains drought tolerance in a tropical foundation tree

1. Plant species with broad climatic ranges might be more vulnerable to climate change than previously appreciated due to intraspecific variation in climatic stress tolerance. In tropical forests, drought is increasingly frequent and severe, causing widespread declines and altering community dynamics. Yet, little is known about whether foundation tropical trees vary in drought tolerance throughout their distributions, and how intraspecific variation in drought tolerance might contribute to their vulnerability to climate change. 2. We tested for local adaptation in seedling emergence and establishment with a full-factorial reciprocal transplant experiment including 27 populations and 109,350 seeds along a 3500 mm precipitation gradient for a widespread tropical foundation tree, Metrosideros polymorpha, in Hawaii. To more precisely relate seedling performance to soil moisture, we conducted a complementary greenhouse experiment to test responses of the same focal populations to simulated drought. 3. In the reciprocal transplant experiment, we observed significant variation among populations and sites in germination and seedling establishment rates. Overall, there was a significant link between historical rainfall of populations and their performance under current rainfall at the study sites consistent with local adaptation. In particular, populations from historically wet sites demonstrated lower germination rates in currently dry sites compared to wet field sites, while populations from historically dry sites germinated well across all sites, with particularly high germination in dry sites. In the greenhouse, seedlings from wet populations survived fewer days without water, and succumbed at wetter soil conditions than populations from historically dry sites, corroborating results from the field experiment. 4. Synthesis. While climate change models project the greatest drying trends for historically dry areas in Hawaii, even moderate drying of wet sites could significantly reduce M. polymorpha recruitment given the sensitivity of seedlings to slight changes in water regimes. Thus, although M. polymorpha demonstrates high seedling drought tolerance in some populations, providing evidence of resilience at the species-scale, there are nonetheless vulnerable populations likely to decline under climate change. Our approach demonstrates that even trees with high dispersal can show significant clines in drought tolerance, and suggests that similar intraspecific variation might be an important consideration for other tropical tree species.See READMe fileField data are seedling counts in reciprocal transplant experiment, with the greatest number of seedlings observed at any census (germinate) and the total number of seedlings left at the end of the experiment (establish). Greenhouse data are performance metrics for focal populations used in the field experiment, subjected to simulated drought