Habitat preference and vulnerability to drought of three Hypericum species of the páramo

Background: Páramos are tropical alpine ecosystems where climate change is expected to cause yet unknown consequences for plant growth, ecosystem structure, ecosystem function and the provision of ecosystem services. Aim: To quantify the relationship between environmental factors (abiotic and biotic) and the spatial distribution of three common Hypericum species in the páramo and their physiological vulnerability to drought. Methods: We recorded soil water content, soil temperature, and vegetation cover in 10 plots along a moisture gradient. Additionally, we measured a series of physiological traits associated with the risk of drought-induced mortality. Results: We found that H. goyanesii and H. juniperinum mainly grew in areas with high soil water content, and similar high vegetation cover. These two species had wider xylem vessels, showed anisohydric behaviour and were equally vulnerable to cavitation with low safety margins against hydraulic failure. H. mexicanum grew in places with less vegetation cover, lower soil water content, and higher soil temperatures. H. mexicanum showed a different strategy that probably allows it to thrive in these conditions; it maintains high values of water potential at noon and has narrower xylem vessels, making it less vulnerable to cavitation. Conclusion: The distribution of the three Hypericum species in the páramo responds essentially to soil water content, in line with the physiological mechanisms of the species to cope with water deficit. Hypericum species from moist habitats could decline if longer drought episodes become more common in the future.