Trade-offs between extreme-temperature tolerance, avoidance and facilitation in tropical alpine plants

Abstract Alpine plants resist extremely high and low temperatures by either tolerance or avoidance, which depend mostly on physiology or growth forms, respectively. Facilitation by neighboring species that buffer extreme temperatures can also be envisaged as avoidance. We tested whether avoidance capacity differs among growth forms; whether there is an avoidance-tolerance trade-off both across species and growth-forms; and if species with low avoidance or tolerance depend most on facilitation. In 35 tropical-alpine species with five growth forms, we measured avoidance as the capacity to buffer high- and low-air temperatures (i.e., as differences between plant and air temperature), and cold tolerance as the temperature that damages half of the leaf surface. We used spatial associations between species as a proxy for facilitation. Thermal-avoidance capacity differed among growth forms. Rather than avoiding high temperatures, leaves warmed above air temperature during the day perhaps enhancing photosynthesis. However, such warming was associated to increased chances of nighttime freezing. The avoidance-tolerance trade-off was confirmed. As expected, the least tolerant species were more associated to nurse plants. However, this was also the case of the best avoiders, which should resist on their own. By linking facilitation with insights from the avoidance-tolerance framework, we found that trade-offs drive alpine-plant strategies to form a continuum between two extremes: Mats warm-up and cool-down rapidly, perhaps increasing photosynthesis but investing more on cold-tolerance mechanisms. At the other extreme, shrubs avoid extreme temperatures but they might not reach optimal photosynthetic temperatures and depend on facilitation during establishment when they are still morphologically immature.