‘Are 3 °C too much?’: thermal niche breadth in Bromeliaceae and global warming

Abstract

Summary By the end of this century, temperature is predicted to increase by about 6 °C at higher latitudes and about 3 °C in the tropics. Although values predicted for tropical latitudes are lower, rising temperatures in the tropics are likely to have more severe consequences for tropical species that are generally assumed to have narrower climatic niches due to a higher degree of climatic stability and higher niche specialization. Even though temperature affects all ontogenetic stages, the regeneration niche of a species is fundamental for overall niche breadth and hence represents a potential major bottleneck for its distribution. We conducted germination experiments along a range of temperatures with 41 epiphytic bromeliad species to determine thermal germination traits (thermal niche breadth, lower and upper thermal limit, thermal optimum). Based on these traits, we asked whether the thermal germination niche breadth of these species is wide enough to cope with the predicted increase in temperature. Furthermore, we conducted phylogenetic comparative analyses to detect possible niche conservatism of these traits in Bromeliaceae. For 93% of all tested bromeliad species, the predicted mean annual temperature range does not exceed the thermal niche breadth. Moreover, for 85% of all tested species, the current mean annual temperature across the distribution range is well below our estimates of their thermal optima. Furthermore, we found evidence for phylogenetic niche conservatism in most assessed traits. Synthesis. Our report represents an important first step to understand and predict present and future responses of epiphytic bromeliads to global warming. At least with regard to seed germination, epiphytic bromeliads should not be negatively affected by the predicted temperature rise of 3 °C. To the contrary, future temperatures are closer to the thermal optima of most species, potentially leading to an increase in performance. However, since niche conservatism in Bromeliaceae may limit their adaptability to novel climatic conditions, a negative effect of increasing temperatures cannot be completely rejected when considering ontogenetic niche shifts.