Research into the thermal tolerance of tropical plants has gotten a renewed interest in recent years as a result of global warming. The upper limit of thermal tolerance is related to the physiological susceptibility of a species to an increase in temperature so that it may reflect its vulnerability to warming. The lower limits of thermal tolerance, such as cold hardiness and frost tolerance, have been widely studied in tropical alpine ecosystems such as the paramo. However, the upper limits of thermal tolerance have never been quantified so we cannot yet predict the response of this tropical alpine ecosystem to global warming. We evaluated thermal tolerance in seven individuals of 21 paramo species using chlorophyll a fluorescence, observing the decline on Fv/Fm after heating leaf disks at seven different temperatures. The critical temperature or upper thermal tolerance (T50) for each species was the temperature causing a reduction in 50% of initial Fv/Fm value, which reflects an irreversible damage to the photosynthetic apparatus. Thermal tolerance was then analyzed regarding the species elevation distribution, growth form (rosettes, shrubs, forbs and grasses) and height. We also calculated the thermal safety margin as the difference between thermal tolerance and the maximum temperature register in the paramos nowadays.
Results/Conclusions
Values of thermal tolerance ranged between 45.4 °C and 54.1 °C, and were not related to the species elevation distribution. However, we found that thermal tolerance was related to growth form, with rosettes having higher heat tolerance than shrubs, grasses and forbs; and to height, with shorter species presenting higher thermal tolerance than taller species, probably because of higher mean temperatures closer to the ground. Regardless of their growth form and height all paramo species analyzed seem to be able to physiologically cope with high temperatures and showed a wide thermal safety margin; species will only reach their upper limit if the maximum current temperature increases by 20 oC. Our results indicate that many tropical alpine paramo species from the Northern Andes could be able to deal physiologically with the high temperatures expected at the end of this century.