High heat tolerance in plants from the Andean highlands: Implications for paramos in a warmer world

Abstract

Tropical plant species are expected to have high heat tolerance reflecting phenotypic adjustments to warm regions or their evolutionary adaptation history. However, tropical highland specialists adapted to the colder temperatures found in the highlands, where short and prostrated vegetation decouples plants from ambient conditions, could exhibit different upper thermal limits than those of their lowland counterparts. Here we evaluated leaf heat tolerance of 21 tropical alpine paramo species to determine: 1) whether species with restricted distribution (i.e., highland specialists) have lower heat tolerance and are more vulnerable to warming than species with widespread distribution; 2) whether different growth forms have different heat tolerance; and 3) whether species height (i.e., microhabitat) influences its heat tolerance. We quantified heat tolerance by evaluating T50, which is the temperature that causes a reduction in 50% of initial Fv/Fm values and reflects an irreversible damage to the photosynthetic apparatus. Additionally, we estimated the thermal safety margins as the difference between T50 and the maximum leaf temperature registered for the species. All species presented high T50 values ranging between 45.4°C and 53.9°C, similar to those found for tropical lowland species. Heat tolerance was not correlated with species distributions or plant height, but showed a strong relationship with growth form, with rosettes having the highest heat tolerance. Thermal safety margins ranged from 12.1 to 31.0°C. High heat tolerance and broad thermal safety margins suggest low vulnerability of paramo species to warming as long as plants are capable of regulating the leaf temperature within this threshold. Whether paramo plants would be able to regulate leaf temperature if drought episodes become more frequent and transpirational cooling is compromised is the next question that needs to be answered.

Weak phylogenetic and climatic signals in plant heat tolerance

“…It has been proposed that heat tolerances vary according to leaf temperatures, but there is mixed support for this hypothesis (Ghouil et al, 2003;Knight & Ackerly, 2002;Leon-Garcia & Lasso, 2019;Nobel et al, 1986;. Nevertheless, large leaves tend to have higher leaf temperatures, possibly explaining why the large-leafed monocots we sampled had high T 50 (Leigh et al, 2017;Nobel, 1999).…”

Living in Drylands: Functional Adaptations of Trees and Shrubs to Cope with High Temperatures and Water Scarcity

“…47 °C. More recently, Leon-Garcia and Lasso [55] stated that physiological heat tolerance is a conserved trait, with a general threshold value ca. 50 ˚C for C3 plants.…”

“…Dreyer et al [54] established that the upper limit of viable leaf temperatures was ca. 47 • C. More recently, Leon-Garcia and Lasso [55] stated that physiological heat tolerance is a conserved trait, with a general threshold value ca. 50 • C for C 3 plants.…”

New insights into the functional ecology of paramo plants: what growth forms can tell us about plant functional types

“…For the tropical high Andes, projections suggest a 3.0 ± 1.5°C rise in the next century (Buytaert, Cuesta and Tobon, 2011), and a decreasing precipitation trend in Northern Colombia and Venezuela, but a small increase in precipitation for Ecuador and Southern Colombia in the next hundred years (Urrutia and Vuille, 2009;Buytaert, Cuesta and Tobon, 2011). While most paramo plants can deal with somewhat higher temperatures (Rada et al , 1992;Körner, 2003;Rada, Briceño and Azocar, 2008;Leon-Garcia and Lasso, 2019), their response to water deficit could differ greatly among groups and future changes in water availability could change the plant community. The combination of lower precipitation, as projected for the Northern Andes, and higher temperatures, which may increase air evaporative demands and reduce soil water content, could greatly affect plant water relations and gas exchange.…”