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.
Highlights
We hypothesized that: 1) species with widespread distribution in elevation that are adapted to broader thermal niches will have higher heat tolerance than species restricted in their elevation to paramos; 2) shorter plants will have higher heat tolerance than taller plants, since plants growing closer to the ground are exposed to more extreme thermal microhabitats [29,34]; and 3) rosette forms, usually pubescent in paramo, will have higher upper thermal tolerance than other growth forms because their insulation structures reduce heat transfer from the leaf to the air
Values of heat tolerance were well above the highest air temperatures currently registered in the paramo, and above the highest leaf temperatures measured on site
The T50 or the critical temperature reflect an irreversible damage to the photosynthetic apparatus and correlates with membrane breakage, electrolyte leakage, necrosis of the leaves and a decline in net carbon assimilation [11,17,22]
Summary
Tropical high elevation Andean ecosystems, locally known as paramos, are said to be vulnerable to climate change due to the large percentage of endemic biota with narrow. We hypothesized that: 1) species with widespread distribution in elevation that are adapted to broader thermal niches will have higher heat tolerance than species restricted in their elevation to paramos (highland specialists); 2) shorter plants will have higher heat tolerance than taller plants, since plants growing closer to the ground are exposed to more extreme thermal microhabitats [29,34]; and 3) rosette forms, usually pubescent in paramo, will have higher upper thermal tolerance than other growth forms because their insulation structures reduce heat transfer from the leaf to the air This is a valuable strategy during freezing periods, but it is a trait that could result in overheated leaves on sunny days [43] and would require these species to have higher heat tolerance
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