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Phenology and plasticity can prevent adaptive clines in thermal tolerance across temperate mountains: The importance of the elevation-time axis.

Luis Miguel Gutiérrez-PesqueraMiguel TejedoAgustín CamachoUrtzi Enriquez-UrzelaiMarco KatzenbergerMagdalena ChodaPol PintanelAlfredo G Nicieza
Published in: Ecology and evolution (2022)
Critical thermal limits (CT max and CT min ) decrease with elevation, with greater change in CT min , and the risk to suffer heat and cold stress increasing at the gradient ends. A central prediction is that populations will adapt to the prevailing climatic conditions. Yet, reliable support for such expectation is scant because of the complexity of integrating phenotypic, molecular divergence and organism exposure. We examined intraspecific variation of CT max and CT min , neutral variation for 11 microsatellite loci, and micro- and macro-temperatures in larvae from 11 populations of the Galician common frog ( Rana parvipalmata ) across an elevational gradient, to assess (1) the existence of local adaptation through a P ST -F ST comparison, (2) the acclimation scope in both thermal limits, and (3) the vulnerability to suffer acute heat and cold thermal stress, measured at both macro- and microclimatic scales. Our study revealed significant microgeographic variation in CT max and CT min , and unexpected elevation gradients in pond temperatures. However, variation in CT max and CT min could not be attributed to selection because critical thermal limits were not correlated to elevation or temperatures. Differences in breeding phenology among populations resulted in exposure to higher and more variable temperatures at mid and high elevations. Accordingly, mid- and high-elevation populations had higher CT max and CT min plasticities than lowland populations, but not more extreme CT max and CT min . Thus, our results support the prediction that plasticity and phenological shifts may hinder local adaptation, promoting thermal niche conservatism. This may simply be a consequence of a coupled variation of reproductive timing with elevation (the "elevation-time axis" for temperature variation). Mid and high mountain populations of R. parvipalmata are more vulnerable to heat and cool impacts than lowland populations during the aquatic phase. All of this contradicts some of the existing predictions on adaptive thermal clines and vulnerability to climate change in elevational gradients.
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