How climate relict trees cope with the Mediterranean climate: the importance of refugial settings and plant adaptive ability

Climate relicts, once widespread over large regions of the globe, are nowadays restricted to refugial areas, where the persistence of single species results from processes that should be analysed from both ecological and evolutionary perspectives. In turn, the physical characteristics of refugia, at both macro- and micro- scales, are key drivers of the environmental adaptability and fitness of such species. In order to investigate these issues, two congeneric relict trees, Zelkova sicula from Sicily (Italy) and Z. abelicea from Crete (Greece), were selected as model species because of their strong affinity in terms of taxonomy, biogeography and evolutionary history. Growth traits and growth form were analysed comparatively with respect to altitudinal and geographical range, microtopographic patterns, and response to main climatic drivers. The results revealed that although the two species share several morphological/physiological traits that facilitate their adaptation to the Mediterranean environment, they show differential responses in terms of growth performance and fitness. In the case of Z. sicula, convergent topographies are basic determinants of hydrological microrefugia in arid environments, playing a key role in decoupling from regional climate and supporting the target species to cope with an unsuitable climatic envelope. For the Cretan relict, an “elevation effect”, involving a significant attenuation of environmental constraints, can be identified as the main offsetting driver for the differential functioning mechanism of low vs. high altitude refugia. Our findings are of major concern for the implementation of in-situ conservation and management action plans of relict plant species.