A comparative study on temperature and water potential thresholds for the germination ofBetula pendulaand two Mediterranean endemic birches,Betula aetnensisandBetula fontqueri

Abstract

AbstractThe influence of temperature and water availability on seed germination can vary across the geographic range of species with a large distribution.Betula pendulais a widely spread European tree that has differentiated into two narrowly distributed taxa, endemic to Mediterranean mountains:Betula aetnensisin Sicily andB. fontqueriin Spain and Morocco. We tested the hypothesis that the regeneration niche, expressed by temperature and water potential thresholds, varies across these species and is influenced by the local climate. Seeds were collected from six populations ofB. pendula, one ofB. fontqueriand two ofB. aetnensis. Germination tests were conducted between 5 and 30°C. The thermal thresholds were calculated before and after cold stratification. The osmotic potential tested ranged from 0 to −1.5 MPa. Time to reach 30 and 50% of germination was calculated by fitting non-linear models. Germination was promoted by high temperatures, but the response to stratification was heterogeneous.Tband Ψbdiffered between and within species.Tbranged between 2.22 and 8.94°C for unstratified seeds. Mediterranean species had higher drought tolerance, whileB. pendulashowed contrasting responses to low water potential. Ψbreached a minimum value of −1.15 MPa inB. fontqueri. High temperatures influenced theTbof unstratified seeds negatively, while, after stratification, theTbincreased with precipitation in the driest month. The heterogeneity observed could reflect higher genetic variability in marginal populations of silver birch. Knowledge of their germination ecology may be useful to mitigate future impacts of climate change on core populations ofB. pendula.