Temperature and drought drive differences in germination responses between congeneric species along altitudinal gradients

Abstract

Temperature and drought effects on germination were explored by comparing species from above and below the treeline of the Central Alps. This study should enhance the understanding of species recruitment under changing environmental conditions. The timing of germination, final germination percentage, and optimum temperature of 14 congeneric species pairs were investigated. We hypothesized that (i) temperature will affect the timing of germination, with species above the treeline having a more rapid germination time; (ii) final germination percentage (FGP) will be generally enhanced by higher temperatures; (iii) species below the treeline will have wider temperature ranges and will be more adapted to drought; (iv) species above the treeline will have higher optimum temperatures of germination, and (v) seed mass will affect the germination rates. Germination experiments were carried out using alternating temperature regimes (25°/10 °C, 20°/10 °C, 15°/5 °C) and decreasing water potentials (−0.3, −0.6, −0.9 MPa), generated using polyethylene glycol. Temperature significantly affected the timing and FGP; species above the treeline exhibited a delayed timing at lower temperatures and a generally lower FGP at all temperature regimes in contrast to their congeners below the treeline. The optimum germination temperatures were significantly higher for species above the treeline. Five of 28 species did not germinate at all. Drought generally affected all species; however, the negative response of the species above the treeline was more pronounced. The seed mass did not affect the FGP. We conclude that the germination traits of the species above the treeline may have major disadvantages in a changing climate, resulting in a weaker performance and delayed growth, whereas the species below the treeline may be considered pre-adapted for potential migration to higher altitudes.