Cardinal temperatures and thermal time in Polaskia Backeb (Cactaceae) species: Effect of projected soil temperature increase and nurse interaction on germination timing

Cardinal temperatures and thermal time are useful tools to quantify the impact of climate change on plant developmental processes such as seed germination. In this study, we quantified the thermal germination behaviour of two cactus species and predicted the effect of increased soil temperature (under a shrub canopy and in unshaded areas) on germination timing under projected climate change scenarios. To achieve these aims, seeds of both species were germinated across a range of temperatures from 5 to 40 °C, and a probit cardinal temperature model was employed to obtain the thermal coefficients. Projected temperature increase by 2090-2099 according to IPCC scenarios B1, A1B and A2 were estimated and environmental and soil heat sum were calculated. We conclude that an increase in soil temperature is predicted to alter the timing of germination but this will not be detrimental to the proportion of seeds which germinate. Thus mitigation of extreme temperatures, such as achieved under a shrub canopy, is not a crucial factor for germination success in these Polaskia species during the 21st century.