Spatial variation in temperature thresholds during seed germination of remnant Festuca hallii populations across the Canadian prairie

Grassland in North America is expected to expand northward as the climate becomes warmer and drier under many climate change scenarios. Successful adaptation of plant species to climate change depends on regeneration success. This paper aims to identify the spatial variation in seed germination characteristics of Festuca hallii populations and to quantify the effect of genetic, geographic, demographic, or climatic factors on the variation. Seeds were collected from 15 populations in five ecoregions over a wide geographic range in Canada. Seeds were germinated at 7 growth chambers with constant temperatures from 5 to 35 °C with 5 °C increments. Thermal time models were constructed to generate germination thresholds. Correlations between germination parameters and genetic variation, longitude, latitude, population size, geographic distance to the nearest neighbour, mean annual temperature (MAT) and mean annual precipitation (MAP) were assessed. Base temperatures (Tb, minimal or base temperature permitting germination) of the 15 populations fell into a narrow range within 2.2 °C with an average of 1.1 °C. Tb was positively correlated with latitude and negatively with longitude and the thermal time requirement for 50% germination was negatively correlated with latitude. Seed mass was negatively correlated with MAT and positively correlated with MAP. High final germination percentage was reached at a wide temperature range from 5 to 20 °C with the highest germination percentage at 10 °C. Germination rate index (GRI) increased with increasing temperature from 5 to 20 °C. Final germination percentage at 5 °C was negatively correlated with MAT and positively correlated with MAP. GRIs at 5 and 10 °C were positively correlated with MAP but not with MAT. GRI at 5 °C was positively correlated with longitude. Germination was not correlated with any genetic and demographic attributes assayed. Population-wise variation in temperature and precipitation affects seed mass, an important attribute determining seed germinability. Sites with greater precipitation and lower temperature had heavier seeds, which usually had lower thermal time requirements for germination and faster germination. The warmer and drier future climate as predicted for the Canadian prairie may reduce regeneration success of this species in the current distribution range, particularly after disturbances followed by sexual regeneration from seeds.