Unravelling the secrets of fern spore germination: what are their thermal thresholds and how will they cope with climate change

Ferns are an important and diverse group of plants. Recruitment from spore to gametophyte represents the first crucial stage in a fern’s development and is responsible for the regeneration of natural populations. However, research into the thermal thresholds for spore germination is deficient and quantitative predictions about how germination may be affected by climate change, limited. This study recorded germination response across a gradient of constant temperatures (5‒40°C) for eleven fern taxa from five different environments. We applied the thermal time model to estimate for each species the parameters: base (Tb), optimum (To) and ceiling (Tc) temperature for germination and the thermal times (θ50). Furthermore, early gametophyte development was recorded, and non-germinated spores were moved to optimal temperatures to study recovery. Results indicate that thermal germination response is strongly linked to the environment of the sporophyte, with ferns from cooler environments generally having lower temperature parameters for germination and higher values for θ50, than those from warmer climates. Moreover, it is shown that in the event of climate change, ferns from the tropics are likely to be affected negatively by global temperature increases of 1‒3.7°C, pushing them above their To, reducing germination rate and increasing the proportion of dead spores and abnormal gametophytes. This could affect global diversity and distribution of ferns.