Minimal temperature of pollen germination controls species distribution along a temperature gradient

Although plant distribution patterns are well documented, our understanding of the ecophysiological mechanisms that control the geographical ranges of plant species remains poor. We used a largely ignored method, the performance of the male gametophyte in vitro, to assess whether the thermal range of pollen germination and tube growth controls species distribution ranges, in this case along an elevational gradient.

Using in vitro pollen germination experiments, we obtained cardinal temperatures (minimal, optimal and maximal) of pollen germination and pollen tube growth for 25 herbaceous species along a mean annual temperature gradient of about 5 °C. These temperatures were correlated with temperatures of the sites where the species were collected. The presence of a phylogenetic signal in the data set as well as an effect of species flowering phenology were also estimated.

We found a strong positive relationship between temperature conditions at our collection sites and the minimum temperature for both pollen germination and pollen tube growth. In addition, a significant correlation between maximum temperature of pollen tube growth and temperature of flowering month was apparent. We conclude that the restriction of pollen germination and growth by low temperatures is an important contributor to the climatic restriction of plant species distributions. Improved knowledge of this thermal precursor to seed production could, from a functional perspective, enhance our understanding of species distributions along climatic gradients and our ability to predict how anthropogenic climate change might affect plant community composition.