Temperature sensitivity of seed germination in four common conifer species from different altitudes in the Pacific Northwest, USA

Background/Question/Methods

Seed germination is an essential process for plants to establish in a new colony and is highly sensitive to temperature. For trees with a long life span, coping with the rapidly changing climate is a challenge because of their slow reproduction and limited dispersal distance. Furthermore, their fitness depends on the probability of reproductive success through pollination and recruitment through germination. Two critical reproductive organs, pollens and seeds, and physiology are sensitive to temperature. Therefore, determining the temperature dependence of seed germination is pivotal to evaluate species response to temperature and predict the range shift in their distribution under climate change. Our focal species were four common conifer species in Pacific Northwest, Pinus contorta (PICO), Picea engelmannii (PIEN), Pinus ponderosa (PIPO), and Pseudotsuga menziesii (PSME). Their seeds were collected at different altitudes ranging from 150 m to 2,600 m from PNW. We applied eight incubation temperatures (5, 10, 15, 20, 25, 30, 35, 40°C) on the seed germination and investigated changes of seed germination percentage (GP) from different elevations for each species. We tested the following two hypotheses: • Seeds of the same species collected from higher elevations would have higher GP at lower temperatures and vice versa because they have adapted to the temperatures corresponding to their habitat. • Wide-range species (PIPO and PSME) are less sensitive to temperature change than narrow-range species (PICO and PIEN), resulting in differential warming effects on GP between wide-ranging and narrow-ranging species.

Results/Conclusions

The preliminary results showed that none of the species we tested showed a consistent trend related to our first hypothesis that seeds from higher elevation have greater GP in lower temperatures and vice versa. However, it is notable that seeds of PIEN and PIPO from the highest elevation did show a greater GP than other lower elevations in 10 and 15°C. The variability in GP was smallest around the optimal temperature regardless of the elevation of their habitat. Wide-ranging species (PIPO and PSME) seem to have a broader range of optimal temperatures, while narrow-ranging species (PICO and PIEN) have a narrower field of optimal temperatures. This resulted in an observation that moderate warming had only minor effects on GP of wide-ranging species. Our findings and data can be valuable for modeling the temperature response of seed germination to provide insights for understanding the reproductive biology associated with the ecological resilience of these species in a changing climate.