Extreme heat accelerates conifer germination but increases seedling mortality under prolonged extreme climate events

초록

The success of natural forest regeneration depends on seed germination and early seedling survival, both of which are largely affected by climate factors. Understanding how future climate affects seed development is therefore essential for predicting regeneration potential. To test regeneration and growth responses to extreme climate events, we developed a unique open-field experimental setup, where a combination of two levels of temperature (+3 degrees C and +6 degrees C) and two levels of rainfall (drought and heavy rainfall) was manipulated along with controlled ambient plots. Based on this setup, we examined seed germination responses of Pinus densiflora (pine) and Larix kaempferi (larch) to the spring extreme climate events, and subsequent seedling survival and growth responses to continued summer events. Pine exhibited a higher germination capacity (74%) than larch (15%), as well as higher survival rates after summer (58% vs. 3%). Germination timing (t50) was shortened linearly with increasing temperature across ambient, + 3 degrees C, and + 6 degrees C treatments, corresponding to reductions of 1.6 days/degrees C in pine and 0.9 days/degrees C in larch. Conversely, decreased soil water content under drought conditions was associated with slower germination speed and lower germination percentage in both species, consistently across temperature treatments. In pine, accelerated germination was associated with increased seedling mortality, without effects on seedling growth. These results demonstrate that increasing spring and summer extreme climate events can decouple germination outcomes from regeneration success by accelerating germination, followed by high subsequent mortality. Our findings highlight the importance of incorporating emergence timing and post-germination survival into predictions of forest regeneration under future climate change.

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