Recalcitrant seeds with physiological epicotyl dormancy may limit seedling recruitment of an endangered subtropical oak species

Castanopsis kawakamii is an endangered relict oak species inhabiting the southern edge of the subtropical region of China. Its recalcitrant acorns (hereafter seeds) exhibit sequential radicle and epicotyl dormancy, requiring prolonged two-phase release, increasing the risk of viability loss and predation before seedling establishment. Seeds of C. kawakamii were collected from the largest population, Castanopsis kawakamii National Nature Reserve, to assess viability under drying and temperature treatments and to identify environmental cues for radicle and epicotyl emergence. Seeds of C. kawakamii lost viability under 4℃ and at a seed moisture content (MC) < 35%. Radicle dormancy was broken by cold stratification (5/15℃) or by subjecting seeds to winter field conditions, but epicotyl physiological dormancy (PD) persisted and required ~ 30 days of warm stratification (15/25℃) following radicle emergence for release. Seeds dispersed from the parent plant in early autumn exhibited deeper PD than those dispersed in late autumn. In the field, radicle and epicotyl emergence occur mainly in the spring following seed dispersal in autumn but with a 1-month lag between the two events. Over 99% seeds with a non-emerged epicotyl died in April. Seed recalcitrance and the requirement for both cold and warm stratification for seedling establishment may be important in limiting plant regeneration in the natural habitat. To enhance seedling establishment under climate stress, we recommend assisted regeneration via protection of late-autumn seeds, moisture retention through burial, and warm stratification to overcome epicotyl dormancy post-radicle emergence.