Germination niche of the endangered dragon tree Dracaena draco (L.) L. subsp. draco of the Macaronesian Islands

The Canary Islands dragon tree (Dracaena draco (L.) L. subsp. draco) is an endangered Tertiary relict species endemic to the Canary and Madeira archipelagos. Based on an ex-situ experiment under controlled conditions, we investigated its germination response to both constant and alternating temperatures, moisture levels (water potential), exposure to light, and desiccation. We found that suitable temperatures for germination (> 5 % of germinated seeds) range from 12.1 degrees C to 32.8 degrees C (thermal germination niche limits) and optimal temperatures (> 95 % of germinated seeds) range from 16.6 degrees C to 28.3 degrees C. Light exposure and water potential below 0 MPa drastically decreased germination performance while seed desiccation pre-treatments and alternating temperatures had slightly negative effects. Comparing the spatial projection of the thermal germination niche with actual occurrences of D. draco subsp. draco revealed that native populations are mainly distributed in areas where temperatures are colder than the optimum for germination. For native populations, germination rate is predicted to reach 78 % on average based on temperature requirements. Our results suggest that niche conservatism and adaptation are involved in shaping the germination requirements of the dragon tree.