Germination of two Erica species along a latitudinal gradient and under various thermal conditions

Background/Question/Methods

Background. In the Mediterranean region, global warming is projected to increase temperatures and reduce precipitation, as well as rainfall temporal patterns. After fire, many species must recover from seeds. Changes in temperature and precipitation patterns may alter germination, hence conducing to community changes.

Question. To what extent the germination of two widespread species in Mediterranean shrublands is sensitive to the temperature? Does this sensitivity change after exposing the seeds to a heat shock, simulating fire? Does the proportion of seeds germinating under simulated fall, winter and spring vary depending on the initial (from cold to warm) conditions? Do germination responses to the above conditions vary along a latitudinal gradient across the Iberian Peninsula?.

Methods. Seeds of Erica australis and Erica umbellata were collected from six populations along a latitudinal gradient through the Iberian Peninsula (600 km). Prior to germination, half of the seeds were subjected to a heat shock treatment (80ºC, 10 min), simulating fire exposure. Later, seeds were set to germinate under four initial temperatures simulating fall conditions along the gradient and global warming (17º/7ºC, 20º/10ºC, 23º/13ºC, 26º/16ºC; 12 hs day/night), during seven weeks, after which temperatures were reduced to simulate winter (8º/4ºC, 11º/7ºC,14º/10ºC, 17º/13ºC) for an additional seven weeks period, and later increased again to simulate spring conditions (15º/6ºC, 18º/9ºC, 21º/12ºC, 24º/15ºC), during seven weeks also.

Results/Conclusions  

Results. Both species were sensitive to the temperature of the germination environment, where by the lowest germination counts were obtained in the highest initial temperature treatment. This was consistent along the gradient. Heat-shock decreased germination of both species but changed the sensitivity to germination temperature for Erica umbellata. For both species the proportion of seeds germinating under different seasons exhibited an idiosyncratic response along the gradient: some populations germinated most under fall conditions, others under spring ones, but this was independent of position along the gradient.

Conclusions. The persistence of Erica australis and Erica umbellata in fire-prone shrublands of the Iberian Peninsula might be threatened by fire, particular if these are intense since both species were negatively affected by heat exposure. In addition, their germination might be hampered by increased ambient temperatures with global warming. Predictions as to which areas are more sensitive to this will be difficult due to the site-specific responses found.