Oaks are the dominant species of tree in many Mediterranean forests. However, the climate of the Mediterranean basin is predicted to change dramatically. Will the make-up of the forests change similarly? To find out, Nabil Amimi and colleagues have been examining the ecophysiology of seed persistence and germination in Tunisian oaks.
There are a number of oaks in Tunisia. The authors compared Quercus canariensis, Q. coccifera, Q. ilex and Q. suber. Normally, altitude, rainfall and temperature give each oak its favoured niche. “It is well established that Q. coccifera becomes dominant under warm and arid conditions. At the opposite end of the scale, Q. suber has strict humidity and soil requirements and is restricted to the warmer humid and sub-humid Mediterranean areas.”
Amimi and colleagues decided to examine the traits of the oak seeds to see if this had an effect on where the oaks could grow. They examined the morphological and chemical traits, seed responses to dessication and freezing and finally how they germinated at a range of constant temperatures.
The team found that there were significant differences in most traits, except in dessication. There appeared to be very little difference in tolerance to dessication, which puzzles the authors. “The ecological significance of this slight but significant difference in desiccation sensitivity remains unclear since seeds of Q. coccifera, which grows in the most xerothermic conditions sampled, did not show a higher degree of desiccation tolerance than seeds of Q. suber, which is restricted to humid and sub-humid Mediterranean areas. Moreover, levels of desiccation sensitivity observed in Tunisian oaks were comparable to those observed in temperate and tropical oaks. Similarly, in a study that included four Quercus species from subtropical China and two species from temperate USA, no correlation was found between the level of desiccation tolerance of embryonic axes and post-shedding rainfall patterns where the samples originated. This contrasts with observations made in other genera, including the genus Coffea, in which the degree of seed desiccation tolerance varies considerably among species and correlates with rain regimes of the species’ native environments.”
Q. ilex and Q. canariensis were the most freezing tolerant seeds, and it’s no surprise that these are the dominant species at higher altitudes. The species found in warmer habitats also had the most cold-sensitive seeds. “Seed freezing tolerance therefore appears to be a major adaptive trait that differentiates Tunisian oak species along altitudinal gradients. Our study also provides evidence for significant interspecific variations in hexoses in embryonic axes, which were found to be correlated with freezing tolerance,” the authors say.
The scientists also found variation in how quickly the seeds germinate, with Q. coccifera being an outlier. Amimi and colleagues have a possible explanation why Q. coccifera seeds would germinate so rapidly. “It is therefore possible that acorns of Q. coccifera germinate in autumn in Tunisia, just after dispersal, making the adaptive strategy of this species different from the other Tunisian oaks. Indeed, Q. coccifera is frequent in warm and arid environments, predominantly on fixed dunes and limestone substrates, which are both subject to rapid water depletion. Furthermore Q. coccifera is more frequent at low elevations where high air and soil temperatures are known to promote evaporation and thus seed water loss. Quercus coccifera seed and acorn germination traits might therefore reflect an adaptive strategy to escape this evaporative stress and enable rapid access to soil water by pivotal roots of young seedlings.”