Reproduction ecology of Pinus halepensis : a monoecious, wind-pollinated and partially serotinous Mediterranean pine tree

Abstract

Fire is an important factor in the evolution and ecology of Mediterranean plant species. The fire frequency has increased in the 20st century. Pines are the most important tree species in the area. Pinus halepensis is the only natural pine in parts of the east Mediterranean basin, such as Israel and Jordan. This pine tree is a monoecious, wind-pollinated tree. Pinus halepensis regenerates in absence of fire as well as after fire, because it is a partially serotinous species. This study aims to explore the reproductive cycle of P. halepensis in relation to fire. Reproductive processes such as seed and pollen production, cone abortion, level of serotiny and germination are examined. These reproductive processes are related to inherent plant factors such as size, as well as to influences from the environment, especially the presence or absence of fire. When combining these reproductive processes in a simple descriptive model, it shows that regeneration of P. halepensis increases with an increase in tree size, without fire as well as after fire. Increase in tree size increases cone production, reduces cone abortion and decreases the level of serotiny. Small trees produce few cones which are mainly serotinous. Big trees produce many cones, of which some are serotinous and are added to the serotinous cones from previous years, increasing the regeneration ability after fire. The increasing proportion of non-serotinous cones in large trees, combined with the improved pollen dispersal distances with the increased height, increase the regeneration in absence of fire. The two life-strategies co-occur in the same individual and the trees shift their relative contribution to these strategies depending on their size. Although most regeneration in absence of fire results from seeds from non-serotinous cones and most post-fire regeneration from seeds from serotinous cones, they can also contribute to the other regeneration process. Serotinous cones do open after several years without fire, and seeds non-serotinous cones contribute to post-fire regeneration if they were still closed at the time of fire. This study shows, however, that seeds from serotinous cones germinate slightly better in the post-fire environment than seeds from non-serotinous cones. We conclude that the dual life strategy complementary rather that competitive. This size dependent dual life strategy optimizes fitness gain under an infrequent fire regime.