pH and osmotic potential of pine ash as post-fire germination inhibitors

The dominant plant species in native pine forests in Israel (Pinus halepensis, Cistus salviifolius and C. creticus) regenerate from seeds after wildfires. The future structure of the regenerating forest is determined largely by the spatial distribution of the seedlings, which depends on the response of the seeds to conditions in the upper layer of the soil. The pH and osmotic potential (pi) of the soil water in this layer is strongly affected by the ash which covers the burned forest floor. The effects of pH and pi on the germination of the wild species noted above and the effect of pH on the germination of two crops, radish (Raphanus sativus radicula) and oat (Avena sativa) were studied in a growth chamber. Bis-Tris propane and CAPS [3-(cyclohexylamino)-1-propanesulfonic acid] buffer solutions (pH 6-11), as well as mannitol solutions (down to -1.5 MPa) were used. The upper soil layer from a recently burned forest had pH 9 and pi -0.08 MPa. Under this pH the germination of P. halepensis and C. creticus was reduced by ca half, and of C. salviifolius by 40%. Germination of radish and oats was reduced by ca 80%. Osmotic potential of -0.1 MPa did not have a significant effect on the germination of any of the species studied. We conclude that the high pH of the soil, caused by ash, is an important environmental factor that controls the regeneration of the forest plant community. Seed adaptation to the high pH may be decisive in determining the plants' fate in fire-prone ecosystems.