Regulation of the germination of Aleppo pine (Pinus halepensis) by nitrate, ammonium, and gibberellin, and its role in post-fire forest regeneration

Abstract

Aleppo pine (Pinus halepensis Mill.) regenerates after wildfires only by germination. This post-fire germination is controlled by temperature, light and the chemical characteristics of the ash-bed. The extremely high pH caused by the ash inhibits germination of Aleppo pine as well as of many other species. Here we report that ash extracts at neutral pH stimulate P. halepensis germination. The effects of ash extracts, nitrate, ammonium (in concentrations present in the ash) and GA 3 on the germination of P. halepensis seeds and on lipase activity at early germination were studied under controlled conditions. Sub-optimal concentrations of nitrate-and ammonium-stimulated germination and the activity of lipase in the seeds. It is proposed that nitrate increases seed sensitivity to GA 3 while ammonium directly affects lipase activity. The balance between the inhibition of germination by the high pH of ash and its stimulation by the nitrate and ammonium present in the ash determines the overall effect of the ash on post-fire germination of Aleppo pine. The pine seedlings appear in high density among the trees but because of the effects of the ash they appear in sparse stands under the projection of the burned pine canopies. These seedlings benefit from weak inter- and intra-specific competition as well as improved mineral nutrition and have good chances to grow into large trees that will form the regenerating forest; therefore, the enhancement of P. halepensis germination by nitrate and ammonium and its inhibition by high pH can he considered as ecophysiological adaptations to post-fire regeneration.