Light and nitrate effects on seed germination of Mediterranean plant species of several functional groups

Post-fire conditions are characterized by enhanced light and the availability of nitrogenous compounds in the soil. It is not known, however, to what extent light or nitrogenous compounds control the germination response of species growing in burned areas and, in particular, whether functional groups of plants differ in their response. The germination response to light and nitrate was tested for 53 species representative of the flora of a Mediterranean recently burned area in Central-Eastern Spain. Differences in germination among species, with and without taking into account their phylogeny, were studied by classifying them according to their life-form (chamaephytes, hemicryptophytes), regeneration strategy (non-sprouters, sprouters) and geographical distribution range (Iberian Peninsula endemics, Mediterranean, widely distributed species). The overall germination mean was not affected by any of the two treatments. There were statistically significant interactions between species and the two treatments. That is, not all species were equally affected, and about 30% of the species were significantly affected by light (half of them positively and the other half negatively) and 25% by nitrate (most of them positively). Species response was related to functional groups. Light response (stimulation vs. non-stimulation) was associated to life-form, regeneration strategy and distribution range. Hemicryptophytes, sprouters and widely distributed species were positively affected by light. No evidence of such an association for nitrate was found. No statistically significant effects of light and nitrate on the mean germination of the various groups (life-form, regeneration strategy, distribution range) were found. Moreover, significant interactions emerged between light and nitrate for all three groups. In summary, the studied set of plants appears to be non-dependent on factors that may change with fire, such as increased light and soil nitrate, for germination. Nonetheless, some species and groups will be affected by such changes. That means that fire will modify the relative balance of germination among species and functional groups. Due to the particular sensitivity of Iberian Peninsula endemics to light, a factor that significantly changes with fire, these species may be at risk under the current fire regime.