Ecology of the fire-dependent forest herbs Geranium bohemicum and G. lanuginosum in Sweden

Geranium bohemicum and G. lanuginosum are the only plant species in Sweden considered fire-dependent, because of their heat-triggered germination. Both are today threatened due to effective fire suppression. An aim with this study was to elucidate their population ecology. These plants have either a summer- or winter-annual life-cycle depending on date of germination. Both at fire sites and in a garden experiment this date occurred in the first half of July, probably determined by day length. Seedlings had a protracted emergence during the fire-year. Winter-annuals in the 15 studied wild populations were more successful than summer-annuals, since they had the whole growing season the year after fire to reproduce. Summer-annuals often germinated too late and were killed by autumn frost before reproduction. Grazing by hare and roe deer was severe during the fire-year and until spring the year after. But plants have an amazing ability to recover successfully. There was large variation in reproductive success between populations, with a positive net return of seeds at all but one site. Almost all seeds were produced by the first generation of plants. Out of this seed crop a few percent germinated within the study period, giving rise to secondary generations. But those plants were small and produced few seeds, probably because of increasing competition in the developing vegetation. Consequently, populations are highly dependent on the first generation, which makes them vulnerable to reproductive failure due to herbivory and winter mortality. But reproductive failure after a fire can be buffered by a remnant seed bank. Geranium seed banks were found in the mineral soil below both unburned and recently deep-burned soil. Most of the recently produced seeds seem to add to the seed bank, since only ca 8 % of seeds were lost during a 2-year observation period. These results suggest that to preserve these threatened Geranium species it is advisable to burn nutrient rich forests, since that is where seed banks are most likely to be present. Further, the time of burning should be during July and August, to promote winter-annuals which perform better than summer-annuals. Burning should be performed when the humus layer is dry enough to be consumed by the fire, to ensure high enough temperature for seed germination to occur