Population size and fire intensity determine post-fire abundance in grassland lichens

Questions: What is the variability in abundance of lichens on grassland soil between and within fields after prescribed fire? Is post-fire lichen abundance an effect of pre-fire population size? Location: Cedar Creek Natural History Area, Minnesota, USA. Methods: Lichen abundance, estimated as ground cover and dominated by Cladonia spp., was mapped in plots in two fields before prescribed burning on 06.10.2003 and 15.10.2003 for the first time since abandonment in the 1950s. The plots were resurveyed one year post-fire. Results: Post-fire cover of Cladonia spp. varied strongly between the fields, most likely due to different weather conditions between the burn events, which resulted in different fire intensities, one of low and one of high intensity. In the field that experienced the low intensity fire, post-fire cover of Cladonia spp. was still relatively high, and showed a positive relationship with pre-fire cover, while no such relationship was found after the high intensity fire. In that field Cladonia spp. experienced high mortality rates irrespective of pre-fire cover. Conclusions: This study provides an example of how species response to disturbance can be a function of population size, but that this relationship can be non-linear; lichens in grassland can survive a low intensity fire proportionally to pre-fire population size, but experience high mortality rates above a fire intensity threshold. The applications of these results are that fire intensity matters to species response to prescribed fire, and that the persistence of climax lichen communities and biodiversity in the study system needs a broad range of fire intervals.