COS 194-2 - Dispersal influences bacterial and fungal community assembly post-fire

Abstract: Given increased wildfire activity, there is growing interest in how microbial communities recover from fire and their roles in post-fire nutrient cycling and plant restoration. Dispersal, or the movement of organisms across space, may be especially important to post-fire succession as biotic communities can be more susceptible to invasion following a disturbance. However, the role of dispersal in the assembly of microbial communities has not been examined experimentally in the field. Here, we directly manipulated dispersal into disturbed leaf litter communities collected from two adjacent ecosystems in Southern California, a semi-arid grassland and coastal sage scrubland (CSS), following a wildfire. We asked three questions: 1) What is the identity of microbes (both bacteria and fungi) dispersing into the soil surface following a fire? 2) How does dispersal influence microbial composition during the post-fire assembly of leaf litter communities? And 3) Specifically, how does dispersal alter alpha- and beta-diversity? Leaf litter bags were constructed out of material which either permitted (“open” bags) or prevented (“closed” bags) microbial cells from immigrating in or out. These bags were filled with charred leaf litter collected post-fire or glass microscope slides to characterize the microbial community dispersing into the soil surface. Using this design, we tracked microbial dispersal and community assembly for one year following the fire. We show that the composition and diversity of microbes immigrating into the soil surface post-fire change across time with seasonal shifts and with the reemergence of above ground vegetation. Further, dispersal significantly contributed to the reassembly of leaf litter microbial communities after the fire, and this effect increased with time. Finally, we found that the effect of dispersal on alpha- and beta-diversity varied between ecosystems but influenced bacterial and fungal communities in a similar manner. Collectively, these results demonstrate that dispersal is important in the early assembly of microbial communities following a wildfire disturbance. These findings also show that dispersal may contribute to post-disturbance succession of bacteria and fungi in predictable ways, despite differing in traits expected to influence dispersal rate and distance.