Fires of differing intensities rapidly select distinct soil fungal communities in a Northwest US ponderosa pine forest ecosystem

Abstract

Environmental change and long-term fire management in the western United States have created conditions that facilitate high-intensity burn areas in forested systems. Such burns may have dramatic effects on the soil microbial communities. In this study, we utilized experimental infrastructure in the Pringle Falls Experimental Forest in Oregon, where ten pairs of sites were assigned to either high burn (HB) or low burn (LB) intensity treatments to examine fungal community responses. In these treatments, understory shrubs and logging debris were masticated and broadcast burned (LB) or piled logs were fully combusted (HB) in a paired design. The burn treatments generated soil temperatures of ∼100°C (LB) or up to 700°C (HB) at the soil surface. We sampled soils (0–10cm) one week before and three weeks after ignition and Illumina MiSeq-analyzed fungal Internal Transcribed Spacer 1 (ITS1) PCR-amplicons to deeply dissect the fungal communities. Our data indicate dramatic and rapid responses in community diversity and evenness in the HB treatment, with similar responses, but to a lesser degree, in the LB treatment. Nonmetric Multidimensional Scaling (NMS) ordinations and analyses of taxon frequencies reveal a substantial community turnover and corresponding replacement of the dominant basidiomycetes by ascomycetes in the HB treatment. Similar trends were visible, but weaker in the LB treatments. These coarse-level taxonomic responses were attributable to a few fire-responsive fungi, particularly Operational Taxonomic Units (OTUs) assigned to Pyronema sp. and Morchella sp., whose frequencies increased more than 100-fold following the HB treatment. Our study highlights the strong and rapid fungal fire responses and differences among fires of different intensities. As the high intensity fires – such as those represented here by the HB treatment – tend to be spatially confined and limited in scale, we emphasize their potential to generate distinct patches that may substantially contribute to beta diversity on small scales. Further, these data lead to questions about fungal community recovery (return to community state preceding the fire) and the importance of patch dynamics following a fire as well as function of post-fire communities and ecosystem services that they may provide.