The absence of frequent surface fires in many dry ponderosa pine (Pinus ponderosa) forests in the western US has led to an accumulation of ladder fuels, increasing burn severity and decreasing understory species diversity. Land managers in many dry ponderosa pine use mechanical treatments to reduce fuels and thus mitigate high severity wildfire effects. Before the 2007 Egley Fire Complex, slash and pile burns were used as fuel reduction treatments in the Malheur National Forest in eastern Oregon. To compare post-fire vegetation recovery between mechanically treated (T) and untreated (U) control areas, 35 T and U paired plots were sampled in 2016. Sites were broadly distributed across elevations, aspects, and the burn severity gradient using satellite images and the Normalized Burn Ratio (NBR). Post-fire understory recovery was monitored remotely using annual Landsat time series images of NBR, and assessed at the 35 paired field sites by sampling understory plant species composition. The objectives of this project are to 1) document to what extent understory plant community composition changes across the fire severity gradient and 2) quantify to what extent pre-fire fuel treatments affect fire severity and long term (9 years) post-fire plant recovery.
Results/Conclusions
A paired t-test confirms significantly lower burn severity in T vs. U plots (n=35, t=4.26, p<0.0001) using the one year post-fire. A blocked multi-response permutation procedure (MRPB) in PC-ORD statistical software showed an overall difference in plant communities between T and U plot pairs (n=35, A=0.008, p=0.099) and a multi-response permutation procedure (MRPP) indicated a significant difference in plant communities between burn severity levels (n=70, A=0.026, p=0.012) 9 years post-fire. Our results suggest that mechanical treatments were effective at reducing burn severity. Analysis of understory vegetation suggests that the cover of grasses and forbs is higher at the high/moderate severity level nine years post-fire. Estimated site recovery time and other implications for management of dry coniferous forests are discussed.