Historical fires in Douglas-fir dominated riparian forests of the southern Cascades, Oregon

Abstract

Despite the ecological importance of fire in Pacific Northwest forests, its role in riparian forests is just beginning to be documented. This study reconstructed the historical occurrence of fire within riparian forests along different stream sizes in coast Douglas-fir (Pseudotsuga menziesii var. menziesii [Mirbel] Franco) dominated forests within the drier western hemlock (Tsuga heterophylla [Raf.] Sarg.) forest series of the Upper Steamboat Creek watershed of the Umpqua National Forest, Oregon. Fire dates were determined from a total of 194 fire-scarred wedges from stumps sampled at 15 riparian and 13 upslope one-hectare plots. Fire was common historically in both the riparian zones and upslope forests of this study area. Riparian Weibull median probability fire return intervals (WMPIs) were somewhat longer (ranging from 35-39 years, with fire return intervals ranging from 4-167 years) than upslope WMPIs (ranging from 27-36 years, with fire return intervals ranging from 2-110 years), but these differences were not significant. Fires were probably mixed in severity and likely patchy, considering the high incidence of fires occurring only at a riparian plot or only at an upslope plot within a pair, but not at both. Finally, fire return intervals showed a non-significant trend of decreasing length from west to east to north aspects. An increased sampling effort may have shown this decrease to be significant. Based on the results from this study, it is evident that restoring fire will be necessary to protect riparian forest health in this study area. Historical recruitment of large woody debris was likely patchy and pulsed for these mixed-severity fire regime forests.