Continuity in fire disturbance between riparian and adjacent sideslope Douglas-fir forests

Abstract

Fire-scar and stand-cohort records were used to estimate the number and timing of fire disturbance events that impacted riparian and adjacent sideslope Douglas-fir ( Pseudotsuga menziesii (Mirbel) Franco) forests. Data were gathered from 49 stream segments on 24 separate streams on the east slope of the Washington Cascade Range. Upslope forests had more “traceable” disturbance events than riparian forests in each of the valley types with a mean difference of 8–62%. Approximately 55–73% of the total traceable fire disturbance for a stream segment occurred on either sideslope and 24–27% in the riparian forest. Plant association groups in the riparian forest had 25–42% fewer fire disturbance events than the same plant association group upslope. Fewer traceable disturbance events in riparian forests may indicate a reduced disturbance frequency or a more severe disturbance regime or both. The two sideslopes on either side of the riparian forest shared the same fire event in 65 and 54% of the recorded fire events on east/west and north/south sideslopes, respectively. Riparian forests shared fire events with adjacent sideslope forests 58–79% among valley types, and 64–76% among aspects. Shared fire events indicate significant continuity in fire disturbance between riparian and adjacent sideslope forests. Fire disturbance regimes of sideslope and riparian forests are quantitatively different, but interconnected through shared fire disturbance events. Disturbance events play a role in maintaining ecosystem integrity and we suggest that disturbance may need to be planned for in administratively defined riparian buffer strips to protect long-term ecological integrity of riparian and adjacent upslope forests.