HOLOCENE FIRE-RELATED ALLUVIAL CHRONOLOGY AND GEOMORPHIC IMPLICATIONS IN THE JEMEZ MOUNTAINS, NEW MEXICO
In fire-prone areas, the geomorphic effects of fire are influenced by fire frequency and severity (i.e. fire regime) and can play a significant role in hillslope evolution. In the Jemez Mountains, tree-ring fire history reconstructions indicate that low-severity fire regimes characterized the ~300 years before Euroamerican settlement, and that human impacts contributed to increased fire severity in recent years. However, other work in the western USA has revealed past episodes of severe fire in similar forest types, therefore, Holocene fire regimes in the Jemez Mountains may also have varied. This work utilized fire-related alluvial fan deposits to assess fire activity, severity and geomorphic response to fire over millennial timescales. It was found that fire activity in the study area may have been influenced by shifts in climate, such that frequent, low-severity fires are promoted by less variable climatic conditions, whereas severe fire may be more likely in forest stands that underwent longer-term wetter than average conditions followed by severe drought. In particular, peaks in fire-related sedimentation occur during severe regional multidecadal droughts around 1800 and 375 calendar years BP. The study area likely experienced somewhat more severe fire events during the parts of the late Holocene than during the period of the pre-Euroamerican tree-ring record. The study area also displays evidence of slope aspect controls on weathering and slope form. These differences may influence the processes and spatial distribution of fire-related erosion and sedimentation in the study area. Morphometric analysis of north and south-facing alluvial fan basins show no clear differences in fan depositional process among these aspects. However, it was found that north-facing alluvial fans are dominated by fire-related sedimentation (77% of stratigraphic thickness measured), but fire-related deposits only make up 39% of the total thickness of south-facing fans. Therefore, it may be that with much greater exposed bedrock and sparser vegetation, south-facing slopes generate substantial runoff and sediment in the absence of fire, whereas denser vegetation and thicker colluvium mean that north-facing slopes produce relatively little fan sedimentation unless impacted by fire. Overall, fire is clearly an important control on geomorphic process in the Jemez Mountains.
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