The Holocene record of fire and erosion in the southern Sacramento Mountains and its relation to climate

As highlighted in this issue’s Gallery of Geology on page 24, large, severe wildfires have become part of the New Mexico late spring and early summer experience in the last few decades. Such fires have considerable relevance to geomorphologists, as erosion rates in mountainous landscapes are often dramatically increased in the several years after severe fires. Erosion and sediment transport often take place during major debris flows and flash floods (Fig. 1). These events are most commonly triggered by intense thunderstorm rainfall, as in New Mexico’s summer monsoon, and very rapid runoff from slopes devoid of vegetation or forest litter. Although water-repellent soils formed by fire effects are often cited as the primary cause of increased runoff, the creation of extensive bare, smooth soil surfaces alone is more than sufficient— for example, consider the erosion that would occur on a plowed, smooth farm field at slope angles of 15–30° or more! The extreme flows that are generated can entrain huge volumes of sediment as they course down slopes and channels. Events of this nature affected a number of small, steep drainages in the Sacramento Mountains southeast of Cloudcroft after the 2002 Penasco fire. Large quantities of mudto boulder-sized sediment may be deposited on alluvial fans along the valley margins, and in some cases deep gullies are also cut in the fans. Major damage to roads, buildings, and property resulted in several locations in the Penasco fire area, as valleyside alluvial fans are common sites for residential and other development. Along with their importance in understanding geologic hazards and watershed impacts, sediments deposited on alluvial fans by postfire debris flows and floods also provide a means of assessing the timing and spatial distribution of past forest fires, and relations between fire and climate, in particular episodes of severe drought. These sediments are often rich in charcoal fragments from the burned area, which allows radiocarbon dating of fire-related sedimentation events thousands of years into the past, providing an important supplement to the more commonly available tree-ring fire histories. Tree-ring dating has provided a wealth of information on low-severity surface fires that scar trees, but leave them living. Such fires swept through the understory of many southwestern forests every few years to a few decades before European settlement and intensified grazing, logging, and fire suppression in the late 1800s (e.g., Brown et al. 2001). However, tree-ring fire-scar records extend back about 500 yrs at most, and do not provide data on severe fires that kill large stands of trees. Standdestroying fires can be dated via the ages of living trees that germinated after fire, but this reveals the last such fire only, and again is limited to about the last 500 yrs. Therefore, alluvial sediments can greatly extend fire histories, albeit with greater uncertainty in ages. Climatic change on time scales of a thousand years or more has strongly affected Earth environments over the Holocene Epoch, the ~12,000 yrs since the last episode of continental glaciation. Thus, the sensitivity of fire activity to climate change over such time scales is of great interest. It is also critical to understanding the potential impacts of future droughts on New Mexico’s mountain forests, given that predicted warming over the next century has no precedent on the short time scales covered by tree-ring fire chronologies.