Fire frequency is often expressed as the fire rotation interval, which is the time required to burn the equivalent of a specified area. However, on many landscapes some areas receive more fires than other areas and thus a more precise measure is the fire return interval, which is the spatially specific time between fires in a specified area. For example, coastal mountains in southern California have a fire rotation interval of about 35 years but some specific sites have a fire return interval of one fire every 5 years and others every 50 years.
Our understanding of historical fire frequency is better for some ecosystems than for others. In surface fire regimes such as ponderosa pine forests of the Southwestern US, low-intensity fires burn and sometimes scar trees (Figure 3) but do not typically kill the trees. Thus, scarred trees can be used as a record of past fires by relating past scars to particular growth rings in the tree. Such trees commonly provide fire records back to 200-300 years. In the giant sequoia forests of Sequoia National Park in California, studies have documented fire frequencies back to more than 2000 years.
Much longer records of past fires have been obtained by examining soil cores from lakes and bogs. Under certain conditions one can obtain an approximate date for a particular layer of a soil core and measure the amount of charcoal, thus providing a relative measure of fire activity over time. While this technique is capable of providing records back to over 10 000 years ago, the resolution and certainty of burning are less than that with tree ring studies. Fire frequency estimates based on charcoal deposition are affected by the pattern of dispersion of charred particles, and fuel type, as well as rainfall that affects sediment movement. Thus, these records can be interpreted
differently by different investigators. Even less direct measures of fires include geological studies of past debris flows that likely resulted from very large and severe wildfires.
For many ecosystems the only reliable source of data on fire history comes from written records maintained by fire management agencies. In the US these often go back to the early decades of the twentieth century. Over time the quality of these records has likely improved by better observations and improved techniques and thus these historical records must be interpreted cautiously.
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