Fire and Vegetation Change in Northern New Mexico (page 4 of 5)
Author: Craig D. Allen. Adapted from: Allen, C.D. 1998. Where have all the grasslands gone? Quivera Coalition Newsletter, Spring/Summer.
Fires in the Forests: Then and Now
Widespread fires occurred about every 5-20 years wherever ponderosa pine grew, with somewhat lower frequencies on the order of 15-40 years in the bracketing piņon-juniper woodlands below and mixed conifer forests above. Although some small, patchy fires certainly occurred, note how in some years almost every tree recorded a fire scar, indicating widespread fire occurrence. Indeed, in many years climate-synchronized fires burned throughout whole mountain ranges, and about four times per century most the mountain ranges across the entire Southwest burned in the same year. With few barriers to spread and without human efforts to contain them, pre-1900 fires may have burned for months in some of these dry years. The position of the fire scars within the annual growth rings indicates that the vast majority of prehistoric fires were occurring in the dry spring period (April-June) before the onset of the summer rains, which is still when most fire activity occurs. Given our dry spring climate and frequent thunderstorms, lightning is believed to have caused the vast majority of these fires. This view is supported by the records of about 4000 lightning-caused fires documented by firefighters in the Jemez Mts. from 1909-1996, and by the over 160,000 lightning strikes recorded over the Jemez country by a lightning detection system between 1985 and1994.
Just like nowadays, the most active fire years occurred after dry winters. The most widespread fire activity in ponderosa pine forests typically occurred in dry periods a year or two after wet years in which herbaceous fuels would have built up (another clue about how widespread and important grassy understories were in these open forests).
Note that crown fires were a natural occurrence in some of the higher elevation, wetter, forest types (such as spruce-fir and some mixed conifer forests) where surface fires were less frequent and fuel loads greater. Places where aspen stands grow today often reflect a history of crown fire. Crown fires took place in particularly dry years, like the spring of 1880 when the spruce forests on Santa Fe Baldy burned.
The widespread surface fires ceased throughout northern New Mexico in the late 1800s. Railroads reached the lands of Rio Arriba in the 1880s, connecting this region to outside markets and capital, which resulted in a massive boom in livestock production. By the end of the 1880s there were over 5 million sheep and more than 1 million cattle in New Mexico, ranging freely over open ranges numbers stayed high into the 1920s (in contrast, today there are only 1.6 million cattle and sheep total in New Mexico). This intense, landscape-wide grazing apparently reduced the grassy fuels to the point that surface fires ceased to spread, inadvertently resulting in de facto fire suppression. Active fire suppression by the US Forest Service became an emphasis after 1910 as woody fuels and forest densities slowly started to build up. Today over a million dollars are spent in an average year to fight fires in the Jemez Mts. alone (the 1996 Dome Fire cost around $10 million), and the cost of fire suppression in the West is now averaging almost $ one billion/year!
Tree rings also record how local forests have changed over the past century since the widespread surface fires ceased. Age studies show increasing numbers of trees establishing during the 20th century in most forest types, ranging from ponderosa pine and mixed conifer forests down through piņon-juniper woodlands. For example, some ponderosa pine stands now have well over 2000 stems/acre, in contrast to 120 years ago when only about 50 stems/acres were present. Piņon-juniper woodlands are also characterized by higher tree densities today, contributing to losses in herbaceous vegetation cover and associated increases in soil erosion (note that recent US Forest Service inventory data indicate the presence of about 1.4 billion piņon trees in New Mexico). These great increases in tree density in multiple vegetation types have caused declines in herbaceous understories, as the grasses are choked by the shade, needle mats, and competition from the dominant trees.
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