Changed Southwestern Forests: Resource effects and management remedies (Page 2 of 3)

Author: Marlin A. Johnson. From a paper presented at the Forest Ecology Working Group session at the Society of American Foresters National Convention held in Albuquerque, New Mexico, November. 9-13, 1996.

Post-European Settlement Changes

A reduction in ecosystem fire occurred by the 1880's or before and about the same time that livestock grazing levels increased substantially. By 1890, cattle numbers were about 1.5 million head in Arizona and New Mexico (Baker et al, 1988). Grazing by domestic livestock removed grasses that had previously carried cool ground fires. This coincided with good moisture in the first few decades of the 20th century, which led to forests with far more trees than before. Without such moisture, grazing and other fire suppression would not have led to the overstocking. This is shown in the Chuska Mountains on the Navajo Reservation where extensive grazing by sheep and goats started in the 1820's, and fires declined. However, forest regeneration did not start until after 1900 as it did in the rest of the Region (Pyne, 1996).

Other factors contributed to reduced fine fuels and, therefore, reduction of fire frequency as the 19th century ended and the 20th began (Pyne, 1996). Loggers began removing mature trees. Bark beetles, fungi, and dwarf mistletoe infested the thickets that, in the absence of grass and fire, sprang up profusely. Severe droughts further reduced light fuels. Roads built by settlers, fixed landownership patterns, reservation of public lands, and establishment of professional forestry also contributed. In combination, the result was suppression of low-intensity fire. Through succession, this encouraged fuel arrays that promote intense fires. These fuel loads are an environmental debt, like toxic dumps. They will take decades of determined action to clean up, and that only if society has the resolve and the money (Pyne, 1996).

The decline in fire frequency was observed in virtually all Southwestern fire scar studies (Weaver, 1951; Dieterich, 1980; Swetnam, 1983 & 1990; Allen, 1989; and Savage and Swetnam, 1991). Exclusion of fire eventually results in elimination (or serious reduction) of ponderosa pine, aspen, and other forest species characteristic of frequent fire regimes. More than anywhere else, this transition can clearly be seen occurring in national parks and wildernesses, and in other areas that have never been logged (Sampson et al, 1993).

Meadows also declined in area as they were invaded by aspen and conifers. This was quantified by Allen (1989, p. 260). He states, "Overall, in the southeast portion of the Jemez Mountains open montane grassland area decreased 55% from 554 ha. in 1935 to 250 ha. in 1981." Comparison of aerial photos for other areas and local knowledge corroborate this statement.

Southwest Forests Today

Change is still occurring. From 1962 to 1985/87, the mixed-conifer forest type increased by 1,040,000 acres, or 81 percent. Ponderosa pine decreased slightly in acreage, and aspen decreased by 46 percent (Johnson, 1994). These changes may have some major impacts on resource values.

Table 1 illustrates the change in forest conditions from 1910, when the first inventories were done, compared to conditions found in the 1985/87 inventories. There has been a substantial increase in total numbers of trees in diameter classes up to 15-inches dbh and also a smaller increase in the 18- and 21-inch classes. In the 24- and 27-inch diameter classes, numbers of trees stayed about the same in 1985/87 as in 1910, and in the class over 30-inches there has been a decline over time. However, the 30-inch-plus class has never been a large percentage of the total ecosystem. In 1910, this class comprised less than one tree per acre (Woolsey, 1910).

What Happens Next?

You might logically ask the question, "Who cares if there are more trees?" After all, trees are beautiful, provide wildlife habitat, and protect the soil. In the Southwest, managing for higher tree densities is unrealistic over the long term. From the resource manager's perspective higher densities should be seen as infeasible. From the firefighter's standpoint, they should be seen as dangerous (Williams, 1996).

There is growing concern among natural resource professionals that disruption of natural fire regimes, increase in tree numbers, and increases in landscape homogeneity are a far greater threat to biological diversity and ecosystem sustainability than is generally realized (Caraher et al, 1992; Kaufmann et al. 1992; Everett et al.1993; Mutch et al. 1993; Covington et al, 1994; Covington and Moore 1994b; Pyne, 1996). Unless concerted actions are taken to reverse ongoing ecosystem degradation, prospects look grim for the quality of life--not only for the region's forest and woodland ecosystems, but also for the human populations that rely on these resources (Covington et al, 1994).

Many changes occur as our forests grow more dense. First, the character of forest fires change. They now burn less frequently, but are high-intensity, stand-replacing fires (Hessburg et al, 1993, Covington and Moore 1994a). Firefighters have noted conditions in recent years that cause grave concern about their safety and their ability to control fires. While crown fires in ponderosa pine were almost unheard of earlier this century (Cooper, 1960), today they are commonplace. Table 2 quantifies changes by showing the numbers acres burned, and average fire size by decade since 1950. Pyne (1996) reports that crown fires increased from 10,127 acres per year in the 1940's to 15,117 acres per year in the 1980's, despite a massive commitment to high-technology firefighting. Society spends considerable resources (sometimes in vain) to stop these fires before they destroy not only natural resources but human developments as well.

Table 2. Acres burned, and average fire size by decade on National Forest lands in Arizona and New Mexico, 1950-1996. (USDA Forest Service Data.)

It is significant that the 3 largest fire years (in terms of acreage burned) since 1950 have occurred since 1993. Seven of the highest 11 years have occurred since 1988; two of the high years were in the 1950's when fire suppression was likely less efficient. The actual increase in fire size and acreage accelerated rapidly in the late 1980's.

Total fire starts and precipitation must also be considered. The number of fires has stayed fairly constant except for the 1970's when it dropped slightly. The 1990's have had drought years, but so did the 1950's and 1970's. Therefore, the recent high average fire size and acreage burned per year cannot be explained by either fire starts or precipitation.

Firefighting technology, road access, and suppression efforts have gradually improved since 1950. Therefore, major factors being about the same, a gradual decrease in acreage burned and average fire size would be expected. However, increasing vegetative density and the resulting fuel laddering appear to be primary reasons for the opposite trend.

Scientists who have studied the changes in vegetation and forest fires feel that fire will burn many forests in the Inland West. Sampson et al, 1993, state that only a few decades remain in which to save our forests, as most of the forests in the Inland West will burn despite elevated fire suppression efforts.

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