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.
Over 150 years of occupancy by northern Europeans has markedly changed vegetative conditions in the Southwest. Less fire due to grazing and fire suppression triggered a shift to forests with very high tree densities, which in turn contributed to destructive forest fires. Options to deal with these changes include prescribed fire, thinning and timber harvest to mimic natural disturbances and conditions. However, there are barriers to implementing these activities on a scale large enough to have a significant benefit.
In this paper, I discuss Southwest forests around the turn of the century, discuss how they looked before then, and then show changes in recent decades. I portray how these vegetative changes affect resource values and propose some actions to avoid future negative consequences.
Thousands of years of human occupation preceded the first accounts, paintings, and photographs of Southwest (SW) landscapes. We can only guess what the earliest occupants saw or thought of their landscape. We know that prehistoric people altered vegetative composition through farming and burning. The view through time's fuzzy lens clears as we near the present.
In the late 19th and early 20th century, visitors to Southwestern forests gave us our first indication of how these forests looked then, primarily from their written observations. Although there were exceptions, most descriptions portray, especially in ponderosa pine (Pinus ponderosa) forests, conditions that are much more open than what we see today (Whipple, 1856, U.S. Geological Survey, 1904, Cooper, 1960). A few pictures are available from this era, and they generally show the same thing. Groups of similarly sized trees with little understory but considerable grass beneath seem to have been the most common condition.
Fires burned frequently (2 to 10 years) at low intensity in lower elevations and less frequently but with moderate intensity at higher elevations (Swetnam and Baisan, 1996). Lightning is common in the Southwest, and Native Americans also contributed to fire frequency. Escaped domestic fires occurred as did intentional burning, although burning by Native Americans has not been documented in the SW to the degree it has further north (Swetnam and Baisan, 1996).
In 1910, Woolsey and also Lang and Stewart gave us the first quantified inventories. Neither inventory was Southwest-wide, and they do not give us a picture of conditions on a same-acreage, statistically sound basis. Nevertheless, they covered several areas and included a large number of plots, so they paint the best quantified picture available today of early 20th-century conditions. They agree with a) early photographs, b) early descriptions mentioned above, c) what a forester or ecologist would expect to find given knowledge of fire regimes, and d) what studies of stumps and other factors indicate was present (Covington, and Moore, 1994a).
Table 1 shows trees per acre by diameter class from Woolsey's 1910 inventory. The table compares his figures with those from USDA - Forest Service inventories made in 1962 and again in 1985/87. Woolsey's inventory was from plots taken in typical stands on three National Forests in Arizona. The other two inventories were based on a plot-grid that covered all of Region-3: both Arizona and New Mexico. The information is grouped by 3-inch diameter classes.
Table 1. Trees per acre, by diameter class, from Southwest inventories.
Woolsey's data were recently converted to basal area (BA) and show that average conditions in 1910 in ponderosa pine forests ranged from BA 22.9 square feet per acre (Tusayan Forest) to 25.4 (Prescott Forest) to 37.9 on the Coconino Forest. Other plots were taken in the best stands that could be found on several forests around Arizona and New Mexico and the average is a BA of 82.0 (Woolsey data converted to BA by the Forest Service). These are all well below today's densities, which are mostly above BA 100 per acre and often up to and even greater than 200 (Forest Service data).