A Burning Question: Can Long-term Data Inform Fire Management in the Twenty-first Century? - Biodiversity Conservation and Environmental Change - page 74

(a)

Regional fire years

30

25

20

15

10

5

1600

1650 1700 1750 1800 1850 1900 1950 2000

Year

(b)

Figure 4.9 Effects of fire suppression in southwestern forests of the United States. (a) Fire history in
southwestern forests and woodlands of the United States from 1600-2000, based on composite fire scar
chronologies from 55 sites in Arizona, New Mexico, and northern Mexico. Most of the sites were
Ponderosa pine or mixed conifer forests. 'Regional Fire Years' are those with 10 or more fires per year.
Before 1900, fire was mainly governed by regional climate, especially variation in El Niño Southern
Oscillation, but after 1900 fire suppression and grazing caused dramatic reductions in fire frequency
(Millar and Woolfenden 1999, Richardson et al. 2007). (b) Repeat photographs from 1909 and 2004, show-
ing expansion of pinyon pine and juniper at the expense of grassland and sagebrush, in Utah, USA, as a
result of decreased anthropogenic burning. 1909 photograph taken by G.B. Richardson (Number 242),
US Geological Survey Photographic Library, Denver, CO. USA2004 Retake by Charles E. Kay (Number
5351-11), Utah State University, Logan, UT. USA. Source: <http://extension.usu.edu/rra/>

In contrast, other US forests are adapted to fire return intervals of several centuries, and are
unlikely to have been affected by active fire suppression, though logging has disrupted forest
structure and function, thereby influencing fire regimes. The forests of the Pacific Northwest
are temperate coniferous forests dominated by species such as Douglas fir ( Pseudotsuga