Abstract:
The effects of fire severity on post-fire succession and shrub demography were
quantified in shrub-steppe grasslands and subalpine forests in the Hells Canyon of the
Payette National Forest, Idaho. Following the 1988 Eagle Bar Fire, species frequency,
community diversity, fire adaptations, and stand ages were quantified at 12 plots in
burned and unburned forests. Pre-fire composition was dominated by Pseudotsuga
menziesii, Pinus contorta, Abies grandis, and Abies lasiocarpa in the overstory. Prefire
basal area ranged from 41 to 80 m 2 ha-1 . Pre-fire stand ages ranged from 71 years in
Abies lasiocarpa forests to > 200 years in Pseudotsuga menziesii stands. Fire scenarios
which were sampled consisted of: (1) complete stand-replacement fires; (2) partial standreplacement fires; and (3) unburned forests (control). Partial stand-replacement forests were characterized by significantly higher mean species diversity and richness (H'=3.16, S=36) than complete stand-replacement (H'=2.78, S=27) or unburned forests (H=2.26, S=15). Vegetation preferentially established in burned areas along a gradient of fire severity according to adapted fire guilds. Single-storied forests dominated by Pinus contorta and Abies lasiocarpa typified stand-replacement fire scenarios, with mean ages ranging from 99 to 159 years corresponding to fire return intervals common in this type. Multi-storied forests of Pseudotsuga menziesii in the overstory and Abies spp. in the mid-story comprised partial stand-replacement forests. Overstory stand ages ranged from 165 to 209 years, while mid-story ages ranged from 77 to 99 years, corresponding to fire-return intervals which characterize the intermediate, partial stand-replacement fire
regime. In the Hells Canyon forests, fire is a dominant ecosystem process in the creation
of landscape mosaics, in governing species distribution, and in the maintenance of
biological diversity. Forests experiencing complete stand-replacement (n=4) were dominated by Salix scouleriana, a classic off-site colonizer. At the fourth post-fire year, Salix scouleriana density ranged from 252,000 shrubs ha' to 569,333 shrubs ha-1 . From the fourth to the fifth post-fire year, mean shrub density showed a significant decrease from 336,667 to 246,000 shrubs ha.4 . During this same period, mean shrub biomass significantly increased from 10.2 g shrub-1 to 33.1 g shrub"', mean aboveground biomass significantly increased from 2.95 to 7.05 Mg ha-1, and mean shrub height significantly increased from 62 to 100 cm shrub-1. Without competing conifers, Salix scouleriana may maintain site dominance for several decades.
The 1992 Windy Ridge Fire burned approximately 2,000 ha of the Purshia
tridentatal Agropyron spicatum plant association at elevations < 1460 m. Because
Purshia tridentata survivorship appeared to be associated with plant age and ecotype, a multiple regression model was developed to partition burned and unburned shrubs into
age class segments at n=10 sites. Pre-fire shrub density ranged from 400 to 1700 shrubs
ha-1. Concomitant with Purshia tridentata decadence and stagnation following a
lengthened fire-free interval were increases in post-fire mortality. Post-fire mortality of Purshia tridentata was closely associated with plant age (R2=0.91). Plants in age classes < 25 years of age exhibited 66% sprouting, while shrubs > 25 years of age had only 20% survival. Shrub mortality in burned communities ranged from 37% to 100%, while
mortality among unburned communities averaged 5%. The reintroduction of fire appears to be a necessity for restoring certain Purshia tridentata ecotypes which have not entered stagnation. Continued fire exclusion may well result in the loss of many at risk Purshia tridentata communities. As an influence on ecosystem function, genetic variation, and landscape mosaics, fire represents a pervasive disturbance in the Hells Canyon ecosystems. Unique successional pathways result from fires which vary in dimension, severity, and periodicity, thereby contributing to diversity at all scales. Intrinsic adaptations to fire testify to the coevolution between flora and fire in the Hells Canyon, and illustrate the resiliency of biota in fire-dominated environments.
