SUMMARY
G. Wayne Minshall, James T. Brock and John D. Varley. 1989. Wildfires and Yellowstone’s
Stream Ecosystems: A temporal perspective shows that aquatic recovery parallels
forest succession. BioScience 39 (10): 707-715.
This article was published just one year after the 1988 Yellowstone fires that burned over 0.57
million hectares of land in the Greater Yellowstone Area (GYA). The authors review past
research on fire effects on aquatic systems and make predictions for the response of the 20 river
basins affected by the GYA fires.
The article is subdivided into four sections:
1) Fire and landscape heterogeneity
2) Temporal responses of stream ecosystems to fire:
a) Immediate effects
b) Midterm effects
c) Long-term effects
3) Potential responses to different degrees of disturbance
4) Research needs and opportunities
1) Fire and landscape heterogeneity
The patchiness of the 1988 fires resulted in a mosaic of areas burned by high intensity canopy
fires, less-intense surface fires, or the area was left unburned. Streams and their drainage basins
differ in area, vegetative cover, slope and soil, among other things. These differences, combined
with differences in fire intensity and severity will likely result in different responses from
different types of stream systems. The authors hypothesize that the effects of fire will be most
severe in small, forested headwater streams that have been completely burned in the upper
portions. In addition, they hypothesize that the effects of the fire will dissipate further
downstream. This is corroborated by the observation that first and second order stream basins
commonly burned entirely, while larger streams rarely did so.
2) Temporal responses of stream ecosystems to fire
Because stream ecosystems are closely linked with the surrounding vegetation, changes in
vegetative structure and composition after wildfire will affect stream dynamics. The authors
hypothesize that stream ecosystems will respond to fire similar to forest ecosystems, with stream
recovery following forest succession. The effects of fire can be broken into immediate, midterm
and long-term.
a) Immediate effects
Immediate effects are those that arise directly from the fire, such as changes in water
chemistry due to ash deposition and abrupt changes in food quality. Changes in water quality
due to wildfire are thought to be minimal and short-lived. However, in some cases increases
in ions or pH following fire can cause fish mortality.
b) Midterm effects
Midterm effects are secondary consequences of fire that occur within a few years post-fire.
These effects can include: increased sedimentation, erosion of stream channels, disruption of
Summary
1
Leah Rogers
nutrient cycling because of increased nutrient runoff, increased stream temperature as a result
of reduced canopy cover and increased algae production.
c) Long-term effects
Long-term responses of streams will most likely be related to forest and understory
vegetation. Large woody debris jams will likely increase post-fire because of fire-killed
snags, but new recruitment of debris will be reduced in subsequent years. In addition,
retention of woody debris (which create pools and habitat for fish) may be decreased postfire because of increased flow.
Suspended sediment and erosion are expected to decrease as the hillslopes are
revegetated after the fire. However, the deposited instream sediment will continue to be
transported downstream for many years after the fire.
Likewise, a pulse in nutrients immediately post-fire will decrease as vegetation returns.
However, primary (algae) production will continue to be elevated for 10-20 yrs post-fire
because of increased solar radiation. As shade producing trees return to the riparian area,
algal production will decrease.
The effects of the fires on benthic macroinvertebrates are expected to be related to the
degree of sedimentation. The macroinvertebrates may experience increased drift as a result
of high runoff, sedimentation, and bed erosion. Recolonization of macroinvertebrates will
depend on the extent of damage in the headwaters, which are seed sources. In addition, the
density of macroinvertebrates that feed on algae (grazers) should increase, while the density
of leaf-eating macroinvertebrates (shredders) should decrease, as algae production is elevated
and leaf-litter input is reduced post-fire. As revegetation occurs, the food source should shift
back to leaf-litter and the density of shredding macroinvertebrates will also increase.
3) Potential responses to different degrees of disturbance
Large, intense fires will have a much greater effect on stream ecology than smaller, lessintense fires. In addition, the size of the watershed burned and the proportion of the burned area
within the watershed will also influence the effects of the fire on stream ecology. The authors
suggest three alternative trajectories for the effects of fire on the biotic features of streams (such
as macroinvertebrate abundance, diversity, etc.)
 Small streams with less burned area within the catchment will recover quickly post-fire.
 Heavily eroded stream catchments with high bed scour will have delayed recovery.
 Repeated disturbances in the same catchment can result in long-term reductions in
abundance and richness.
4) Research needs and opportunities
The authors outline the need for more research on the effects of fire (primarily midterm and
long-term effects) on streams. They emphasize that the recent fires in Yellowstone provide(d) a
great opportunity to study these effects, because of the heterogeneity and the spatial extent of the
fire.
Summary
2
Leah Rogers