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Fire in Southwestern Riparian Habitats:
Functional and Community Responses
D.E. Busch 1
FIRE OCCURRENCE
There was little difference between the structural
classification of burned vegetation and the genernl population
of riparian vegetation that had not burned. Comparisons of
burned areas within each dominance type indicated that
significantly less cottonwood/willow (Populus fremontii/Salix
gooddingii) and mesquite (Prosopis glandulosa and P.
pubescens) dominated habitat- burned than would be expected
based on the representation of these taxa in the riparian
community. Fires in habitats dominated by saltcedar (Tamarix
ramosissima) appeared to be disproportionately latge, but this
relationship was not statistically significant presumably due to
the small subsample of stands that could be evaluated using GIS.
Assuming that the period evaluated is representative, fire
could be expected to burn approximately 25% of the riparian
vegetation, and 40% of the saltcedar-dominated habitat, each
decade. This implies a distmbance interval shorter than that from
fire in most forest associations, and one that is insufficient for
full maturation of historically dominant cottonwood, willow, and
mesquite. It appears that shrubby species that are less valuable
from a wildlife habitat standpoint may benefit from riparian zone
fires.
Hydrogeological perturbations and exotic species
introductions are among the factors indirectly contributing to
shifts in riparian ecosystem processes. Among these altered
processes is the incidence of :Qre, which appears to occur at
greatly elevated frequencies in pertw:bed low-elevation riparian
ecosystems and may be a novel fonn of disturbance in these
systems.
- Between 1981 and 1992, 166 fires burned 11,846 ha (27%)
of the riparian vegetation in the lower Colorado and Bill
Williams River floodplains (Fig. 1). The area burned annually
and fire frequency were correlated between 1981 and 1988 (r =
0.85, P < 0.01).
40
3000
--AREA
2500
-----__ • NUMBER
30
cu 2000
.c:
-
<C
~ 1500
<C
w
a::
u:
FUNCTIONAL RESPONSES
a::
20
w
m
:E
=>
Z
W
a::
u:
1000
10
500
o
1980
1982
1984
1986
1988
1990
1992
YEAR
Figure 1. - Area and number of fires occurring between 1981
and 1991 in the lower Colorado and Bill Williams River
riparian zones.
1Bureau of Reclamation, Division of Environment, Boulder City,
Nevada, USA.
304
There is evidence for mechanisms which would facilitate
increased community dominance of saltcedar and arrowweed
(Tessaria sericea) following riparian zone fires. Rapid recovery
in these taxa appears to be attributable to salinity tolerance and
to mechanisms pennitting efficient water uptake and use (Busch
and Smith 1993).
Soil elemental analyses indicated that concentrations of nearly
all soil constituents increased following fire. This contributed to
a potential nutrient abundance, but also was a manifestation of
elevated alluvium salinity. Boron was elevated in riparian soils
following fire and lower leaf tissue concentrations suggest that
saltcedar may derive tolerance to this element by an effective
elimination mechanism. Based on leaf sodium concentrations, a
dichotomy between halophytic (saltcedar and arrowweed) and
glycophytic (cottonwood and willow) taxa was detectable, but
no clear fire-related differences were apparent. There was little
interspecific variation in leaf nitrogen concentration, but burned
arrowweed had elevated concentrations of this element. This
may contribute to the higher water use efficiency following fire
that was observed in recovering burned arrowweed relative to
unburned controls.
Higher leaf stomatal conductance in all taxa and in both
ecosystems was an indication of vigorous post-fIre recovety.
This may, in part, be attributable to increased rndiation loads
associated with the reduction of plant canopy cover following
fire. However, decreases in water potential accompanying an
increased transpiration load may signify water stress as a result
of fire injury or inefficient recovery. Examination of
trnnspiration-water potential regression parameters provided
evidence for reduced post-fire hydraulic efficiency in burned
Colorado River willows.
saltcedar. Marked increases in arrowweed abundance
accompanied cottonwood declines in the cottonwood/willow
dominance type. Willow maintained its importance in burned
cottonwoodlwillow vegetation, but did not become established
in other dominance types in the manner of arrowweed. Similarly,
mesquite and other shrubs were present in burned riparian
stands, but did not dominate even those areas that were classified
as mesquite-<iominated prior to fire.
As was expected of plants that recover from fIre via
resprouting, multiple linear regression analysis of burned
riparian comnmnity ordination data revealed that the species that
were dominant prior I to burning were strongly associated with
post-fire community composition. Subtle evidence of a
successional trend was shown by the positive relationship of
community structure with fire year, and the tendency of younger
tree demographic classes and ruderal taxa to be found near the
opposite extremes of Detrended Correspondence Analysis axis
3 from older demographic classes and slower growing species.
COMMUNITY RESPONSES
In agreement with functional, analyses of fIre recovety
efficiency, the community importance of saltcedar and
arrowweed was high following fire in all pre-fire dominance
types. Fire also appears to contribute to the catastrophic declines
in cottonwood populations that are in evidence in pertUJbed
desert riparian ecosystems.
Relative cover and frequency of saltcedar was high in three
burned riparian vegetation dominance types while arrowweed
importance in burned riparian vegetation approached that of
REFERENCE
Busch, D.E. and S.D. Smith. 1993. Effects of fire on water and
salinity relations of riparian woody taxa. Oecologia
94:186-194.
305