FIRE EFFECTS
SPECIES: Aristida purpurea | Purple Threeawn 



IMMEDIATE FIRE EFFECT ON PLANT : 
Threeawns (Aristida spp.) are readily harmed by fire because their
rootcrowns are close to or above the soil surface [63].  Purple threeawn
cover is generally reduced by fire [52,62,63].  Purple threeawn produces
relatively large, densely culmed, fine-leaved bunches.  Since it is
grazed very little, litter usually accumulates around bunches.  Wright
and Bailey [63] stated that bunchgrasses with this growth habit may
continue to burn for 2 or 3 hours after fire has passed.  Such prolonged
burning transfers heat downward and damages roots.  Basal cover may be
greatly reduced or plants may be killed.  Fire-induced mortality varies,
however.  Evans [15] reported that only a few purple threeawn were
killed by spring prescribed fire in southern Idaho.  Small purple
threeawn bunches were killed by a second fire 2 years later.  Large
bunches were not killed by repeated burning but split into several small
bunches.


DISCUSSION AND QUALIFICATION OF FIRE EFFECT : 
NO-ENTRY


PLANT RESPONSE TO FIRE : 
Fire reduced blue threeawn in a desert mountain shrub community near Big
Bend National Park, Texas.  Wildfire burned the community in November,
1975.  Prefire vegetation was dominated by sotol (Dasylirion
leiophyllum), lechuguilla (Agave lechuguilla), blue threeawn, and
sideoats grama (Bouteloua curtipendula). Precipitation was above average
in postfire years 1 and 2.  At postfire year 2, blue threeawn cover on
burned plots averaged 0.42 percent while blue threeawn cover on adjacent
unburned plots averaged 4.09 percent.  Grasses and succulents had
decreased at the expense of forbs and subshrubs.  Forb and subshrub
cover on burned plots was 650 percent greater than on unburned plots
[3].

Wright's and purple threeawn (Aristida purpurea var. purpurea) were
slightly to greatly reduced for at least two growing seasons following
chaining and burning in a redberry juniper (Juniperus erythro- 
carpa)-mixed grassland community in King County, Texas.  In 1979
or 1980, plots were chained, chained and broadcast burned in March, or
left untreated (control).  Fine fuel loads averaged 2,264 lbs/acre
(2,573 kg/ha) in 1979 and 1,327 lbs/acre (1,508 kg/ha) in 1980.  Weather
conditions were:  relative humidity 25 to 40 percent, temperature 68 to
78 degrees Fahrenheit (20-26 deg C), and winds 7.7 to 14.4 miles/hour
(12-24 km/hr).  Fire spread was "excellent" in both years, with nearly 100
percent coverage.  Precipitation following the 1979 fire was above
average; drought followed the 1980 fire.  Combined yield of Wright's and
purple threeawn was [47]:

_________________________________________________________________________
|          1979 Treatments (wet year)       | 1980 Treatments (dry year)|
|___________________________________________|___________________________|
|                1st growing   2nd growing  |  1st growing   2nd growing|
|                  season        season     |    season        season   |
|___________________________________________|___________________________| 
|control             63a           63a*     |      57a           25b    |
|chained             92a          113a      |      85a           37b    |
|chained & burned    47a           56a      |      12a            6b    |
|_______________________________________________________________________|
*Values within a treatment year identified by the same letter are not
significantly different (p > 0.05).

Prescribed fire on the Texas Technological College Research Farm near
Amarillo had little effect on Fendler threeawn size but stimulated
seedstalk production.  Six plots were used.  Two plots were burned in
fall, 1965 (1 with and 1 against the wind); two plots were burned in
spring, 1966 (1 with and 1 against the wind); one plot was burned
summer, 1966 (with the wind); and one plot was not burned (control).
There were no significant differences in soil moisture between plots at
the time of the fall or spring fires.  Basal diameters of Fendler
threeawn plants decreased between 1966 and 1967 on plots burned 2 years
in succession, but either maintained or increased in size on the
unburned control plot and plots burned only once the previous year.
Height and seedstalk production decreased between 1966 and 1967
regardless of treatment.  Relative to the control plot, fire had little
effect on seedstalk numbers the first growing season after fire.
However, in the second growing season (1967), Fendler threeawn seedstalk
numbers were greater on all burned plots compared to the control plot.
Average seedstalk production and average maximum height of Fendler
threeawn was [52]:

                         Number of Fendler Threeawn Seedstalks
               |----------------------Treatment-------------------------|
               |Control   Fall     Fall     Spring     Spring     Summer|
Year           |          1965  1965 & 66    1966    1966 & 67     1966 |
_______________|________________________________________________________|
1965*          |  115      135      107       55         115        127 |
1966           |   40      105       82       25          67        37  |
1967           |   39       45       55       43          48        55  |
_______________|________________________________________________________|
*before burning

                                     Height (cm)
               |----------------------Treatment-------------------------|
               |Control    Fall     Fall     Spring    Spring     Summer|
Year           |           1965  1965 & 66    1966   1966 & 67     1966 |
_______________|________________________________________________________|
1965*          |  33        32       30        27         27        35  |  
1966           |  25        23       20        20         20        20  |
1967           |  12        11       10        11         11        11  |
_______________|________________________________________________________|
*before burning

Fendler threeawn was reduced following wildfire in a cheatgrass stand in
north-central Utah.  Vegetation had been sampled at the end of the 1955
growing season.  The wildfire occurred in summer 1956.  Precipitation in
1957 (the only year for which precipitation data are given) was near
average.  Fendler threeawn cover and importance percentages were as
follows [6]:

                                    Year
                  ----------------------------------------
                  1955     1957     1958     1960     1961
                  ----------------------------------------
cover (%)         11.5      3.4      6.2      9.9      5.0
importance (%)    17.7      4.2      8.6     11.6      5.9 


DISCUSSION AND QUALIFICATION OF PLANT RESPONSE : 
NO-ENTRY


FIRE MANAGEMENT CONSIDERATIONS : 
NO-ENTRY

Related categories for Species: Aristida purpurea | Purple Threeawn