FIRE EFFECTS
SPECIES: Pleuraphis  mutica | Tobosa 



IMMEDIATE FIRE EFFECT ON PLANT : 
Tobosa is very resistant to fire mortality.  New growth begins about two
weeks after the burn [3], with resprouting occuring from extensive
rhizomes and basal root crowns.  In Arizona and New Mexico tobosa grows
on both upland and lowland sites, but only in the lowland swales are
stands dense enough to carry a fire.  In Texas where moisture is not as
limiting, tobosa tends to grow more often in pure stands.  When found in
these pure stands it accumulates large amounts of litter (3,000 to 6,000
pounds per acre of continuous grass fuel [5]) which carries a fire quite
easily.  This litter and old growth are virtually entirely consumed as
was demonstrated in a Texas study where 97% was removed by fire, leaving
ash and small amounts of charred vegetation [42].






DISCUSSION AND QUALIFICATION OF FIRE EFFECT : 
Accumulation of tobosa litter with its compact and somewhat woody
growth form usually insures a complete burn.  Compared with other grass
species tobosa burns hot when in pure stands, upto 1260 degrees F (682
C), with a duration over five minutes with temperatures over 150 degrees
F (66 C) [45].  Although tobosa normally burns hot, it has been shown
that the intensity of the fire does not influence plant response
following fire.  In Texas, it was shown that high intensity fires do no
more harm than low intensity fires, as yields following fire are the
same for different fire intensities [39].  Soil temperatures after
burning have been found to be 39 to 41 degrees F (4-5 C) warmer than
control plots at a soil depth of 3 inches (7.5 cm) [42].  During the year
after burning, it has been shown that soil bacterial growth increases
ten fold, which is apparently due to higher soil temperatures and increased
organic carbon levels [9].


PLANT RESPONSE TO FIRE : 
The response of tobosa following fire is dependent upon site
characteristics, geographic location, soil moisture, season of burn, and
precipitation in the months following burning [15,35,52].  Generally early
spring burns during moist years will show the greatest increase in
herbage production [3,53].  Tobosa quickly revegetates the bare ground
after fire, dominating the community by the middle of the first growing
season.  Precipitation following burning seems to be the most important
factor.  In western Texas where mean annual precipitation is
approximately 19 inches (48 cm), tobosa yields can be expected to
increase 2 to 3 fold within the year following burning if burning is
conducted during a moist spring [23,35,52,53].  But in contrast, in this
same area of Texas, burning during a dry spring will decrease tobosa
yields slightly during the current growing season [35,53].  In southern
New Mexico where the mean annual precipitation is only 8.86 inches (22
cm) herbage production following burning declined no matter what time of
year burning took place [15].

Yields of tobosa on concave sites following burning is greater than on
convex sites.  In Texas concave sites yielded 1462 pounds per acre (1646
kg/ha) while convex sites yielded 893 pounds per acre (1005 kg/ha).
This is probably due to the fact that lowland sites generally have
denser stands of tobosa due to accumulation of surface runoff.  But the
average increase in yield was 103.7% on concave sites and 232% on convex
sites.  On these drier convex sites, fire increased tobosa
frequency, entending plant distribution from the convex sites into the
more mesic midslope and concave sites [35].

Culms produced following fire will stand erect for several years before
they die, break off, and contribute to the litter layer.  At this time
litter consists mostly of dead leaf blades, but by the end of the fourth
or fifth growing season following fire there are large amounts of
standing dead material and litter from stems that have died and fallen
[43].  If precipitation is average or above average, then yields should
dramatically increase (2 to 3 times that of unburned sites) in the
current growing season following early spring burning, and then
successively decrease until equilibrium with unburned areas is reached
about 5 years later [35,53].  Studying the nitrogen levels of tobosa in
west Texas, postfire nitrogen levels reached prefire levels in five
years on concave sites, but convex sites were slower to recover, and
eight years is the estimate to reach prefire nitrogen levels on these
upland sites [43].


DISCUSSION AND QUALIFICATION OF PLANT RESPONSE : 
NO-ENTRY


FIRE MANAGEMENT CONSIDERATIONS : 
Burning is a very effective tool for stimulating forage production in
stagnant tobosa stands.  Tobosa range is considered one of the simplest
to burn because of the nonvolatile nature of tobosa and mesquite.
Burning should take place in the spring while plants are still dormant
and the soil is wet [54].  At this time it can be determined whether
ranchers need to use tobosa as a reserve feed for cattle as they
sometimes do [53].

Guidelines for burning tobosa range in Texas are as follows [5].

A dozed line cut out of mineral soil should surround the area to be burned.
A second dozed line is cut 100 feet inside the perimeter line on the
north and east sides.  This 100 foot strip should be burned when the
following weather conditions prevail:

   1. 40 to 60 % relative humidity
   2. 40 to 60 degrees F (4.4 to 15.5 degrees C) air temperature
   3. wind speed less than 10 miles per hour (16 kilometers/hour)

This creates a fireline that will contain the head fire to be ignited at
a later date.  Firelines should be burned between late January and early
March.  

The main area should be burned in early spring (March) before green up
of tobosa.  Weather conditions for head fires should be as follows:

   1. 25 to 40 % relative humidity
   2. 70 to 80 degrees F (21 to 26.7 degrees C) air temperature
   3. wind speed 8 to 15 miles per hour (12.8 to 24 kilometers/hour)

Tobosa is a important forage species for Southwest ranchers.  Burning
tobosa stands is also able to reduce some undesirable plants that are
often found there.  Prickly pear (Opuntia engelmannii) and cholla (O.
imbricata) are moderately harmed the first year after burning, and
continue to die in years following the burn (82 % mortality at the end
of the second growing season) [53].  Annual broomweed (Gutierrizia
dracunculoides) is a major winter annual in tobosa stands and is
esssentially eliminated for the first two years after spring burning
[52].  But by the second or third year after burning, broomweed
infestations are similar to preburn levels.  Mesquite (Prosopis
glandulosa) which is commonly associated with tobosa stands, is only
moderately harmed by fire alone; plants less than 3.5 years of age are
easily killed by fire, but plants over 3.5 years old are virtually
impossible to kill with fire alone [5].

Broomweeds and firewhirls are the two primary dangers to be concerned
with when burning tobosa stands [53,56].  Broomweeds burn off at the
base and tumble across pastures having the potential to ignite nontarget
areas. Firewhirls generally develop where windshears occur such as a
head fire running into a backfire.  With wind speeds of more than eight
miles an hour fire whirls are unlikely to occur unless there is a
windshear.

Prescribed burning can be used on tobosa stands on a five to eight year
frequency.  Bottomland sites should not be burned more frequently than
every five years while upland sites may be burned every eight years
[5,43].  For grazing purposes burn areas on a rotational basis.  One
fifth to one eighth of each tobosa pasture can be burned each year [55].

Related categories for Species: Pleuraphis mutica | Tobosa