FIRE EFFECTS
SPECIES: Prosopis velutina | Velvet Mesquite 



IMMEDIATE FIRE EFFECT ON PLANT : 
Fire mortality of velvet mesquite is generally low.  Following most
fires, plants sustain little or no damage, have their aboveground
foliage partially destroyed, or are completely top-killed.  Only a small
percentage are killed.


DISCUSSION AND QUALIFICATION OF FIRE EFFECT : 
Based on observations following prescribed fires and wildfires, top-kill
and mortality appear to be most influenced by the size of the velvet
mesquite plant and the intensity of the fire.  

Size of the plant:  Percent top-kill and mortality of velvet mesquite
following fire are inversely related to plant height and basal stem
diameter.  At the Santa Rita Experimental Range in southern Arizona,
fall burning on areas with 303 pounds per acre (340 kg/ha) of herbaceous
fuel and about two burroweed plants per meter square resulted in greater
than 80 percent top-kill of velvet mesquite plants up to 2 inches (5 cm)
in basal stem diameter.  The percentage of plants top-killed, on a size
class basis, gradually decreased to near zero as basal stem diameter
increased to 10 inches (25 cm) [79].  Evaluation of several other
prescribed fires at Santa Rita showed that young velvet mesquites less
than 0.5 inch (1.25 cm) in basal stem diameter suffered about 50 percent
mortality, but only 8 to 15 percent of plants greater than 0.5 inch in
basal stem diameter died [51].  In southern Arizona, an accidental June
fire in Lehman lovegrass (Eragrostis lehmanniana) resulted in 30 percent
mortality of velvet mesquite less than 0.5 inch in basal stem diameter
and 18 percent mortality of plants with basal stem diameters from 2 to 6
inches (5-15 cm) [20].  At Santa Rita, 4- to 6-inch-tall (10-15.5 cm)
seedlings were burned in a "good stand" of grass.  Of the 8-month-old
seedlings burned in March, 67 percent were killed and 23 percent were
top-killed only.  Of the 12-month-old seedlings burned in June, 65
percent were killed and 25 percent were top-killed only [19].

The effects of fire on velvet mesquite 10 months following a June
prescribed fire (mesquites in bloom at time of burn) in an area with 40
percent grass cover dominated by curly mesquite (Hilaria belangerii) is
summarized below [8]:

                             basal stem diameter (inches)
fire effects         0-.5    .5-1    1-2    2-3    3-4    >4    total
                                  (number of plants)
no crown damage       1        4      5      7      8     12     37
partial top-kill     16       14      8      1      0      1     40
total top-kill       73       11      1      0      0      0     85
dead                  8        1      0      0      0      0      9
             total   98       30     14      8      8     13    171

Fire intensity:  In some locations, velvet mesquite has displaced
grasses to the extent that the herbaceous fuels are insufficient to
carry anything but a cool or spotty fire [104].  Low intensity fires
occur under these circumstances and usually inflict no damage or only
partially kill the aboveground crown.  High intensity fires may kill
many young plants, but mature plants are often only top-killed and then
resprout.  Fire intensity is influenced most by the amount of herbaceous
fuel and the time of year burning takes place.  Plants seem to be more
susceptible to burning during late spring and early summer, which is the
hottest and driest time of the year in Arizona, than during other times
of the year [22].  At the Santa Rita Experimental Range, about 29
percent of velvet mesquites were killed on areas burned in June,
compared to about 10 and 4 percent on areas burned in November and
February [51].  Following a June fire at Santa Rita, velvet mesquite
suffered 25 percent mortality in an area with 4,480 pounds per acre of
herbaceous fuel (4,973 kg/ha) dominated by Lehman lovegrass, but on
areas with 2,200 pounds per acre (2,465 kg/ha) of herbaceous fuel
dominated by black gramma (Bouteloua eriopoda), velvet mesquite suffered
only 8 percent mortality [20].

The fire effects on velvet mesquite on the Santa Rita Experimental Range
as influenced by plant size and season of burn are summarized below
[51].  Since herbaceous fuels were lacking, barley hay was added to
increase the fine fuel to 800 pounds per acre (896 kg/ha).

basal stem diameter                       Month
and fire effect
                                Feb     June    Nov     Oct 
up to 0.5 inch                
 top alive                       3%      1%     13%     ---
 top-kill                       74%     39%     66%     ---
 dead                           23%     60%     21%     ---

0.5 to 1.0 inch            
 top alive                      35%      5%     12%      5%
 top-kill                       50%     62%     73%     90%
 dead                           15%     33%     15%      5%

1.0 to 2.0 inches
 top alive                      50%      6%     41%     28%
 top-kill                       50%     74%     52%     62%
 dead                            0%     20%      7%     10%

2.0 to 5.0 inches
 top alive                      78%     20%     63%     62%
 top-kill                       22%     65%     32%     31%
 dead                            0%     15%      5%      7%

over 5.0 inches 
 top alive                      82%     40%     82%     90%
 top-kill                       18%     49%     18%     10%
 dead                            0%     11%      0%      0%


PLANT RESPONSE TO FIRE : 
The response of velvet mesquite following fire depends on the amount of
damage the fire inflicted on the plant.  Plants may initiate new growth
from either aerial crown or underground stem buds.  Following
low-intensity fires which only partially top-kill plants, velvet
mesquite often sprouts from axillary buds on the branches of the crown.
Following severe, top-killing fires which char or completely consume the
crown, plants survive by producing numerous basal stem sprouts [20,126].
Some plants which are lightly or moderately damaged survive by both
refoliating undamaged portions of the crown and by producing basal
sprouts.  In general, smaller plants receive more crown damage and thus
tend to recover by basal sprouting, while larger plants tend to produce
crown sprouts.  Cable [20] observed that basal sprouting predominated on
trees with a stem diameter less than 2 inches (5 cm), while crown
sprouting predominated on plants with basal stem diameters less than 2
inches.

Following a fall burn at Santa Rita, heights velvet mesquite resprouts
were 105 percent of preburn levels in 4 years.  Six years after burning,
some resprouts flowered and set seed [79].


DISCUSSION AND QUALIFICATION OF PLANT RESPONSE : 
NO-ENTRY


FIRE MANAGEMENT CONSIDERATIONS : 
Prescribed burning has not been effective in controlling velvet mesquite
because of the species' fire survival strategies.  On grasslands in good
condition with a low density of velvet mesquite, repeated prescribed
fires have the potential to kill small plants under 0.5 inch (1.25 cm)
in basal stem diameter and keep others low in stature and from producing
seed [22].  However, on some sites velvet mesquite has reduced the
native grass cover to the extent that there is now insufficient fuel to
carry anything more than a "spotty" or "cool fire" [42].  In general,
fire will not carry in Southwestern grasslands having less than 600
pounds per acre of herbaceous fuel (654 kg/ha) unless there is a good
stand of burroweed present [127].  When there is less than 892 pounds
per acre (1,000 kg/ha), a windspeed of 8 miles per hour (12.8 km/hr) is
needed to carry the fire [127].

Related categories for Species: Prosopis velutina | Velvet Mesquite