Vegetation dynamics following seasonal fires in mixed mesquite/acacia savannas

  • Published source details Owens M.K., Mackley J.W. & Carroll C.J. (2002) Vegetation dynamics following seasonal fires in mixed mesquite/acacia savannas. Journal of Range Management, 55, 509-516.


Fires were a natural part of many savanna ecosystems but suppression policy has altered fire frequency and seasonality. Reintroducing fires simulating natural wildfires has been proposed as a method to reduce woody cover and encourage grassland vegetation. This study compared the effects of annual and biennial fires on the vegetation of a mixed mesquite Prosopis/acacia Acacia savanna at Lyles Experimental Ranch near the city of Uvalde, Texas (29°03’N, 95°54'W), south USA.

In a semi-arid honey mesquite P.glandulosa/mixed brush community (livestock grazing ended in 1988), an 80 ha area was divided into two for growing season (July-August) or dormant season (January-February) prescribed burning. Each 40 ha area was subdivided into three 13 ha plots, each plot randomly assigned one of three treatments applied 1991 through 1995:

1) unburned control;
2) burned every year;
3) burned every two years.
Plots were large enough for ‘normal’ fire behavior but this resulted in a non-replicated experiment. Prior to burns, shrub and tree canopy cover was measured (line intercept method) along 12, 15-m transects in each plot, and grass density in 30 (1m²) quadrats. For each grass species, the number of plants within one of three basal area cover classes (<25, 25-50, and >50 cm²) was counted. Forb cover was estimated using Daubenmire frames (20 x 50 cm) placed in each grass quadrat. Sampling was undertaken in June or July from 1990 through 1994.

Twenty-four woody shrub and tree species (cover 40 to 65%) were present within the plots; honey mesquite was dominant (17-44% cover). Although each plot had somewhat different species composition and cover, there was little change in woody vegetation in response to any treatment. Shrubs in this community typically resprout after fire. The only mortality was of some small shrubs or saplings (diameter < 3 cm), and large trees with woodrat Neotoma micropus nests at the base which allowed burning throughout the root crown.

Thirty-three grass species (2 annuals, 31 perennials) were recorded. All plots showed a trend away from short-grass species towards mid-grasses regardless of fire season or frequency. The population size structure of perennial grasses was unaffected (basal area of most plants < 25 cm²) by fire. Burning during the growing season when conditions were hotter and drier did not accelerate succession toward grass-dominated communities.
Over 100 forb species were recorded, composition was unaffected by either the season or frequency of fire, but was related to the year (probably attributable to weather variations).
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