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Research Project Summary: Effects of prescribed fires in semidesert plant communities in southeastern Arizona

• RESEARCH PROJECT SUMMARY CITATION
• SPECIES INCLUDED IN THE SUMMARY
• STUDY LOCATION
• SITE DESCRIPTION
• PREFIRE PLANT COMMUNITY
• PLANT PHENOLOGY
• FIRE SEASON/SEVERITY CLASSIFICATION
• FIRE DESCRIPTION
• FIRE EFFECTS ON PLANT COMMUNITY
• FIRE MANAGEMENT IMPLICATIONS
• REFERENCES

1974—Wildfires in February and April:
Bock, Jane H.; Bock, Carl E.; McKnight, J. Robert. 1976. A study of the effects of grassland fires at the research ranch in southeastern Arizona. Arizona Academy of Science. 11(3): 49-57 [9].

1975-1976—Wildfires in mid-May and mid-June, 1975 , and February 1976 :
Bock, Carl E.; Bock, Jane H. 1978. Response of birds, small mammals, and vegetation to burning sacaton grasslands in southeastern Arizona. Journal of Range Management. 31(4): 296-300 [1].

1984—Prescribed fires conducted in July:
Bock, Jane H.; Bock, Carl E. 1987. Fire effects following prescribed burning in two desert ecosystems. Final Report: Cooperative Agreement No. 28-03-278. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 20 p. [7].

Bock, Jane H.; Bock, Carl E. 1992. Short-term reduction in plant densities following prescribed fire in an ungrazed semidesert shrub-grassland. The Southwestern Naturalist. 37(1): 49-53 [8].

1987—Wildfire in July:
Bock, Carl E.; Bock, Jane H. 1991. Response of grasshoppers (Orthoptera: Acrididae) to wildfire in a southeastern Arizona grassland. The American Midland Naturalist. 125: 162-167 [2].

Bock, Carl E.; Bock, Jane H. 1992. Response of birds to wildfire in native versus exotic Arizona grassland. The Southwestern Naturalist. 37(1): 73-81 [3].

Bock, Carl E.; Bock, Jane H. 1997. Shrub densities in relation to fire, livestock grazing, and precipitation in an Arizona desert grassland. The Southwestern Naturalist. 42(2): 188-193 [4].

Bock, J. H.; Bock, C. E. 1992. Vegetation responses to wildfire in native versus exotic Arizona grassland. Journal of Vegetation Science. 3: 439-446 [6].

Based on the description provided by Bock and Bock [2,3,7,8], vegetation on the study sites could be classified in the plant communities and probably historically experienced the fire regimes described in Table 5.

Fire behavior: Fire behavior is not described for the 1974, 1975, or 1976 wildfires.

The 1984 prescribed fires occurred when weather was hot and dry (Table 6) [7].

The 1987 wildfire occurred during hot, dry, very windy weather. Maximum temperature on the 2 days of the fire was 29 °C, and minimum relative humidity was 21%. Winds averaged about 15 mph, gusting to 30 mph. Most aboveground vegetation was consumed, with the exception of the basal stems of some shrubs and the trunks and large branches of honey mesquite [2,3,6].

1974 February and April wildfires: The February wildfire occurred in savanna, and the April fire occurred in grassland. In the first year after the fires, grass cover in both savanna and grassland habitat was significantly less on burned than unburned plots, and forb cover was significantly greater (P<0.01). A calculation of similarity between burned and unburned plots indicated that, in postfire year 1, burned plots were approximately 80% similar to control plots; by postfire year 2, the similarity had increased to approximately 90% [9].

Some species of small mammals decreased during the time of postfire observations (6-19 months after the February fire, 3-16 months after the April fire); no small mammal populations increased significantly in burned areas during the study. Western white-throated woodrats were significantly (P<0.005) less abundant on burned than control plots (burned in the April fire), likely because their ground nests were destroyed by fire. Hispid cotton rats were significantly (P<0.005) reduced on February-burned plots, probably because of reduced grass cover. Pocket mice were usually found in "relatively open areas", but they were sparse on the February burn (P<0.005), which may have been too open to be suitable habitat. Other mammals observed on both burned and unburned plots, with no significant differences, included spotted ground squirrels, western harvest mice, North American deermice and white-footed mice (combined in this report), northern pygmy mice, and southern grasshopper mice [9].

Birds were more abundant in burned than unburned habitat during the first 16 to 19 months after these wildfires. In the savanna (February burn), this was due mainly to increases in mourning doves and chipping sparrows. These 2 species were also abundant after fire in the grassland (April burn), as were some flycatchers and seed-eating species. Only the grasshopper sparrow, which relied upon shrubs for singing perches, was significantly reduced on both burned areas [9] (Table 7).

1975 mid-May and mid-June wildfires, 1976 February wildfires: In the first 2 years after these summer wildfires in big sacaton grassland, forbs were significantly more common on burned plots than on unburned plots. Big sacaton cover was significantly reduced in the 1st postfire growing season but recovered to prefire levels in the 2nd season. Other grasses, especially vine-mesquite, were significantly more abundant 1 year after the winter burn than in any other plots. Cover of dead vegetation and bare ground was significantly greater 1 year after the summer fires than after the winter fires or on control plots [1]. (P≤0.005 for all of these significant results.)

The summer-burned plots had significantly (P<0.05) more total birds than the control plots. Eleven species were found only in burned plots, whereas 4 species were found only in control plots (Table 8). The first year after fire, summer-burned plots supported significantly (P<0.05) more raptors than other habitats, probably because visibility of prey had increased. Several species of seed-eating songbirds, doves, and quail had more birds in summer-burned plots (both postfire years) than in control plots, probably because the abundance of seeds and annual forbs was greater in summer-burned than other plots [1].

Fires reduced small mammal populations on big sacaton grasslands, more on summer-burned than winter-burned plots. Hispid cotton rats, which feed mainly on green vegetation, declined after fire, whereas hispid pocket mice and kangaroo rats, which are seed-eating species, increased. Numbers of North American deermice, white-footed mice, and western harvest mice were unaffected by the fires [1] (Table 9).

1984, 25 May and 12 June prescribed fires: These prescribed fires were conducted in semidesert grassland with some shrub cover, and in woodland with Emory oak and Arizona oak in the overstory and a grass-dominated understory with some shrubs. Bare ground was more prevalent in the 1st year after fire on both sites (Table 10). The difference was smaller, but still significant, in the 2nd postfire year [7].

The 1984 prescribed burns reduced total aboveground herbaceous biomass on both grassland and woodland sites. On the grassland site, biomass on burned plots was reduced by more than 60% in postfire year 1 and recovered to about 50% of the original herbage in postfire year 2. Not all of this change was attributable to fire, since biomass on control plots declined about 30% in postfire year 1 and was only 90% of the prefire level in postfire year 2. On the woodland site, biomass on burned plots declined by about 50% in postfire year 1 but exceeded that on controls in postfire year 2 [7,8] (Table 11).

Fire effects on grasses were mostly limited to the first year after fire. In both grassland and woodland, grass density was significantly reduced relative to density on unburned control plots in the 1st postfire year. In the 2nd postfire year, grass density on burned plots was significantly greater than in the previous year and similar to that on control plots (Table 1 and Table 3). By the 2nd postfire year, the only grasses with significantly lower density on burned than control plots were threeawn species on the grassland site; the only grasses with significantly higher density on burned plots were sprucetop grama on the grassland site and curly-mesquite on the woodland site [7,8].

Fire effects on forbs, like those on grasses, were mostly limited to the 1st postfire year. On the grassland site, forb density on burned plots was significantly reduced relative to that on control plots in postfire year 1 and similar to that on control plots in postfire year 2. On woodland plots the effect was opposite in the first year: Forb density on burned plots was significantly greater than that on control plots in postfire year 1 and similar to that on control plots in postfire year 2 (Table 1 and Table 3). By the 2nd postfire year, the only forb with significantly higher density on burned plots was toothleaf goldeneye on the woodland site; no forbs had significantly lower density on burned than control plots [7,8].

Fire effects on woody species were short lived. On grassland sites, density of catclaw mimosa and velvetpod mimosa was significantly less on burned than control plots in postfire year 1. Density increased in postfire year 2, though not to the level on control plots. Density of yerba de pasmo showed little response to fire (Table 2). On woodland sites, density of shrubs and trees showed no significant responses to fire (Table 4). On both grassland and woodland sites, heights of all 3 shrub species were significantly less on burned than control plots in postfire year 1; shrubs were taller in postfire year 2, although only yerba de pasmo recovered to prefire height [7].

1987 16-17 July wildfire: This summer wildfire caused extensive but largely ephemeral alterations in grassland vegetation on stands dominated by native grasses and on stands dominated by weeping lovegrass, a nonnative species. Litter on burned sites, both native and exotic, was significantly (P<0.001) less than on unburned sites from postfire season 1 through postfire season 4, the last year reported [3]. Wildfire significantly (P<0.0001) reduced grass cover for the 1st postfire year on both native- and nonnative-dominated sites. By the 3rd postfire year, burned and unburned plots had similar grass cover [6]. Fire did not alter the proportion of nonnative grass cover on the nonnative-dominated site, which averaged about 90% [3]. Forb cover increased significantly (P≤0.002) on burned plots in the 1st postfire year, after which it was similar on burned and unburned plots. Table 12 describes the responses of individual herb species to fire [6].

Shrub cover was reduced significantly (P<0.001) on burned sites dominated by native grasses in the 1st postfire growing season [3]. Burroweed density declined from approximately 60 stems/200 m² to near zero in postfire year 1; it had recovered to some extent by postfire year 8, but not to prefire levels. to prefire density within 1 year [4].

Honey mesquite trees were top-killed by fire in both native and nonnative grassland. By postfire year 3, 94% had sprouted from the trunk and/or root crown [6].

Avian responses to this summer wildfire [3] reflected alterations in cover and resources. Many birds that winter in the grasslands of southeastern Arizona depend upon seed crops. Burning probably increased seed production by both native and nonnative grasses, as occurred after the 1974 wildfires described above [9]. Fire-caused reductions of litter and grass cover made the abundant seed easily available on a bare soil surface for 2 postfire years, and bird populations increased "dramatically" in both native and nonnative sites during these years [3]. Correlations suggested that species responding positively to the burn did not require grass and litter cover, whereas species responding negatively to the burn may have required shrub cover (Table 13). Seed crops depend upon adequate precipitation, and bird populations in fall of 1989, the only year of this study with rainfall substantially below average, were lower than in fall of any other year.

Grasshopper densities were reduced more than 60% on burned plots (both native and exotic stands), but responses of particular species varied depending on mobility and diet requirements: In the first 2 weeks after fire, charred grasshopper remains were abundant and obvious on burned plots; particularly conspicuous were dead Dactylotum variegatum, a large flightless grasshopper. This species was absent from the burned plots during 1987 and occurred in low numbers in the following 2 years. Most grass feeders were sparse in the 1st postfire year, including the Gompocerinae and Phoetaliotes nebrascensis. Melanoplus gladstoni, a forb feeder, increased in the 1st postfire year and then declined. Trimerotropis pallidipennis and other band-winged species (Oediponidae), which are usually limited to areas with substantial bare ground, outnumbered other subfamilies in the 1st month after fire; density remained relatively high 1 year later (Table 14).

Fire frequency: After observing that most species recovered to prefire or control levels the 2nd year after spring wildfires, the authors speculate that burning at approximately 5-year intervals may have few negative effects on native flora and fauna [9].

Nonnative grasses: Fire "offers little hope" as a tool for reducing South African lovegrasses, such as weeping lovegrass, from semidesert grasslands. These species are not reduced by burning. The studies reported here did not indicate that these grasses increased substantially after fire in native-dominated grasslands [3,6]. A 2008 review by Rice and others [13] reports increases in lovegrass cover and density in semidesert and desert grasslands after fire.

Season of burning: Summer burns are likely to be more severe than winter burns and thus may have more "dramatic" effects on vegetation and wildlife [1]. This variation could be partly due to plant phenological stage. The first year after the 1984 prescribed burns, forbs decreased on in oak woodland and increased in grasslands. The woodland burn occurred 19 days prior to the grassland burn, so the grassland plots may have reached a more vulnerable phenological stage by the later burning date [7]. The research summarized here was conducted in ungrazed grasslands. Bock and Bock [1] note that grazing after fire could result in reduction of plant cover and severe erosion.

Animals: Animal responses to fire are closely related to habitat change. Early-summer fire in grassland can increase native forbs and thus benefit many bird species, especially raptors, but a mosaic of different-aged stands is needed to support diverse wildlife [1].

Responses of insects to fire are likely to be specific to the taxa present and their requirements for food and shelter. Insect density and species composition after fire are also likely to be affected by insect populations in the surrounding area that are in a mobile life stage [2].

Fire behavior measurements: Burning conditions and fire behavior were quantified for the 1984 prescribed fire, meeting the 2nd objective of this body of research (see Fire Description).

REFERENCES:

1. Bock, Carl E.; Bock, Jane H. 1978. Response of birds, small mammals, and vegetation to burning sacaton grasslands in southeastern Arizona. Journal of Range Management. 31(4): 296-300. [3075]

2. Bock, Carl E.; Bock, Jane H. 1991. Response of grasshoppers (Orthoptera: Acrididae) to wildfire in a southeastern Arizona grassland. The American Midland Naturalist. 125: 162-167. [15598]

3. Bock, Carl E.; Bock, Jane H. 1992. Response of birds to wildfire in native versus exotic Arizona grassland. The Southwestern Naturalist. 37(1): 73-81. [18594]

4. Bock, Carl E.; Bock, Jane H. 1997. Shrub densities in relation to fire, livestock grazing, and precipitation in an Arizona desert grassland. The Southwestern Naturalist. 42(2): 188-193. [27525]

5. Bock, Carl E.; Bock, Jane H.; Jepson, Karen L.; Ortega, Joseph C. 1986. Ecological effects of planting African lovegrasses in Arizona. National Geographic Research. 2(4): 456-463. [48085]

6. Bock, J. H.; Bock, C. E. 1992. Vegetation responses to wildfire in native versus exotic Arizona grassland. Journal of Vegetation Science. 3: 439-446. [20082]

7. Bock, Jane H.; Bock, Carl E. 1987. Fire effects following prescribed burning in two desert ecosystems. Final Report: Cooperative Agreement No. 28-03-278. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 20 p. [12321]

8. Bock, Jane H.; Bock, Carl E. 1992. Short-term reduction in plant densities following prescribed fire in an ungrazed semidesert shrub-grassland. The Southwestern Naturalist. 37(1): 49-53. [18651]

9. Bock, Jane H.; Bock, Carl E.; McKnight, J. Robert. 1976. A study of the effects of grassland fires at the research ranch in southeastern Arizona. Arizona Academy of Science. 11(3): 49-57. [4537]

10. Hann, Wendel; Havlina, Doug; Shlisky, Ayn; [and others]. 2005. Interagency fire regime condition class guidebook. Version 1.2, [Online]. In: Interagency fire regime condition class website. U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior; The Nature Conservancy; Systems for Environmental Management (Producer). Variously paginated [+ appendices]. Available: http://www.frcc.gov/docs/1.2.2.2/Complete_Guidebook_V1.2.pdf [2007, May 23]. [66734]

11. LANDFIRE Rapid Assessment. 2005. Reference condition modeling manual (Version 2.1), [Online]. In: LANDFIRE. Cooperative Agreement 04-CA-11132543-189. Boulder, CO: The Nature Conservancy; U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior (Producers). 72 p. Available: http://www.landfire.gov/downloadfile.php?file=RA_Modeling_Manual_v2_1.pdf [2007, May 24]. [66741]

12. LANDFIRE Rapid Assessment. 2007. Rapid assessment reference condition models, [Online]. In: LANDFIRE. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Lab; U.S. Geological Survey; The Nature Conservancy (Producers). Available: http://www.landfire.gov/models_EW.php [2008, April 18] [66533]

13. Rice, Peter M.; McPherson, Guy R.; Rew, Lisa J. 2008. Fire and nonnative invasive plants in the Interior West bioregion. In: Zouhar, Kristin; Smith, Jane Kapler; Sutherland, Steve; Brooks, Matthew L., eds. Wildland fire in ecosystems: fire and nonnative invasive plants. Gen. Tech. Rep. RMRS-GTR-42-vol. 6. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 141-173. [70332]