Soil and Vegetation Changes in a
Pinyon-Juniper
Area in Central
Arizona
after Prescribed Fire
Steven T. Overby, Will H. Moir, and George T. Robertson
Abstract
Prescribed fire has been used as an inexpensive and rapid method for disposing of slash following fuelwood sales in pinyon-juniper sites. Soil heating during a fire has a direct effect on soil nutrients and microbial activity. The potential for understory cover quantity and quality, along with soil nutrient changes should be the determining factors in management decisions to use prescribed fire for slash disposal. Our investigation measured soil nitrogen and phosphorus changes, and the understory community following a prescribed fire in a pinyon-juniper site in cental Arizona.
Study Site
Hogg Pasture is located in central Arizona on the Coconino National Forest. The soils are Typic Haplustalf, fine, smectic, mesic. The site before Harvesting consisted of Juniperus osteosperma and Pinus edulis. Sampling data was collected in May 1997, almost 2 years after a prescribed fire was used to remove slach following a fuelwood sale.
| Vegetation and Ground Cover in 1997 at Hogg Pasturea | ||
| Vegetation (% cover) | Burn (n=5) | Control (n=3) |
| Perrenial grasses | 6 (2.5) | 2 (1.9) |
| Perrenial forbes | 2 (1.4) | 0.03 (0.3) |
| Annuals | 12 (10.4) | 0.03 (0.03) |
| Shrubs | 7 (2.7) | 9 (3.2) |
| Unidentified herbs | 2 (1.1) | 0.05 (0.05) |
| Species (number/transect) | 24 (1.9) | 16 (5.5) |
| Ground Surface (% cover) | ||
| Litter | 10 (3.2) | 6 (4.1) |
| Rock-gravel | 38 (7.5) | 33 (14.8) |
| Soil | 49 (10.9) | 59 (14.6) |
| Basal Area | 2.3 (0.8) | 0.7 (0.5) |
| Cryptograms | na | 0.4 (na) |
| Coarse woody debris (CWD) | 1.4 (0.9) | 1.98 (na) |
aValues in parenthesis are standard error of the mean, n=number of transects
| Soil anlaysis (0-10cm) from Hogg Pasture in Central Arizona following prescribed firea | |||||||
| Nutrient | Mean Burned canopy | Mean Unburned canopy | Mean Burned interspace | Mean Unburned interspace | |||
| % Total Organic Carbon | 2.72a | 2.61a | 1.63b | 1.09c | |||
| % Total Nitrogen | 0.39a | 0.27ab | 0.12bc | 0.07c | |||
| Mineralizable nitrogen (ug/g) | 2.354b | 28.76a | 14.77ab | 6.27b | |||
| KCl-extractable ammonia (ug/g) | 28.25a | <0.01b | 1.67b | 0.99b | |||
| KCl-extractable nitrate (ug/g) | 25.67a | 1.29b | 1.85b | 1.03b | |||
| % Total phosphorus | 0.0068a | 0.0057a | 0.0045a | 0.0045a | |||
| Extractable phosphorus (ug/g) | 15.02a | 3.06a | 3.88a | 0.96a | |||
aMeans with same letter were not significantly different using ANOVA with Tukey’s
Studentized Range test (alpha=0.05, df=16).
Studentized Range test (alpha=0.05, df=16).
| Analysis of Litter from from Hogg Pasture in Central Arizona following prescribed firea | |||||||
| Nutrient | Mean Burned canopy | Mean Unburned canopy | Mean Burned interspace | Mean Unburned interspace | |||
| Total litter biomass (g/m2) | 37.50b | 4805.00a | 102.5b | 165b | |||
| Total phosphorus (g/m2) | 0.002b | 0.27a | 0.0044b | 0.0017b | |||
| Total nitrogen (g/m2) | 0.90b | 48.80a | 0.75b | 0.60b | |||
a Means with same letter were not significantly different using ANOVA with Tukey's
Studentized Range test (alpha=0.05, df=8).
Studentized Range test (alpha=0.05, df=8).
Discussion
Higher available nitrogen and phosphorus indicates that burned sites are more fertile, but the loss of mineralizable nitrogen indicates otherwise. An increase in forage is needed to justify the expense of treatment, but unpalatable, noxious, or ephemeral plants were the main respondents.
Conclusion
The goal of this prescribed burn was to increase forage for livestock and wintering elk, and to improve the watershed condition by increasing vegetative cover. Slightly higher amounts of palatable species produced little overall increase in forage. Before fuelwood cutting, the overstory provided protection from raindrop impact, but following harvesting and prescribed fire, almost 50% of the soil was exposed. This degree of exposed soil poses a high potential for surface erosion, compounded with the loss in total and mineralizable nitrogen further degrading the soil resource.
Recommendation
When the understory community is sparse with little perennial grass cover, slash should remain on site following fuelwood cutting until establishment of a herbaceous understory.
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