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Wildlife, Animals, and Plants
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FIRE EFFECTS
SPECIES: Juniperus osteosperma
| Utah Juniper
Utah juniper is usually killed by fire [10,45], especially when shorter than 3 to 4 feet (0.9-1.2 m) [35,125]. Larger trees, above 4 feet (1.2 m) tall, are capable of surviving surface fires [24,114]. Mortality occurs when 60% or more of the crown is scorched [69,114]. Surface fires will thin a juniper stand to large trees and trees growing on rocks and in other refugia [35].
Tausch and West [117] studied fire scars of Utah junipers in a stand in southwestern Utah. Thirty-eight percent of the Utah junipers sampled were older than an approximately 146 year-old fire, and 4 trees had fire scars from a second fire 317 years before sampling.
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Barney and Frischknecht [10] evaluated 28 different burns in west-central Utah to assess vegetation changes following fire in pinyon-juniper communities. The effects of fire on Utah juniper over time were as follows:
| Approximate age of burn (yrs) |
Crown cover (%) |
Basal area (ft2/acre) |
| 3 |
-- |
-- |
| 6 |
-- |
-- |
| 11 |
trace |
trace |
| 22 |
0.5 |
0.6 |
| 36 |
1.2 |
1.9 |
| 46 |
1.3 |
1.6 |
| 71 |
16.0 |
21.5 |
| 86 |
17.2 |
33.4 |
| 100+ |
31.8 |
142.6 |
Trees that established on burned sites immediately after the fire were adjacent to or underneath burned trees, suggesting they had grown from residual seed on the site. Utah junipers dominated these sites 46 to 71 years following the burn [10].
Rate of re-establishment of juniper depends on the age of the burned stand. More mature trees produce more seed, thereby increasing the rate at which a new stand is established [10,35]. Large rodent populations and unburned seeds or unburned patches within a burned stand will speed up re-establishment of Utah junipers [125].
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Juniper habitats are often burned to increase herbaceous cover for grazing or wildlife. Junipers are often difficult to ignite, and burning has been most successful when the trees themselves were lit and managers did not depend on understory fire to carry into the crowns. Often the conditions necessary to get a fire to burn in a dense juniper stand--hot, dry, windy weather--are too dangerous to allow burning [26,114].
One technique for determining whether a prescribed fire in a juniper stand is likely to succeed, at temperatures below 75° Fahrenheit (24°C) and windspeeds above 5 miles/h, was published in 1979. Add together the maximum windspeed (miles/h), air temperature (°F), and percent vegetation cover. When the total of those three numbers exceeds 110, a burn is likely to succeed, with some retorching and some mosaic burning; when the number exceeds 130, conditions are too hazardous for burning. Ideal conditions for a carrying (self-sustaining) fire exist at a score of 126-130 [26].
Vegetative recovery following a fire in a mature juniper site may be slow, since the prefire herbaceous cover is often sparse [45,46,48]. During this intervening period, soil erosion may be a problem [29,57]. However, Roundy and others [101]studied erosion and infiltration rates following prescribed burns in Nevada and concluded that erosion rates would increase on interspaces, but that on coppice dunes (areas around vegetation with higher infiltration rates) erosion is not a problem.
Prediction of postfire succession is affected by prefire vegetation and its fire survivability, soil seedbank, immigrating propagules, and postfire precipitation [46,48,49,51]. Succession following fire in a climax pinyon-juniper woodland often proceeds as follows: skeleton forest and bare soil; annual stage (2-3 years); annual-perennial forb stage (3-4 years); perennial forb-grass-half-shrub phase (4-6 years); shrub stage or perennial grass stage; eventual pinyon-juniper climax [4,10,41]. However, Everett and Ward [51] studied 6 burned sites to determine successional pathways, and they concluded that succession starts from multiple points along a hypothetical pathway, and that early postfire communities vary considerably.
In the years following a fire, burned pinyon-juniper and juniper sites are preferred by wildlife species such as pronghorn, elk, bighorn sheep, and mule deer, due to increased understory forage [106]. McCulloch [85] evaluated the effects of wildfire and prescribed burns on mule deer use of pinyon-juniper woodlands. During a mild winter, there was no significant difference between use of the burned and unburned plots, but during the following winter, a harsh one, mule deer use was significantly higher (p<0.10) on the burned than unburned sites.
Severe fires that result in soil temperatures above 122° Fahrenheit (50oC) reduce the vesicular-arbuscular mycorrhizae propagules in the soil and may restrict the ability of juniper to recolonize the site [77,76].
Utah junipers, like many other arid and semi-arid shrubs and trees, concentrate soil nutrients underneath their canopies by withdrawing them from a large area around the tree through extensive roots. Burning may result in a volatilization loss of nitrogen from a nutrient poor site. Natural nitrogen replenishment rates on these sites are low [119].
Related categories for
SPECIES: Juniperus osteosperma
| Utah Juniper
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