Chapter 3 Pinyon-Juniper and Oak Woodlands Authors: Corrine Dolan and Alix Rogstad Cover photo: Corva Prescribed Burn, July 2000. Courtesy of USDA Forest Service, Kaibab National Forest Chapter 3 34 PINYON-JUNIPER AND OAK WOODLANDS Description Pinyon-juniper and oak (PJO) woodlands are found in the lower elevations that skirt the sky islands of southeastern Arizona and are widely distributed throughout the Colorado Plateau north and south of the Mogollon Rim (Figure 3.1). The PJO vegetation community is scattered in a wide belt between ponderosa pine forests and grasslands. There are approximately 15 million acres of pinyon- juniper and oak woodlands combined in Arizona, the vast majority of which are pinyon-juniper woodland (USGS 2004). PJO occurs from 4,000 to 6,500 feet. PJO woodlands receive between 10 and 24 inches of annual precipitation, making these woodlands drier than the ponderosa pine forests that border their upper elevations. Photo by Guy McPherson Some of Arizona’s fastest-growing residential communities are located in PJO woodlands. These Pinyon-juniper stand include towns such as Sierra Vista, Patagonia, Sonoita (Bahre 1991), Sedona, and Prescott. PJO woodlands are valued by humans for a variety of other reasons as well, including livestock grazing, Fire regime information is limited for PJO hiking, camping, hunting, watershed protection, woodlands because this vegetation community has and habitat for threatened and endangered wildlife been studied less than ponderosa pine or mixed (Kruse et al. 1996). conifer forests. The most current information suggests that wildland fires in PJO woodlands occurred with more variability than the neighboring ponderosa pine forests because the PJO woodlands are themselves more variable. For example, as opposed to ponderosa pine forests that have only two major overstory species, PJO woodlands have many different species of pinyon, juniper, and oak trees that make up the overstory. In addition, PJO woodlands have a wider distribution than the ponderosa pine forests in Arizona. Distribution of PJO woodlands can vary by soil type, topography, and geography. Plant species and variability woodlands and individual plant distribution create a more variable fire regime in PJO woodlands than in the forest communities described in this Photo by Guy McPherson publication. Boundary between oak woodland and grassland Chapter 3 35 Pinyon-Juniper and Oak Woodland in Arizona Figure 3.1 Pinyon-Juniper and Oak Woodlands in Arizona Chapter 3 36 Associated Species The most common species associated with pinyon-juniper and oak woodlands are listed below. Detailed PLANT FACT SHEETS with pictures of each species listed are located in Appendix C. Box 3.1 Species Commonly Associated with Pinyon-Juniper and Oak Woodlands Major Tree Species Understory Shrubs and Grasses Alligator juniper Rocky Mountain juniper (Juniperus deppeana) (Juniperus scopulorum) Big sagebrush (Artemisia tridentata) Arizona white oak Shrub live oak (Quercus arizonica) (Quercus turbinella) Blue grama (Bouteloua gracilis) Emory oak Single leaf pinyon (Quercus emoryi) (Pinus monophylla) Pinyon ricegrass (Piptochaetium fimbriatum) Mexican blue Utah juniper (Quercus oblongifolia) (Juniperus osteosperma) Sideoats grama (Bouteloua curtipendula) Pinyon pine Three awns (Pinus edulis) (Aristida spp.) Chapter 3 37 Historic Fire Regime Compared to ponderosa pine and mixed conifer forests, there is relatively little known about historic fire regimes in PJO woodlands, particularly in those areas dominated by evergreen oak species. Tree species in these woodlands, especially oaks, do not have fire scars or annual rings as distinct as other trees. This factor complicates the accurate reconstruction of fire history. Although subjective at this point, fire ecologists have been able to piece together some aspects of the historic fire regime in PJO woodlands. Before major European-American settlement, pinyon-juniper and oak woodlands are thought to have had a mixed-severity fire regime. In other words, low, moderate and high-intensity, stand- Photo by Guy McPherson replacing fires occurred (Swetnam et al. 1992). Historic woodlands were open and grassy Fires may have burned for months at a time and burned thousands of acres (Swetnam 1988). On wetter sites, fine herbaceous fuels carried low- intensity fires while drier sites saw more stand- replacing fires carried by shrubby understories. The majority of these fires occurred in the summer (between May and late July) when thunderstorms Historically, pinyon-juniper and oak brought lightning ignitions. Historically, wildland woodlands developed with a mixed- fires are thought to have occurred every 5 to 40 severity fire regime. In other words, years on most sites dominated by PJO woodlands while others went a century or more without fire low, moderate, and high-intensity, (Zouhar 2001). The key point to remember is that stand-replacing fires all occurred at wildland fires in PJO woodlands occurred with more variability than in any of the other vegetation varying intervals. communities described in this publication. Current Fire Regime The current fire regime in PJO woodlands is vastly different from what is thought to have occurred Improper grazing management and full historically. Improper livestock grazing management and fire suppression have altered the naturally fire suppression have almost excluded complex PJO stands. Currently, stands are mostly wildfire in PJO woodlands. When uniform with respect to plant species, age, and wildfires do occur, they are usually size. Improper livestock grazing management in conjunction with full fire suppression management high-intensity and stand-replacing. strategies have decreased the amount of grasses that would naturally be present on a given site. As a consequence, the growth of woody species has Chapter 3 38 been promoted. At present, the extremely dense natural mixed-severity regime that was present on woodlands are hard to ignite and do not carry fire the landscape before major European settlement as well without the grassy understory. This leads has all but disappeared (Allen 1996; Heyerdahl and to infrequent wildland fires occurring within this Alvarado 2003). vegetation community. Periods of dry and hot weather, however, can create conditions in which There are some special cases where fire might behave the abundant woody fuel does ignite. Once it does, differently or more unpredictably and are therefore the exceptionally crowded woodlands can produce of particular concern to fire managers. See Box 3.2 to a high-severity, stand-replacing wildland fire. The learn more. Box 3.2 Special Cases SLOPE Fire spreads more quickly on slopes than on flat terrain. A fire moving uphill preheats the fuels above it, moving the fire quickly up the slope. In addition, burning embers on slopes can roll down slopes and ignite fires in other areas. RIDGE TOPS AND SOUTH FACING SLOPES Oak woodlands on ridge tops and south-facing slopes are more open and have more grass in the understory and in openings than those found on slopes facing other direc- tions. Not only are these areas subject to rapid fire spread due to the slope, but there is also more fine fuel available to burn and carry the fire on south-facing slopes. RIPARIAN AREAS Riparian areas in PJO include more broad leaf tree species and different understory shrubs than the surrounding woodland. Riparian areas typically have denser vegeta- tion that, during dry years when fuel moistures are low, could increase fire intensities. On the other hand, during wet periods, riparian areas retain more moisture than upland areas and act as fuelbreaks when fire moves through an area. Photos by Guy McPherson Chapter 3 39 Fire Effects establishment is generally delayed (sometimes 20-30 years) until a shrub layer becomes reestablished (Minnich 1999). Consequently, juniper species will dominate over pinyon species on a recently burned site. Photo by Steven J. Baskauf Mature Utah Juniper (Juniperus osteosperma) Plants Mature pinyon trees (Pinus spp.) are susceptible to College of California Photo courtesy of Saint Mary’s fire because they are short with large amounts of Young Pinyon Pine (Pinus edulis) foliage and do not self-prune their dead branches. Most, however, can survive low-intensity surface fires. Juniper trees (Juniperus spp.) are also susceptible to surface fires, but less so than pinyon species. Both pinyon and juniper trees over four feet tall are more fire resistant than shorter trees because the foliage is higher and the bark tends to be thicker (Scher 2002). Smaller trees are more vulnerable and can be killed by even low-intensity fires. Low-severity fires in mature pinyon-juniper stands will usually remove trees in the understory and a few overstory trees, while moderate-severity fires remove more of the overstory (Zouhar 2001). Alligator juniper (Juniperus deppeana) is the exception because it has the ability to resprout and can come back rapidly after a fire. Pinyon trees reestablish on a burned site by Photo by Cori Dolan seed. Seedlings require nurse plants (primarily Gambel Oak leaves (Quercus gambelii) sagebrush) for establishment. Therefore, seedling Chapter 3 40 The aboveground portion of oak trees (Quercus spp.) is usually killed by fire, but that does not necessarily mean the whole tree dies. Oak trees can resprout vigorously from the stump (Pavek 1993). Large trees can survive fires of low severity, although the acorns are frequently killed unless covered by a layer of soil (Bock and Bock 1990;