Petran Subalpine Conifer Forests and Alpine Tundra

Petran subalpine conifer forests are at the southern end of their range in Arizona. Typically above 8,500 ft. above sea level, these forests are variable and complex. They occur at the highest elevations of plateaus and mountain ranges in northern Arizona (e.g., Kaibab Plateau, San Francisco Mountain, White Mountains) as well as in the southern reaches of the state (e.g., Pinaleño, Chiricahua and Santa Catalina mountains).

A number of coniferous and deciduous species characterize these forests. At higher elevations and on north facing slopes the dominant conifers are usually Engelmann spruce and subalpine fir. On drier and more exposed sites, limber and bristle-cone pines may dominate. At lower elevations these forests are composed of a broad mix of coniferous species including blue spruce, white fir, and Douglas fir. This elevational band is often referred to as the cool/moist mixed conifer forest (Margolis et al. 2013). Quaking aspen is the dominant deciduous species and is found intermixed with the conifers, however pure aspen stands may be found post-fire. Dense overstories limit or prevent growth of herbaceous vegetation except in aspen stands and along forest edges where bunchgrasses are common. Otherwise, understory vegetation is limited to mosses, lichens, and prostrate junipers (Brown et al. 2020; Margolis et al. 2013).

Historically, Petran subalpine conifer forests were subject to a high-severity fire regime where fires were infrequent and stand-replacing. Patch sizes varied from as small as 74 acres to greater than 2,400 acres. After a high-severity fire, these forests followed a fairly predictable successional pathway beginning with aspen regeneration, followed by shade-tolerant conifer regeneration, and eventually forming an aspen/mixed conifer forest. Due to the patchiness of high-severity fires, irregular forest structures were common across stands and at landscape scales. Stands were multi-storied due to the shade tolerance of mixed-conifer tree species and the infrequency of high-severity fires that did not have the effect of “thinning from below.” While high-severity fire patches from recent fires may be larger than historical ones, the fire regime and resulting forest structures have not changed as dramatically in these forests as they have in drier forest communities at lower elevations (e.g., ponderosa pine-dominated forests). Petran subalpine conifer forests may be within their natural range of variability and do not necessarily indicate a need for restoration (Margolis et al. 2011; Margolis et al. 2013).

However, due to their limited distribution in Arizona, Petran subalpine conifer forests have been disproportionately affected by a small number of development projects such as ski runs, communication towers, and observatories (Patten and Stromberg 1995; Dahms and Geils 1997). They have also been degraded by a century of livestock grazing and fire suppression and have experienced significant tree mortality due to drought and insect infestation. In 2019, beetle-caused tree mortality in Douglas fir, white fir, and subalpine fir increased in Arizona and affected about 35,000 acres, primarily on the Apache-Sitgreaves and Coconino national forests and on the Navajo Nation. However, spruce beetle mortality has been consistently low in Arizona (120 acres in 2019) with most of the activity observed on White Mountain Apache lands (USFS 2020). In addition, invasive pests affect Engelmann and blue spruce (spruce aphid) and more recently aspen (oystershell scale). Recent monitoring indicates the emergent oystershell scale is widespread in northern Arizona and is contributing to aspen mortality on the Coconino, Kaibab, Apache-Sitgreaves, and Prescott national forests. A new effort to evaluate aspen decline was initiated in 2019 (USFS 2020). 

Alpine tundra is found above 11,500 ft above sea level. Restricted to a few thousand acres above timberline on the highest peaks of San Francisco Mountain and on Mount Baldy, it is one of the rarest biotic communities in Arizona. This community is extremely isolated from any other alpine tundra, with the closest being in central New Mexico. Extreme cold temperatures exclude trees. Dominant plants are ground-hugging forbs and grasses (Brown 1990). Some recreational activity and past sheep grazing have affected this community. However, the largest threat is climate change which could lead to the elimination of alpine tundra habitats in Arizona.

Significant Habitat Features

The following describes habitat features or microhabitats in this habitat type that are of particular importance to certain species:

• Sinkholes are important as they can provide important sources of water and feeding and nesting habitat for many species of mammals, birds, reptiles, and amphibians in an otherwise xeric environment.

• Wet meadows and vernal pools typically have surface water only during the wetter periods of the year and do not usually support fish unless they are connected to perennial water. These habitats are important for amphibians such as leopard frogs and tiger salamanders, because their early life stages can thrive free of fish and other predators.

• Talus slopes and boulder piles along hillsides and canyon drainages provide essential sheltering habitat for native land snails like talussnails and mountainsnails during hot, dry periods of the year. These mollusks aestivate for months in the cool and damp interstitial spaces in talus, under boulders, and deep rock crevices.

• Aspen stands provide important habitats for many species at these high altitudes, including mule deer and red-naped sapsucker. Many aspen stands are in decline thanks to fire-suppression, conifer encroachment, and over-browsing by elk.

• Maple draws provide important breeding and foraging habitats for many neotropical migrant bird species, such as red-faced warbler and hermit thrush.