Climate, soil, topography, natural disturbances and vegetation dynamics all help shape the montane forest ecosystems of the American Southwest.
The climate of the montane forests of the American Southwest is characterized by cold winters and warm to cool summers. Precipitation is bimodal with peaks in winter and mid-summer. Winter precipitation falls as snow, and January is usually the coldest, snowiest month. A substantial snowpack develops, especially in the subalpine zone, and lasts through March and often into April, depending on elevation and accumulation during winter. As the region becomes influenced by high pressure in April, the temperatures warm and precipitation decreases, causing April through mid-June to be the driest time of the year. The monsoon season, which is characterized by scattered afternoon thunderstorms, starts in late June or early July as warm, moist air masses move into the region from the Gulf of Mexico and Gulf of California. July is the warmest month; August is typically the wettest. Precipitation declines in September, but increases again in October-November with the beginning of the snow season. Lightning, which ignites many forest fires, occurs frequently in the Southwest, especially during the dry months of May and June.
The climate varies both spatially and temporally. Spatially, temperature and length of the growing season decrease with increasing elevation, but precipitation increases up to the subalpine zone and decreases above that point. Temporally, annual weather conditions are influenced by the El Niño-Southern Oscillation. El Niño events occur on an average of once every 3-4 years and typically last 12-18 months. They bring cooler, wetter winters to the Southwest, and rainfall during the monsoon season tends to be normal or above normal. In contrast, El Niño’s counterpart, La Niña, tends to bring drier winters. These spatial and temporal variations markedly affect the frequency, duration, and intensity of forest disturbances such as fires.
Topography is a key factor influencing montane forests of the Southwest. The importance of large-scale variation in topography is illustrated by the restriction of montane forests to upper-elevation plateaus and mountains. Montane forests occur at elevations of 1,850 to 2,900 meters (approximately 6,000 to 9,500 feet), but it depends on latitude, with southern forests being at somewhat higher elevations than northern forests. Immediately below this elevational range is pinyon-juniper vegetation and in some areas, interior chaparral shrubland. Alpine tundra occurs at elevations higher than 2,900 m (~9,500 ft.) on mountains such as northern New Mexico’s Wheeler Peak (4,011 meters; 13,161 feet) and northern Arizona’s San Francisco Peaks (3,852 meters; 12,637 feet).
On a smaller spatial scale, the topography of drainages, slopes, and ridgetops within the upper-elevation plateaus and mountains affects forest stand structure, species composition, and fire regime. This occurs because topographic position and slope exposure (aspect) affect site moisture. For example, moist drainages and moist north-facing slopes may have denser forests and burn less frequently but ultimately more intensively than the forests on drier ridgetops or on drier southfacing slopes.
Soils of montane forests in the Southwest are highly variable because they are products of diverse climate, topography, and vegetation. Soils have formed from volcanic materials or sandstone, limestone, or igneous rocks; however, parent material tends to have less influence on montane soils than soils at lower elevations (Klemmedson and Smith 1979, Maker and Saugherty 1986). In general, upper-elevation soils tend to be leached, acidic, well-developed, well-drained, shallow to deep, and fine to moderately coarse-textured (Hendricks 1985, Maker and Saugherty 1986). Soil moisture varies greatly by topographic position and slope exposure, as well as the time of year.
Key natural disturbances in montane forests of the American Southwest include past climate variability, fire, and native insects, parasites, and pathogens. Understanding their independent and interactive effects is essential for interpreting the stand structure, species composition, and other characteristics of these forests.
Past climate variability
Climate is a driver of the general distribution of montane forests, but climate variability is a disturbance factor. Montane forests of the Southwest have been subject to shifts in climate throughout their existence (shifts that were independent of human influence are treated in this section; those related to humans are covered in a following section on recent climate change). Drought is an important aspect of climate variability that has dramatically affected montane forests. It not only affects vegetation directly, but also indirectly by altering other disturbance factors. For example, drier periods have been characterized by a higher frequency of forest fires (Grissino- Mayer and Swetnam 2000) and more extensive fires (Margolis et al. 2007), and drought years have been associated with insect-caused mortality (e.g., Worrall et al. 2008).
Because both the dry climate and high frequency of lightning in the Southwest encourage fires, fire has been a major influence on the montane forests. Also, Native Americans historically set fires to aid in hunting and increasing wild food crops; however, these fires were only important in relatively small areas, not throughout southwestern montane forests (Allen 2002). Historically, there were surface, crown, and mixed-severity fire regimes. A surface fire regime is characterized by frequent, low-intensity, low-severity fires with short flame heights (above left). In contrast, a crown fire regime has infrequent, high-intensity, high-severity fires that burn the tops (crowns) of trees (above right); such fires are also known as stand-replacing fires. A mixed-severity fire regime has both surface and crown fires, typically with surface fires being far more common and widespread than crown fires.
Native insects, parasites, and pathogens
The major insects, parasites, and pathogens impacting the montane forests of the Southwest are native species of bark beetles, defoliating insects, dwarf mistletoes, and root decay fungi (Dahms and Geils 1997). Several species of bark beetles are usually present in low numbers in different tree species, persisting in freshly fallen trees and in less productive living trees. These small populations only occasionally result in tree deaths, but outbreaks occur periodically, especially with drought, and cause high mortality rates among larger, older, stressed trees. Among the defoliating insects, western spruce budworm primarily impacts understory white fir and Douglas-fir in the Southwest, and western tent caterpillar affects aspen. Outbreaks lasting several years reduce tree vigor and can cause mortality. Dwarf mistletoes occur on most conifers and increase the likelihood and impacts of insect outbreaks and fire. Root decay fungi are widespread, weakening trees and thereby increasing the chances of bark beetle infestation.
Vegetation dynamics are the responses of vegetation to environmental changes. Because the environment is always changing, from small changes in weather to large disturbances such as crown fire, vegetation dynamics occur continuously. They range from small shifts in species’ population sizes and distributions to reforestation following disturbance. A specific type of vegetation dynamics is the process of succession. It often occurs after major disturbances such as logging and involves a series of species replacements over time that is independent of additional changes in the regional environment.