The fact that woodlands have increased in extent and density over the past 150 years in the Interior West of America is universally agreed upon. However, the root cause of this phenomenon continues to be debated in the scientific community. Here we provide the two predominant perspectives on this issue, which we term the Encroachment Hypothesis and the Colonization Hypothesis. Further research is needed to determine which hypothesis is correct, or which is applicable to specific sites. A need for additional research is emphasized because often the suspected root cause of a phenomenon such as this may help to guide managers in their management goals and priorities.

Encroachment Hypothesis
     The encroachment of woodlands and shrubs into grassland ecosystems is of global interest to land and wildlife managers, as this phenomenon has been documented in semi-arid grasslands of North America, South America, Africa and Australia (Van Auken 2000). In the past 150 years, extensive tracts of grassland and savanna in the Interior West of America have been affected by brush and woodland encroachment and densification. This trend was observed in Arizona by Aldo Leopold as early as 1924. Leopold (1924) remarked, “In northern Arizona there are great areas where removal of grass by grazing has caused spectacular encroachment of juniper on park areas”. This same phenomenon has been subsequently verified throughout the Southwest by contemporary researchers, who report that the composition and structure grasslands of the region have changed due to both the increase of density and cover of woody species, and the spread of various species of juniper down-slope into adjacent grasslands (Allen and Breshears 1998, Jacobs et al. 1999, Van Auken 2000, Brockway et al. 2002, Jacobs et al. 2002, Hastings et al. 2003; Albert et al. 2004). However, other research indicates that increases in the acreage occupied by woodlands in some areas may be attributed to canopy infill of what were once woodland savannas that existed prior to Euro-American settlement, rather than to novel establishment of woodland outside of their previous range (Shaw 1999, Ffolliot and Gottfried 2002). Evidence also suggests that woodlands were more common on hillsides and rocky slopes in the era just prior to settlement, and did not regularly occur in valley bottoms and other flat and open areas (Albert et al. 2004, Burkhardt and Tisdale 1976).

Colonization Hypothesis

Historical Context

     Because the root causes of the presence of woody species may be due to multiple factors, the historical context of the area's ecology and management is very important. Thus, there is not a one-size fits all prescription to address this phenomenon. There is active debate in the scientific community with regard to the root cause(s) of woodland and shrub colonization and densification in the semi-arid grasslands of the Southwest. One of the leading theories suggests that the driving force behind woodland colonization is historically high levels of herbivory by domestic livestock, which reduced competition from grasses and thus reduced production of the fine fuels necessary to carry ground fires. This in turn altered or eliminated grassland fire disturbance regimes (Leopold 1924, Brockway et al. 2002, Yoakum 2002, USDA 2004). This combination of factors favors the establishment, survival and growth of woody plants (Brown et al. 1997, Van Auken 2000).

     Woodland colonization that has occurred in recent history should be distinguished from climate-induced shifts and expansions in the ranges of woodland species over geologic time. This distinction is difficult to make amongst the tangle of natural and human-induced ecosystem changes, including pre-historic changes caused by native Americans such as fuel harvesting. For instance, in some areas such as Chaco Canyon, the documented increase in woodland extent and cover in the past century may actually represent woodlands that are recovering from over-harvesting by the Anasazi some 800-1000 years ago (Betancourt and Van Devender 1981, Samuels and Betancourt 1982). Some researchers have suggested that special consideration should be given to those areas that were heavily populated by Native Americans within the last 1,000 years (Samuels and Betancourt 1982, Kohler 1988). In another example, an isolated stand of piñon at Owl Canyon, north of Ft. Collins Colorado is at the northern terminus of this species northward migration since the end of the last ice age (Betancourt et al. 1991). This stand was colonized by piñon less that 500 years ago, possibly from accidental plantings by Cheyenne and Arapaho, who carried piñon nuts on their journeys along the Front Range. Without this historical context, this expansion of piñon could be mistakenly ascribed to more recent human impacts such as fire suppression and overgrazing (Allen et al. online). This illustrates the importance of historical context and the need to distinguish between natural range shifts, influences from Native Americans, and colonization and woodland expansion due to relatively novel land management practices of Euro-American settlers. Thus, it is understandable why the management of these dynamic vegetation mosaics has been a source of debate in the scientific community and the public at large.

     Paleobotanical research shows that during the late Pleistocene, Piñon (Pinus edulis) and juniper covered what are now the hot deserts of the southwestern United States, mostly below 1500 m elevation (Swetnam et al. 1999, Betanacourt et al. 1993). After 11000 14C yr BP, data from packrat middens shows that piñon was extirpated from desert lowlands and dramatically expanded its range northward and upward in elevation in response to the warming climate conditions of the Holocene (Swetnam et al. 1999, Betanacourt 1996). This implies that the fact that woodlands even exist on the Colorado Plateau at all is a function of warming climate trends that have occurred in the past 10,000 years. In his thorough review on the causes and dilemmas related to modern woodland encroachment, Van Auken (2000) states, “…in the semi-arid southwestern United States, warming would presumably cause juniper to move northward and up in elevation. This did not occur, juniper populations seemed to move down in elevation…Recent climactic or precipitation changes do not seem connected to these vegetation changes in semiarid grasslands either directly or indirectly”. The logic that continued warming would hypothetically result in woodlands continuing to move upward in elevation in response to further warming is salient. However, the claim that climate, including cyclical drought, is “apparently not connected” with the dynamic processes that have led to brush and woodland colonization is refutable. The question is not if climate has played a role, but rather exactly what role it has and continues to play. Interdecadal climate variability has been cited as one plausible cause of woodland and shrub colinization (Neilson 1986). In addition, the widespread die-back of piñon in the past decade across the Southwest is a direct result of drought-related stress and associated insect infestations (Citation).

     Research has demonstrated that junipers, and to a lesser degree piñon, are a fairly drought-tolerant species; junipers have been shown to do relatively well during drought conditions, while competing vegetation may not survive (Albert et al. 2004, Miller and Rose 1995). Research also demonstrates that piñon establishment has been sustained by a string of warm, wet springs associated with anomalous warming of the tropical Pacific between 1976 and the late 1990's (Swetnam et al. 1999, Swetnam and Betancourt 1998). And while atmospheric carbon dioxide levels may be positively correlated with periods of colonization and closure of woodland stands, results of some research has discounted a cause and effect relationship (Archer et al. 1995). Untangling the causal mechanism(s) of woodland colonization and densification is the first step in identifying appropriate management actions. This presents a major challenge for managers, who are charged with improving range and habitat conditions, managing wildlife populations and multiple human uses in the grassland/ woodland interface. The sustainability of these ecosystems, and indeed management practices, is of significant local and national interest.

     In the Land Use History of North America, Allen et al. provide the following salient advice with regard to establishing a solid historical context for management: "Historical data can be used to discriminate between natural and cultural causes of environmental change. Environmental variability and trends have regional and local components. One effective approach to determining causation is to identify synchronous regional responses of biotic systems (which are often climate-driven) and asynchronous, disparate responses observed at local scales (which are often attributable to human land uses and other local disturbances). Additionally, comparison of multiple lines of evidence from different types of ecological reconstructions (e.g., photographs, tree ages, fire scars, cultural histories, climate records) can be the key to identifying causal factors." Within a well-researched historical context, the appropriate management prescription is likely to become apparent.

What About the Grasses?

Wildlife Habitat Impacts

     Woodland colonization into grassland ecosystems has been implicated in the degradation of habitat quality and increased predation risk for grassland-obligate songbirds (Rosenstock 1999), and the American pronghorn (Antilocapra americana) (Ockenfels et al. 1994, Yoakum 2002).  Woodland expansion and densification has also been blamed for watershed degredation, although there is little peer-reviewed research to support this assertion (Citation).

     The increase of woodland density and cover in some areas has reduced habitat
quality for grassland obligate species such as pronghorn, specifically by increasing visual obstruction and decreasing vegetative biomass in the understory (Yoakum 2002). Woodland encroachment and drought have been accompanied by a simultaneous reduction of native plant species richness in the greater Anderson Mesa area in northern Arizona (Phillips 2001, Ockenfels et al. 1996, Yoakum 2002) and in other locals of the Southwest (Pieper 1990, Tausch et al. 1995). Such factors may influence the long-term viability of pronghorn populations in some areas, as this ungulate not only prefers open habitats, but also requires sufficient forb and shrub diversity to fulfill its dietary requirements (Yoakum 2002). Woodland encroachment may also negatively impact pronghorn because a lack of visibility makes pronghorn more susceptible to predation. Yoakum (2002) clarifies that pronghorn do use forests, but open stands with sparse understory shrubs or young trees are needed for visibility and freedom of movement.

     Pronghorn favor ranges with a high diversity of grasses, forbs, and shrubs. O’Gara and Yoakum (2004) note that, “Pronghorn are highly opportunistic feeders, taking the most palatable and succulent forage available at all seasons”. Thus, pronghorn often travel long distances to access areas with preferred range conditions. If sufficient quantity and quality of preferred forage plants, rich in nutrients essential for healthy fetal development, are not available during the does’ third trimester, this could lead to improper nutrition and contribute to low fawn recruitment (Ellis 1970, Schwartz and Ellis 1981, Yoakum 2002). The Anderson Mesa pronghorn herd has a recent history of low fawn:doe ratios. Thus, a management strategy implementing selective vegetation treatments, followed by prescribed fire, could positively affect understory composition and productivity (Brockway et al. 2002), which could in turn influence pronghorn fitness, recruitment and population trends – a “bottom-up” approach.

Discuss potentially negative effects of treatments on woodland communities/wildlife.

Anderson Mesa Case Study

     Grasslands in central and northern Arizona have been extensively affected by woodland colonization. An unpublished analysis by Loeser and Miller completed in 2005 compared historical vegetation maps produced by the U.S. Forest Service (USFS) in 1912 with remotely sensed canopy cover maps from 1997, and found that woodlands and forests have increased in extent by approximately 12% on Anderson Mesa. This is a conservative estimate, as this analysis classified areas of up to 30% canopy cover as “grassland”, based on 1997 imagery. Regardless, this analysis corroborates the assertion that woodlands have expanded into large expanses of what were open grasslands in the late 19th and early 20th centuries. Even at the time of the USFS 1912 Survey, the expansion of woodlands on Anderson Mesa was likely well underway, as the pulse of heavy grazing and the concomitant elimination of fire had already set colinization into motion some 50 years earlier (circa 1860), with the arrival of Euro-American settlers and their large cattle herds.

     Historical stand reconstruction work completed on Anderson Mesa in northern Arizona by Landis and Bailey (2005) found that, during the 20-year period between 1860 and 1880, there was a distinct pulse in the establishment and survival of Utah juniper (Juniperus osteosperma) trees on both sandstone and basalt-derived soils that has continued at high levels until the present. These researchers also found that piñon trees had established on basalt and limestone-derived soils for at least the past 300 years. A marked increase in density of piñon was observed on both soil types beginning around 1860, with highest rates of establishment between 1940 and 1960. This provides another credible line of evidence that woodland colonization began, or at least accelerated concurrent with settlement (circa 1860) and the ecosystem changes that resulted from altered land use practices. Piñon establishment in the period between 1940-1960 reported by Landis and Bailey (2005) may have been driven in part by this species’ ability to rebound in wet post-drought periods, which would be consistent with other documented pulses in piñon recruitment, whereby anomalous seedling survivorship followed the 1950s drought (Swetnam et al. 1999, Swetnam and Betancourt 1998). Therefore, certain stands of piñon established during these periods, as well as future pulses of piñon establishment and recruitment that may follow the cessation of the current drought, may not actually be colonization, but rather might represent a long-standing and successful adaptation to warm and arid environments that experience cyclical drought.

     In 2005, the Coconino National Forest (CNF) released a Final Environmental Impact Statement (FEIS) for the Bar T Bar and Anderson Springs Allotment Management Plans that cover a large portion of Anderson Mesa. The FEIS preferred alternative (Alternative 6) approved approximately 49,462 acres of vegetation treatments in piñon-juniper woodland and Ponderosa pine (Pinus ponderosa) to be harvested in varying intensities with the goal of grassland, woodland and forest maintenance and restoration, with prescribed burns on the same acres only when soil conditions are satisfactory. This includes treatments in wildlife travel corridors to encourage the movement of elk, deer, and antelope between summer and winter range (USDA 2005). Planned and ongoing grassland restoration treatments, implemented in conjunction with the Arizona Game and Fish Department, primarily target areas with Mollisol (grassland-like) soils in areas with less than 15% slope and less than 15% canopy cover.