Causes of conifer encroachment include a combination of human-induced interruptions to natural wildfire cycles, historic overgrazing by domestic livestock, and favorable climatic periods (increased precipitation, possibly increased CO₂ levels).

There have been cycles of expansion and retraction throughout the past several thousand years. But, most recent expansion occurred in early 1900’s (1905-1920) due to a very wet period coupled with a reduction in fine fuels due to grazing.

The rate of expansion and seedling establishment has declined since then, but is still going on.

In the video on the left, Rick Miller explains the causes of conifer encroachment.

When discussing encroachment, it is important to distinguish true woodland sites (sometimes called “old growth” or “ancient”) from sites experiencing conifer encroachment. Sagebrush ecological sites, which were or should be predominantly sagebrush, have few to no old trees, stumps, downed wood, or snags, and often have deeper soils with higher herbaceous production. Persistent woodland ecological sites are often characterized by the presence of ‘old-growth’ trees (i.e., those more than 150 years old) in stands or savannas, and the presence of scattered downed wood, snags, and stumps. Persistent woodland ecological stands have ecological value for certain species of wildlife and should be conserved.

Watch the video to learn more about old-growth juniper.

For more information, see Miller et al. (2014), Appendices 4a and 4b, and Miller et al. (2007) for characteristics to distinguish between pre-settlement (old-growth) and post-settlement expansion pinyon-juniper woodlands.

The invasion of juniper and other conifers into sagebrush rangelands has a variety of negative ecosystem effects and degrades habitat for sage-grouse. Ecosystem effects include transpiration of soil and in-stream water, alteration of soil acidity, shading out of other plants, competition with understory grasses and herbs for water and nutrients, reduction of food and cover for sagebrush-obligate species, and elimination of forage for other wildlife and livestock. Habitat quality for sage-grouse decreases through increasing the density and canopy cover of trees (which grouse avoid) and reducing grass and sagebrush cover. Conifers also serve as perches and roosts for hawks, ravens, crows and other birds that prey on sage grouse eggs and nestlings.

In the video on the left, Jeremy Maestas describes the ecosystem problems resulting from from conifer expansion.

Millions more acres within the sagebrush steppe are considered at risk. The probability of juniper or other conifer invasion increases where juniper/pinyon seed sources are present and fire intervals increase beyond their historical range.

Approximately 12% of the current distribution of sagebrush is predicted to be replaced by expansion of other woody vegetation for each 1°C increase in temperature and under the worst case warming scenario, the area of sagebrush would be reduced to 20% of its current extent (Neilson et al. 2005, Miller et al. 2011). In one modeling exercise, there are only a few small areas in the Great Basin where sagebrush is simulated to persist under both current and all future climate simulations.

The largest is in southern Wyoming in the gap between the northern and central Rocky Mountains, followed by areas along the northern edge of the Snake River Plateau and small areas in Washington, Oregon and Nevada (Neilson et al. 2005, Figure 3).

Fortunately, aggressive actions to reduce and/or remove conifers in appropriate places can have a positive effect in improving habitat for sage-grouse, especially in Phase I and Phase II stands which are still in the Prevention phase.

For example, below are before and after photos of the same landscape that has been successfully treated to remove encroaching conifers (from the Sage Grouse Initiative, Science to Solutions).

A key concept available for dealing with conifer encroachment is resistance and resilience which helps inform the type, location, and tradeoffs associated with treatments applied to control conifer expansion.

In the video on the right, Rick Miller describes resistance and resilience concepts.

The figure below (Chambers et al. 2014, Figure 6) illustrates several basic principles about vegetation occurrence, productivity, and fuels that are widespread across the Great Basin and the entire distribution of sagebrush. Lower elevation sites are generally warmer and drier and have less overall vegetation than higher elevation sites, which are cooler, moister, and have more vegetation.

The elevation/moisture/temperature gradient is tied to the occurrence of sagebrush subspecies, as shown in the figure, and to the distribution of conifers and the probability of encroachment. Conifer encroachment is more likely on cool, moist sites and/or higher elevation/higher productivity sites.

Next explore the Land Management Tools section to learn about tools for combating conifer encroachment.