This section introduces you to the science behind conifer encroached systems. Scoll down the page to read each sub-section, or click the Science drop-down navigation to go directly to a sub-section.
In the following video Rick Miller (Professor Emeritus, Oregon State University) describes the conifer encroachment problem. Click the Play button to watch.
Juniper and pinyon woodlands have increased in area across the sagebrush steppe of the Intermountain West over the past 150 years by up to 625%. As much as 90% of the current area of these woodlands consisted of sagebrush plant communities prior to the European American settlement (Davies et al. 2011, Miller et al. 2011). Numerous examples of this increase are available from all over the Great Basin, for example the photo series at right (Miller et al. 2005, Figure 2a and 2b.
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.
Woodland succession can be separated into three transitional phases, differing in their overall cover of juniper (increasing from Phase I > Phase II > Phase III) and cover of sagebrush (decreasing from Phase I > Phase II > Phase III). Select a tab below to learn about the characteristics of each phase and thresholds.
Junipers are present but shrubs and herbs are the dominant vegetation that influence ecological processes (hydrologic, nutrient, and energy cycles) on the site (Figure 22a, Miller et al. 2005), (with young scattered trees, < 10% conifer canopy cover and intact sagebrush and understory vegetation).
Junipers are codominant with shrubs and herbs and all three vegetation layers influence ecological processes on the site (Figure 22b, Miller et al. 2005), (trees co-dominate with sagebrush, understory grasses and herbaceous plants are declining).
Junipers are the dominant vegetation and the primary plant layer influencing ecological processes on the site (Figure, http://www.sagegrouseinitiative.com/), (trees dominant, shrubs absent). In this video, Rick Miller describes the three phases of conifer encroachment. Photo credit: BLM.
As shown in the figure below from Miller et al. (2005), approximate thresholds between Phase I and II can be identified, though this is hard to detect and varies from place to place. Another key threshold is the point at which a stand progresses from Phase II to Phase III; at this point, prevention actions will no longer work to improve the stand, a full-blown restoration project must be implemented.
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.
These changes are illustrated by this figure from "Great Basin Factsheet Series Number 4" which plots the cover of shrubs, tall grasses, and short grasses as tree cover increases from left to right. Note especially how quickly shrub cover, for example sagebrush, decreases with even small increases in tree canopy cover.
The density and distribution of conifers has been directly linked to the amount of sage-grouse lek activity and to the presence of other species of wildlife associated with sagebrush. Click the tabs below to explore these impacts.
Four percent conifer cover is generally regarded as a key threshold for Sage-grouse, as shown by this figure from the Sage Grouse Initiative, Science to Solutions, and Baruch-Mordo et al. (2013.) The figure shows the relationship between the probability of lek activity at a lek and the cover of conifers in the surrounding landscape. Leks are basically inactive when conifer cover is 4% or greater on the landscape (right hand part of the graph) and the probability of lek activity declines steadily as conifer cover increases from 0%. Four percent conifer canopy cover is generally regarded as a threshold beyond which sage-grouse will not persist on a landscape.
Leks are more likely to be active in areas where trees were clustered or clumped and large clear patches of sagebrush remain (even at different canopy cover levels). For example, this graphic from Baruch-Mordo et al. (2013), Figure 4, shows that regardless of the level of conifer cover (low, medium, and high from top to bottom), active leks are present when the conifers are clumped versus spread across the landscape. The probability of lek activity drops off even where small trees are widely scattered across the landscape (top right in this figure), suggesting that grouse avoid areas with active encroachment as well as established woodlands.
Conifer treatments also have effects on numerous other species of wildlife, particularly those most directly associated with sagebrush habitats. The effect of increased conifer canopy cover in reducing habitat quality for the sage-grouse and other sage-associated birds is affirmed by a recent study (Knick et al. 2014), where it was found that prescribed fire or mechanical treatments which reduced pinyon/juniper cover down to 5.9 – 24.1% did not have sagebrush obligate bird communities. Only those sites where conifer cover was reduced to 0.2% had sagebrush obligate bird communities.
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.