The Big Creep

Keeping Conifers from Encroaching on Sagebrush and Grassland Habitats

We talk a lot about invasive plants that crowd out native species. But, sometimes, the invader is a native. That’s the case with a phenomenon called “conifer encroachment.” It can occur in streamside areas, and along the edge of grasslands or sagebrush steppe where conifers (evergreen trees such as juniper and Douglas-fir) gradually move in and muscle out the plants that would dominate under natural conditions. As that happens, we’re losing valuable wildlife habitat. The risk of extreme fires is also increasing.

Sagebrush steppe area  in 1965 with very few conifers within the sage habitat.

Same overview in 2001 shows significant encroachment by conifers into the sagebrush steppe.


One of the biggest causes of conifer encroachment is human suppression of natural fire. For centuries, occasional blazes swept through and culled younger trees, without killing larger more mature trees. It kept the forest boundaries in check and maintained a diverse and more resilient patchwork of vegetation and habitat. After the big fires of 1910, people tried to put out every wildfire that was sparked. Today, the policy is changing and we want to return to that more natural cycle, but nature needs a little help.

In Montana’s High Divide Headwaters, the Conservancy is thinning trees and restoring the sagebrush habitat essential to wildlife such as greater sage-grouse. In places, we’re putting fire back on the land with prescribed burns, which greatly extend the lifespan of the restoration and minimizes the risks when natural fires do occur. And we aren’t working alone, we’re teaming up with public and private partners to do the job. 

To get the most bang for our buck, we’re focusing on areas where our efforts will produce more than one benefit -- places where sage-grouse are found and where forage can be improved. We also consider the degree of encroachment. There are three different phases.


In phase 1, the trees are just beginning to creep into the open land. © Jim Berkey

In phase 2, the threes are getting thicker and are starting to establish themselves. © Jim Berkey

By phase 3, thinning out the trees would be a difficult and costly endeavor. © Jim Berkey

We’re concentrating our efforts on phase 1 and 2 conditions, where we can still make a significant impact. We’re also using science to help us understand how much sagebrush steppe we have already lost to encroachment and to identify sites where wildlife of highest concern would be most affected by future encroachment. 

Read more of the science 


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