What Drives the Spread of Cedar Trees-The Most Important Predictor


The Great Plains are facing increasing cover by woody species.  While Nebraska’s challenge is eastern red cedar trees, other species like junipers in North Dakota and mesquite in Texas are also spreading.  This encroachment is more than just a change in scenery: woody plants outcompete grass, displace wildlife, reduce forage production for grazing, increase wildfire risks, and impact water quality/quantity.

The Key Factor in Cedar Encroachment

While cedar trees do have natural limitations on where they grow, what is the most important predictor of where encroachment occurs in Nebraska?

Precipitation levels, soil type, and range management can all contribute to the encroachment of woody plants.  However, research has shown that proximity to seed sources is the most important predictor of where cedar encroachment occurs.

How Far Do Cedar Seeds Travel?

Research from Drs. Dillon Fogarty and Dirac Twidwell at the University of Nebraska - Lincoln has shown that the cedar seeds are transferred from local seed sources into grasslands (see links to resources below).  Birds play a critical role in this seed dispersal. They consume the seeds from female cedar trees, and spread the seeds as they fly, depositing seeds into pastures.  Seed sources include tree rows, naturally occurring trees in river breaks, and mature female cedar trees.

What is the distance between most new seedlings in grasslands and the seed source?  95% of seedlings are found within 200 yards of a mature female tree- the seed source.

Take for example, a mature cedar windbreak at a ranch headquarters, surrounded by treeless pastures.  Initially, the “cedar creep” begins slowly, with new seedlings appearing within 200 yards. That 200-yard radius around a mature female tree is equal to 26 acres of infected grassland. 


Occasionally, a new seedling is established beyond 200 yards, but this is less than 5% of cases in uninvaded landscapes.  These outliers eventually mature, producing their own seeds, which continue the seed spreading cycle.

The Cycle of Cedar Encroachment

During the first 1 to 2 years of growth, cedar seedlings are nearly invisible, blending into the grass.  Each year, cedar seedlings grow about 1 foot per year, so eventually the trees become obvious above the prairie grasses.  After several years, once the female trees reach 5 feet tall (6 to 7 years), they begin producing seeds.  At this stage, each new mature tree creates its own 200-yard seed dispersal zone, thus accelerating the spread.  Over time, the scattered smaller trees appear farther and farther from the original seed source.

When Should a Land Manager Take Action?

Knowing that proximity to seed sources is the biggest predictor of encroachment, when should a land manager start controlling cedars? 

  • Act early- before a cedar problem is visible. For two to three years, seedlings often remain hidden below grass height.
  • Remove unplanted cedar seedlings immediately to prevent further spread. Once you see a cedar seedling that wasn’t planted, that cedar tree should be terminated.
  • Begin monitoring areas around mature cedars, particularly pastures near windbreaks.
  • Regularly check around mature windbreaks.  Most new seedlings should be within 200 yards of the seed-producing trees.  Trees as short as 5 feet tall can begin producing seeds.
  • Eliminate rogue cedar trees beyond 200 yards before they mature and produce seeds.

By staying vigilant and taking early action, land managers can gain back grass by curbing the spread of woody encroachment.

Looking Ahead

Next time, we will discuss management strategies that you can implement in your pastures, based on the three stages of woody encroachment.

Key Research Findings on Cedar Trees:

  • A mature female cedar tree can produce up to 1.5 million seeds every year.
  • Cedars have a high germination rate- up to 70%.
  • Luckily, the seeds are short lived in the soil.  Only 5% are viable at 14 months, and just 3% viable after 28 months.
  • Cedars are not rhizomatous- cutting below the lowest branch prevents resprouting.
  • Cedar seedlings grow about 1 foot per year.
  • Female trees begin producing seeds at 6-7 years old (or about 5 feet tall). However, female trees may not produce seeds every year.

Resources:



Source: Bethany Johnston, Nebraska Extension Educator