A lot of people in this region of the country have some commonly stated misconceptions about Juniper species, colloquially referred to as “cedar”, though there are no true cedar (Cedrus spp.) native to North America.
The first misconception is usually along the lines of, “Juniper sucks water from the soil and is a water hog.” Not quite so. Juniper is very shallow-rooted and is not considered a phreatophyte plant, so therefore that line of thought does not hold water (pun intended).
However, Juniper leaves have much more surface area than leaves of other plants, especially broadleaved plants like Oaks (Quercus spp.), and can intercept a lot of precipitation with its canopy; that precipitation will evaporate before it ever reaches the ground. In many ecosystems where Juniper is present, especially arid and semi-arid rangelands, Juniper interception can cause as much as 50% water loss during each precipitation event. In areas that have an eternal soil moisture deficit, the negative implications of Juniper interception can set in motion a decline in perennial grass cover, accelerate the many effects of overgrazing, cause negative changes in watershed hydrology, and even lead to the early stages of desertification. This situation can happen to tallgrass prairies in the East where juniper is an invader, and all the way through the Western states into the Bluebunch Wheatgrass (Pseudoroegneria spicata) prairies of Eastern Washington.
There are many reasons for the increase in Juniper density throughout the Midwestern and Western United States. The top two commonly cited reasons are overgrazing and fire suppression. Another reason is the increase in atmospheric carbon dioxide (CO2), which favors an increase in plants that utilize the C3 photosynthetic pathway (woody plants and cool season herbaceous species). The compounding trickle-down effects of overgrazing, fire suppression, and the increase in CO2, give aggressive C3 plants a favorable competitive advantage over many other plants. All three of the above mentioned reasons cause a decline in water quality, especially fire suppression.
While Juniper may be important for grassland birds that overwinter in the Southern Great Plains, generally only 3-5 Juniper plants per acre are needed to meet their habitat needs. After all, they are grassland birds that like wide open spaces, and Juniper encroachment negatively affects their habitat as well.
Side note: The Trans-Pecos region of Far West Texas has the unfortunate distinction of having 7 Juniper species, 2 of which are moderate to strong re-sprouters after being top-killed by fire or cutting.
The need for reducing Juniper and other aggressive woody grassland invaders is tremendous in the face of rising CO2 levels, increasing land fragmentation, and a growing word population that needs more and more water by the day. Decreasing woody plant species and increasing native herbaceous species (grasses and forbs) can positively affect our watersheds by increasing precipitation infiltration into the soil, and into the water tables, as they are the hardest to replenish.
Because water quality always follows soil carbon levels, decreasing woody plant species coverage in grasslands and increasing perennial, herbaceous, deep-rooted species can, over time, increase the amount of carbon within the soil. The more carbon in the soil, the more filtration of the infiltrated precipitation that occurs. Prescribed fire can really help to add carbon to the soil by depositing a layer of fine nutrient-rich ash after each burn. Also, as perennial grasses go dormant at the end of the growing season, a portion of their fibrous root system dies each year and adds to the organic content of the soil. The higher the organic content, the higher the carbon content, and the higher the rate of precipitation infiltration is likely to be. By planting native prairies and managing them with prescribed fire, you are positively affecting the water quality within that portion of the watershed.