Grazing History & Plant Succession

Introduction

After the Apache Wars in the 1880's & 1890's, homesteaders began to occupy and lightly graze the Rincon Valley, Tanque Verde Valley, and Cactus Forest area (see map). The Great Depression brought upon many homesteaders selling their land to wealthy cattle ranchers from Texas and further east, intending to graze heavily upon the land and increase their wealth. Intense grazing followed through the 1950's and 1960's as cattle grazed upon the vegetation year round (which was a violation of the Taylor Grazing Act). Grazing ceased in the Cactus Forest Allotment (around the scenic loop drive) at the request of park staff and the Rincon Valley Allotment followed suit in 1958. However, through the 1960's and into the 1970's the permittee of the Tanque Verde Allotment continued grazing intensely through protections from politicians in the area. Eventually, ecological abuse became evident enough that the park hired ecologist Warren F. Steenbergh to initiate a study showing the abuse to not only the saguaro cactus, but even more so palatable (edible) vegetation of the park. Ultimately, the park was allowed to remove cattle grazing from the park in 1978 after the court saw evidence of abuse.

 

Park employees surveying the 9th out of 10 plots in this post-grazing, vegetation succession study.

NPS Photo

The Study

As stated, this study began in 1976 prior to removal of grazing and then repeated in 2007 & 2018. Ten plots (four pairs, two plots unpaired) were surveyed in different parts of the formerly abused Tanque Verde Allotment. Each plot is 20x50 meters and we measured absolute canopy cover (how much each plant covers the ground total) & density (how many total of each species per plot).

Our largest findings indicate that as recovery of the park has occurred, grass canopy cover increased 6-fold over the past 11 years (2018). This is directly attributed to increasing autumn rains (1995-2018) from tropical activity and likely a result of climate change. We also find that unlike many other land agencies in the west, woody species encroachment (such as velvet mesquite, Prosopis velutina) has not occurred here and that a semi-desert grassland is instead thriving. This can directly be attributed to fires from 1989-1999 that were very intense, killing even the woody species of the area that re-sprout (start to grow again from the crown) during normal fires. The reason for such high levels of fire intensity were exotic grasses such as red brome (Bromus rubens), crimson fountain grass (Pennisetum setaceum), and buffelgrass (Cenchrus ciliare).

Another interesting finding related to climate change is that Encelia farinosa, or brittlebush has encroached and began to dominate in the east side of the park below 3,600 feet in elevation where in the early 1900's it was not found on this side of the park. This is because average winter minimums have increased some 10-15 degrees (F) during the past 100 years, allowing cold-sensitive species to encroach from the warmer southwestern Arizona Sonoran Desert. This is significant because brittlebush has toxins found in its leaves that once monsoon season starts, can infiltrate the soil and kill any plants growing beneath them (allelopathy).

 

This image shows total canopy cover (percentage of total meters measured) between all 10 plots in 1976 (orange), 2007 (dark blue) and 2018 (light blue).  This shows the large increases in grasses and indeed all vegetation groups (guilds) with only trees not increasing overall.

NPS photo

Conclusions

As we continue to monitor this low-mid elevation plot study, we will see if current climate and fire trends continue in the area. If grasses continue to increase from increased tropical autumn precipitation, these desert semi-grasslands and oak savanna ecosystems will require larger herbivores and in higher number than historical levels to control the fine fuels that create intense wildfires. The largest herbivore we are missing in the park is the desert bighorn sheep (Ovis canadensis nelsoni), which consumes primarily grasses just as cattle do. This is why vegetation compositional and succession-oriented studies are so critical to our wildlands!