Graduate Thesis Or Dissertation
 

The Ecological Recovery of Saguaro National Park after Removal of Cattle Grazing and Associated Succession

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  • In the area that is now Saguaro National Park East, grazing began in the 1880's. Because of the impact to the iconic saguaro cactus as well as all palatable plant species in the area, ecological damage in the park were determined to be so great that anti-grazing conservationists challenged the right of grazing to continue in what was Saguaro National Monument. Ultimately, this group won a court case eliminating grazing in 1978. The Rincon Mountains, located within the park are one of many Madrean Sky Island ecosystems connecting the Rocky Mountains to the Sierra Madre mountain ranges to the south in Mexico. This area is one of the most biodiverse locations on the planet, with over 6,000 plant species (USFS, N.D.). This area is also poorly understood when considering succession and the impacts of disturbance to compositional guilds of plant species. Surveys of ten plots began in 1976 and were replicated in 2007 and 2018 in an effort to understand vegetation changes as affected by grazing management, as well as fire disturbance. The study plots were arranged into intensely grazed and lightly grazed to compare the composition of the plant community through time. Ultimately, little significance was found when considering the composition of paired plots through time, the original aim of the study. However, plant canopy cover, density and diversity continued to increase significantly from 1976 to 2007 and 2018. Major implications of our work identified perennial grasses increasing at a significant rate by 2007 and even more so by 2018, while tree species such as the commonly identified encroacher Prosopis velutina decreased significantly by 2007 and stayed steady by 2018. Grass re-establishment can be directly attributed to increased tropical moisture creating uncommon survivability conditions (speaking from historical precipitation means) in recent years previous to 2007 (1996, 1998, 2000, 2003 & 2006) and dramatically increase in the years prior to 2019 (2014-2016 & 2018). Implications here identified that past fire intensity (1989, 1994 & 1999 fires, see fire history) were likely much more intense than previously thought, and supported the scientific community’s knowledge that fire control woody species and supports perennial grasses re-establishment such as is occurring in our work. Our findings also documented that increasing winter minimums have allowed previously excluded, cold-sensitive species such as Encelia farinosa to move into the area, dominating a post-fire ecosystem. The means of succession when considering disturbance history proved quite interesting particularly when combined with climate change impacts in our research.
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