Joshua Tree National Park is the site of the intersection of two deserts, the southern part of the Mojave and the northern end of the Colorado. Mormon pioneers named the unique yuccas in the Mojave region “Joshua Trees” after a Christian prophet.
In the 1930s, after hearing about Californians shooting guns and burning down various Joshua trees, Minerva Hamilton Hoyt lobbied for the preservation of those trees with the National Park Service Director Arno Cammerer.
Minerva Hoyt heard of the burning down of an 80 foot tall “Giant Joshua Tree” and had the Desert Conservation League, which she helped form, put out an award for the capture of the arsonist. 
Minerva Hoyt traveled down to Mexico to visit other desert plants and spoke with the Mexican President Pascual Ortiz Rubio, at the Chapultepec Castle, about preserving the cactus forest at Tehuacán. President Pascual Rubio called her “the Apostle of the Cacti” during the course of their meeting. 
After her encounter with the Mexican President, Minerva Hoyt spoke with President Franklin Delanor Roosevelt, after being introduced to him by California Governor James Rolph, about making part of the Mojave and Colorado desert a National Monument.
Franklin Roosevelt had spent time, as President, visiting various natural wonders such as the Smoky Mountains, Yellowstone, and Yosemite and was supportive of preserving the Joshua Trees. 
In 1930, Franklin Roosevelt visited various national wonders while embarking on a 4,000 mile cross country trip around the United States, and continued his cousin’s Theodore Roosevelt’s legacy of conservation during his presidentcy. https://youtu.be/pFp_PxunW6E
Minerva Hoyt worked with botanist Susan Adams Delano McKelvey, a cousin of Franklin Roosevelt, to convince Henry de Forest, a board member of the Southern Pacific Railroad, to get the Railroad to sell the lands they held to be included in a Desert Plants National Park which would include the areas of Hidden Springs Canyon, Corn Springs and the Salton Sea. 
The Southern Pacific refused to sell the land and in 1936 Congress passed a bill making part of the Mojave and Colorado desert, without the Salton Sea, near the land owned by the Southern Pacifc owned as part of a Desert Plants National Park. On August 10, 1936, President Franklin Delanor Roosevelt passed a bill making Joshua Tree a National Monument. While the Joshua Tree National Monument was smaller than the park Minerva Hoyt originally proposed, the National Park Service agreed to expand the park south if more private land was available for public acquisition in the future.
Author’s Note:
In 1936, Colonel Charles Goff Thomson, a superintendent at Yosemite National Park, took charge of Joshua Tree National Park after it formed. In 1937, Charles Thomson passed away from a heart attack.
Hamilton Historical Records 