The narrator explains that there's no camp or lake at Camp Green Lake. 100 years ago, it was the largest lake in Texas, but the lake and the town dried up. Temperatures hover at 95 degrees, and the only shade is under two oak trees with a hammock strung between—though that shade is "owned" by the Warden. There are rattlesnakes and scorpions around, but being bitten or stung by them isn't the worst thing that could happen. Campers who are bitten spend a day recovering and then return to digging holes. However, if someone is bitten by a yellow-spotted lizard, death is guaranteed.
Using the word "owned" to describe the Warden's relationship to the shade shows that she's a person who seeks to dominate the natural world and use it for her own means, rather than coexist with it. It also suggests a domineering nature, which introduces the Warden herself as a dangerous and controlling character. The mention that bitten campers have only a day to rest implies that the camp pays little attention to the welfare of the campers there.