Final answer:
Controlled burns face challenges due to rapidly changing environmental conditions and costs, and are necessary in temperate grasslands for suppressing tree growth to maintain the grassy ecosystem.
Step-by-step explanation:
The main issue with controlled burns is the unpredictability and variability of conditions that influence fire behavior and the safety margin for conducting such burns. Controlled burns are used in temperate grasslands to suppress tree growth and maintain grasses, as without fire, grasslands may convert to scrub or forests. The risks and limitations include rapidly changing moisture levels caused by light rain or warm weather that can reduce the 'safe' burning period to mere hours instead of days. Additionally, other factors such as costs for effective control, the potential for burns to intensify in the afternoon, and areas that may burn irregularly pose challenges in managing fire regimes effectively and ensuring public safety in the face of changing climate and environmental conditions.