Producers are organisms that can make their own food. Consumers are organisms that eat other organisms for food. By and large, it is ONLY the plants that are producers (they make food by harvesting the energy of sunlight). As trees and grass are both plants then they are producers.
The producers are at the bottom of the food chain because they can use sunlight and water to make their own food. There is a large amount of producers too. Producers make their own food by photosynthesis.