ISA Certified Arborist Practice Exam

Trees are classified as primary producers because they are capable of photosynthesis, which allows them to convert sunlight into energy. This process involves using sunlight, carbon dioxide from the atmosphere, and water from the soil to produce glucose and oxygen. As primary producers, trees serve as the foundational source of energy and nutrients for various ecosystems, supporting both herbivores that feed on them and, subsequently, carnivores that may prey on those herbivores.

Regarding the other options, decomposers break down dead organic material, secondarily managing nutrients within an ecosystem, while consumers rely on other organisms for energy and nutrients. Secondary producers, on the other hand, generally refer to organisms that consume primary producers, which differentiates them from trees as they directly utilize sunlight and inorganic materials to sustain themselves. Hence, trees' role in directly harnessing light energy qualifies them as primary producers in ecological terms.