Which of the following can be considered a primary indicator of forest health?

The number of different tree species is a vital indicator of forest health because biodiversity plays a significant role in the overall resilience and stability of forest ecosystems. A diverse range of species can enhance the ability of a forest to withstand pests, diseases, and environmental changes. This variety ensures that the ecosystem can perform essential functions, such as nutrient cycling and habitat provision, which contribute to overall forest health.

While tree trunk diameter, location of the forest, and age of the trees can provide some insights into specific aspects of a forest’s conditions, they do not offer as comprehensive an understanding of the ecological dynamics at play. For instance, tree trunk diameter might indicate growth rates or maturity, but it doesn't necessarily reflect the diversity and resilience of the ecosystem. The location could pertain to geographical conditions affecting the forest, but it doesn't directly indicate health. Similarly, the age of trees may provide information about forest maturity but doesn't capture the ecological interactions that diverse species can foster. Thus, the number of different tree species is a more holistic measure of forest health.