TREES is an acronym that stands for:
Technology
Regulatory
Environment / Infrastructure
Economy
Social / Cultural Trends
PESTLE analyses look at the macro environment (i.e. the external environment that the business operates in)
Using the library, you will be able to find sources that address these factors - sometimes a source will address more than one factor since they are interdependent (i.e. technological changes can affect social factors).
Under each component of PESTLE, you'll find a list of relevant databases. Start at the top of the list and work your way down. The best places to start are: