The process of choosing keywords is important no matter what database you will use. Begin by breaking your research question into its component parts, and then brainstorm synonyms for each of those concepts. You may need to do some background research to find out basic information about the topic to figure out appropriate terms.
How is climate change affecting the health of deciduous trees in the Pacific Northwest?
May expand to the following in a search:
Climate change OR global warming
health OR phenology
"Pacific Northwest" OR Washington OR Oregon