As you might expect, you have a lot of choices in developing your plan for planting trees to grow into a "natural forest" condition. Interpreting your environment - soils, topography & climate - is a key early step to help decide what tree species are suitable. Some of the soils and topography in your area can limit your choices - wet soils, dry soils, shallow soils, rocky soils, etc. You might start with a visit to the Clackamas Soil and Water Conservation District to get some help with this https://conservationdistrict.org/. They may also have some programs to assist you in making your plan.

Once you have assessed your site environment, you can look at suitable species lists for that environment. Take a look at this publication on Establishing and Managing Forest Trees in Western Oregon to get some ideas. Since you are starting with an open grassy area, it will take a long time to get back to a forest condition and you have some choices to make about staging your introduction of trees, shrubs, and herbs that would make up a more natural forest condition. Controlling the grass and other vegetation that competes with little trees for moisture and sunlight will be very important in early stages, so we often focus on establishing the trees first and then look at shrubs and herbs after that.

After you have done a little research, feel free to contact me directly via email for further consultation.
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