Which platform are you using- ArcGIS Desktop, ArcGIS Pro, ArcGIS Online? Using Desktop or Pro, right click on your feature layer and click properties then click on the Definition Query tab. Click on the Query Builder.. or New definition query button. Create a query that looks something like this (for ArcGIS Desktop):
STAND DIAMETER = "SEED/SAPLING" OR TREE TYPE = "ASPEN"
Your polygon layer should now only display features that contain these values.
In the properties dialog box, click on the symbology pane & under show choose Categories -> Unique values, many fields. Using the Value Fields drop down, select STAND DIAMETER and TREE TYPE then click Add All Values. You can then create symbol groups by holding the ctrl key and clicking on values that you want display using the same symbology then right clicking and pressing group (ie SEED SAPLING, ASPEN; SEED SAPLING, BIRCH; SEED SAPLING, MAPLE)