You should link these tables in a relationship. Neither the botanical nor the common name is the best option to use as a match field in the relationship.
Define an auto-entered serial number field in Tree Info: __pkTreeID
Link it to a number field, _fkTreeID in Site Data
Tree Info::__pkTreeID = Site Data::_fkTreeID
Remove the common and botanical names from Site Data, you do not need them. With this relationship, you can simply add these names fields from Tree Info to your Site Data layout.
Format a value list for _fkTreeID to list __pkTreeID as the first field and either the common or the botanical name as the second field. Selecting a value from this value list can then be done by name, but the value list enters the ID number. When you select a tree from this value list, the name fields from Tree Info will automatically display the names for that tree.
This is a "beginner level" solution. Other options exist that make working with large numbers of names easier to manage than you get with an ID number based value list.