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This linden was planted on May 30, 2002 to commemorate...
Explanation: ...the outstanding level of cooperation between X and Y.
May 30, 2002
So, you just put the date under the statement, but add the power of presence to the tree by using the present tense.
Please not that the form 2002-05-30 is yet another international standard that goes largely ignored in English and creates an obstacle to smooth reading, thereby distracting the reader from the message.