The multiset partition C = {{1,1},{1,2,3},{2,3,3}} is not a tree but has the cap {{1,1},{1,2,3,3}} which is a tree, so C is not counted under a(8).
Non-isomorphic representatives of the a(2) = 2 through a(6) = 29 multiset partitions:
{{1,1}} {{1,1,1}} {{1,1,1,1}} {{1,1,1,1,1}} {{1,1,1,1,1,1}}
{{1,2}} {{1,2,2}} {{1,1,2,2}} {{1,1,2,2,2}} {{1,1,1,2,2,2}}
{{1,2,3}} {{1,2,2,2}} {{1,2,2,2,2}} {{1,1,2,2,2,2}}
{{1,2,3,3}} {{1,2,2,3,3}} {{1,1,2,2,3,3}}
{{1,2,3,4}} {{1,2,3,3,3}} {{1,2,2,2,2,2}}
{{1,1},{1,1}} {{1,2,3,4,4}} {{1,2,2,3,3,3}}
{{1,2},{1,2}} {{1,2,3,4,5}} {{1,2,3,3,3,3}}
{{1,1},{1,1,1}} {{1,2,3,3,4,4}}
{{1,2},{1,2,2}} {{1,2,3,4,4,4}}
{{2,2},{1,2,2}} {{1,2,3,4,5,5}}
{{2,3},{1,2,3}} {{1,2,3,4,5,6}}
{{1,1},{1,1,1,1}}
{{1,1,1},{1,1,1}}
{{1,1,2},{1,2,2}}
{{1,2},{1,1,2,2}}
{{1,2},{1,2,2,2}}
{{1,2},{1,2,3,3}}
{{1,2,2},{1,2,2}}
{{1,2,3},{1,2,3}}
{{1,2,3},{2,3,3}}
{{1,3,4},{2,3,4}}
{{2,2},{1,1,2,2}}
{{2,2},{1,2,2,2}}
{{2,3},{1,2,3,3}}
{{3,3},{1,2,3,3}}
{{3,4},{1,2,3,4}}
{{1,1},{1,1},{1,1}}
{{1,2},{1,2},{1,2}}
{{1,2},{1,3},{2,3}}
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