On Tu B'Shvat, which is Rosh Hashana L'Ilon, the new year for trees (Rosh Hashana 2a) – there is a custom to eat fruits of the tree. On Shavuos, which is Yom Din, judgment day, for fruits of the tree (Rosh Hashana 16a) – the Mishna Berura 494:10 says in the name of the Magain Avraham that we put trees in the synagogue, and in houses, as a remembrance that we are being judged on the fruit.
Now this seems to be upside down. When it is the new year for trees (note: it is not the new year for fruits of the tree) – Tu B'Shvat – we are eating fruit, and when we it is a Yom Din for fruit – Shavuos – we do something with the trees. Should it not be the other way around, that on Tu B'Shvat we should do something with the trees, and on Shavuos we should be eating fruit?