The various minhagim of Tu B'Shevat have almost no mention in the early sources, other than a brief sentence in the Rama. The very idea of signifying a day whose entire relevance is halachic (the New Year for trees in regards to tithes, orlah, etc.) with practices of joy in something unusual, and we do not find anything similar regarding the other New Years mentioned in the beginning of Masechet Rosh Hashana (other than Rosh Hashana itself, of course).

While we cannot explain the reason for this practice, we will attempt to find some allusions in it.

We must first ask: Why are fruits singled out to have their own New Year, in contrast to all other crops, whose ma'aser year is determined as Rosh Hashana, without any consideration as to the growth cycle of the fruit? Furthermore, the Torah relates specifically to fruit trees in regards to the prohibition of bal tashchit ("Do not destroy"). While the prohibition includes all destructive acts for no purpose, the Rambam indicates that the punishment of lashes is limited to cutting down fruit trees. In addition, it is permissible to ruin all other things for constructive purposes, while fruit trees may be cut down only under specific circumstances!

In answering these questions, we find that Adam, before sinning, was to eat, "from the fruit of the trees of the Garden," whereas after sinning he was told, "you shall eat the grass of the field."

In Avot D'Rabbi Natan (1:8) Chazal teach:

When Adam Harishon heard that G-d told him, "You shall eat the grass of the field," he immediately trembled. He said before him: Master of the Universe, shall I and my animals eat from one feeding trough? G-d said to him: Since you trembled -- "By the sweat of the brow shall you eat bread."

Perhaps we can explain as follows: Before the sin, Adam was to receive his food without effort, and the food was to grow in an edible manner, whereas the animals eat grass. (Note that in fact most fruits are edible without effort, unlike food that grows from the ground.) After sinning, Adam lost his special standing and was also commanded to eat from the grass of the ground. Thus, before the sin, the difference between man and animal was that man ate from fruit trees, and animals from grasses, whereas after the sin -- when both eat the produce of the ground -- what differentiates man from animals is that man eats prepared food, while animals eat as is.

The need for labor has an aspect of punishment, as explained in the Mishna in the end of Masechet Kiddushin (82a), "I corrupted my actions and I ruined my sustenance." Prior to the sin, man's toil and perfection was only in the spiritual realm, as Chazal comment on the pasuk, "to work it and to guard it" (Bereishit 2:15) -- with positive commandments and prohibitions, whereas after the sin he labors and toils in preparing his physical needs. However, within the punishment there is also an element of rectification, since through this toil and labor he is singled as man and not animal.

Thus, the relationship between fruits and produce of the ground parallels the relationship between man and animals. As such, just as an animal has no individual importance, but only as part of the entire Creation -- in contrast to man who was created singular and forms a world to himself -- so, too, produce of the ground have no individual importance, but rather are subservient and considered part of the ground -- unlike the tree that is dealt with by itself.

I heard that this is the reason that G-d completes the days and years of a righteous person, because he lives an independent life, and has personal, independent importance. Therefore, his life is counted in relation to himself. The life of the wicked and simple person, however, is counted as part of the general existence. The righteous person is compared to a tree, as Chazal comment in Parshat Shelach, "Are there trees in it or not?" -- is there a righteous person whose merit protects them, while the wicked -- "When the wicked bloom like grass." (Tehillim 92:8)

Establishing a special new year for trees expresses the idea that the tree is an independent entity, and therefore its years are counted in relation to itself, like man.

However, when man sins he declines to the level of animals, and is therefore nourished from produce of the ground. Only through toil and effort is he able to renew his human supremacy, as we mentioned earlier. (Perhaps an additional allusion is that together with Adam's sin we find that the earth sinned, in that the tree was not in its perfection, and the taste of the tree was not like the taste of the fruit.) On the New Year of trees, on Tu B'Shevat, we signify the hope to return and be nourished again from the tree -- "For the lifetime of My people will be like the lifetime of the tree." (Yeshaya 65:22)