The fifteenth day of the Hebrew month of Shevat, or Tu BeShevat, is known as the "New Year for Trees." Since man is likened by the Torah to "a tree of the field," this day is - by extension - celebrated by man as well.
A tiny seedling's germination and development into a full-fledged fruit-producing tree is one of the most inspiring transformations in all of G-d's creation. First and foremost comes the development of the tree's root system. Thereafter the trunk and body of the tree as well as the branches and leaves come into being. Finally there comes the time when the tree bears fruit.
The roots are, for the most part, concealed from the eyes of the beholder. Nevertheless, it is from them that the tree derives its main life-force. While it is true that the leaves also nurture the tree by absorbing sunlight and so on, still, the roots are the tree's mainstay; sever them and the tree will soon wither and die.
Furthermore, the roots enable the tree to be firmly embedded in the earth and to withstand strong gusts of wind or other elements that seek to uproot it.
The trunk and body of the tree, including the leaves, constitute the overwhelming majority of the actual mass of the tree. This part of the tree is generally in a constant state of growth, witnessed by thicker trunk and boughs, additional leaves, and so on. Furthermore, the age of the tree may be ascertained from its trunk and body, especially from its annual rings.
The physical predominance of the trunk and body of the tree notwithstanding, the tree attains a state of completion only when it bears fruit. This is so to an even greater degree when the seed contained within the fruit serves as the forebear and seed for future trees in coming generations.
Man, too, has roots, possesses a trunk and body, and produces fruit. In many aspects there is a remarkable degree of similarity between man's development - even his spiritual development - and that of a tree.
Man's roots are his faith. It is a person's faith that unites and binds him with G-d, the source and wellspring of his existence. Even after the Jew grows in Torah knowledge and in the performance of Divine commandments, he still derives his life-force through his belief in G-d, Judaism, and Torah.
Conversely, a weakening in one's spiritual root system of faith can have dire consequences even on an otherwise spiritually well-developed individual.
Having attained the level of achieving viable roots of faith, a person may be inclined to rest on his laurels. Here the tree comes and teaches us that it is composed predominantly of trunk, branches and leaves. Man, too, should be predominantly composed of Torah study and good deeds. In spiritual terms this means that a Jew can never be satisfied with faith alone, for he would then be like a tree that laid down roots but never developed a trunk, branches and leaves. Such a "tree" is in reality no tree at all - its roots are there, but nothing else. In addition to healthy roots, a Jew must have the full complement of trunk, branches, leaves, and so on.
A Jew's trunk, branches and leaves are the study of Torah, the performance of Divine commandments, and good deeds. They should comprise the overwhelming majority of his activities. Indeed, one can tell a Jew's "age" by measuring his "rings" - how many of his years have been spent in pursuit of spiritual knowledge and substantive deeds.
Furthermore, just as a tree's body grows constantly, so, too, should there be constant growth in the Jew's trunk, branches and leaves - in the study of Torah, and in the performance of Divine commands and good deeds.
Yet as laudable as all these things are, man attains his state of wholeness only when - like a tree - he bears fruit, affecting his friends and neighbors in such a manner that they, too, fulfill the purpose of their creation. By doing so he bears an endless yield of fruit, generation after generation.
Based on Likkutei Sichos, Vol. VI, pp. 308-309
1. Rosh HaShanah 1:1.
2. Devarim 20:19 and commentary of Rashi.