Tu B'Shevat is the New Year for the trees, and this has ramifications with regard to the following Mitzvot as they relate to the fruit of the trees: Terumot (Priestly tithes), Maasrot (Levittical tithes and other tithes, the type of which is determined by the year), Orla (the fruits of the first three years, which are forbidden for consumption) and Sheviyit ("Shemittah" - the seventh, Sabbatical year). Rosh Hashanah in Tishrei determines the halachic year for Maasrot or Sheviyit, which, like the Jewish calendar year, begins in Tishrei and ends when the following year's Tishrei begins. This is what determines regarding Terumot and Maasrot of vegetables, and the prohibition of working the land on the seventh, Shemittah year, which also begins in Tishrei. However, as opposed to vegetables, where what determines is the time of their harvest, in the case of fruits of the tree, what determines is the time of the fruit's ripening. For example, citrus fruits, which generally are picked after Rosh Hashanah, belong to the previous year, according to the time of the ripening of the fruit. And according to the Rambam, "ripening" means that they have reached the time of Maasrot, as will be explained further on. The significance is: every fruit which has ripened from Tu B'Shevat and afterwards, even if the fruits are picked only after Tishrei of the following year, and even though a new year has already begun for Maasrot for vegetables, the aforementioned fruits belong to the previous year. Similarly, Tu B'Shevat determines regarding Orla. For example, fruit trees that were planted before the 15th of Av (that is, the last date for planting so that the remainder of that year is already counted as a year when Rosh Hashanah comes in Tishrei), after three years have passed (three times Rosh Hashanah of Tishrei. After that point, the plant is no longer considered a "sapling," whose new year is Tishrei with relation to Orla. It is now a "tree," whose new year is Tu B'Shevat), then we could think that the years of Orla are over. But if the fruits ripen before Tu B'Shevat (of the fourth year of the tree), they are still Orla, meaning that they are still categorized as Orla of the previous year. Only fruits which ripen after Tu B'Shevat of the fourth year are considered to be after the years of Orla. (For an in-depth explanation, it is recommended to learn the subject in Masechet Rosh Hashanah, page 10.)
But with the Etrog this isn't so, since the Etrog grows on all water, and therefore there is an opinion that it belongs to the year it is picked, like a vegetable. But at the moment we won't go into all the opinions and ramifications regarding the Etrog.
With regard to Maasrot, HaRav Eliyahu Gerditz and also HaRav Kook explain according to the Rambam, that the ripening which is mentioned is the time of the Maasrot, meaning the elementary growth of the fruit. The fruit's further growth is only in size, in volume, but its critical structural development was in this year. If the fruit is ripe enough to at least be edible, even though its size is minimal, then the fruit has reached its time of Maasrot, but if it is still unripe and inedible, for example, if the fruit is sour, this is still a deficiency. The fruit has to develop a minimum basis in size, but if it is totally unripe, then it is not something to be taken into account regarding Maasrot, as it has not yet ripened. Trees, "Renew"
The determining date for trees is Tu B'Shevat, at that time there is a renewal. G-d created the world with an element of cycling; one cycle ends, and another, new cycle begins, and the same is true regarding Tishrei. Obviously, on a deeper level, inside of every renewal is a new vitality, new manifestations in creation. Just as we say that on Rosh Hashanah the man renews, and for him the significance is that he needs to adopt a better outlook on reality, similarly the renewal of the entire universe, the vegetation, and the fruit of the trees has its significance for them. This is the determining date for the New Year. The Torah established everything according to natural events: the new day begins with the emergence of the stars; we see that in nature a different day is beginning. The new month begins with the new moon; toward the end of the month the moon becomes veiled. And the New Year: the summer has ended, the hot days have passed, the days are getting shorter, and we see a change in nature. From then onwards we see something new.
All the laws of the Torah, Terumot and Maasrot of the different years of Maasrot - everything is determined according to the climate of the Land of Israel. The same is with the appearance of the new moon, and also the horizon is gauged (according to certain opinions) according to that of Jerusalem.
In the summer, in the "blessing of the years" we say, "And give a blessing on the face of the earth" (as opposed to asking for rain, which we do in the winter). Even if an entire country needs rain specifically during the summer, like France or Spain, the Shulchan Aruch rules that even in this case the Jews residing there shouldn't request rain in the "blessing of the years" but as a private petition - even for an entire country - in the blessing "You...who hears prayer" (where one is permitted to add private prayers). The reason for this is that our Sages formulated the eighteen blessings of the Amidah according to the needs of the Land of Israel, and since the Land of Israel needs rain in the winter and not in the summer, the universal prayer goes according to this, even if all of Europe needs rain in the summer.
The nations of the world, on the other hand, established their timetable arbitrarily. According to them, the new day begins at 12:00 at night. Why? Because by then they have finished the night's partying and can take the time to say Tikkun Hatzot...they don't take the moon into account at all, and they created the months arbitrarily.
The ancient Romans had the wisdom to begin their new year in March, because then all can see that the winter has passed and there is a change in nature. Afterwards came idolaters who believed that the "holy spirit" entered a certain individual in April and left him in the end of December. According to their "logic," nine are the months of the "holy spirit!" And because of that, they arbitrarily set their New Year in the dead of winter.
By the nations of the world, the year is set up arbitrarily, and by us, according to nature. There is a renewal for the trees beginning from Tu B'Shevat, as our Sages say, most of the year's rains have fallen and the trees have absorbed the precipitation and now they are exuding resin, with new vitality.
The idea which accompanies Tu B'Shevat is to see the renewal. He who thinks that everything is static doesn't get excited about anything. The rejuvenation in nature gives the man motivation for his own renewal, with a new way of thinking, and this awakens the man from his slumber. That is the way it is in nature, and that is the way it is regarding Rosh Hashanah in Tishrei, which is the Day of Judgment, and men must mend their ways, start a new page, renew. The general guidance is that a man is to be defined as "walking," not to stand in one place and not to stop at the level he has attained, but to continually advance. If there is advancement, the man ascends, but if he stays in the same place, he retreats backwards.
The "S'fat Emet" mentions over and over the subject of new vitality that a man receives on Rosh Hashanah for the entire year, and the trees undergo a similar process on Tu B'Shevat. In the Diaspora, Jews ate carob from the Land of Israel, because they didn't have anything else...