The Solar Tree is a powerful tree rich in the power of both the earth and sun. Originally planted in San Miguel, the tree is capable of purifying Dark Matter, particularly when it has permeated the soil and landscape. Its purifying abilities are such that Master Otenko has speculated that a fully-grown Solar Tree could purify even Istrakan, the City of Death.
Solar Tree in Istrakan.
Despite it's great power, however, the tree is not invulnerable. In the events prior to the first Boktai, the tree and its caretaker, Lita, were kidnapped and taken to Istrakan. The process of the transfer and the corrupted soil and air of the region severely damaged the tree, leaving it a rotted husk of its former self. Even in this decrepit state, however, the tree was still rich enough in solar energy to support the growth of Solar Fruit by combining existing fruits within special hollows within its roots.
The Solar Tree was not beyond recovery, either, as it could still absorb sunlight and use it to purify itself. Consequently, the more Solar Energy picked up by the Solar Sensor, the healthier the tree would become. Eventually, the tree would reach its original size and shape; giving it even more Solar Energy, however, would eventually cause it to become the "Pink" Solar Tree and break out in blossoms. (In Boktai, the Pink Solar Tree is needed to obtain the Astro Card, which in turn allows one to obtain the Astro Battery.)
In Boktai 2, it is shown that the Solar Tree can produce fertile seeds, as Lita plants one in San Miguel. After the defeat of Durathror and her "Darkness Tree", a tiny sapling sprouts in this area, and from there can be grown in much the same fashion as its parent.
In Boktai 3, it is revealed that both Solar Trees are descended from the Ancient Tree.