There are about eight species of chestnuts, grouped in the Genus Castanea. Some species are also called "chinkapins". Chestnut tree grow in temperate areas of the Northern Hemisphere, occurring in North America, Europe, and Asia. Depending on the species, chestnut trees can grow anywhere between 32 feet and about 196 feet. Unfortunately, the tallest species - Castanea dentata or American Chestnut - has nearly been wiped out by Chestnut Blight, a fungal disease, in the early to mid 1900's.
The edible portion of the trees are the nuts they produce. The nut is surrounded by a spiky out covering called a "burr". Inside the nut has another covering called the "pellicle". The color called 'chestnut' is based on the color of this pellicle. The nut inside can vary in sweetness, depending on species.
In both the Book series and in actuality, the chestnut fruit is a source of food for people and other creatures, including dragons in the Books. Chestnut trees are also harvested for their wood, which has a nice red color. The wood is high in tannins, which can be used in leather-making.
In the Book series, chestnuts are also used as a projectile weapon, lit on fire.
|“||He found a bowl of chestnuts, sucked up a whole mouthful so that his cheeks were bulging like a hamster, and zoomed over the heads of the Warriors, spitting out fire and red-hot roasted chestnuts like a barrage of flaming bullets.||”|
|— Toothless in Book 4|