If there are valuable minerals discovered on the land, that increases its market value, and if it is sold, there is a capital gain. After all, the gain is essentially a sale of the minerals, discounted by the cost of extracting them. Now consider that minerals have been extracted for a while, and they are being depleted. That will cause a drop in the market value of the land. If it is sold at that point, the sale price will represent a gain of only the minerals expected to remain extractable, minus projected extraction costs.
Similar considerations apply to land used for agricultural production.
After all, farming and ranching is a kind of extraction of a scarce resource, with an extraction cost, and potentially depletion. There can be both an earned income from its production, and an unearned income from its increase in market value, but the market value is based in part on its potential for income production, and that can decline. Now consider capital improvements, such as a factory, or a house offered for sale, or a house, office, or apartment offered for rent, or a theater, restaurant, or bank. There are a lot of ways taxes can be imposed on such operations. On sales, on rents, on services, on products, on gains.