Though it is a very broad term and encompasses different periods and stylistic trends, modernism is the most basic principle I would name regarding this painting (and others) by Van Gogh.
Modernism has as its goal the portrayal of the artist's inner self rather than a literal representation of the outside world. Especially in his late paintings, of which this is one, Van Gogh creates a scene on canvas that projects his own feelings upon things external to himself. He once said something to the effect that he dreams a picture, and then paints the dream. The olive trees are painted in lines that are like jagged waves of a turbulent sea, and their roots are made visible as if they are whirlpools anchored in the earth. Above them is a harsh but still somehow life-giving sun that radiates outward with a preternatural symmetry, in concentric lines bathing the landscape in an extraordinary yellow. Some people have suggested that, due to his mental disorder and his anguish, Van Gogh literally saw the outer world this way.
Whether he did or not, the principle of projecting one's own inner self onto canvas is realized here in the most startling and extraordinary way. As in his even more famous "The Starry Night," Van Gogh depicts a scene that is both frightening in intensity and yet benevolent. In his night scene the stars become huge whirlpools—somehow resembling the transfigured roots of his olive trees—seemingly destructive but hypnotically peaceful at once. His olive trees beneath a bright sun are equally beyond literal reality, but the whole concept is one of a strange harmony the artist has discovered in both himself and in nature as he sees it.