From Christie's:
Painted in 1903, Birch Forest is filled with the stillness and mystery that make Klimt's landscape paintings so absorbing. This strange and isolated slice of nature leaves the viewer immersed in a world in which there is no sky, no hint of daylight in the upper sections; instead, there is an affecting sense of near-suffocation in the completeness of the surrounding trees, in the dense wall of wood that confronts us. As in almost all of Klimt's landscapes, Birch Forest is marked by the strange and complete absence of people and animals, removing any sense of movement or of time passing. The clock appears to have stopped entirely, nature has paused eerily, allowing us to glimpse the woodland in the spiritual fullness to which the artist himself was witness.
In part, this removal from people and from any sense of time reflects Klimt's enjoyment of a genre that was utterly removed from the portraiture and allegory that filled so many of his works in Vienna. Landscape provided solace and solitude for Klimt, and these are reflected in the still and absorbing isolation in which Birch Forest places the viewer. Despite only really beginning to paint landscapes a handful of years before Birch Forest, by the time he died the landscape genre accounted for almost a quarter of his oils. This reflects the extent to which these subjects allowed him to distill his own unique view of the world, his wonder at the forms of nature, in oils. Among the most celebrated of these landscapes are his rare woodland scenes. In Birch Forest, the dense mesh of striated vertical and horizontal strokes, contrasting with the scattered leaves--a series of bright and burning dots of color--reflects Klimt's ability to distill the view before him to a point that borders on abstraction. This is accentuated by the absence of any single point of focus. The woodland in its absorbing vastness is the subject, every square inch of the canvas the focus.