The art of Caspar David Friedrich (1774-1840), mysterious and spiritual as it was, depended on an intense engagement with nature. On the long hikes that he took through his native north Germany, and further south in the Bohemian mountains, he drew landscapes, buildings, people and, most intently of all perhaps, trees. Half of Friedrich’s surviving drawings come from the sketchbooks that he compiled on his journeys and referred to during the whole of his career. A handful of these sketchbooks survive intact. The one known as The Oslo Sketchbook of 1807 was used for just two months, from April to June of that year. Its 23 pages of drawings record, with almost hallucinatory simplicity and clarity, trees that Friedrich would use in his paintings for years to come.