After a hot dry summer, older branchlets of Thuja plicata (western red cedar) turn brown and drop, a phenomenon known as cladoptosis or flagging.
Cladoptosis (from the Greek, clados = branch, ptosis = fall) is analogous to leaf-drop of dicotyledonous trees in the autumn. Cedar leaves are tiny, however, and it is whole branchlets that are shed. Cladoptosis occurs in a variety of trees, including Salix and Araucaria: in all, a periderm (bark layer) forms across the stem, cutting off the vascular supply. Cedar blight fungus (Didymascella thujina) also causes browning in cedar, but small black fruiting bodies distinguish it.