Grease bottom and sides of a 15" x 10" x 1" jellyroll pan; line bottom with wax paper. Grease and flour wax paper.
Beat egg yolks at high speed of an electric mixer 5 minutes or until thick and pale. Gradually add 1/3 cup sugar, beating well. Add water and vanilla. Fold in 1/2 cup ground pistachios. Gradually fold in cake flour and cocoa.
Beat egg whites at high speed until foamy. Add cream of tartar and salt; beat until soft peaks form. Add remaining 1/3 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Fold into yolk mixture. Spread into pan. Bake at 375° for 10 minutes or until top springs back when touched.
Sift powdered sugar in a 15" x 10" rectangle on a cloth towel. When cake is done, immediately loosen from sides of pan, and turn out onto sugared towel. Peel off wax paper. Starting at narrow end, roll up cake and towel together; cool completely on a wire rack, seam side down.
Unroll cake; remove towel. Spread cake with half of Rich Chocolate Buttercream; carefully reroll. Cover and chill. Cut a 1" thick diagonal slice from 1 end of cake roll. Place cake roll on a serving plate, seam side down; position cut piece on top of cake to resemble a knot. Spread remaining buttercream over cake.
Score frosting with the tines of a fork to resemble tree bark. Top cake with chopped pistachios for moss. Garnish sides of cake with Meringue Mushrooms, bay leaves, and crab apples.