Defects due to Natural Forces: The following defects are caused by natural forces are following:

(1) Knots 

(2) Shakes

(3) Wind cracks       

(4) Upsets

(a) Knots: many of its branches fall and the stump of these branches in the trunk is covered When a tree grows. The stump of fallen branches appears as knots in the sawn pieces of timber. Knots are dark and hard pieces. In this portion Grains are distorted. Given figure shows some varieties of knots. If the knot is intact with surrounding wood, it is known live knot. If it is not held firmly it is dead knot.

                  Live knot         Decayed knots

                              Knots

(b) Shakes: In the timber the shakes are cracks which appear due to excessive heat, frost or twisting due to  wind during the growth of a tree. Depending upon the shape and the positions shakes may be classified as star shake, ring shakes ,cup shake, and heart shakes .

               Cup shakes                       Heart Shakes                          Ring shake                                 Star shakes

                                            Shakes

(c) Wind Cracks: due to the shrinkage of the exterior surface these are the cracks on the outside of a log. They appear as shown in given figure

                             Wind cracks

(d) Upsets: In given shows a typical upset in a timber. In the tree when it was young this type of defect is due to excessive compression. Upset is an injury by crushing. It is also known as rupture.

                                         Upset