How European Beeches manage the radial tensile forces.

If curved structural parts of a tree are straightened by bending lateral fo rces would lead to longitudinal splitting if the internal structure of the wood would not be optimized and load-adapted. For the example of a leaning beech tree (F. sylvatica L.) different ways of internal self-optimization are shown which allow the tree to live with lat eral forces without failure: 1. Radial rays act like tensile ropes growing larger in cross-section towar ds the bark and finally may even split into many smaller rays. 2. The late wood of the individual tree rings acts as an anchor-plate behin d the ,,knots" of the rays. 3. By inserting secondary rays in between primary rays their distance in ci rcumferential direction will be limited. 4. The brittle-stiff late wood bridges in between two rays are curved into the shape of a bow with the convex side to the tree center. This is to bett er resist the radial forces pushing the tension wood fibres against the lat e wood bridges (Fig. 5). It compares to the tensile stress release of old b ridges. Also the knots made up by the blade-sheath-connections of the rays act like compressive wedges and compensate the circumferential tensile forc es due to bending of the late wood bridges. 5. The ray of beeches itself has optimized weldings at the transition from late to early wood. This done by inserting the blade of the early wood ray into the sheath of the stiff hollow late wood ray. Both are glued together by pectin. Regarding all this one has one more reason to believe that trees with small rays provide less public safety in cities than trees with large multi-seri ate rays.