Reaction wood

     Ever look at a big, old, native pecan tree and wonder how the branches can hold up all the weight of leaves and nuts. Trees respond to the force of gravity by producing what has been termed, "reaction wood". Reaction wood is basically the production of wider growth rings in response to the pull of gravity.
   The photo above is a cross section of a lateral branch cut from a large native pecan tree. When this branch was young, growth rings around the pith are fairly uniform in thickness. But as the branch grew larger (and heavier), the growth rings along the bottom of the branch became much wider than the same rings at the top of the branch. This is especially true for the last three years of growth.
   Reaction wood is also the reason crooked graft unions, so common in young trees, seem to grow into nice straight trunks with age. The tree simply grows thicker growth rings is areas under gravitational stress, eventually growing into a well balanced trunk.