Wood has been used in the making of organ pipes for centuries. The tone qualities that come from wood are hard to describe, but most builders and organists agree that wood pipes have their own tonal character and special use within the organ.
Contemporary organbuilding does not use a high percentage of wood pipes as compared to metal pipes, and the ratio will vary from builder to builder and from organ to organ. Some pipe organs are built with no wood pipes, but the only organ with all wood pipes of which I know is the famous Compenius organ (1609) in Denmark. Even the tiny one-rank mixture is all wood. and the instrument is a tour de force in wood pipe making.
In the 19th and early 20th centuries, it was common to make many ranks of pipes in wood, and their names are familiar to us. The Melodia, Lieblich Gedackt, Concert Flute, Stopped Diapason, Flute d'Amour, Flute Triangular, Tibia and many others are almost always associated with pipes of wood construction. Even ranks of strings and diapasons were occasionally made of wood, and the percentage of wood pipes in this era might exceed 25% of the total pipes in the organ.
Current usage of wood for pipe making centers on a few areas. The stopped flute of 16' pitch in the pedal is usually wood, though in smaller instruments it may be metal. A Holzregal (Holz is the German word for wood) should be a rank of reed pipes with wood resonators, and a Holzgedackt should also be a stopped flute made of wood. The 16' pedal reed may have wood resonators, but even this is far from universal. There are occasions when unusual divisions or unusual ranks are made from wood (wood principals, open wood flutes, etc.), and wood remains as an optional material for the organbuilder to use as the situation suggests.
The use of wood for pipes gives rise to several different tonal characteristics. Where a strength of fundamental and fullness of tone is needed in the 16' line in the pedal, the thick sturdy walls of a wood Subbass or Posaune provide a firm bass line for the organ. The thicker the wall, the firmer and more powerful the tone can be made, but perhaps at the loss of the clarity provided by the upper harmonics of the sound. It is this ability to give a full fundamental to the bass line which is so valuable in the large wood pipes of the pedal division.
When wood pipes are used for manual ranks, it is not always for the fullness of the fundamental, but rather for an overall difference in the tonal quality which is quite apart from that supplied by metal pipes, if hardwoods are used, the sound is usually precise, clear and sometimes very chiffy. Oak, maple, walnut, ash, poplar and other hardwoods have this clarity of line and promptness of speech which may be enhanced or subdued by the voicing and wind pressure used. The softwoods (pine, fir, etc.) give a more relaxed tone, somewhat indefinite in pitch and mellow. Current usage of wood pipes is usually aimed at achieving a noticeable difference in sound for a particular rank or series of ranks from the overall sound of the metal pipes in the instrument.
Most builders select a type of wood for a rank of pipes based on the tone desired, but always with the consideration of how that wood will react to changes in humidity and temperature in the organ. The wood must be of the best grade, and well seasoned with first-rate joinery and gluing so that it will last and stay in tune. The tuning problem is one which limits the use of wood pipes, since they seem to go their own way in pitch compared to the metal pipes, and some builders find this problematic. Some wood pipes stay in tune very well, while others float off pitch. This may have its own charm and musical usage in the right hands.
There is no telling how many hundreds of types of wood have been used over the years for making organ pipes, and some woods have come and gone in favor as musical tastes changed. Exotic woods have been used, but their high price was more an indication of scarcity than stability, as the wood warped and split and left the pipe speechless.
Recent experiments with high-grade plywood indicates that its materials are very useful in the construction of wood pipes. Some plywoods are far more stable than many solid woods, and the ability of plywood to resist splitting and cracking makes It highly suitable for pipe construction. Many types of plywood are good for this work, and the sound of the finished pipe is indistinguishable from that of solid wood. Some builders would not use plywood for pipes because of the stigma against such a material, but many others have done so for many years with considerable success. I predict wider usage of laminated materials for pipes.
Many of us have forgotten that America once had a prodigious industry in harvesting some of the most magnificent large-scale woods of premium quality. A very large pipe organ from the early part of the 20th century will usually have 32' pipes made from awe-inspiring pieces of lumber, 2"-3" thick, 2' wide and 30' long. Such wood is quite rare now and of incredible price. It is unfortunate that a variety of factors has made such wonderful wood less plentiful and dearly priced, but fortunate the client and the builder with such lumber. America will not run out of wood, but its availability in the quality and size needed is another issue.
A pipe organ need not have wood pipes, and there is nothing about the sound of wood pipes that mandates their use. They are simply another item in the vast catalog of options which the organbuilder has available to build the King of Instruments.