The three parts of a guitar that carry the stress most are the neck, front and bridge. Guitar makers have settled on a few types of wood for those parts based on literally centuries of trial and error. The neck may be the most important.
In the last few years good quality mahogany has become harder to find for the makers and because of the flattened shape of the top where the tuners are installed and that slight angle so the strings will press firmly on the nut, a much larger piece of mahogany is required than one would think looking at the long, narrow shape of a guitar neck. This has forced makers to make compromises. Martin makes some guitars with what players refer to as "wings" : small pieces of flat mahogany bonded to each side of the head. I've heard that this small alteration saves something like 25% of the wood needed to make a single piece neck. Although it is generally recognized that the use of wings has no affect on the sound or the strength of the guitar, some players object to the look of them - although Martin goes to great lengths to stain the wood and match the grain so that the wings are not obvious. Wings are a big subject of discussion on the guitar forums such as the Unofficial Martin Guitar Forum and and The Acoustic Guitar Forum and to my mind, the jury is still out on that subject. I do know that Martins with wings are often less desirable on the used guitar market. Time will tell whether that will always be the case.
Taylor Guitars often gets around this by using a "finger joint" at the point where the angle changes between the head and fingerboard. Personally, I don't like the way that looks at all and it might even be a deal breaker for me if I was considering buying a Taylor. Not all Taylors have the finger joint however; some are a single piece, some have a tapered oval shaped joint. I have read about both types of joints failing on a few Taylors but the company must feel this is not a big problem because they continue to use those solutions to wood shortage.
Some companies have begun using a man made composite for the necks of some of their less expensive guitars, including Martin with their new and very popular 1-series instruments. They also use it in their popular travel guitars. They call the stuff HPL and while it is quite different in appearance and some have complained about the weight of necks made with the stuff it is apparently all but impervious to changes in humidity and is much stronger than mahogany. Does it affect the sound? Most say no. This may very well be the future of less expensive acoustic guitars.
Speaking of the affects of temperature and humidity, although mahogany is extremely strong and quite durable, it can be prone to changes as the seasons progress. This is somewhat annoying but should not be a huge cause of alarm if your "plays like butter" guitar suddenly has strings that are much higher off the fingerboard, or worse, buzzes begin to appear as you play because the action is too low. In most cases these things can be dealt with by turning the inset screw mechanism that will be found either just under the upper bout inside the body or under a plastic plate up on the head. What happens is a long rod that is installed between the fingerboard and the neck will slightly change the angle of the neck- this is called the "relief." A big, big word of caution here. Keeping in mind that turning in a clockwise direction will result in less relief (lower the action) and turning counter clockwise will do the opposite, ALWAYS loosen the strings, look down the neck to get an idea of the relief before adjustment, then make about a quarter to a half turn, tighten up the strings and check the effect. If it isn't what you want, go through the process again. Resist the temptation to give it a few cranks right off the bat because it is highly unlikely you'll accomplish what you set out to do and you could potentially do severe or even fatal damage to the neck. If you have any trepidation about the process, bring your guitar to an accomplished repair person.
Sometimes though a more serious neck issue can develop. If a neck "goes out of set," the action will get higher and higher until it is virtually unplayable, the result of that steel rod not giving the degree of support it should and the wood reacting to the stress of the strings. You can make a basic determination if this is happening by looking down the neck. If you see an abrupt change of angle at some point, rather than a curve, it could very well mean a net reset is called for. This is a big decision because a neck reset is essentially open heart surgery for a guitar - the neck will have to be removed, the rod reset and the heel may have to be sanded or shaved to get the right angle again and low action. Luthiers can charge anywhere from a couple hundred to $400 or more for this job depending on the complexity. You then have to decide whether your guitar is worth that investment. For what its worth, guitars with a mortise and tenon neck are less expensive to reset than ones with a dovetail joint. This is because M&T neck attachments usually are nothing more than a large bolt that is hiding under a plate of wood inside the guitar. It's worth considering this job carefully - before I even knew what a neck reset was, many years ago, I traded my first Martin, a D-35, for a ...... gulp...... Yamaha because I thought my D-35 had a warped neck! Live and learn.
Next time we'll look at fingerboards and bridges.
Peace & good music,