Pipes are most commonly made from a species of heath (Erica arborea), which grows at elevations of 500 to 1,000 meters all around the Mediterranean. The best material for pipes is generally found on remote rocky slopes and hillsides, where the heath can grow undisturbed for decades or even centuries. On fertile soil, it does not survive in competition with other vegetation and if readily accessible it all too soon becomes firewood or is used for some other purpose.
Corsica was the earliest major producer of briar for pipes, and still produces some. Italian production is pretty much used by pipe makers within Italy, and little if any is exported. Greece produces and exports considerable quantities. Algeria was once the preeminent producer of briar, but when the French were ousted in the 1960s export of briar was prohibited, and the craftsmen who knew the business of selecting and cutting briar left, leaving behind no one capable of restarting the industry. Morocco replaced Algeria as the main supplier to the French pipe making industry, and I assume it still is. Albania has been rumored to have notable stocks of briar growing on its hills, but I am not aware of any being harvested and exported. (Of course, there is no reason I should be aware, so who knows?)
The wood used for pipes comes from the burl. The rocky hill sides where the heath grows will not allow a large tap root to form and grow straight down, yet high winds blowing around the hills would blow over any tree-size plant with a root system that simply spread out from the base of the trunk. The heath survives because it forms a sizable ball, or burl, between the roots and the trunk. This ball-shaped mass sends out a sprangle of roots into the ground, which anchor it and feed water and nutrients to it, and it supports and holds the trunk which rises above it.
Due to arid conditons and infertility of the soil, it usually take 30 years or more for a burl of 5 or 10 pounds to form -- large enough to make perhaps a half-dozen pipes. Anything smaller than this is hardly worth digging up, and larger burls are of course more highly valued. My understanding is that current production for quality pipes is mainly from burls of age 60 or more. There was a time (pre-WW II) when this figure was 120 to 150, and burls 350 years old and more were not unknown, but most of these old stocks have long been exhausted.
This type of wood is used for a combination of reasons. First, it is tough enough to withstand the heat of burning tobacco, which commonly reaches 700 degrees F and under hard puffing may go much higher. Briar from burls that grew under arduous conditions and required decades to centuries to form has the tightest grain structure and performs best in this regard. And the very best is from such a burl when the plant itself has died and the burl has aged for years in the ground. This is the true so-called "dead root" of great fame.
Second, it has a grain structure that allows it to absorb moisture originally contained in the tobacco and vaporized during combustion but that starts condensing as soon as it is drawn away from the combustion zone into cooler areas of tobacco, the stem, and the mouthpiece. This absorption of moisture enables a drier smoke, much prized by those who truly enjoy the taste of tobacco. The best grain structure for such absorption of moisture is the grain structure that carried water and nutrients from root to trunk -- the exterior portion of the burl. Wood from this area is known as plateau briar, and the the exterior surface of the burl is always left untouched by the sawyer as proof to the pipe maker that such a piece is indeed plateau briar.
Wood from the interior of the burl generally has lost much of its capacity to transport or absorb liquids, and is of markedly lower quality for pipe making. In large burls, the center of the burl has often died and rotted out, and if not the sawyer normally cuts it out and discards it. With small burls, the sawyer cannot afford such a luxury, as he might have little left after discarding the center.
Third, the wood must not impart an unpleasant taste to the smoke. Briar is one of the better woods in this regard, but sap, resins, tannin, and other constituents of the wood must be removed or neutralized to make a pipe truly fit to smoke. After the burl is dug up, it may be stored under moist earth or straw for a few months before being cut into ebauchon, or blocks. The ebauchon are then boiled in water for 12 to 24 hours, dried for a few months, and finally aged for a few years before they are fit to be made into a pipe. All this, just to eliminate vestiges of unpleasant taste. And even then, the pipe maker may go on to boil the ebauchon in oil, or boil oil in the bowl of the unfinished pipe, or apply other treatments to improve the performance and taste of the final product.