How Bristlecone Pine Trees Are Able to Survive

The combination of high altitude; dry, cool air; low rainfall; high winds; and poor soil has provided life support for the oldest known organism on Earth-the bristlecone pine tree -whose great age was unknown until 1953. Bristlecone pines flourish atop arid mountains in an area stretching from Colorado to California in the United States of America. The oldest-almost 5,000 years old-is located in the Ancient Bristlecone Pine Forest high in the White Mountains of California. Some of the trees standing today were seedlings when the great pyramids of Egypt were built, were middle-aged trees at the beginning of the common era, and today are hoary (ancient) patriarchs standing in fields of stone.

There are two species of bristlecones, one living in the westernmost regions (Pinus longaeva) and the other inhabiting the eastern regions (Pinus aristata). The trees do not grow very tall, with none over 60 feet and many much shorter. The bristlecone named the Patriarch has a girth (distance around the tree) that is typical of these pine trees; it is just over 36.5 feet around, but it is a relative youngster at 1,500 years. With the short summer growing season high in these mountains, bristlecones typically grow 1/100th of an inch or less in diameter in any given year. The trees stand in isolation, each appearing as a guard, overlooking an otherwise barren rock-strewn landscape. Their needles are remarkable. While occurring five per bundle and about 1 to 1.5 inches in length, they can live for 20 to 30 years before being cast off. This extraordinary leaf longevity gives the trees a stable photosynthetic output and sustains the trees during years of unusual stress when producing new leaf tissue would be difficult. The trees must generate new leaves and cones as well as produce enough reserves for the long winter months, all on scant annual precipitation of about 10 inches.

Ecologists have long noted the peculiar distribution and growth habits of certain plants. Why do plants grow where they do? What adaptations permit them to survive in life-threatening environments? The bristlecones' age and habitat offer several insights. These trees grow in places on earth where no other plant can grow. One answer appears to be the type of soil in which they are anchored. Stands of bristlecone pine grow on outcrops of dolomite, an alkaline limestone substrate low in nutrients but of higher moisture content than the surrounding sandstone. The granite and sandstone formations surrounding the dolomite outcroppings support sagebrush and limber pine, but not bristlecones. At these altitudes, the radiant sunlight is extreme. Dolomite reflects more sunlight than other rocks and thus keeps the root zones cooler and more moisture laden during the important growing season.

Another survival tactic is revealed by taking small core samples of the wood.  These trees have large amounts of dieback (dead wood that is no longer functional), reducing the amount of tissue that the leaves need to supply with food.  For example, one bristlecone over 4,000 years old is nearly 4 feet in diameter, but its functioning conducting tissue is a ring only 10 inches wide. Other characteristics provide bristlecones with survival advantages.  Because their dense resin-filled wood renders them inhospitable sites for colonization by pathogenic fungi or bacteria, they are relatively free from these attacks.  These trees are often struck by lightning, but with the absence of ground cover and decaying leaves on the ground, fire rarely spreads from tree to tree.

As with other trees, the age of these living trees is determined by an instrument called an increment borer, a type of drill that is inserted into the tree trunk at its widest girth. Using a hollow drill bit, a linear core of wood is obtained that can be "read" for the number of annual rings. In this way, the age of a tree can be obtained without cutting the tree down to examine the stump. Each year, a tree will add a layer of wood to its trunk, and these layers become the annual rings that can be observed in a cross section of the trunk. During spring growth of wood, large diameter water-conducting cells are formed; later in the summer, the water-conducting cells produced have a smaller diameter. This difference in appearance between early (spring) wood and late (summer) wood is sufficient to make each growth increment distinctive, and thus counting of the rings possible.