Cross section of stem crown of early segmenting plant; note the hollow center; the adjacent dark, dead wood; the growth increments in lobes of active growth at peripheral areas, and the necrotic wood or spaces separating arcs of active growth.
Cross section of stem crown of segmenting plant; note cracks in wood and necrotic areas separating lobes of active growth (these lobes are sections through the peripheral branches; later branches would have arisen at the peripheral edge of these lobes.) |
stem crown thus
disappear. The plant has now become a done, composed of several independent
stem crowns derived from one original seedling and, hence, genetically
identical. As the several stem crowns grow, new branches continue to be
produced at the peripheral arc of a stem crown segment, older branches die back
at the interior or central part of the clone where eventually a bare area is
developed. The clone, therefore, spreads across the ground much like a circle
across a smooth water surface. However, the creosote clone does not grow at the
same rate in all directions. Older clones more often become elliptical or
elongated tear drop shapes. Usually, a mound of sand accumulate in the central
area to a depth of a half meter or so. The mounds of sand, and hence the
central bare area of the clonal creosote ring, commonly attain diameters of
several meters. Occurrence and Age of Large Clones In a few areas of the Mojave Desert of California, the clonal creosote rings get even larger. For example, on the lower west slopes of the Argus mountains, many hundreds of creosote rings exceed 10 meters in diameter and some exceed 20 meters across. Near Lucerne Valley, California, one large clone has a long axis of about 23 meters and an average radius of 7.8 meters. This particular clone has been named "King Clone" and was the subject of a study which showed that the satellite plants around the circumference of the clone bad identical lsoenzyme patterns and hence are genetically identical as would be expected in a clone. Another large clone near Kramer Junction, has a long axis of 36 meters and has called "Emperor Clone." But what does large size mean in terms of plant age? Developmental studies of young seedlings show that stems grow radially at the average rate of 0.73 mm per year. Does this slow, average growth rate persist indefinitely? Evidence from radiocarbon dating of old wood does indicate that slow growth rates persist over extended periods of time. On this method, chunks of old wood are located under the central mound of sand and are carefully excavated after measuring the distance to the nearest living stem crown and to the geometric center of the clone. The distance from the wood sample to the living crown, divided by the age of the wood (determined by radiocarbon analysis), gives an average radial growth rate per year over recent centuries. Assuming a substantially constant average growth rate, back through time, the entire distance from the clone center to the living stern crown can be divided by the average growth rate to obtain an estimate of the total age of a done. On this basis, the average radial growth rate for 23 clones ranged from 0.34 mm per year to 1.37 mm per year, with a grand average of 0.66 mm per year.' Applying the grand average growth rate to the average radius of King Clone, yields an age estimate of 11,700 years. This estimate is probably conservative, since nearby clones had growth rates below the grand average, in contrast, clones in more sandy or more mesic areas had growth rates above the grand average Emperor clone is situated in a sandy, more mesic area and, therefore, probably grows more rapidly. Despite its larger size, it probably is not older than King Clone. Nevertheless, this line of evidence indicates that the large clonal creosote rings have been continuous residents of the Mojave Desert since the last ice age. |
