Joshua Tree - Yucca brevifolia (Asparagaceae) ThunderSky (c) 2014 Weve all heard of charismatic megafauna, you know things like cheetahs, pandas and Galapagos tortioses. Well, the Joshua Tree certainly must be numbered among the charismatic megaflora. Its gnarled and twisted branches terminated by tight round clusters of swordlike leaves look for all the world like fists raised in anger to the heavens. Although most people have never seen a Joshua tree in the phloem sort if speak, most are familiar with it nonetheless. Its other worldly appearance and its abundance within a 90 minute drive of the major Hollywood studios virtually assured that the Mohave Desert, where it is king, would figure prominently in a scifi and monster movies of the 50s and 60s, such as in the 50s blockbuster Them! Form a botanical standpoint, Yucca brevifolia is a signature species of the Mojave Desert, an elevated plateau just to the north if the Sonoran Desert, stretching from Southern California east across Nevada, northwestern Arizona and a small corner of extreme sw Utah. The Mojave is still considered a warm desert, temperatures routine exceed 100°F for five months if the year and can reach 120°F. But because much of the desert lies above 1,000 meters, winters are cold at night with frequent freezes and occasional snow. Im fact, Joshua Tree requires cold stratification and will not flower unless it has been exposed to an extended period of nights below 32°F. The number and density of trees depends upon moisture availability which is a function if a combination of annual rainfall, elevation, exposure and soil type. In some areas, such as Joshua Tree National Monument near Cima Dome, California, the species can form a dense forest. More typically, one finds Joshua Trees scattered as widely spaced individuals among creosote bush (Larrya tridentata) below 1,000 meters and western Juniper (Juniperus occidentalis) above 1500 meters. Although tree-like in habit, Joshua trees are monocots and as such do not produce true wood in the way an oak or conifer makes wood. Instead, the saplings pass their youth looking much like other yuccas with a flush if leaves at soul level as the stem increases in girth. Instead of rings of woody xylem produced by a cambium, Joshua trees produce a stem consisting of ground tissue through which numerous fibrous vascular bundles are scattered. Palms attain a tree-like habit in a similar fashion. Because Joshua trees dont produce annual rings, the age of any given individual can be difficult to ascertain. Nevertheless, annual growth rates in stem length are more or less consistent from year to year so that it is possible to estimate the age of a given individual with some accuracy. In good conditions, most Joshua trees live to be several hundred years old, while a few of the largest trees have estimated ages exceeding a thousand years. Although not currently threatened, Joshua tree populations at lower elevations are potentially threatened by climate change. As the climate in the southwest warns are dries, many populations of Joshua trees below 1,000 meters may not receive sufficient rainfall nor the necessary cold in enter to reproduce successfully. Ultimately the tree may disappear from many if the areas in which it currently occurs, most notable Joshua Tree National Park and National Monument. There is also concern about the ability of Joshua Tree to migrate to wetter, cooler areas as the climate changes. Currently, there are no known animal vectors that disperse the trees heavy seeds. This was not always the case. During the Pliocene and Pleistocene, the Shasta Giant Ground Sloth (Nothrotheriops shastensi) traveled much of the West, including the Mojave. Fossilized dung and stomach contents yield chewed leaves, bits if trunk and most importantly, seed-bearing fruits of Joshua Tree. The Sloth went extinct 18,000 years ago along with most of the test of North Americas megafauna. Whether Yucca brevifolia can disperse successfully to new regions without the sloth is anyones guess. The common name Joshua Tree derives from Mormon settlers who crossed the Mojave Desert in the mid-19th century. The trees unique shape reminded them of a Biblical story in which Joshua reaches his hands up to the sky in prayer.
Posted on: Wed, 13 Aug 2014 21:58:30 +0000
Trending Topics
Recently Viewed Topics
© 2015