Yucca brevifolia
Joshua Tree
I chose this plant because it shares the same first name as me. (That is Joshua, not Yucca.) When I was a toddler, my family would drive to Arizona at least once a year to visit relatives and I got a chance to visit the Joshua Tree National Park and have my picture taken by a few specimens. It was then that I discovered their uniqueness in living in the desert as a tree and for such long periods of time. Later I discovered the album Joshua Tree by U2 and my fascination grew. This project has given me a chance to nuture my early curiosity and learn more about this fascinating plant.
U2's Joshua Tree album cover and tree (Biringer 2007)
Habitat
This plant is found in deserts and xeric shrublands. It is native to the Mojave Desert regions of southern California, Arizona, Utah and Nevada and is the only natural distribution of the plant so it is invasive nowhere (Clark 2008). In this area, it is distributed at elevations from 2,000 to 6,000 ft. These restrictions make it common in the open grasslands of Queen Valley and Lost Horse Valley of Joshua Tree National Park. The Joshua Tree is actually used to demarcate the boundaries of the Mojave Desert since it only grows here.
Distribution map of Yucca brevifolia (Flora of North America 2008)
Life History
Yucca brevifolia is a perennial, monocotyledonous tree although it sometimes grow more like a shrub. (Plants Database 2008) Also, according to Clark et al. (2008), this tree is capable of living for over 200 years because of its slow and highly regulated growth style it uses to cope with the desert environment. This slow growth can start with either a seed or from an underground rhizome, the latter being why you often find them in stands or clumps in an otherwise open desert. The new plant can grow 10-20 cm in its first year and about 10 cm each following year. Another desert adaptation is a shallow root system to soak up the sporadic rains which seems counter-intuitive when one looks at the trees top heavy branches. Even though they are a perennial plant, they don't bloom every year. Instead, they must wait for the proper amount of rainfall to come at the proper time and after a winter freeze. This vital biological step in sexual reproduction is shown its importance by the Joshua tree's strategy of not branching, and therefore allocating resources elsewhere, until after the bloom. This is only over-ruled when their growing tips are destroyed by the yucca weevil. If the proper stimuli are presented, Y. brevifolia produces large inflorescences of white flowers which are strictly pollinated by the yucca or pronuba moth. This moth has co-evolved with many yucca plants so that it is the only one able to pollinate the flowers while laying its larvae on the plants seeds. These larvae and young insects feed on the subsequent fruit in which they are developing inside of. This co-evolution has gone so far that they Joshua tree is able to detect any ovaries that have too many eggs laid in them and abort them so as not to waste resources on a "fruitless" task.
Yucca brevifolia preparing to bloom (Shebs 2005)
Yucca brevifolia in bloom (DeLange 2005)
Biotic Interactions
Because Y. brevifolia is one of the few woody plants to be found for miles in the areas it inhabits, many animals have developed close relationships with these trees. (Clark et al. 2008). These include birds, mammals, reptiles and insects which all might depend on the Joshua tree for food and shelter. In addition, humans have learned to utilize it when sharing the same habitat as Y. brevifolia. The Cahuilla Native Americans use the plant's leaves to weave things like sandals and baskets as well as depending on the seeds and flower buds for available food for harvest. These, of course, are shadowed in comparison to the mutualism with the moths stated above.
Conservation
According to Clark et al. (2008), climate change is a cause for major concern when it comes to Y. brevifolia. This is because it could drastically damage the ecosystems of the parks that the Joshua tree inhabits. This problem is accentuated by the inability for the plant to migrate long distances to more favorable climates due to the extinction of the giant ground sloth, around 13,000 years ago, which is the only animal able to transport the seeds for the plant. This problem was examined more closely in a study by Dole et. al (2003) in which they predicted the distribution of Y. brevifolia after a climate change and then repeated with a simoultaneous increase in CO2. They predicted that with only a climate change, the distribution of the Joshua tree would change dramatically; however, the total area occupied by the tree would only decrease slightly. When looking at an increased CO2 concentration, the researchers found that this not only helps to increase the freezing tolerance of the plant, but also affects photosynthetic assimilation, stomatal conductance, water use efficiency and nitrogen use efficiency. These other physiological changes would probably have a feedback into freezing tolerance and cold acclimation or seedling recruitment. Based on this information, the researchers were able to predict a different, and slightly larger, distribution as that predicted from only a climate change. This study makes the future for Y. brevifolia to seem less bleek; nevertheless, the tree is limited in its migration because of its lack of seed dispersal and proximity required for rhizome growth.
Deserts & Xeric Shrublands
These habitats are most known for their dry climate. According to Wikipedia’s article on this habitat, deserts receive ten or less inches of rain per year on average. This qualifies them as an arid, where sparse grasslands, shrublands or woodlands can thrive, or hyper arid climate, usually devoid of life, that has a strong moisture deficit. This means that the annual potential loss of water from evapotranspiration exceeds rainfall. Most deserts solve this problem by having their own water reserves, which usually come in the form of distant glacial melt water (Graham-Rowe 2006). Wikipedia states that these habitats appear in tropical, subtropical and temperate areas of the world. Deserts are also characterized by sandy or rocky soils low in organic material and that are often saline or alkaline. All of these hindrances do not stop life from invading these “barren wastelands,” however. Xerophytes such as succulent plants, geophytes, sclerophyll, and annual plants all grow in deserts and animals like insects, reptiles, arachnids, birds and mammals can live here, although many must utilize a nocturnal lifestyle to avoid heat and water loss. These plants and animals have all evolved adaptations to help them survive in low moisture conditions. Because of its characteristics, the major disturbance even that shapes this habitat is drought. The long stages of drought have formed the arid landscape and pushed its inhabitants down their evolutionary paths to survive.
Deserts can be found all over the world. There are approximately 3.7 million square kilometers of deserts on our planet, one-quarter of earth’s land area (Graham-Rowe). Most deserts are found north and south of the equator because of Hadley cells dropping hot dry air on top of these areas. In addition, many deserts are found near mountain ranges and are formed by the rain shadow effect keeping all the moisture on the far side of the range. The range of a particular desert is often demarcated by the presence of a unique native plant that is restricted to that desert in that part of the world (Graham-Rowe). Some examples are the saguaro cactus found in the Sonoran Desert and Joshua Trees found in the Mojave. The importance of these habitats is often overlooked, nevertheless, because of their wide distribution, they are more important to our biosphere than they might appear. Deserts are almost always teeming with life that is specially adapted to survive under such harsh conditions.
Threats to these habitats are dissimilar to those seen to lush environments as forests and wetlands. In an article by Graham-Rowe (2006), a report by the UN Environment Programme states that deserts are in danger of becoming hotter and drier that ever before. They report that the temperature will rise by seven degrees Celsius and rainfall will drop twenty percent. If this happens, even those organisms that have spent centuries adapting to the harsh desert habitat will find difficult, if not impossible, to continue thriving here. If our climate continues to change in the direction it is going, the deserts will not only become harsher but their distribution will probably expand. Current deserts will extend their grip and new deserts will form where other habitats once had a foothold. Our best chance of stopping this is to continue to fight global warming in every way possible. We can also find other sources of obtaining water than tapping into ground water or other reserves that may be the life line for a distant desert.
Works Cited
Biringer, Brad [Internet]. [updated 2007 Nov 2]. Joshua Tree National Park; [cited 2008 Feb 18]. Available from: http://www.joshuatreenationalpark.net/u2tree.htm
Clark, Curtis et. al [Internet]. [updated 2008 Feb 15]. Wikipedia; [cited 2008 Feb 18]. Available from: http://www.wikipedia.org/wiki/Yucca_brevifolia
Delange, George. Delange, Audrey [Internet]. [updated 2005 Mar 16]. Delange; [cited 2008 Feb 18]. Available from: http://www.delange.org/JoshuaTree/JoshuaTree.htm
Dole KP, Loik ME, Sloan LC. 2003. The relative importance of climate change and the physiological effects of CO2 on freezing tolerance for the future distribution of Yucca brevifolia. Global and Planetary Change [Internet]. [cited 2008 Feb 18] 36(1-2): 137-146. Available from: http://journals.ohiolink.edu/ejc/pdf.cgi/Dole_Krishna_P.pdf?issn=09218181&issue=u36i1-2&article=137_trioccifdoyb
Graham-Rowe D. 2006. The heat is on for desert life. New Scientist [Internet]. [cited 2008 Mar 2] 190 (2555): 20-20. Available from: http://web.ebscohost.com/ehost/detail?vid=4&hid=13&sid=9583559f-3d36-467f-b411-f5dda75843d3%40sessionmgr102
Shebs, Stan [Internet]. [updated 2005 Mar]. Wikimedia Commons; [cited 2008 Feb 18]. Available from: http://commons.wikimedia.org/wiki/Image:Yucca_brevifolia_bud.jpg
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