Friday, October 1, 2004

Plant for the Day: Joshua Tree, Yucca brevifolia

Joshua trees occur in certain parts of the Mojave Desert between 2,000 and 6,000 feet elevation. They are members of the lily family, and, with trunks made of thousands of tiny fibers, are one of the few tall tree species in the Mojave Desert. Their creamy white flowers are pollinated exclusively by the yucca moth—a service for which the moth is repaid. The adults lay eggs in the ovary of the flower, then the larvae feed on the seeds once the fruit ripens. The Joshua tree is a valuable component of the ecosystem where it occurs. Many animals depend on it for food and shelter. The leaves were woven into shoes and baskets, and flower buds and seeds were eaten by native peoples of this area. Later, Mormon settlers used trunks and limbs for fencing and fuel.

We started our ride today right after midnight. We were fortunate that the air quality wasn’t too bad in Los Angeles. The topography, the typical climatic conditions at this time of the year, and the millions of cars on the roads around LA set the stage for frequent air pollution problems. The LA metropolitan area is in a basin, and when high pressure dominates, air tends to stagnate under an inversion layer that traps pollutants within the basin. Although this pattern is very common in the summer, there has been a bit more atmospheric instability this year than there is most years.

The earlier rains in southern California have also diminished somewhat the threat of wildfires, which are common in this area in the fall. The native vegetation in this area, chaparral, is dominated by shrub species whose life cycles depend on recurrent fire. Fire opens up the vegetation, thereby increasing light levels, and stimulates sprouting in some species and seed set in others. Although blackened skeletons of shrubs are all that’s visible right after fire, once the winter rains come, plants regenerate, bringing new life to the chaparral. The fall is a time when the vegetation has a low moisture content, having endured the long dry summer typical of these Mediterranean climates, and will burn readily if ignited. Santa Anna winds, which are hot, dry easterly winds that blow towards the coast, are common in the fall and facilitate the spread of wildfires in this area.

We eventually turned to the north just west of Desert Hot Springs on our way up to Las Vegas. We had to skirt around Joshua Tree National Monument (JTNM). JTNM is a beautiful chunk of Mojave Desert. I spent a lot of time there back in the early 1980s, since it was one of the study areas for my doctoral dissertation research. It’s a fascinating place both geologically and biologically. Parts of the monument are visually dominated by large, rounded, fractured boulders. They are made of a granitic-type rock, monzonite, which formed from molten material that solidified below the earth’s surface millions of years ago. Several sets of fractures, or joints, formed in the rock; and once the overlying bedrock was eroded away, these joints were avenues for weathering and erosion that allowed water to smooth the angular rock into the rounded forms that are so prominent in this landscape. The monument is also home to a diversity of desert plants and animals. The Colorado Desert, a simpler type of vegetation occupying relatively low elevations, and the more floristically diverse Mojave Desert, occurring at higher elevations where there’s more moisture, come together in JTNM. Each species has its own way of coping with the harsh physical environment found here. Some perennial plants are drought deciduous, dropping their leaves when the soil is dry. Others have small, waxy, evergreen leaves that retard water loss. Cacti and other succulent plants take up water quickly after the rare rains and store it in their tissues for later use. Annual plants dodge the dry conditions completely, becoming active only when sufficient winter rainfall triggers germination. Then they transform the bare desert soil to a carpet of color for a brief period before they set seed and die.

After we circumnavigated JTNM, we turned north again and crossed Bristol Dry Lake. The lake bed occupies the floor of an internally drained basin—a common feature in the Mojave Desert. This area is part of the Basin and Range, a region of the West whose topography has been formed by normal faulting, as the earth’s crust has been stretched in an east-west direction. North-south trending upfaulted mountain ranges, or horsts, alternate with downfaulted valleys, or grabens. Because many of the basins lack external drainage, streams in the basin deliver water to the basin floor where it accumulates in a wet period (some winters), only to evaporate during the more typical dry periods. Impurities in the water are typically left behind as precipitates, and in Bristol Dry Lake, this process has formed the mineral halite, or sodium choride, and other salts. The halite deposits are thick enough that they are mined for table salt—we could see the salt evaporators right along the road as we rode across the lake bed.

From there we cycled by Amboy Crater, to the junction with old Route 66. Amboy Crater is a basalt cinder cone embedded within a lava field that covers about 30 square miles. Both features formed from volcanic eruptions occurring on the northern edge of Bristol Dry Lake about 10,000 years ago. Cinder cones are small hills that are built during moderately explosive eruptions when fountains spew lava into the air, the lava solidifies instantly to form cinders and volcanic bombs, then these fragments accumulate in a cone-shaped landform around the vent. Amboy Crater is actually a group of nested cones, surrounded by pahoehoe lava—a type of lava flow that has a smooth ropey surface.

North of Amboy Crater, we crossed through Mojave National Preserve (MNP), just as the sun was setting. What a beautiful landscape when it’s bathed in the golden evening light! MNP was created in 1973 to protect the variety of desert ecosystems the preserve encompasses. It includes scenes similar to those we saw earlier in the day—granite boulders like those of Joshua Tree National Monument, and cinder cones similar to Amboy Crater but older—as well as some new landscapes. The mountains in the northeastern part of the preserve are high enough (nearly 7500’), hence cool and moist enough, to support white fir forests. In the cooler wetter climates of the last glacial period, white fir forests were more widespread throughout this area; but currently this far south, this species is restricted to isolated populations occurring on just the highest mountain ranges.

Darkness had fallen by the time we reached the northern part of the preserve and rode the rest of the way into Las Vegas. What a stark contrast to leave the quiet darkness of the desert, where the howling coyotes were the only sounds and the waning moon provided the only light, and come into Las Vegas, with its flashing neon lights and the hubbub of a city that never sleeps!