Discussion of Existing Broadcast Native Seeds
Beginning in 1992 and then again in 1994, seeds of various types of native vegetation and grasses were broadcast in various areas of the western Antelope Valley. The 1992 activity was known as the Emergency Watershed Protection (EWP) program where 2500 acres of severely eroded land were seeded. Additionally, seeds were applied using a variety of application techniques to smaller plots in three different locations in 1994. The technical steering committee of the DustBusters Taskforce has decided to evaluate this vegetation every 2-3 years in the future rather than annually. Because of the age and maturity of much of this vegetation, the vegetation growth rate and mix is slowing considerably compared to earlier years. Because of abundant and above average rainfall in 2004-2005, we expect the “annuals” to be plentiful and the “barren/senescent” to be markedly lower. We reported on this native seeded vegetation in lasts year’s A&WMA annual conference paper.
Seedlings
The San Diego State University Soil Ecology and Restoration Group (SERG) collected seeds from native plants in the Antelope Valley in the spring of 1998. In their greenhouses in San Diego, they germinated and grew these seedlings for planting in the Antelope Valley in January and February 1999. Seedlings were planted at two locations on Los Angeles World Airports (LAWA) Palmdale Regional Airport land using a variety of irrigation, soil amendment and geographical configurations. Site names were based on proximity to nearby roads, thus creating the 50th and 90th Street sites (Figure 1). After a few years, we noted that the 50th Street site had non-natural elevated soil nutrient levels which emanated from secondary effluent flooding of this land. We abandoned this site in the fall of 2003 and created a replacement site where soil nutrient levels typically found in the Antelope Valley would be better represented. This third site was planted according to the same configurations as the first two sites, in November 2002, and was named 85th Street. Land for the 85th Street site was provided by Phil Giba, of Giba Farms. Each location contains several different plant species including three species of A triplex, Mesquite (Prosopis glandulosa), Larrea (Larrea tridentate), and Indian rice grass (Oryzopsis hymenoides). These plants have two types of irrigation, surface watering and deep pipe. Three types of surface-applied soil amendments are being used including bark (wood chips), compost (mulch) and control. A total of 2,012 seedlings were planted at the three sites. During the first 18 months following planting, these seedlings were protected from herbivory by structural barriers (tree-pees and tubex).
Until this latest winter (i.e. 2004-2005), because of La Niña conditions, rainfall has been sparse since these 10-13 cm seedlings were planted. Annual precipitation data in the Antelope Valley from 1999 through 2004 are shown in Table 1. Monthly irrigation of 0.5 gal per plant occurred at the 50th Street site through July 2000. At the 90th Street site, high winds blew off many of the protective barriers resulting in significant herbivory damage. Seedlings were replanted at this site in March 2000 with supplemental watering occurring until January 2001. At the 85th Street site, monthly irrigation was performed for 14 months following planting and ceased in January of 2003. Typically, in a normal precipitation year, supplemental watering need only occur through the first summer. In dry years, watering through the second summer may be necessary.
The story of these seedlings is told in Tables 2 through 5. Monitoring of the 50th Street was performed for the last time in June of 2003. Table 2 displays the survival rates for all plants. Table 3 provides the amount of annual growth by treatment at the three sites. Table 4 illustrates the annual increase in growth at the 85th and 90th Street sites from time of planting to June 2004. Table 5 gives the chronological history of the soil nutrient levels at the 85th and 90th Street sites and the undisturbed “control” site.
In October 2003 and March 2004, soil samples were collected at the 85th and 90th Street sites and from an undisturbed “control” site nearby. The results of the March 2004 soil samples taken at 90th Street show a decrease in organic matter. A decrease in organic matter indicates that it is being broken down (by fungi and bacteria) and made available to the plants in the form of nitrates. Nitrogen levels often go up following organic matter break down, something which is indicated in the data. Average sulfate levels at 90th Street have increased from 13.2 ppm in the spring of 2003, to 22.6 ppm in the spring of 2004. It is very common, particularly in arid climates, for there to be wide fluctuations in sulfate concentrations as environmental conditions play a large role in their mineralization process. During the winter season sulfates usually drop as they are leached out by rainfall. In extremely dry years they begin to build up, but seldom reach levels that are harmful to plants. Sulfate levels in desert soils can exceed 20 ppm without any adverse effect on the plants. The below average rainfall observed during the 2003 -2004 rainy season is likely to have contributed to the high sulfate levels.
Soil nutrient levels on 85th street are typical for recently farmed land. Soil samples have only been analyzed four times for this site, once before planting, and three times after planting. The site was used for growing onions between the first and second data collecting occasion. Some expected changes seen in the data collected following seedling planting were decreases of organic matter, nitrates, and sulfates. Further decreases are expected as the data still illustrate that nutrient levels are comparable to the high levels characteristic of agricultural soil.