Project Objectives

The hydroecology of selected vegetative species in the Tierra del Sol watershed in San Diego County, California, will be studied. Hydroecology studies the water affinities of vegetative species and their adaptations to moisture gradients. The latter are present in the environment due to local geology and geomorphology. The Tierra del Sol watershed is a 2.75 square-mile watershed located immediately north of the U.S.-Mexican border and east of the Campo Indian reservation, in the San Diego backcountry.

The water affinities of two vegetative species present in the Tierra del Sol watershed will be studied. These are the blue elderberry (Sambucus Mexicana) and the red shank (Adenostoma sparsifolium). In Tierra del Sol, both of these species appear to be aligned along readily identifiable lineaments, revealing the presence of clearly defined moisture gradients (Ponce, 2006). For red shank, in particular, the lineaments can be observed in digitally enhanced infrared images.

A cause-effect relation between the lineaments and the prevailing soil moisture will be sought. The outcome will be the seasonal and mean annual water supply of these two species in their respective lineaments, as measured by the soil-moisture content at the root level. The relation between selected individual specimens and their soil-moisture environment will be analyzed, as well as a comparison with a suitable control species such as the chamise (Adenostoma fasciculatum) (Marion, 1943). Four sites for each of three species (two target species and one control species), with a total of twelve measuring sites areconsidered in the testing program. Biweekly measurements will span a period of 18 months.

The benefit of this project will be a better understanding of the relation between vegetative lineaments and rock fractures in the Tierra del Sol watershed. The objective is to show that the lineaments exist because of the presence of the rock fractures; or, in other words, that the latter provide the moisture gradients which facilitate the lineaments' existence.

Background

Two closely related chaparral species, chamise (Adenostoma fasciculatum) and red shank (Adenostoma sparsifolium) are significantly represented in the Tierra del Sol watershed. Although congeners, these two species are not at all alike in appearance. Stands of chamise chaparral are dull, dark-green in color and uniform in appearance. The mature chamise plant is a medium-sized shrub 2 to 8 feet tall, with sparse leaf litter. In contrast, red shank is a tall, round-topped arborescent plant 6 to 20 feet high, with thick, naked stems and considerable leaf litter, 0.5 to 2 inches in depth. Red shank appears to dominate over chamise on sites with higher moisture content, organic matter, and nutrient availability (Beatty, 1984).

Hanes (1965) noted that though both Adenostoma species resumed growth in early winter, chamise displayed a sudden flush of growth in the spring, whereas red shank grew more steadily, with continued growth throughout the summer and fall. Chamise flowered in April and May, whereas red shank flowered abundantly in August. While chamise has an apparent enforced dormancy in the fall, red shank has been known to experience substantial autumnal growth, suggesting a sustained moisture source.

Despite being sclerophyllous, red shank can be shown to violate several definitions of sclerophyllous plants. First, it remains physiologically active during summer drought; thus, it is drought tolerant without being drought dormant (Hanes, 1965). Secondly, its root morphology is unique among the chaparral. Its shallow root system suggests that its moisture for summer growth must come from the top layers of the substrate. Red shank seems to be a type of shrub well adapted to drought conditions, but lacking the obvious morphological characteristics suggesting such adaptability (Shreve, 1934). Thus, the water affinities of red shank lie in between those of the xerophytes, which are well adapted to drought, and those of the mesophytes, which habitually require a more sustained moisture source. In semiarid regions, this ecological niche must be connected to the presence of groundwater, or, at the very least, to plenty of vadose moisture, which suggests the close proximity to groundwater.

The blue elderberry is present in the flood plains and other mesic locations of the Tierra del Sol watershed. The water affinities of the blue elderberry and its preeminent role as an indicator of groundwater have been known for almost one hundred years. Early references to the blue elderberry's moisture habits are scattered throughout the literature. Ball (1907) has stated that in Southwestern Nevada and Eastern California, the elderberry tree is unknown, except in the vicinity of water. Spalding (1909) has noted that the blue elderberry, while being structurally a flood-plain mesophyte, is nevertheless very limited in its range, growing where there is an ample supply of moisture, such as near irrigation ditches.

In the Southwestern United States, particularly in Southern California's inland hillslopes, the blue elderberry may be thought of as a meso-hygrophyte, to indicate that while structurally a mesophyte (Meinzer, 1927), its water habits resemble those of a hygrophyte, which thrives on wet soil and is more or less restricted to wet sites. In semiarid regions such as the San Diego backcountry, this unique ecological niche appears to be connected to the presence of groundwater.

Plan

The study will be divided into two phases:

1. Field data collection

   Twelve soil-moisture sites will be established in the field, four sites for each of three species. The sites for the blue elderberry will be selected in the vicinity of the Morning Star Ranch, in Tierra del Sol. The owners of the ranch, Mr. and Mrs. Ed Tisdale, have agreed to facilitate the field data collection on their premises. The red shank sites will be selected in the vicinity of the existing lineaments shown. There are hundreds of specimens of red shank on these lineament. Many of them would be suitable for this study. The lineament crosses the Willoughby and Hope ranches. The owners, Helen Willoughby and Jerry Hope, have agreed to facilitate the field data collection on their premises. The sites for the chamise will be chosen off the lineaments in the Tierra del Sol watershed, within the Morning Star, Willoughby, and Hope ranches. The measurements will be fortnightly, for a period of 18 months. Local experience suggests specific attention, in the field mesurements as well as in the data analysis, to the effect of the gravitational pull of the moon (a full moon) on the shallow groundwater levels in the vicinity of the fractures.
   

2. Data analysis and report writing

   The analysis will seek to support the hypothesis that the study sites, corresponding to the blue elderberry and red shank, have a sustained moisture source which is weakly dependent of the season. On the other hand, the control site, corresponding to the chamise, would have a soil moisture which is strongly dependent on the season. The differences in moisture would be correlated with the presence or absence of vegetative lineaments, the fractures on which they are sustained, and the local moisture gradients that these fractures enable. A report will be written with the documented research findings. A version of the report will be submitted for publication in a recognized journal, such as the Journal of Biogeography or the International Journal of Ecohydrology and Hydrobiology.

The soil moisture will be measured with an Aqua-Pro "capacitance" moisture sensor. The moisture probe transmits a very low powered radio frequency through the soil to measure moisture. There are two copper bands (radio antennas) on the end of the sensor. One antenna transmits a low powered radio frequency signal that is received by the other antenna. The proprietary microprocessor can determine the moisture by the change in frequency of the signal it receives. The more moisture in the soil, the greater the shift in the frequency of the signal. The moisture sensor has a repeatable accuracy of plus or minus 2%, and the moisture meter has a resolution of 1% (dry is 0% and water is 100%).

Aqua-Pro sensors measure soil moisture every inch of the way from the soil surface to the bottom of the access tubes. The profiling probe is lowered into a polycarbonate tube that has previously been inserted into the soil. The polycarbonate tubes come in 1-meter lengths but can be extended to 2-meter or greater lengths if necessary, by connecting them together. The readout from the radio frequency sensor is percent available moisture. The field procedure is: (a) drill a hole to the maximum depth of moisture measurement desired, estimated at 2 m at this time; (b) insert the polycarbonate access tube in the hole; and (c) insert the probe in the access tube and lower it to the bottom to record moisture.

Timelines

The project duration is 18 months, starting January 1, 2009 and ending June 30, 2010. A breakdown of tasks and estimated duration is given below.

1. Field data collection: 18 months, January 2009 - June 2010.

2. Data analysis and report writing: 3 months, April - June 2010.

Ball, S. H. (1907). A geologic reconnaisance of Southwestern Nevada and Eastern California. U.S. Geological Survey Bulletin No. 308, United States Government Printing Office, Washington, D.C.

Beatty, S. W. (1984). Vegetation and soil patterns in Southern California chaparral communities. In B. Dell, editor, Medecos IV: Proceedings, 4th International Conference on Mediterranean Ecosystems, August 13-17, 1984, The Botany Department, University of Western Australia, Nedlands, Australia, 4-5.

Hanes, T. L. (1965). Ecological studies of two closely related chaparral shrubs in Southern California. Ecological Monographs, 35(2), 213-235.

Marion, L. H. (1943). The distribution of Adenostoma sparsifolium. American Midland Naturalist, 29(1), January, 106-116.

Meinzer, O. E. (1927). Plants as indicators of ground water. U.S. Geological Survey Water-Supply Paper No. 577, United States Government Printing Office, Washington, D.C.

Ponce, V. M. (2006). Impact of the proposed Campo landfill on the hydrology of the Tierra del Sol watershed. Online report. http://tierradelsol.sdsu.edu.

Shreve, F. (1934). The problems of the desert. The Scientific Monthly, 38(3), March, 199-209.

Spalding, V. M. (1909). Distribution and movement of desert plants. Publication No. 113, Carnegie Institution of Washington, Washington, D.C., 5-17.