Zonation in Californian Chaparral:
A Multivariate Analysis Based on a Review of the Ecology Literature
John V. Richardson Jr. (email@example.com)
“One of the most striking natural phenomena of this shrubby grassland area is the zonation of herbs around thickets of shrubs which dominate this flora [i.e., Californian chaparral],” according to Jeffrey B. Harborne (1982, p. 249). Yet, the factors which can account for this phenomenon are poorly understood at best. Hence, a clearer understanding of the nature and respective influence of the following variables would be a significant contribution to the ecology literature: shade, soil, drought (including temperature and humidity), nutrient conditions, elevation and slope of ground, root competition, insect and animal predation, and water-soluble components among others. Furthermore, justification for the adoption a multi-variate approach comes from Swank and Oechel (1991, p. 104) who state: “no single factor can be regarded as causing the absence of herbs in chaparral.”
Hence, the purpose of this paper is to review (as well as to identify, describe and analyze) the ecological literature for evidence which might account for the situation mentioned above. The ultimate goal is to predict the presence or absence of zonation by constructing a testable factor analytic model that takes auto-correlations into consideration. Two research questions are driving this report: what concepts are employed and how are they operationalized as variables? But, most importantly, what might a statistical model for a factor analysis look like?
Eight Potential Variables
First, this section identifies numerous concepts and prior attempts to operationalize these concepts as variables. The following eight variables are listed in no particular order, other than Harborne’s listing (1993); all are potential candidates for causation.
1. Shade (or Light)
Conceptually, levels of light (i.e., electromagnetic radiation) is characterized as heavy, medium, light, and scattered; sometimes the word “patchy” is used as well to describe the understory. This concept has been operationalized as 75-100% shade, 50-75%, 25-50%, and 25% or less respectively (Plummer, 1911, p. 27.) and scored as 1-4 where “direct sunlight impinged on >95, 50-95, 5-50, or <5 percent of the soil surface” by Williams et al. (1991).
Soil consists of porous material including clay, sand, silt, and organic matter over bedrock (which may be highly fractured). Chaparral can be characterized as having poor soil conditions or poor soil development. Poor (i.e., thin or shallow) can be defined as soil which is 60cm or less above granitic rock (Kummerow, 1977, p. 163). Furthermore, some of the literature simply calls the soil—rocky or coarsely textured. Operationally, the uppermost soil layer is defined as 0-20cm (Kummerow, 1977, p. 169). In addition, soil scientists also refer to a soil’s profile as typically possessing layers or horizons divided into three levels: A, B, and C. The humus or topsoil being A while C, at the bottom, is weatherized rock, but can be 30-40 feet deep (Young and Casler, 2001). Between the topmost layer and the bottom is a middle layer (called B, or subsoil), but in chaparral it is “commonly absent” (Young and Casler, 2001). Next, the phrase “edaphic climax community” is also used to describe a chaparral community determined by its soil factors such as alkalinity, poor drainage (i.e., low holding capacity), or salinity rather than its physiographic (i.e., physical geography) situation. In this case, neutral pH is defined as 7 at 25 degrees Celsius where alkalinity is measured as a pH of greater than 7. Internal soil drainage is measured by a percolation test and can be characterized as fast or slow (in the latter case, there is an imbalance between fine and coarse-grained materials); the presence of moisture in the soil should be considered especially in light of fire, whether natural ignition, accidental or by prescription and its subsequent suppression (Minnich, 2001).
3. Drought as well as Temperature and Humidity
Conceptually speaking, drought means a lack of rainfall over an extended period (of months or even years) making for abnormally dry weather in a region; this concept is often measured in terms of annual mean rainfall (see the interactive US Drought Portal at drought.gov) and is further characterized as a moderate, severe, extreme, or exceptional drought (labeled D0-D4, respectively). Discussions of temperature include: a) heat waves (maximum 40˚C), b) thermal shock (i.e., sudden, and rapid change above 120°C) and c) the presence of light, moderate, or intense fires. Soil temperature has been measured at the soil’s surface as well as at various levels, but standardized at 1, 2, and 4cm intervals below the surface. Smoke from fire plays a role in germination as well (see findings below).
4. Nutrient conditions
Organic and inorganic nutrients “are produced through microbial decomposition processes (mineralization),” according to Lynch and Poole (1979). Nutrients are composed of carbon (C) as well as nitrogen (N). Organic nutrients take the form of leaf litter (i.e., dead leaves, bark, and twigs) due to N bacteria fixation. Furthermore, litter can be either fresh or decomposed, but both are measured for their ash-free dry mass of C and N; some attempts have been made to create a litter quality index (Quideau, 2005). Litter deposition rates may be characterized as high or low.
5. Topography (including elevation and slope of ground)
Topography includes elevation and slope. Studies have measured elevations below 2,000 feet; 2,000 to 5,000 feet; and above 5,000 feet (Plummer, 1911, p. 27ff), facing north, south, east, and west. “The position on a slope relative to north is called slope aspect,” according to SBCC (2002); a slope’s declination or inclination is measured in degrees. A “steep” slope may be approaching the perpendicular.
6. Root competition
Chaparral plant/herb/shrub roots (i.e., the below ground biomass) may exist in layers (hence, no competition) or at specific depth zones where there is strong competition for resources (i.e., water and nutrients); similarly, chaparral may have horizontal root extension.
7. Insect and animal predation
The presence of water and plant seeds in chaparral support insects (e.g., the Harvester ant or Argentine ant, gall wasps, yucca moths, and darkling beetles) which in turn attracts animals (e.g., lizards including the Coast Horned Lizard, Phrynosoma coronatum, or the Western Fence Lizard, Sceloporus occidentalis). Their presence attracts other animals including birds, such as the Wrentit and California Thrasher, which live among chaparral for the seeds as well. And, of course, rodents, especially wood rats as well as reptiles, such as the Western Rattlesnake, which are present, all according to Quinn and Keeley (2006, chapter 5).
8. Water-soluble components
According to Millar and Haynes (1998, p. 189ff), volatile or water-soluble chemical inhibitors (i.e., allelopathic agents) may be given off by chaparral plant communities in their resinous foliage or leaf litter; transportation may be atmospheric or by surface leaching. Toxic (in as few as 100 ppm) to germinating seeds, these terpenes include 1,8-cineole, α-pinene, artemisia ketone, α-thujone, β-pinene, borneol, camphor, caryophyllene, dipentene, isothujone (all of which are derived from two C5 isoprene units, or ten carbon monoterpenes phytotoxins: (C5H8)n. The monoterpenes fraction has been measured as being present, with results up to 80% of the total oil.
Based on the above cited studies, one can summarize the research as follows; attention is focused on simple correlations which should not be used in discussions of causation.
Light shade increases seedling survival (Williams et al., 1991). The elevation and slope may be related to shade density; in addition, slope and elevation appear correlated—between 2,000 to 3,000, chaparral is denser on north facing slopes while at 3-5,000 foot elevations, chaparral is denser on the east, south, and west facing slopes. Steep slopes are in the 25° - 70° range and 60-70% slopes are “common” (Young and Casler, 2001). Shallow soils have poor drainage; subsurface drainage can be improved with increased organic matter, especially after fires. The deep rooting pattern in chaparral relates to a Mediterranean climate (of wet winters and dry summers) and layering is present among certain species (i.e., A. fasciculatum roots are found in the uppermost while C. Greggii can be found in lower layers). Nutrient absorption appears to occur in fine roots at upper levels of soil. In addition, low N levels are reported in chaparral tissues (Herman and Rundel, 1989), but periodic brush fires may mineralize nutrients (Miller, 1981) and “C- and N-rich ash resulted in rapid mineralization of N in the burned soil,” during the first year after a fire, according to Herman and Rundel (1989, p. 1229). Otherwise, Schlesinger and Hasey (1981) found “decomposition of leaf litter during the interval between natural fires may be a substantial source of plant nutrients for chaparral growth.” Decomposition rates are higher during the winter; lower during summers and droughts (Peterson et al., 2001). Certain species such as “Ceanothus, which is a notable N fixer exhibited lower N concentration in its litter than manzanita probably because of efficient N reabsorption before abscission” (Quideau et al., 2005). Also, “summer drought may limit soil biological activity and inhibit decomposition processes,” according to Peterson et al. (2001). Deep dormancy or germination is related to temperature and/or smoke in twenty-five species (Keeley and Fotheringham, 1998) and much literature has been generated about heat cues from fires, whether natural, prescribed or accidental (Halsey, 2004). During fires, “micro-organisms were affected lethally at much lower temperatures than those necessary to change nonliving organic matter” (DeBano, 1979, p. 4). Furthermore, “large temperature gradients develop between the surface [538˚C typically, but as high as 716˚C] and underlying soil layers [174˚C at 2.5 cm] because soil is a poor conductor of heat and only a small portion of the energy released during burning is transferred downward into the soil; soil litter does not seem to insulate subsurfaces (DeBano, 1979, p. 6). Notably, “foraging herbivores may eat the seeds and seedlings under or near the shrubs” (Young and Casler, 2001). Finally, “defined ground cover layer” is lacking in chaparral communities and may be due to volatile or water-soluble chemicals (Young and Casler, 2001). Finally, water-soluble components in the form of terpenes are present in A. Californica, M. sativa, Salvia mellifera and Salvia leucophylla (Miller and Haynes, 1998, p. 190) and S. greggii and may be toxic at less than 100 ppm (Weidenhamer et al., 1993).
In short, based on the preceding findings the following statistical model might be tested in the future:
DV IV1 + IV2 + IV3 + IV4 + IV5 + IV6 + IV7 + IV8 where
the dependent variable (DV) is the presence or absence of zonation in Californian chaparral and the independent variables (IVn) are shade, soil, drought as well as temperature and humidity, nutrient conditions, slope of ground, root competition, insect and animal predation, and water-soluble components as operationalized in the ecology literature as discussed above.
ACKNOWLEDGMENT AND REFERENCES
This paper was originally prepared as the final examination in UCR Botany X426 “Chaparral of Southern California,” and taught by Michael Wangler, Geography Instructor at Cuyamaca College. Citations are arranged alphabetically by first author’s last name and were identified using Botany X426 class handouts, Google Scholar, and full-text database searching with citation pearl building techniques via VPN to UCLA YRL’s TLC. Labeled photographs from the May 2010 class taught by Michael Wangler are available at http://picasaweb.google.com/jvrjvr/ChaparralOfCaliforniaMay2010#.
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