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Oakmead Herbarium: Floristic References

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"The vegetation of Jasper Ridge was originally mainly chaparral. Large portions have been cleared of the bushy growth, quite certainly within a century, and the cleared areas are now expanses of wild oats (Avena fatua and A. barbata), with scattered oaks, especially Quercus douglasii, which is peculiarly characteristic of such secondary areas." -- Cooper (1922), p. 32.

"The ridgetop is bisected by serpentine soils, which are distinct from adjacent soils in being shallow, nutrient-poor, and high in some heavy metals . . . The serpentine soils support a grassland community that is dominated by a diverse array of native annual forbs, but also includes perennial forbs, bunchgrasses, and annual grasses, most of which are also native. Similar grassland communities occur on serpentine throughout west-central California and are considered remnants of native grassland that once occurred on more fertile soils." -- Moloney et al. (1992).

For additional references search the Research Publications database. Geological Time Scale.

Serpentine Prairie Area H, April 1969; Herb Dengler Collection
Serpentine Prairie Area H, April 1969; Herb Dengler Collection


.1) "The abrupt transition from serpentine to greenstone substratum was most clearly displayed by a change in species composition among the non-native annual grasses. Of the eleven species of non-native annual grasses found, only three occurred on both serpentine and greenstone grasslands. Hordeum sp. [H. marinum], Bromus mollis and Lolium multiflorum showed a disproportionate decline in frequency in the serpentine grassland during the drought relative to Vulpia microstachys (Table 4) but also declined to a lesser extent in the greenstone grassland. Lolium multiflorum, unlike the other two alien grass species, was affected by the drought to roughly the same extent in greenstone and serpentine grassland. The most common native annual grass, Vulpia microstachys, tolerated the combination of drought and serpentine substratum much better than did the non-native annual grasses." (p.225)

.3) The influential ecologist F.E. Clements hypothesis first presented in 1920 was that the bunchgrass Stipa pulchra dominated a climax, perennial grassland that constituted the pre-European vegetation of the Central Valley, valleys of southern California, and many areas of the Coast Ranges. See Hamilton (1997) for a critique of Clements' bunchgrass climax theory and differing views of the extent and composition of California's pre-European contact grasslands, including Cooper's (1922).

1) Herb Dengler wrote in a postscript to a letter dated April 18, 1962:

Fifty years ago, Dr. W. S. Cooper, then of Carnegie Institute, began his studies of broad-sclerophyll (Oakwood land-Chaparral) vegetation of  California on Jasper Ridge.  His work stands today as a classic. The  detailed instrumental studies -- the basis for his findings, were in 15 meter square quadrats (I'm not referring to the fenced quadrats seen on the Ridge today).  Anyway, I was fortunate in relocating these, and with photos supplied by Dr. Cooper, I have been able to make some interesting comparisons.  Almost plant for plant Chamise (Adenostoma fasciculatum), and Manzanita (Arctostaphylos crustacea) remain as they were then.  Some Adenostoma that were 18 inches tall then are still 18 inches tall, and little "scrub" oaks (Quercus durata) which were present then are only 3 feet tall. There is a whole sequence of surprising circumstances surrounding this comparison.  I've just about finished this problem.  Last month Dr. Cooper sent me all of his notes relating to a brush fire in the chaparral in 1912.  Twenty-three stumps of Adenostoma and one burned clump of Toyon (Heteromeles arbutifolia) were left standing in one 15 meter square quadrat.  These all sprouted shortly, and flourished. The second year, the bare ground was covered with over 3000 herbs, but with succeeding years these did not appear because of the dense growth of Ceanothus, seedlings which gained ascendancy.  In 12 years the Ceanothus plants were 8 to 10 feet tall, seeming to surpass all else, but as is their habit, they died off so that by the end of the 15th year substantially all that was left were the original shrubs of Chamise and Toyon -- the same plants that existed before the fire. The course of this was in the complete absence of human disturbance. Hillside brush that composes the chaparral may not appear imposing, but the vitality of these plants is very great when left undisturbed and may vie with oaks and redwoods for length of life. (source: Nona Chiariello, email, 18 April 2007).

2) Google Scholar shows 80 citations to Hamilton's paper and the most recent edition of the Barbour (2007) Terrestrial Vegetation of California incorporates his deconstruction of the bunchgrass paradigm: Google Books Result

2.5) "We conclude that effects of interference from non native annuals are important through all life stages of the native perennial N. pulchra. Our results suggest that persistence of native bunchgrasses may be enhanced by greater mortality of annual than perennial seedlings during drought, and possibly by reduced competition for water in wet years because of increased resource availability." Compare with findings of Armstrong, J.; Huenneke, L. 1993.

2.6) Abstract:

We studied the dynamics of serpentine annual grassland in northern California over the period 1983–2002 in a replicated series of experimental plots comprising controls, gopher exclosures, and aboveground herbivore exclosures. Annual rainfall amount varied greatly during the study period, which included two major El Nino events and a period of prolonged below-average rainfall. Gopher disturbance was highly variable both spatially and temporally but was positively correlated with soil depth. Disturbance was reduced but not eliminated from the gopher exclosures and was significantly increased in the aboveground herbivore exclosures. Grassland dynamics were driven by rainfall amounts and distributions that had the most pronounced effects on the dominant plant species, while gopher disturbance had additional effects on the rarer species. Effects of excluding aboveground herbivores were swamped by a large increase in gopher disturbance within aboveground exclosures. Overall species numbers were reduced during a period of below-average rainfall but recovered in subsequent years. There was a large array of different responses of individual plant species to both rainfall and disturbance. Our results provide support for the "insurance" hypothesis, which suggests that biodiversity buffers ecosystem processes against environmental changes because different species (or phenotypes) respond differently to these changes, leading to functional compensations among species. Here, a species that was at very low abundance levels at the start of the study (Microseris douglasii) temporarily increased in abundance to become one of the dominant species in the grassland following a period of prolonged below-average rainfall. We also observed the repeated invasion of the serpentine grassland by the nonnative grass Bromus hordeaceus, which increased greatly in abundance following both of the major El Nino events. The results emphasize the importance of long-term observations in providing a context for shorter-term studies and allowing analysis of plant community responses to climate variation and disturbance, particularly in the face of ongoing global change.

2.7) "Fertilization with nitrogen and phosphorus increased biomass of the resident vegetation substantially in the first season, and within two years allowed the invasion and dominance of non-native annual grasses in patches originally dominated by native annual forbs. Species richness declined with fertilization, as the increased biomass production by invaders sup- pressed some native forbs. Increased macronutrient availability can increase production on serpentine-derived soil, even when other serpentine characteristics (such as low Ca/Mg ratios and high heavy-metal concentrations) have not been mitigated. Observed changes in community structure and composition demonstrate that the invasibility of plant communities may be directly influenced by nutrient availability, independent of physical disturbance".

The implication is that the large size (and presumed competitive vigor) of annual grasses frequently allow them to exclude smaller forb species, unless grass biomass is severely limited, either by resource limitation or by frequent disturbance.

3) Methods and results. [Stipa pulchra was a dominant on his serpentine plots, constituting 30% of "total [above-ground] standing crop" (biomass), determined by harvesting, drying, and weighing. Bromus hordeaceus was 27% of the total biomass on serpentine. California poppy was next, 9%. On non-serpentine grassland the three most abundant species made up 77% of the standing crop: Bromus diandrus, 42%; B. hordeaceus, 22%; and Avena fatua, 13%. He finds dominance and diversity are inversely related and increased productivity is generated by increased dominance. "The grasslands, however, show no relationship between stability and productivity, dominance, or diversity", which I take to mean that the grasslands are essentially recreated de novo each season, and composition and abundance varies, often unpredictably. This is essentially the finding of Hobbs and Mooney (2007). What is particularly interesting is McNaughton's statement, or intuition, that the current annual grasslands are likely more similar to the vegetation that would dominate in the elimination of further disturbance (such as grazing and fire) than the presumed "original prairies" of perennial grasses. McNaughton has been subsequently cited by writers (e.g., Wester, 1981) expressing doubt that the bunch grasslands were as extensive as had been assumed (Burcham, 1957; Heady, 1977).

3.5) James Smith writes, "I do not want to discourage you, but you will not be able to identify every grass that you run through the key in this book or any other, for that matter. Let us stipulate that your unknown grass is included in the key. Here are the potential problems. Your plant is too immature or too old to show you the features used in the key, or you may have top- snatched it so that annual versus perennial or rhizomes- stolons present or absent cannot be determined. Furthermore, you are dealing with living plants that exhibit biological variation. Your plant may be unusually large or stunted, so its spikelet size does not quite fit the description; its lemmas are awned, but they are not "supposed" to be. It seems to have the characters of two species— which may very well be the case because it is a hybrid, and the family is notorious for this phenomenon. I was told a wonderful story about Agnes Chase of the Smithsonian Institution and Ledyard Stebbins of the University of California at Davis. Both were acknowledged experts on grasses. Chase is reported to have said, "Oh, Ledyard, every time you can’t identify a grass, you claim that it is a hybrid!" We also have to admit that the fault might lie with you or me. You misread the key or you were sloppy in your observations or measurements. Or I may be the culprit by constructing key steps that simply do not work properly or by describing the plants incorrectly."

4) Prof. Thomas discusses "one experiment to illustrate the current work at the Preserve and to emphasize the interdependency of all scholarship." The example being the Microseris study begun by Dr. James Price and Dr. Kenton Chambers.

5) Vestal wrote in 1926: "Invasions of the weedy grassland by Baccharis and Rhus diversiloba can be observed, [on Jasper Ridge] and the frequent germination of acorns would result in establishment of many young trees if it were not for the intense competition of wild oats, and the repeated burning, cutting, grazing, and trampling to which the fields are subject". Also see Bocek and Reese, 1992, "Ranching and Farming" in  Land Use History of Jasper Ridge Biological Preserve, pp. 46-87; Franco, 1976, Grazing Effects on Oak Distribution in Jasper Ridge and Adjacent Areas. Vestal left Stanford in 1929 for the University of Illinois, to take the position of assistant professor of plant ecology. The Stanford Daily (11 July 1929) reported:

"For the past few years Dr. Vestal has conducted experiments at Stanford regarding the results of grazing and variations of water supply on different pasture lands. The purpose of this is to find methods of restoring valuable regions that have become barren. On Jasper ridge, Vestal has erected instruments to measure rainfall and has fenced off pasture lands for the purpose of experiments and observation. He intends to return year after year and note the process and conditions of rehabilitating on these purposely over-grazed sections. The results will be assembled, and it is hoped that a better understanding of the methods of protecting verdant grazing land and restoring over-grazed country will result."

The literature on sheep and cattle grazing effects on plant communities, their removal, adaptation of native grasses to large grazing mammals, and related questions is extensive and frequently the conclusions are contradictory. There is also an extensive literature on the destructive consequences of overgrazing (Fleischner, 1994). Naveh & Whittaker (1979) describe a continuum of floristic changes and species richness (in shrub and open woodland) under different grazing regimes, with the lowest values for their functional groups (annual forbs, annual legumes, perennial herbs, annual grasses) at the extremes of the grazing gradient. This phenomena also applies to some grasslands. As noted above, Stuart Weiss believes grazing, fire, and mowing are important management tools for preserving or recovering native plant diversity in grasslands, though any particular method may not be suitable for any specific circumstance. This view is supported by other workers (Edwards, 1992, 1995; Murphy & Ehrlich, 1989; Marty, 2005). Weiss writes (1999, p. 1485) in reference to Bay Area serpentine grasslands:

"The invasive grasses that have dramatically changed California's grasslands are poised to dominate the last refugia [serpentine grasslands] for the native grassland flora and fauna, given the chance. That chance is provided by smog-induced fertilization, but only with the additional land-use change of removing grazing . . . It is ironic that grazing, which has contributed so greatly to the transformation of California's native grasslands, may prove necessary for their maintenance . . ."

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