Annual grasslands characteristically experience a succession of seasonal phases which provide for the coexistence of various species by allowing them to make their major demands upon environmental resources at different times. Thus, an ecosystem furnishing certain resources at a limited rate is enabled to support a larger total biomass.
The grasslands of southern California consist principally of annual grasses coexisting with the few remaining native perennial grasses. Characteristic also is the inclusion of forbs which are distinctly either early spring or late summer plants. Many spring forbs, annual grasses, and summer forbs are observed to germinate in approximAte unison shortly after the first winter rains. In spring, however, differing individual responses to environmental opportunities and stresses segregate these species along a time gradient. The effect is that the grassland then passes successively through a spring phase dominated by filaree (Erodium spp., Geraniaceae), an intermediate phase dominated by annual grasses (e.g. Avena spp., Poaceae), and a summer phase characterized by tarweed (iloll4marpha spp., Hemizonia spp., Asteraceae).
A problem of intense interest to plant ecologists is the means by which this segregation into phases is accomplished. If segregation is achieved independently of environmental influences, then what are the mechanisms? If not, then which environmental factors serve as cues indicating to each species when its time for action has come?
To determine the value of specific environmental factors in segregating annual grassland plants along a time gradient, representative species were studied in field and controlled-environment conditions. Growth of the plant species in field plots was observed over a three year period, and the individual and collective patterns characterized and quantitatively described. Climatic and microenvironmental factors were simultaneously monitored and then correlated with plant growth. Effects of intra- and inter-specific competition were examined by sowing all possible combinations of the three species into sterilized soil of field plots, each combination repeated in three different densities. Effects of temperature, day length, and soil moisture tension were determined by analyzing development of the species grown in controlled environmental conditions.
The fundamental contributions of this study appear to fall naturally into two groupings. First, temporal phase segregation of representative species in the southern California grassland is quantitatively and graphically described. We believe this description is long overdue. Second, controlled-environment laboratory experiments, examining hypotheses suggested by field correlation studies, indicate that filaree, slender oat, and tarweed are segregated along a temporal gradient by their differing individual responses to particular combinations of temperature, dayiength, and soil moisture tension.
Bearing in mind the fact that results obtained in controlled environments are not directly transposable to the field, where a multitude of possibly unrecognized factors may be effective, certain limited conclusions appear to be warranted by the present study. Phenologic advancement of filaree appears to be favored by lower temperatures, shorter days and higher soil moisture tension. Slender oat is advanced more rapidly by moderate temperatures and soil moisture tension, and by long days. Tarweed shows marked advancement at higher temperatures and longer days, when soil moisture tensions are moderate.
Statistical correlation of seasonal growth or phenological development with local environmental factors, although intuitively appealing, is of little use in determining causal relationships. One .reason is that the dependent variable (plant growth or phenology) is an expression of physiological processes that may not be the same from time to time throughout the season. Also, the questionable assumption is made that successive phenological events are independent. For these reasons, correlative results obtained from limited ecological studies are most profitably used only to suggest hypotheses for subsequent experimental testing.
Intra-specific and inter-specific competition, while slightly affecting individual species' development, are not significantly important in temporal phase segregation of southern California annual grassland species. But if these factors are not gignificant, others exist which definitely are. This study indicates that the time a southern California grassland species spends in the vegetative stage is not an inherent constant. Rather, it is determined by the individual species' physiological and phenological responses to the quantitive availability of water, heat and light. It is the synergy of individual responses which accomplishes the temporal segregation described by this study.
Earl W. Lathrop
Leonard R. Brand
Conard D. Clausen
Norman L. Mitchell
Paul Y. Yahiku
Doctor of Philosophy (PhD)
Year Degree Awarded
Date (Title Page)
Library of Congress/MESH Subject Headings
Grasslands -- California, Southern; Grassland ecology
Loma Linda University Libraries
This title appears here courtesy of the author, who has granted Loma Linda University a limited, non-exclusive right to make this publication available to the public. The author retains all other copyrights.
Johnson, George E., "Environmental Cues in the Seasonal Development of the Southern California Annual Grassland" (1978). Loma Linda University Electronic Theses, Dissertations & Projects. 634.
Loma Linda University Electronic Theses and Dissertations
Loma Linda University. Del E. Webb Memorial Library. University Archives