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In this study, the influence of elevated carbon dioxide (CO2) and nitrogen (N) deposition on gastropod herbivory was investigated for six annual species in a California annual grassland community. These experimentally simulated global changes increased availability of important resources for plant growth, leading to the hypothesis that species with the most positive growth and foliar nutrient responses would experience the greatest increase in herbivory. Counter to the expectations, shifts in tissue N and growth rates caused by N deposition did not predict shifts in herbivore consumption rates. N deposition increased seedling N concentrations and growth rates but did not increase herbivore consumption overall, or for any individual species. Elevated CO2 did not influence growth rates nor have a statistically significant influence on seedling N concentrations. Elevated CO2 at ambient N levels caused a decline in the number of seedlings consumed, but the interaction between CO2 and N addition differed among species. The results of this study indicate that shifting patterns of herbivory will likely influence species composition as environmental conditions change in the future; however, a simple trade-off between shifting growth rates and palatability is not evident.

We studied the dynamics of a serpentine annual grassland in northern Cal- ifornia in relation to disturbance by pocket gophers (Thomomys bottae) and interannual variation in rainfall over the period 1982-1988. Mapping of gopher mound formation indicated that the probability that gophers would disturb any given area during the 6-yr study period had a near normal distribution with a peak at two disturbances during that period. Disturbance levels varied considerably from year to year and spatially, and thus the disturbance regime is complex. Exclosure experiments indicated that gopher disturbance had a significant effect on the abundances of many of the grassland annuals, perennial grasses, and cormaceous species. Aboveground herbivores had no detectable effects. Over the period of the study annual rainfall varied threshold and plant community dynamics were strongly affected by this. In particular, Plantago erecta decreased and Las- thenia californica increased in abundance with increasing rainfall. Abnormally high rainfall in 1982-1984 allowed the buildup of populations of Bromus mollis, the only nonnative species that invades the serpentine significantly. B. mollis was, however, subsequently virtually eliminated from the serpentine grassland by two consecutive years of severe drought. Invasion of B. mollis was found only on gopher mounds formed in 1983. In subsequent years recolonization of gopher mounds was predominantly by other species, in particular Lotus subpinnatus. Interannual variability in species abundances was most apparent on disturbed microsites. Our results provide a longer term perspective from which to view shorter term exper- imental and modelling work and highlight the need for long-term observations of ecological systems.

Non-native annual grasses dominate most Californian mediterranean-cli- mate grasslands today. However, native Californian grassland flora persists in grasslands on serpentine-derived soils. We manipulated soil nutrient resources to explore the links between nutrient availability, plant productivity, and the relative abundances of native and non-native species in serpentine grassland. Factorial combinations of nitrogen, phos- phorus, and other components of a nutritionally complete formula were added to field plots over two growing seasons. 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 com- munities may be directly influenced by nutrient availability, independent of physical dis- turbance.

The photosynthetic characteristics of the C3 desert winter annual Camissonia clavi- formis and the microclimate of the Camissonia habitat in Death Valley, California on 21 March 1976 are presented. This species has one of the highest photosynthetic capacities of any higher plant, with rates exceeding 6 nmole CO2 cm-2 sec1 at midday irradiances and optimum leaf temperatures. The photosynthetic temperature optimum is at 20'C, in concert with the prevailing cool winter tempera- tures of this desert habitat. The daily totals of primary production and water loss for Camissonia on 21 March are calculated using leaf energy budget and CO2 diffusion photosynthetic equations. Mea- sured and predicted leaf temperatures were 2-40C below air temperatures during the day. The rate of primary production is predicted to be 68.1 g CO2 m-2 day-' under high stomatal conductances, while daily transpiration at these stomatal conductances will be 5.54 kg m-2 day-1. The effects of decreases in stomatal conductance on daily primary production, transpiration, and water-use effi- ciency are calculated and show that the values of these 3 parameters diminish rapidly with decreasing stomatal conductance.

Photosynthetic rates were measured for five species of intertidal marine algae, in the air and submerged. Ulva expansa and Prionitis lanceolata from the lower intertidal show reduced photosynthetic capacity air in air compared to submergedrates. In contrast, species from the middle and upper littoral (Iridaea flaccida, Porphyra perforata, Fucus distichus, and Endocladia muricata) reach maximum photosynthesis after some degree of drying. For these latter species, photosynthetic rates can be 1.6 to 6.6 times greater in air than in water at the same illumination and temperature. Desiccation rates under natural conditions are slow enough that these algae are capable of continuing a high rate of photosynthetic activity for extended periods while exposed and may fix the bulk of their carbon at this time. The capacity of these algae for sustained photosynthesis in air vary according to their intertidal zonation. It is suggested that these relationships may be partially responsible for the vertical distribution of intertidal marine algae.

An ecological comparison was made between an alpine population of Thalictrum alpinum L. from the White Mountains of California and diploid and triploid plants of an arctic population from Ogotoruk Creek, Alaska. In a uniform environment plants of the alpine population were found to differ physiologically from the arctic plants in their photoperiod requirement, temperature of optimum photosynthesis, and photosynthetic light efficiency. Arctic triploid differed from diploid plants primarily in their greater vigor and photosynthetic capacity. Certain of the physiological differences between populations correlate with structural leaf characteristics, and are comparable to those previously found between arctic—alpine populations of Oxyria digyna. The ecological differences between the arctic chromosome races are discussed in relation to polyploid frequency in the arctic. See full-text article at JSTOR

On soils derived from a uniform bedrock two closely related species of Erigeron replace each other elevationally in the White Mountains of California. Populations of the two species occur side—by—side at places at the same elevation but on dissimilar soils because of compensations provided by the interactions of substrate color, temperature, moisture, and chemistry. The altitudinal distribution of the species is controlled by temperature at the upper limits and moisture at the lower. See full-text article at JSTOR

The summer drought—deciduous coastal sage in Southern California near Los Angeles has a bimodal, coastal and interior, distribution at low elevations (sea level to 300 m), whereas the evergreen sclerophyllous chaparral is developed at higher elevations. Winter rainfall is 300—350 mm at the low elevations and 500—600 mm at the higher altitudes. Photosynthesis and transpiration in response to artificial drying were examined in field—collected leaves of species from the two vegetation types. The fully hydrated mesomorphic leaves of the coastal sage species had initially higher photosynthesis and transpiration rates which declined rapidly during the first 20—30 min of drying, presumably because of hydroactive stomatal closure. At zero net photosynthesis cuticular loss of water was high, nearly 25% of the maximum rate for these species. In the chaparral species photosynthesis and transpiration rates were one—fourth to one—third those of the coastal sage species. Decline of rates with drying time was not nearly as rapid. Net photosynthesis continued at 20—30% of maximum after 70—80 min with lower cuticular losses of water vapor. During cool spring periods with sufficient soil moisture, high transpiration and assimilation rates allow for rapid growth, flowering, and fruiting in plants of the coastal sage community. They are drought—evading by virtue of their summer deciduous habit and are better adapted to prolonged summer drought in areas of low rainfall. Chaparral species, at slightly higher altitudes with higher precipitation and with deeper and more extensive root systems and xeromorphic leaf characteristics, are less sensitive to conditions of high evaporative demand and can maintain net assimilation, although low at times, throughout the long summer drought period.

Winter rates of photosynthesis and respiration in bristlecone pine were measured at controlled temperatures at 3,094 m elevation. Following the onset of severely cold temperatures in autumn, net photosynthesis declined sharply. By midwinter net photosynthesis became zero and remained there through early spring; during this time only low gross photosynthesis could be measured. Respiration rates remained high until late winter and then dropped significantly in early spring when temperatures increased and the soil thawed. The absence of net photosynthesis and the continued high respiration results in a large negative CO2 balance by the end of the winter. Approximately 117 hr of photosynthesis during summertime at peak rates are needed to equalize the winter loss of 140 mg CO2 per gram dry weight of photosynthetically active tissue. These findings are related to the growth of bristlecone pine at timberline. See full-text article at JSTOR

Sugar and starch contents of roots or rhizomes of Polygonum bistortoides, Saxifraga rhomboidea, and Calyptridium umbellatum were measured at different altitudes in the Sierra Nevada of California and in the Medicine Bow Mountains of Wyoming. Carbohydrate reserves appear to be higher near the lower altitudinal limits of a species. Ploygonum bistortoides, Caltha howellii, and Sibbaldia procumbens transplanted to lower and warmer elevations used up stored carbohydrates more rapidly than at the elevation of origin. The higher carbohydrate content of individuals in natural populations near the lower altitudinal limits of an alpine species thus appears to be an ecotypic or ecoclinal adaptation. See full-text article at JSTOR

Plants of Erigeron clokeyi, Haplopappus apargioides, Chamaebatiaria millefolium, and Encelia californica were grown at elevations of 1,250 and 3,094 m for at least 4 weeks in the White Mountains of California. During this period the photosynthetic temperature response was different at the two elevations. However, there was no apparent photosynthetic acclimation to differences in the CO2 tensions of the two growth environments. See full-text article at JSTOR