Search

The article offers information on a study of ecosystem processes recovery in annual grasslands on Santa Cruz Island, California. In the examination of the influence of shrub colonization on nutrient cycling using the native shrubs Artemisia californica and Eriogonum arborescens, Artemisia increased nitrogen pools while Eriogonum depressed nitrogen pools. It also reveals in the determination of shrub effects that Eriogonum colonization is harmful while Artemisia favors grasses over the shrub.

An alpine fellfield community on granite substrate (elevation 3750 m) near the University of California Barcroft Laboratory in the White Mountains of eastern California was studied during the 2000 growing season to determine whether classic perennial life forms can be treated as plant functional groups. A series of 1-m² quadrat samples were measured to determine common species. The four species with greatest cover were Penstemon heterodoxus var. heterodoxus, Trifolium andersonii var. beatleyae, Poa glauca subsp. rupicola, and Eriogonum ovalifolium var. nivale. These and four additional common perennial species were selected for ecophysiological studies representing four distinct ecological life forms: chamaephytes, cushion plants (including mat formers), herbaceous dicot perennials, and graminoid perennials. Summer midday leaf temperatures for species with foliage held close to ground surface were up to 20°C higher than air temperatures, whereas on upright species, leaves away from the ground surface closely matched ambient temperatures. For the eight species, peak values of mean maximum photosynthetic rates ranged from 11.5-25.5 μmol CO2 m-2 s-1, typical of published values, although chamaephytes in the study showed higher rates comparable to herbaceous perennials. Water-use efficiency, as estimated by a ratio of internal to ambient CO2, was relatively high (ci:ca ratios of 0.43-0.59) compared to published data. During the stressful end of the growing season, neither predawn nor midday shoot water potentials ever reached low levels, presenting conflicting evidence for the role of soil moisture as a limiting factor. Overall, the data on plant functional attributes showed no strong patterns of differences between categories of life forms in the feIlfield community, suggesting that classical life forms in this habitat do not represent plant functional groups. [ABSTRACT FROM AUTHOR]

Question: What changes in species composition and cover have occurred in chaparral as a function of fire history across an ecoregion? Location: San Diego County, California, USA. Methods: Stands in which 40 mid-elevation chaparral vegetation plots (each 400 m² in area) were located in the 1930s were resurveyed in 2001. We stratified the stands into Infrequently versus Frequently burned (0-1 versus 2 or more fires recorded in the 91-yr period), and Immature versus Mature (≤31 yr versus >31 yr since last fire), resulting in four groups. Ten stands were randomly selected from each of these groups for survey. Results: There were no major shifts in life form composition, e.g., live oak trees were not invading chaparral that had experienced little or no fire, nor were subshrubs or herbaceous species replacing shrubs in areas that had experienced more frequent fires. However, there was a notable increase in the frequency of the subshrub Eriogonum fasciculatum across all fire history groups. In the mature stands with infrequent fire, average cover of resprouting shrubs increased (from 72 to 91%) and cover of obligate seeding shrubs (species with firecued germination) decreased (from 21 to 6%) significantly. Mature stands with frequent fire showed a significant decrease in resprouter cover (from 87 to 80%) and increase in obligate seeders (from 10 to 16%). Conclusions: While the tremendous changes in land use in southern California have been predicted to cause shifts in chaparral composition, these shifts are difficult to detect because species longevity and fire cycles are on the order of decades to a century. In this study, the expected trends could only be detected in groups that were mature at the time of the second survey. [ABSTRACT FROM AUTHOR]

Annual and short-lived perennial plant performance during wet years is important for long-term persistence in the Mojave Desert. Additionally, the effects of elevated CO2 on desert plants may be relatively greater during years of high resource availability compared to dry years. Therefore, during an El Niño year at the Nevada Desert FACE Facility (a whole-ecosystem CO2 manipulation), we characterized photosynthetic investment (by assimilation rate-internal CO2 concentration relationships) and evaluated the seasonal pattern of net photosynthesis (Anet) and stomatal conductance (gs) for an invasive annual grass, Bromus madritensis ssp. rubens and a native herbaceous perennial, Eriogonum inflatum. Prior to and following flowering, Bromus showed consistent increases in both the maximum rate of carboxylation by Rubisco (VCmax) and the light-saturated rate of electron flow (Jmax) at elevated CO2. This resulted in greater Anet at elevated CO2 throughout most of the life cycle and a decrease in the seasonal decline of maximum midday Anet upon flowering as compared to ambient CO2. Eriogonum showed significant photosynthetic down-regulation to elevated CO2 late in the season, but the overall pattern of maximum midday Anet was not altered with respect to phenology. For Eriogonum, this resulted in similar levels of Anet on a leaf area basis as the season progressed between CO2 treatments, but greater photosynthetic activity over a typical diurnal period. While gs did not consistently vary with CO2 in Bromus, it did decrease in Eriogonum at elevated CO2 throughout much of the season. Since the biomass of both plants increased significantly at elevated CO2, these patterns of gas exchange highlight the differential mechanisms for increased plant growth. The species-specific interaction between CO2 and phenology in different growth forms suggests that important plant strategies may be altered by elevated CO2 in natural settings. These results indicate the importance of evaluating the effects of elevated CO2 at all life cycle stages to better understand the effects of elevated CO2 on whole-plant performance in natural ecosystems. [ABSTRACT FROM AUTHOR]

ABSTRACT.--We asked: Is Lycium californicum an ecotone species and what factors restrict it from overlapping more broadly with dominant plants of the salt marsh (downslope) and the coastal sage scrub (upslope)? Field sampling at Tijuana Estuary revealed that Lycium grows within a 1.1-m elevation range intermediate between that of Eriogonum fasciculatum (a subshrub of the coastal sage scrub) and Salicornia subterminalis (an obligate wetland subshrub of tidal marshes). Hence, Lycium was confined to the wetland-upland ecotone. In greenhouse experiments salt water wetting and soil moisture influenced these three species as follows: Salicornia required saturated soil to establish, restricting it to the wetland. Eriogonum failed to tolerate salt water wetting of the soil, both as seedlings (100% mortality) and as adults (100% mortality), likely restricting it to the upland. Lycium adults tolerated seawater wetting of the soil (100% survival) but seedlings did not (100% mortality). Both Lycium and Eriogonum should be able to establish seedlings near the wetland when soil salinity is low, but only Lycium would survive subsequent tidal surges (seawater wetting). Lycium showed evidence of drought tolerance but no requirement for groundwater (which would restrict it to the lowland); hence, its absence from the coastal sage scrub community may be due to competition with less salt-tolerant species. The few remaining populations of Lycium are restricted by the narrow range of suitable habitat and continuing pressure for coastal development. Thus, the transition from salt marsh to coastal sage scrub should be protected and restored to support this rare ecotone species. [ABSTRACT FROM AUTHOR]

Rainfall interception shelters are frequently used to study the ecological consequences of drought. One common shelter design employs V‐shaped plastic troughs spaced on a supporting frame to intercept rainfall. Shading, reflection, and infrared radiation may alter the radiative environment under shelters in ways independent of their intended effect on soil moisture. We measured microclimate and several photosynthetic variables for watered, potted plants under rain‐out shelters and in open‐air, unsheltered plots. We tested whether the shelter infrastructure altered aboveground micrometeorology and photosynthesis for watered, potted plants of native Californian species: Elymus glaucus, Eriogonum latifolium, Mimulus aurantiacus, and Morella californica. We quantified the effects on photosynthesis in terms of light harvesting by photosystem II (PSII) and leaf‐level gas exchange on open‐air and shelter plots, the quantum yield of PSII for darkened leaves, dark respiration, and nocturnal stomatal conductance. The rain‐out shelter reduced daily integrated photosynthetically active radiation by 20%. Air temperature, leaf temperature, and leaf‐to‐air vapour pressure difference were not different under shelters compared with controls during the day. Likewise, there were no effects of shelters on net CO2 assimilation, stomatal conductance to water vapour (gs), internal leaf (CO2), or electron transport rate through PSII during the daytime. At night, Tair was 0.6°C higher under shelters, but there were no effects on dark respiration or stomatal conductance. Despite some differences in micrometeorology under rain‐out shelters compared with open‐air plots, there were little or no aboveground nondrought effects of the shelters on leaf‐level photosynthesis for watered, potted plants of these California native plant species. [ABSTRACT FROM AUTHOR]