Endomycorrhizal role for interspecific transfer of phosphorus in a community of annual plants.

Phosphorus-32 applied to leaves of Plantago erecta in a serpentine annual grassland reached the shoots of about 20 percent of the close neighbors. Vesicular-arbuscular mycorrhizae connect the root systems of neighbors of different species and probably mediate nutrient transfers among them. Spatial patterns of transfer show that taxonomic affinity, distance from donor, and size of recipient do not serve as predictors of transfer and that models of transfer by simple diffusion are not appropriate. No alternative predictor was discovered. The results underscore the importance of belowground interactions in explaining neighbor effects, but the factors controlling nutrient transfer and its consequences for community structure appear complex.

Responses of hawaiian plants to volcanic sulfur dioxide: stomatal behavior and foliar injury.

Hawaiian plants exposed to volcanic sulfur dioxide showed interspecific differences in leaf injury that are related to sulfur dioxide-induced changes in stomatal condutance. Species with leaves that did not close stomata developed either chlorosis or necrosis, whereas leaves of Metrosideros collina closed stomata and showed no visual symptoms of sulfur dioxide stress.

High photosynthetic capacity of a winter annual in death valley.

Camissonia claviformis, a winter annual of Death Valley, California, that fixes carbon dioxide by the C(3) mechanism, has an in situ photosynthetic rate at midday in spring of nearly 6 nanomoles of carbon dioxide per square centimeter per second-an exceptionally high rate. Camissonia fixes absorbed noon sunlight in the 400- to 700-nanometer region into chemical energy with an efficiency of 8.5 percent, which is 80 percent of that theoretically possible for intact leaves. This performance is primarily due to an unusual capacity to utilize high irradiances. Factors associated with this include a high stomatal conductance to carbon dioxide and high levels of soluble protein and ribulose-1,5-diphosphate carboxylase.

Leaf pubescence: effects on absorptance and photosynthesis in a desert shrub.

The presence of leaf pubescence (leaf hairs) in Encelia farinosa, a desert species of the Composite family, reduces the absorptance of photosynthetically active radiation (400 to 700 nanometers) by as much as 56 percent more than a closely related but nonpubescent species, E. californica, a native of the relatively moist southern California coast. Pubescence in E. farinosa, which increases through the growing season, modifies the leaf energy balance and dramatically reduces the photosynthetic rate. The reduction in the photosynthetic rate is caused by decreased light absorption rather than decreased carbon dioxide conductance through the boundary layer.

Exchange of materials between terrestrial ecosystems and the atmosphere.

Many biogenic trace gases are increasing in concentration or flux or both in the atmosphere as a consequence of human activities. Most of these gases have demonstrated or potential effects on atmospheric chemistry, climate, and the functioning of terrestrial ecosystems. Focused studies of the interactions between the atmosphere and the biosphere that regulate trace gases can improve both our understanding of terrestrial ecosystems and our ability to predict regional-and global-scale canges in atmospheric chemistry.

Photosynthetic Mechanisms and Paleoecology from Carbon Isotope Ratios in Ancient Specimens of C4 and CAM Plants.

Carbon istotope ratios of modern, 10,000-year-old, and more than 40,000-year-old Atriplex confertifolia (C(4)) material from Nevada caves indicate that the C(4) photosynthetic pathway was operating in these plants over that period. Samples of a plant with crassulacean acid metabolism, Opuntia polyacantha, were also measured, and a shift in the 8(13)C value from -21.9 per mil (more than 40,000 years ago) to -13.9 per mil (10,000 years ago) was observed. This provides unique physiological evidence to support the hypothesis that the late Pleistocene pluvial climate in the region already had become drier about 10,000 years ago.

Atmospheric water uptake by an atacama desert shrub.

Nolana mollis, a succulent-leaved shrub of the extreme coastal desert of Chile, has the capacity to condense water on its leaves out of unsaturated atmospheres, Metabolic energy would have to be expended to move this water either from the leaf surface directly to the mesophyll or, when dripped to the soil, from there into the roots. Because of the unusual aridity of its habitat and of the utilization of water-use-efficient metabolism by Nolana, at least during certain periods, such an energy expenditure could be effective.

Global biodiversity scenarios for the year 2100.

Scenarios of changes in biodiversity for the year 2100 can now be developed based on scenarios of changes in atmospheric carbon dioxide, climate, vegetation, and land use and the known sensitivity of biodiversity to these changes. This study identified a ranking of the importance of drivers of change, a ranking of the biomes with respect to expected changes, and the major sources of uncertainties. For terrestrial ecosystems, land-use change probably will have the largest effect, followed by climate change, nitrogen deposition, biotic exchange, and elevated carbon dioxide concentration. For freshwater ecosystems, biotic exchange is much more important. Mediterranean climate and grassland ecosystems likely will experience the greatest proportional change in biodiversity because of the substantial influence of all drivers of biodiversity change. Northern temperate ecosystems are estimated to experience the least biodiversity change because major land-use change has already occurred. Plausible changes in biodiversity in other biomes depend on interactions among the causes of biodiversity change. These interactions represent one of the largest uncertainties in projections of future biodiversity change.

Response of radish to multiple stresses: I. Physiological and growth responses to changes in ozone and nitrogen.

Experiments were conducted to determine the impact of nitrogen and ozone (O3 ) stress on the growth of domestic radish Raphanus sativus L. cv. Cherry Belle. Plants were grown in field chambers with sub-, optimal and supra-optimal levels of nitrogenous fertilizer. Chamber air was either charcoal-filtered, or supplemented with one of two levels of O3 . The highest O3 treatment resulted in significant reduction in weight of hypocotyls and roots while elevated nitrogen treatments resulted in increased weight of all plant parts. Ozone did not affect the weight of plant foliage at any nitrogen level. Plants grown with lower levels of nitrogen had less leaf biomass but the tissue accounted for a greater percentage total weight than did the foliage of higher nitrogen treatments. Relative growth rate of whole plants was not affected by O3 or nitrogen treatments reflecting compensation in response to both stresses. Ozone-induced depression in biomass was observed in O3 -treated plants grown with higher nitrogen supply but not in those grown with limiting nitrogen. This observation could reflect compensation at the lower levels of nitrogen supply or inability to detect changes in biomass due to reduced weights of plants grown at the lowest nitrogen supply. The dry weight ratio of sink organs (hypocotyl plus root)/shoot was significantly correlated with the total non-structural carbohydrate (TNC) content of these organs, regardless of treatment. Initially, O3 induced a significant decrease and nitrogen an increase in percent TNC of sink organs. At later sampling times, plants adjusted to stress as effects on percent TNC were no longer evident.

Biological Response to Climate Change: An Agenda for Research.

Our knowledge of the structure and functioning of terrestrial ecosystems on a global scale is not developed to a sufficient degree to understand-much less predict-the consequences of climate change either on the systems themselves or on subsequent atmospheric interactions. In many regards we have lagged behind the atmospheric scientists, and to a certain degree the oceanographers, in establishing a global understanding of the dynamics of our respective systems. This is due in part to the inherently greater complexity of biotic systems, but also to the lack of appropriate tools to measure regional biotic processes. These tools are now becoming available and with them a better understanding of terrestrial and atmospheric interactions. Even as these capabilities become a reality we must be realistic in recognizing that we have so far to go along the road to understanding that useful predictive capacity may elude us for a long time to come. What we need to do is act on the recommendations that have been emerging over the past few years and develop a global program to document more precisely the distribution, structure, and quantity of the earth's biotic systems, their principal functional properties, and-most difficult of all-their changing nature. In order to do this we will have to: (1) perfect some of the emerging new tools for assessing these properties, (2) fill some of the gaps in our knowledge about the relevant processes, and (3) establish an international network of long-term observations and large-scale ecosystem manipulations. We have been aware of these needs and shortcomings for some time and we must move from plans to concerted international action.

Emergence of the Study of Global Ecology: Is Terrestrial Ecology an Impediment to Progress?

The emergence of the study of how the earth system operates and is responding to global change has seen the development of large-scale cross-disciplinary research efforts in addition to progress in traditional single-discipline, single-investigator approaches. Although terrestrial (I use this word in the broad sense to include continental systems encompassing terrestrial, wetland, lake, and river ecosystems) ecology is a central area of research for understanding earth system functioning, this field has not engaged in, nor has it the mechanisms for, strategic research planning, and thus it has not provided the momentum apparent in the allied earth sciences. The development and execution of the International Geosphere-Biosphere Program provides one forum for more integrated research planning by ecologists, as well as research opportunities along the entire spectrum of concern of this discipline. However, there needs to be a national focal point for continuing strategic planning for research in terrestrial ecology.

Photosynthetic plasticity of populations of Heliotropium curassavicum L. originating from differing thermal regimes.

Plants of the widely distributed species Heliotropium curassavicum L. have a large photosynthetic acclimation potential to temperature. There are, however, some differences among the acclimation potentials of populations occupying dissimilar thermal regimes. Plants of populations originating from a cool maritime climate have a greater acclimation potential than plants of populations originating from a desert habitat, which is characterized by large seasonal changes in temperature.

Community changes following shrub invasion of grassland.

We studied the development of the shrub Baccharis pilularis ssp consanguinea and its effects on herbs of the annual grassland in Northern California. A series of shands of Baccharis was sampled of ages ranging from 1 yr to>9 yr, representing most of the life cycle of the shrub. In each stand we examined shrub biomass, structure and litterfall. We also determined cover and biomass of all herbaceous species and estimated seed production, seed rain and storage of seed in the soil. Abundances of all herbaceous species declined greatly after Baccharis formed a closed canopy at 2-3 yr, and little seed of herbaceous species was either dispersed into shrub stands or stored in the soil. Exclosures suggested that herbivory by small mammals in the closed shrub stands may be important in reducing the abundance of herbaceous species following shrub invasion of grassland.

Tissue water relations of four co-occurring chaparral shrubs.

Chaparral shrubs of California have a suite of morphological and physiological adaptations to withstand the prolonged summer droughts of a mediterranean climate. Not all species of chaparral have the same rooting depth and there is some evidence that those with shallow roots have tissue that is most tolerant to water stress. We tested this notion by comparing the tissue water relations of four co-occurring chaparral shrubs: Quercus durata, Heteromeles arbutifolia, Adenostoma fasciculatum, and Rhamnus californica. We used a pressure-volume technique and a dew-point hygrometer to metsure seasonal changes in osmotic potential when plant tissue was fully hydrated and osmotic potential at predawn, midday, and the turgor loss point. We also calculated seasonal changes in the minimum daily turgor potential, saturated weight/dry weight ratio of leaf tissue, and the bulk modulus of elasticity. We had information on the seasonal water use patterns and apparent rooting depths of these same four shrubs from a previous study (Davis and Mooney 1986). All evidence indicated that Rhamnus had shallow roots and Quercus deep roots. Our results indicated that the tissue water relations of our four co-occurring chaparral shrubs were not alike. Even though Rhamnus had shallow roots, it had the least xerophytic tissue. Seasonal osmotic potential and saturated weight/dry weight ratios were relatively high and bulk modulus of elasticity and minimum daily turgor potentials were low. Furthermore, even though Quercus had deep roots and experienced no seasonal water stress at our study site, its tissue water relations indicated relatively high tolerance to water stress. We conclude that seasonal drought tolerance of stem and leaf tissue of co-occurring chaparral shrubs does not necessarily correspond to rooting depth, to soil moisture resources available to the shrub, or to the degree of seasonal water stress experienced by the shrub.

Water use patterns of four co-occurring chaparral shrubs.

Mixed stands of chaparral in California usually contain several species of shrubs growing close to each other so that aerial branches and subterranean roots overlap. There is some evidence that roots are stratified relative to depth. It may be that root stratification promotes sharing of soil moisture resources. We examined this possibility by comparing seasonal water use patterns in a mixed stand of chaparral dominated by four species of shrubs: Quercus durata, Heteromeles arbutifolia, Adenostoma fasciculatum, and Rhamnus californica. We used a neutron probe and soil phychrometers to follow seasonal depletion and recharging of soil moisture and compared these patterns to seasonal patterns of predawn water potentials, diurnal leaf conductances, and diurnal leaf water potentials. Our results indicated that 1) Quercus was deeply rooted, having high water potentials and high leaf conductances throughout the summer drought period, 2) Heteromeles/Adenostoma were intermediate in rooting depth, water potentials, and leaf conductances, and 3) Rhamnus was shallow rooted, having the lowest water potentials and leaf conductances. During the peak of the drought, predawn water potentials for Quercus corresponded to soil water potentials at or below a depth of 2 m, predawn water potentials of Heteromeles/ Adenostoma corresponded to a depth of 0.75 m, and predawn water potentials of Rhamnus corresponded to a depth of 0.5 m. This study supports the concept that co-occurring shrubs of chaparral in California utilize a different base of soil moisture resources.

Resource sharing among ramets in the clonal herb, Fragaria chiloensis.

The herbaceous perennial, Fragaria chiloensis, reproduces vegetatively on coastal sand dunes in California by growth of stolons that bear rosettes. Movement of water and photosynthates through stolons integrates water and carbon metabolism of rosettes both before and after they root. New, unrooted rosettes import sufficient water and nitrogen to maintain levels near those of established rosettes; yet support of an unrooted rosette did not decrease growth of a connected, rooted sibling given abundant light, water, and soil nutrients. Under such conditions strings of unrooted rosettes with the associated stolon appeared self-sufficient for carbon; shade and drought induced import of photosynthates. New rosettes produced and maintained a limited root mass upon contact with dry sand, which could increase probability of establishment. Rooting did not induce senescence of stolons. Connection between two established rosettes prevented death by drought and shade, even when neither rosette could have survived singly. Results suggest that physiological integration of connected rosettes may increase total growth of clones of F. chiloensis through sharing of resources among ramets, especially when resource availability is changeable or patchy.

Physiological adaptation and plasticity to water stress of coastal and desert populations of Heliotropium curassavicum L.

In spite of the ten times higher evaporative demand in a desert versus a coastal habitat, plants of populations of Heliotropium curassavicum from both show similar stomatal conductances in the field as well as under controlled conditions. The desert plants however have a plastic stomatal response to dry air growing conditions which results in a greater photosynthetic performance at negative water potentials. The root and stem resistance to water flow is lower in the desert plants resulting in the maintenance of a high transpiration rate without a large reduction in water potential.