Plant Physiological Ecology: Linking the Organism to Scales Above and Below

ABSTRACT:
In 1987, a series of papers on plant physiological ecology were published in BioScience. These seminal papers set forth a set of themes that essentially defined the field and communicated some of the more important questions in need of further research in the decade to come. Since their publication these papers have also helped scientists, graduate students and funding agencies see where the field was headed or needed to head. Over the last 13 years, plant physiological ecology has gradually broadened and diversified beyond these central themes. Today the field is challenged with redefining is goals and future directions; the forthright and concise agenda it set forth in 1987 has evolved and needs to be revisited.

Today, physiological ecologists still hold tightly to the idea that the "organism" remains the cornerstone for all of its investigations. Organisms define the boundary and ultimate product of molecular and cellular processes, the unit of selection, the key elements comprising populations and communities and it is organisms that drive or shape ecosystem functions. Not surprisingly, therefore, scaling-up from organismal level processes to populations, communities and ecosystems and scaling-down to molecular and cellular phenomena as well as over evolutionary time seems to be what, today, best defines where the field is headed.

The ultimate goal of this symposium is to clearly communicate a future vision of plant physiological ecology to students, scientists, funding agencies and the public. More specifically, the symposium will address the following questions: (1) What path has the field taken to get where it is today?; (2) How well did we answer the questions posed in the BioScience series?; and (3) What is the path for the future? Our goal is that Snowbird will foster discussion that will culminate with a workshop in 2001, and ultimately a series of papers like those in BioScience that so clearly communicate a scientific agenda.

Tuesday, August 8

Historical Perspectives

1:00 MOONEY, H.A. Where are we, how did we get here and where are we going in plant physiological ecology?
1:25 BAZZAZ, F.A. AND R.W. PEARCY. Directions in plant physiological ecology since the 1984 Asilomar meeting.

Directions for the Future: Plant Physiological Ecology at Different Scales

1:50 BALDWIN, I.T. and J. BERGELSON. Using molecular techniques to ground ecological processes in physiological mechanisms.
2:15 DAWSON, T.E., D. ACKERLY, AND M. LECHOWICZ. Evolutionary perspectives in plant ecophysiology.
2:40 SCHMITT, J., and M.A. GEBER. Microevolution of physiological traits in natural populations.
3:05 BREAK
3:15 JACKSON, R.B. AND R. MONSON. Physiological ecology and global change: adding mechanism to the madness of predicting the future
3:40 EVINER, V. AND F.S. CHAPIN, III. Linking suites of plant physiological traits to ecosystem dynamics and feedbacks.
4:05 FIELD, C.B. AND M.L. GOULDEN. Physiological ecology and biosphere/atmosphere interactions

4:30 EHLERINGER, J. AND M.M. CALDWELL. Scaling Physiological ecology in the future
4:55 COLEMAN, J.S. AND E. DELUCIA. Summary: A look ahead

Discussion to continue at the Physiological Ecology Business Meeting and Mixer.

ABSTRACTS FOR PLANT PHYSIOLOGICAL ECOLOGY SYMPOSIUM

Coleman, Dawson and Jackson

MOONEY, H.A. Stanford University, Stanford, CA. Where are we, how did we get here and where are we going in plant physiological ecology?

A decade ago it was noted that the field of plant physiology consolidated around models of carbon, water and energy balances, growth models and optimization theory. Since that time there has been a shift in research activity, away from continuing to build this foundation, toward the application of these tools for predicting the consequences of global change. Although there is intense activity by physiological ecologists in global change research there has been a relative lack of development of molecular plant physiological ecology, an area of enormous potential. The basis for this dichotomy will be discussed along with a discussion of the consequences of the relative loss of the foundation fields of plant physiology and plant biochemistry. Finally, some predictions about the next decade of activity for this field will be made.

BAZZAZ, F.A.¹ and R. W. PEARCY² Harvard University, Cambridge MA¹ and University of California, Davis CA ². Directions in plant physiological ecology since the 1984 Asilomar meeting.

Plant physiological ecology has undergone many changes since the 1984 Asilomar meeting that considered the state of the field at that time and made recommendations regarding its future development (See Ehleringer, JR. Pearcy, RW, and Mooney, HA. 1986. Bull. Ecol. Soc. Amer. 67:48-58). Up to then much emphasis had been on process studies of resource acquisition and stress responses. These are still important areas of research, but increasingly the emphasis has shifted to understanding how these processes at the organ and whole-plant level scale to ecosystem function. Both the development of new techniques for scaling, such as stable isotope and whole-plant water flux methodology, and research imperatives arising from environmental change issues are important drivers of this trend. In addition, linkages with molecular biology, though still in their infancy, are leading to a better understanding of cellular and metabolic functions and their integration into whole-plant environmental responses. Studies at the organismal level have increasingly focused on the integration of plant processes. For example, investigation of root function and its integration into whole plant responses was in its infancy in 1984 but now is a major area of research. During this period studies of responses to neighbors as mediated by signals and resources also developed, leading to a greater appreciation of the role of plasticity in the short-term, and of the evolutionary responses to it in the long term.

BALDWIN, I.T.¹ and J. BERGELSON². Max Plank Institute for Chemical Ecology, Jenna, Germany¹ and University of Chicago, Chicago, IL². Using molecular techniques to ground ecological processes in physiological mechanisms.

The expression of resistance characters is generally thought to involve a fitness cost, and the presumption of such costs is the cornerstone of explanations for the maintenance of variation in resistance. Costs of resistance are often difficult to detect, perhaps due in part to their sensitivity to ecological conditions and their interaction with genetic background. Model systems which are amendable to genetic transformation allow for the expression of new characters and the repression of old ones, and allow ecologists to explore the (complicated) physiological linkage between gene expression and its fitness consequences. Examples of expressing a new resistance character (herbicide resistance in Arabidopsis thaliana) and repressing an old one (nicotine production in native Nicotiana) will be used illustrate the two approaches.

DAWSON, T.E.¹, D. ACKERLY² and M. LECHOWICZ³. University of California, Berkeley, CA¹, Stanford University, Stanford, CA² and McGill University, Montreal, Quebec Canada³. Evolutionary perspectives in plant ecophysiology.

The historical roots of physiological ecology are in comparative biology, particularly comparative physiology. When set in a phylogenetic context, comparisons among species that differ in form and function can reveal the degree to which traits are adaptive responses to an environment versus legacies of past events in a lineage. This evolutionary perspective provides new insights and strengthens the conclusions that can be drawn about the interactions between plants and their environment. We present a brief historical review and perspective on this sort of research at the interface between plant ecophysiology and evolutionary ecology. With data we show the promise, the pitfalls and some potential directions that this research agenda might fruitfully take. Specifically, we show how traits, measured on plants in the field and in common gardens, are being used to examine the origin of adaptations, how specific traits or suites of traits that confer some functional benefit to particular species within a clade may have evolved, and how an understanding of macroevolutionary patterns can help in identifying the possible agents of natural selection that may have influenced trait evolution. We draw our examples from a range of plant families, taxa and life forms growing across a wide range of conditions. We conclude that evolutionary perspectives can enrich and reinvigorate studies in physiological ecology that focus on how species do, or might, adapt to different environments.

SCHMITT, J.¹, and M.A. GEBER². Brown University, Provdence, RI¹ and Cornell University, Ithaca, NY ². Microevolution of physiological traits in natural populations.

For physiological traits to evolve, they must be genetically variable and they must affect fitness. We will focus on the dual themes of genetic variation and fitness consequences of physiological and developmental traits in natural populations. There is increasing evidence that physiological and developmental traits, such as gas exchange physiology, photomorphogenesis, and development rate, are under selection in the wild. The fitness consequences of these traits are often manifested through intermediate fitness components, such as resource allocation, plant size and flowering time. Their expression and effects are also environment-dependent. There is also evidence that these traits are genetically variable within populations, providing the substrate for natural selection to act on, and differ among populations with different histories of selection. However, response to selection may be constrained by genetic trade-offs among traits and among environments. An exciting prospect for the future is identifying the loci underlying variable phenotypes and detecting selection directly on these loci.

JACKSON, R. B. ¹ and R. K. MONSON². Duke University, Durham, NC ¹; University of Colorado, Boulder, CO ². Physiological ecology and global change: adding mechanism to the madness of predicting the future.

The physiological ecology of plants and animals provides a mechanistic framework for predicting the consequences of global change and understanding feedbacks with the biosphere. Whether the driver of global change is rising atmospheric CO2, increasing temperature, or altered nitrogen deposition, organism physiology is altered in fundamental and predictable ways. Nonetheless, such physiological changes must also be integrated with changes at scales above and below the organism. We discuss the role of physiological ecology in predicting the consequences of global change, including the effects of elevated temperature and carbon dioxide, interpreting satellite and eddy flux data, and understanding feedbacks with atmospheric composition. Elevated CO2 affects photosynthesis and stomatal conductance differently for plant functional types (e.g., angiosperms vs. gymnosperms), leading to different predictions geographically and latitudinally. Similarly, the distribution and abundance of C3 and C4 plants can be predicted fairly well from physiological responses to such environmental variables as temperature, light, and water, with projected shifts for the coming century. Physiological ecology also plays an important role in understanding the controls over the exchange of reactive chemical species with the atmosphere. Global isoprene emissions cause approximately a 20% increase in the tropospheric lifetime of methane; such modeling results use algorithms developed primarily through ecophysiological research and help us develop prognostic scenarios of how climate warming will influence forest isoprene emissions and perturb the lifetime of atmospheric methane.

EVINER, V.T.¹, and F.S. CHAPIN III ². University of California, Berkeley, CA¹ and University of Alaska, Fairbanks, AK². Linking suites of plant physiological traits to ecosystem dynamics and feedbacks.

Plant physiological traits actively influence ecosystem properties such as resource availability and dynamics, disturbance regime, and trophic dynamics. Most previous research has focused on the direct effects of a single plant trait on ecosystem processes. Suites of traits provide the basis for a broader set of generalizations about the effects of changes in species composition on ecosystem dynamics. We present evidence for the following hypotheses: (1)There are suites of ecophysiological traits which lead to predictable constellations of ecosystem consequences. Due to the consistent correlations among resource-dependent processes and disturbance-dependent processes, traits which trigger changes in resource supply or disturbance have multiple, predictable consequences. (2) However, there are other plant traits that vary independently of these suites of traits, and can alter the overall plant's effect on ecosystem processes. The interaction of suites of traits with these other traits can be predictable by focusing on a functional matrix of traits, rather than simply functional groups. (3) The relative impact of different physiological traits depends on: the strength of effect (at low vs. high plant density, short term vs. long term plant presence), and the duration of the effect after the plant species has been removed. (4) Plant physiological traits vary with abiotic factors, interaction with other biota, and through time. The sensitivity of a trait to change and the corresponding change in a plant's effect on ecosystem processes varies among traits.

FIELD, C. B.¹ and M. L. GOULDEN². Carnegie Institution of Washington, Stanford, CA¹ and University of California, Irvine, CA². Physiological ecology and biosphere/atmosphere interactions.

As the scale of ecosystem research has expanded to address regional to global questions, it is increasingly important to incorporate the bi-directional nature of ecosystem/atmosphere coupling. The atmosphere and climate clearly have large influences on ecosystems, but, locally and at the global scale, ecosystems also have a large influence on climate and the atmosphere. Aspects of this coupling have been studied by specialists approaching the issue from an atmospheric or an ecological perspective, but, until recently, few studies have taken the intermediate approach, with nearly equal emphasis on both of the major suites of processes. The exchange of carbon between ecosystems and the atmosphere is the best studied component of the bi-directional interaction, but recent studies still focus on a small subset of ecosystem types in a relatively narrow range of developmental stages. For the future, the premier challenges in ecology and in biosphere/atmosphere interactions are broadly overlapping. They include (1) integrating time scales that range from seconds to centuries, (2) dealing effectively with spatial and temporal heterogeneity, and (3) accounting for ecological interactions and biological diversity. The emergence of a vibrant sub-discipline in biosphere/atmosphere interactions does not decrease the need for approaching scientific problems in this area with the maximum possible ecological sophistication.

EHLERINGER, J.R. ¹ and M. M. CALDWELL ². University of Utah, Salt Lake City, UT ¹, and Utah State University, Logan, UT ². Scaling physiological ecology in the future.

Physiological ecology has a rich tradition investigating mechanisms of adaptation between organisms and their environment. Increasingly, our understanding of physiological ecology has proved valuable for addressing larger scale questions related to ecosystem dynamics and biosphere-atmosphere fluxes. The extension of physiological ecology to longer time scales and to larger spatial scales is desirable, but challenging. While the nonlinearities in scaling of processes from individual to ecosystem are appreciated , rules for scaling are elusive. No where is the need for process scaling more apparent than in global change studies, where the focus is on determining how ecosystems are responding to atmospheric, hydrologic, and land-use changes. In this presentation, we examine 3 examples of the melding of physiological ecology and ecosystem ecology: (a) C3/C4 photosynthesis in response to atmospheric carbon dioxide and its impacts on productivity and animal diversity, (b) water-resource partitioning, competition, and ecosystem dynamics in arid ecosystems in response to monsoon-boundary shifts, and (c) stable isotope ratios in carbon dioxide fluxes between the biosphere and the atmosphere that allow determination of carbon sources and sinks in different ecosystems and under land-use change activities.

The Water Limitation: Issues in Plant, Community, and Ecosystem Water Use

Abstract:
Water is a limiting resource in most ecological systems. The symposium will present an up-to-date overview of the fast changing field of ecological plant water relations, integrating individual plant, community and ecosystem level approaches. At the individual plant level, there have been recent highly publicized challenges to the theory of how water moves through plants (cohesion theory) and the mechanisms of how they recover from impairment of the water transport capacity (xylem refilling). Two of the speakers will critique these challenges, pointing out that the cohesion theory still stands, but that we need more research into the mechanisms of xylem refilling under conditions where plants are transpiring. Also at the individual plant level, two speakers will present the latest interpretations of stomatal responses to hydraulic vs. chemical signals from roots. Understanding the mechanism of stomatal responses to soil and atmospheric drought increase our ability to predict and manage plant water use. The remaining speakers will address the impact of the individual plant water use on community and ecosystem level responses. Hydraulic lift is an apparently widespread phenomenon that redistributes water from one soil region to another via plant roots, and it has major implications for competition and total water use of plant communities. Hydraulic constraints on stomatal conductance may play a central role in the decline of productivity with forest age. Ecosystem level studies of water use indicate the extent to which controls on whole plant water use scale up to regional patterns and also reveal new controls operating at higher levels. Overall, the symposium will provide a summary of current debates and advances in the field of ecological plant water relations for ecologists with interests ranging from physiology to ecosystem biology.

Monday August 7, 8 a.m. - 12 noon

8:00 HINCKLEY, T. M.* and J. ROMBOLD. Water movement in plants: tension among the ranks.
8:25 HOLBROOK, N. M.*, M. A. ZWIENIECKI and P. J. MELCHER. Embolism repair: Can we exorcise Maxwell's Demon?
8:50 SPERRY, J. S.*, R. OREN and J. P. COMSTOCK. Hydraulic architecture and stomatal action.
9:15 DAVIES, W. J.* and S. WILKINSON. Soil drying and atmospheric stress can combine to affect shoot growth and functioning through chemical signalling.
9:40 BREAK
9:55 CALDWELL, M. M.* and R. RYEL. Hydraulic lift and uptake of water and nutrients.
10:20 DONOVAN, L. A.*, J. H. RICHARDS and M. J. LINTON. Disequilibrium between predawn plant and soil water potentials.
10:45 MEINZER, F. C. and G. GOLDSTEIN. Ecophysiology of plant water use: convergence from leaf to tree.
11:10 BOND, B. J.*, M. G. RYAN, N. PHILLIPS and N. G. McDOWELL. Hydraulic constraints on productivity of old growth trees.
11:35 OREN, R.*, K. V. R. SHÄFER, G. G. KATUL, K. LIU and J. S. SPERRY. Controls on ecosystem water use.

Principal Organizer: Lisa A. Donovan, Department of Botany, 2502 Plant Science, Athens, GA 30602-7271 phone 706-542-2969, FAX 706-542-1805, donovan@dogwood.botany.uga.edu

Co-organizer: John Sperry Biology Department, University of Utah, Salt Lake City UT, 84112 phone 801-585-0379, FAX 801-581-4668, sperry@biology.utah.edu

The Rhizosphere--Top-down and Bottom-up Approaches

Sponsored by the Soil Ecology Section of ESA.

Organized by: CARDON, Z. G.¹ and J. L. WHITBECK²
University of Connecticut, Storrs, CT¹, and Tulane University, New Orleans, LA².

This symposium focuses on recent technical developments and conceptual syntheses in rhizosphere research. Study of the ecology of the rhizosphere is by necessity interdisciplinary, depending on investigation of (1) plant biology, (2) soil physical and chemical properties and their interactions with roots, and (3) soil community structure, dynamics, and function. By bringing together experts from these three general backgrounds, working in natural, managed, and agricultural systems, we hope to foster a stimulating symposial conversation of interest to plant, soil, microbial, mesofaunal, and global change ecologists. Speakers will present new approaches for analyzing key biotic and abiotic processes in the rhizosphere, discuss links between rhizodeposition and soil organic matter dynamics, and explore novel connections between soil food webs, ecosystem management, NPP, and biodiversity.

8:00 CARDON, Z. G. and J. L. WHITBECK. Introduction.

8:15 JAEGER, C. H. Peering into the rhizosphere using genetically engineered bacteria as microscopic sensors.
The rhizosphere is subject to its own Heisenberg uncertainty principle. It has proven difficult to study this system without changing it in the process. In the rhizosphere, the interaction of roots, soil and microorganisms occurs in a three dimensional physical and biological matrix with structure down to the microscopic scale. This matrix is disrupted by the introduction of sensors or the harvesting of samples. While difficult to study, the rhizosphere is the site of plant - microbial interactions that are central to nutrient cycling in ecosystems. Genetically engineered bacteria that "report" on their surroundings make it possible to examine the chemistry of the rhizosphere without physically disrupting it and without removing the influence of physical soil properties or native soil microbial flora and fauna. We have used this approach to map the pattern of root exudation of sugars and amino acids into the rhizosphere. This information is crucial to understanding how roots influence rhizosphere microorganisms involved in nitrogen cycling. Other researchers have used this approach to map bacterial phosphatase induction and to determine iron availability in the rhizosphere. In the future, genetically engineered bacteria could be used to evaluate other parameters such as micro-site redox potentials or to study the induction of bacterial enzymes involved in nitrogen transformations or in biodegradation of pollutants in rhizosphere soil.

8:45 FIRESTONE, M. K. Plant C meets microbial N-transformations in rhizosphere soil.
Microbial populations and activities are elevated in soil near plant roots due primarily to enhanced availability of labile carbon from root exudates and debris. The effects of plant-originated carbon on N-immobilization, mineralization, and nitrification are of dominant importance in this soil compartment, yet are temporally and spatially highly dynamic. Prevailing theories state that under high carbon availability, gross immobilization of N into microbial biomass would predominate; under conditions of low carbon availability, microbial starvation processes will result in release of NH4+. However, a variety of alternative mechanisms complicates an already spatially and temporally complex picture. Microfaunal grazing of bacteria/fungi and drying/wetting cycles can also result in release of NH4+ due to utilization of nitrogen-containing organic compounds for energy generation. If rhizosphere microbes access organic N from soil sources, then this would also contribute to the localized N-cycle in rhizosphere soil. Recent work on root carbon flow, bacterial populations, and protozoal biomass, begins to relate the spatial and temporal dynamics of these rhizosphere players to rates of N-transformations in this important soil compartment.

9:15 EISSENSTAT, D. M. Root foraging in nutrient-rich patches: interactions with rhizospheric organisms.
The roots of plants commonly proliferate in nutrient-rich patches, increasing nutrient acquisition and general plant performance. This can result in an accumulation of young, densely populated roots that may be very susceptible to root-feeding organisms. Interactions among soil organisms and roots can have dramatic effects on root longevity. For example, application of fungicide and insecticide more than doubled median lifespan of fine roots in a 60-year-old sugar maple stand. Maple roots that had grown into locally fertilized patches also had much longer lifespans than untreated roots, suggesting that active production of defense compounds of healthy roots helps protect them from root-feeding organisms. Other evidence that plants can exert controls over root longevity comes from a greenhouse study where apple seedlings were grown in split-pots containing unsterilized soil. Roots in the high-nutrient pot received 4 mM nitrate while those in the low-nutrient pot received 1.6 mM nitrate twice weekly. Control seedlings had uniform low- or uniform high-nitrate addition. Root longevity was longest in the uniform-high and heterogeneous-high pots and lowest in the heterogeneous-low pots with longevity in the uniform-low pot intermediate. For the plants exposed to heterogeneous nitrate supply, patterns of longevity were similar to patterns of nitrogen acquisition efficiency of the roots. We conclude that rhizospheric organisms may exert a major effect on patterns of root longevity. Plants foraging for nutrients not only must expend energy on root growth and nutrient uptake processes, but also on defense from the abundance of root-feeding organisms in the rhizosphere.

9:45 JASTROW, J. D. and R. M. MILLER. Soil aggregation in the rhizosphere: optimal conditions for multiple mechanisms.
Soil aggregate structure together with its influence on porosity is a major controlling factor for virtually all soil processes. Moreover, the physical protection of organic debris within stable aggregates is an important mechanism enabling the accumulation of soil organic matter and nutrient reserves. In soils with a legacy of long-term exploration by roots, an aggregate hierarchy often develops, where primary particles and clay microstructures are bound into increasingly larger aggregates. Nearly ideal conditions for aggregate formation and stabilization exist in the rhizosphere. As fibrous roots grow, they exert pressures and locally dry the soil causing soil particles to be pushed and drawn together at the same time that root exudates and decomposition products are converted into polysaccharides and humic substances by a diverse microbial and faunal community. Roots and the hyphae of mycorrhizal fungi enmesh primary particles and microaggregates to form macroaggregates. The deposition of plant and microbially derived compounds and of glomalin (a glycoproteinaceous substance exuded by mycorrhizal hyphae) cements soil particles, organic debris, and smaller aggregates together to stabilize larger aggregates. As roots and hyphae die, particulate organic matter is deposited throughout the aggregate structure of the soil, where its decomposition stabilizes the pores within both macroaggregates and microaggregates. In a chronosequence of tallgrass prairie restorations, we observed that many binding agents are correlated with the formation of stable soil aggregates. However, by using path analysis to investigate the causal interrelationships of these agents, we find that the direct and indirect effects of roots dominate the effects of other binding mechanisms and thereby demonstrate the overall importance of rhizospheric processes to aggregation in grassland soils.

10:15 BREAK

10:30 CHENG, W. Elevated CO2 and rhizospheric carbon fluxes: controversies of concepts and numbers.
Rhizosphere processes are intimately linked with nutrient cycling, plant growth, and soil carbon dynamics. Carbon fluxes in the rhizosphere constitute a significant component in the terrestrial carbon cycle. Therefore understanding rhizosphere carbon fluxes in relation to increasing atmospheric CO2 concentrations is important for predicting ecosystem responses to environmental changes. This synthesis emphasizes on four issues: (1) connections between rhizosphere respiration and photosynthesis, (2) categories of carbon fluxes in the rhizosphere, (3) potential causes of locally missing carbon, and (4) consequences of altered rhizosphere carbon fluxes by increasing atmospheric CO2. It is not surprising that many short-term studies actually show strong connections between rhizosphere respiration and photosynthesis. Rhizosphere respiration is often much enhanced by elevated CO2 primarily due to significantly increased photosynthesis. But what happen to those experiments that do not show such strong connections? What categories of rhizosphere carbon fluxes are affected by increasing CO2 concentrations? Why should there be locally missing carbon? What are the short-term and long-term implications if rhizosphere carbon is altered by a CO2-richer atmosphere? These questions will be discussed and addressed.

11:00 COLEMAN, D. C. and S. FU. Soil rhizosphere food webs in agroecosystems: impacts of herbivory and tillage management.
Soil rhizospheres are one of the principal "hotspots" of terrestrial ecosystems. Using isotopic Carbon tracer techniques, we measured impacts of aboveground herbivory on rhizosphere microbial growth and subsequent repercussions in an agroecosystem detrital food web. With a simulation model of decomposition of 14C-labelled litter, we measured relative contributions of bottom-up and top-down forces in the detrital foodweb. Microbial biomass was more resource-regulated, and microbivorous fauna was more sensitive to second- and third-order predators in the system. Implications for biodiversity and resource conservation are discussed.

11:30 MOORE, J. C. NPP and biodiversity: contrasting traditional community ecological approaches with those developed from soils.

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