Todd E. Dawson

Hydraulic lift: consequences of water efflux from the roots of plants

Abstract Hydraulic lift is the passive movement of water from roots into soil layers with lower water potential, while other parts of the root system in moister soil layers, usually at depth, are absorbing water. Here, we review the brief history of laboratory and field evidence supporting this phenomenon and discuss some of the consequences of this below-ground behavior for the ecology of plants. Hydraulic lift has been shown in a relatively small number of species (27 species of herbs, grasses, shrubs, and trees), but there is no fundamental reason why it should not be more common as long as active root systems are spanning a gradient in soil water potential (Ψs) and that the resistance to water loss from roots is low. While the majority of documented cases of hydraulic lift in the field are for semiarid and arid land species inhabiting desert and steppe environments, recent studies indicate that hydraulic lift is not restricted to these species or regions. Large quantities of water, amounting to an appreciable fraction of daily transpiration, are lifted at night. This temporary partial rehydration of upper soil layers provides a source of water, along with soil moisture deeper in the profile, for transpiration the following day and, under conditions of high atmospheric demand, can substantially facilitate water movement through the soil-plant-atmosphere system. Release of water into the upper soil layers has been shown to afford the opportunity for neighboring plants to utilize this source of water. Also, because soils tend to dry from the surface downward and nutrients are usually most plentiful in the upper soil layers, lifted water may provide moisture that facilitates favorable biogeochemical conditions for enhancing mineral nutrient availability, microbial processes, and the acquisition of nutrients by roots. Hydraulic lift may also prolong or enhance fine-root activity by keeping them hydrated. Such indirect benefits of hydraulic lift may have been the primary selective force in the evolution of this process. Alternatively, hydraulic lift may simply be the consequence of roots not possessing true rectifying properties (i.e., roots are leaky to water). Finally, the direction of water movement may also be downward or horizontal if the prevailing Ψs gradient so dictates, i.e., inverse, or lateral, hydraulic lift. Such downward movement through the root system may allow growth of roots in otherwise dry soil at depth, permitting the establishment of many phreatophytic species.

Hydraulic lift and water use by plants: implications for water balance, performance and plant-plant interactions

During drought periods, sugar maple (Acer saccharum) demonstrates “hydraulic lift”; nocturnal uptake of water by roots from deep soil layers that is released from shallow roots into upper soil layers. Using standard water relations methods and stable hydrogen isotope analysis of both source-water and plant-water, I investigated (1) the magnitude and radial extent of hydraulic lift by mature, relatively open-grown trees, of A. saccharum, (2) the proportion of hydraulically-lifted water (HLW) used by shallow-rooted neighbors growing at different distances from target trees, and (3) the influence that this water source had on stomatal conductance to water vapor (g), water balance and growth of these neighbors. Soil water potentials (ψs) at −20 and −35 cm showed a distinct diel fluctuation. Soil pits dug beneath three mature trees revealed a distinct hard-pan (e.g. fragipan) layer at a depth of approximately 50 cm. Examination of root distributions obtained from soil cores and soil pits revealed that some larger diameter roots (1.9–3.7 cm) did penetrate the fragipan and were established in the ground water table. The presence of the fragipan indicated that the “rewetting” of the upper soil layer during the night could not be explained by capillary rise from the shallow water table; it was the trees that were taking up ground water and then redepositing it at night into the upper 35 cm of soil, above the fragipan. The greatest fluctuations in ψs occurred within 2.5 m of trees and only extended out to approximately 5 m. Application of a two-end-member linear mixing model which used stable hydrogen isotopic data obtained from environmental water sources and xylem-sap demonstrated that all neighbors used some fraction (3–60%) of HLW supplied by sugar maple trees. Plants that used a high proportion of HLW (e.g. rhizomatous or stoloniferous perennials) maintained significantly higher leaf water potentials and g, and showed greater aboveground growth when compared with (i) neighbors that used little or no HLW or (ii) conspecifics found growing at distances greater than about 3 m away from maple trees. Three important conclusions can be drawn from the results of this investigation that have not been demonstrated before: (1) hydraulic lift need not only occur in arid or semi-arid environments where chronic water deficits prevail, but can be important in relatively mesic environments when subjected to periodic soil water deficits, (2) that plants neighboring trees which conduct hydraulic lift can use a significant proportion of this water source, and (3) that the HLW source can effectively ameliorate the influence of drought on the performance and growth of neighboring vegetation. The results are also discussed in terms of their influence on plant nutrient relations (including plant-mycorrhizal associations), the nature of plant-plant interactions and the water balance of individuals, communities and floristic regions.

Global patterns of foliar nitrogen isotopes and their relationships with climate, mycorrhizal fungi, foliar nutrient concentrations, and nitrogen availability.

Ratios of nitrogen (N) isotopes in leaves could elucidate underlying patterns of N cycling across ecological gradients. To better understand global-scale patterns of N cycling, we compiled data on foliar N isotope ratios (delta(15)N), foliar N concentrations, mycorrhizal type and climate for over 11,000 plants worldwide. Arbuscular mycorrhizal, ectomycorrhizal, and ericoid mycorrhizal plants were depleted in foliar delta(15)N by 2 per thousand, 3.2 per thousand, 5.9 per thousand, respectively, relative to nonmycorrhizal plants. Foliar delta(15)N increased with decreasing mean annual precipitation and with increasing mean annual temperature (MAT) across sites with MAT >or= -0.5 degrees C, but was invariant with MAT across sites with MAT < -0.5 degrees C. In independent landscape-level to regional-level studies, foliar delta(15)N increased with increasing N availability; at the global scale, foliar delta(15)N increased with increasing foliar N concentrations and decreasing foliar phosphorus (P) concentrations. Together, these results suggest that warm, dry ecosystems have the highest N availability, while plants with high N concentrations, on average, occupy sites with higher N availability than plants with low N concentrations. Global-scale comparisons of other components of the N cycle are still required for better mechanistic understanding of the determinants of variation in foliar delta(15)N and ultimately global patterns in N cycling.

Root water uptake and transport: using physiological processes in global predictions

Plant water loss, regulated by stomata and driven by atmospheric demand, cannot exceed the maximum steady-state supply through roots. Just as an electric circuit breaks when carrying excess current, the soil-plant continuum breaks if forced to transport water beyond its capacity. Exciting new molecular, biophysical and ecological research suggests that roots are the weakest link along this hydraulic flow path. We attempt here to predict rooting depth and water uptake using the hydraulic properties of plants and the soil, and also to suggest how new physiological tools might contribute to larger-scale studies of hydraulic lift, the water balance and biosphere-atmosphere interactions.

Fog in the California redwood forest: ecosystem inputs and use by plants

Abstract Fog has been viewed as an important source of moisture in many coastal ecosystems, yet its importance for the plants which inhabit these ecosystems is virtually unknown. Here, I report the results of a 3-year investigation of fog inputs and the use of fog water by plants inhabiting the heavily fog inundated coastal redwood (Sequoia sempervirens) forests of northern California. During the study period, 34%, on average, of the annual hydrologic input was from fog drip off the redwood trees themselves (interception input). When trees were absent, the average annual input from fog was only 17%, demonstrating that the trees significantly influence the magnitude of fog water input to the ecosystem. Stable hydrogen and oxygen isotope analyses of water from fog, rain, soil water, and xylem water extracted from the dominant plant species were used to characterize the water sources used by the plants. An isotopic mixing model was employed to then quantify how much fog water each plant used each month during the 3-year study. In summer, when fog was most frequent, ∼19% of the water within S.sempervirens, and ∼66% of the water within the understory plants came from fog after it had dripped from tree foliage into the soil; for S.sempervirens, this fog water input comprised 13–45% of its annual transpiration. For all plants, there was a significant reliance on fog as a water source, especially in summer when rainfall was absent. Dependence on fog as a moisture source was highest in the year when rainfall was lowest but fog inputs normal. Interestingly, during the mild El Niño year of 1993, when the ratio of rainfall to fog water input was significantly higher and fog inputs were lower, both the proportion and coefficient of variation in how much fog water was used by plants increased. An explanation for this is that while fog inputs were lower than normal in this El Niño year, they came at a time when plant demand for water was highest (summer). Therefore, proportional use of fog water by plants increased. The results presented suggest that fog, as a meteorological factor, plays an important role in the water relations of the plants and in the hydrology of the forest. These results demonstrate the importance of understanding the impacts of climatic factors and their oscillations on the biota. The results have important implications for ecologists, hydrologists, and forest managers interested in fog-inundated ecosystems and the plants which inhabit them.

Gender and sexual dimorphism in flowering plants

1 Gender and Sexual Dimorphism in Flowering Plants: A Review of Terminology, Biogeographic patterns, Ecological Correlates, and Phylogenetic Approaches.- 1.1 Introduction.- 1.2 Terminology.- 1.3 Incidence of Dioecy.- 1.3.1 Overview.- 1.3.2 Ecological Associations.- 1.3.3 Geographic Patterns.- 1.4 Importance of Phylogenetic Approaches.- 1.5 Using Phylogenies to Understand Process and Pattern.- 1.5.1 Phylogenetic Distributions.- 1.5.2 Self-Incompatibility and Dioecy.- 1.5.3 Dioecy and Fleshy Fruits.- 1.5.4 Habitat Shifts, Pollination Biology, and Changes in Outcrossing Rates.- 1.6 Conclusions.- References.- 2 Theories of the Evolution of Dioecy.- 2.1 Introduction.- 2.2 Importance of Theoretical Models.- 2.3 Pathways to Dioecy.- 2.4 Theoretical Relationships Between Allocation of Reproductive Resources and Invasion of Populations by New Sex Morphs.- 2.4.1 Fitness in Outcrossing and Partially Selfing Cosexes and Allocation in Cosexes.- 2.4.2 Invasion of Populations by Females and Males.- 2.4.3 Effect of Cosex Allocations on Invasion by Unisexuals or Partially Sterile Types.- 2.4.4 Effects of Unisexuals on Cosex Allocations.- 2.4.5 Other Possible Routes to Dioecy.- 2.5 Testing the Theory.- 2.5.1 Comparative Tests.- 2.5.2 Gain Curves.- 2.5.3 Intraspecific Data.- 2.5.4 Genetic Data.- 2.6 Conclusions.- 2.7 References.- 3 Empirical Studies: Evolution and Maintenance of Dimorphic Breeding Systems.- 3.1 Introduction.- 3.2 Evolutionary Pathways to Gender Dimorphism.- 3.2.1 Approaches to the Study of Gender.- 3.2.1.1 Quantitative Description of Plant Gender.- 3.2.1.2 Theoretical Modelling.- 3.2.1.3 Phylogenetic Analysis.- 3.2.2 Overview of Pathways.- 3.2.3 From Cosexuality Via Gynodioecy to Dioecy.- 3.2.4 From Monoecy Via Paradioecy to Dioecy.- 3.2.5 From Cosexuality Via Androdioecy to Dioecy.- 3.2.6 From Heterostyly to Dioecy.- 3.2.7 From Duodichogamy or Heterodichogamy to Dioecy.- 3.2.8 The Evolution of Trioecy.- 3.3 Maintenance of Gender Dimorphism in Natural Populations.- 3.3.1 Sex Ratios.- 3.3.2 Evidence for an Outcrossing Advantage: Rates of Selfing and Levels of Inbreeding Depression.- 3.3.3 Relative Seed Fecundity of the Two Sexes.- 3.3.4 Relative Pollen Fecundity of the Two Sexes.- 3.3.5 Case Studies: Tests of Theoretical Models.- 3.3.5.1 Female Frequency and Habitat in Plantago lanceolata.- 3.3.5.2 Plant Vigour, Fruit Production and the Sex Ratio in Hebe strictissima.- 3.3.5.3 Rates of Selfing, Inbreeding Depression and the Sex Ratio.- 3.3.5.4 The Breakdown of Outcrossing Mechanism in Aralia.- 3.4 Directions for Future Research.- 3.4.1 Testable Predictions from Ecological Correlations.- 3.4.2 Other Research Gaps.- 3.5 Conclusions.- References.- 4 Theories of the Evolution of Sexual Dimorphism.- 4.1 Introduction.- 4.2 Models of Sexual Dimorphism.- 4.2.1 Types of Models.- 4.2.2 General Features.- 4.2.3 Sexual Dimorphism in a Dioecious Organism.- 4.2.3.1 Genetic Models.- 4.2.3.2 ESS Models.- 4.2.4 The Evolution of Gender and Sexual Dimorphism.- 4.2.4.1 ESS Models.- 4.2.4.2 Genetic Models.- 4.3 The Biology of Sexual Dimorphism.- 4.3.1 Disruptive Selection on Homologous Characters.- 4.3.1.1 Biological Circumstances.- 4.3.1.2 Theory on Disruptive Selection in Dioecious Organisms.- 4.3.1.3 Theory on the Evolution of Gender and Sexual Dimorphism.- 4.3.1.4 Disruptive Selection and Sexual Dimorphism in Plants.- 4.3.2 Ecological Competition.- 4.3.2.1 Biological Circumstances.- 4.3.2.2 Theory on Character Displacement Due to Intraspecific Competition.- 4.3.2.3 Competitive Character Displacement and SSS in Dioecious Plants.- 4.3.3 Intersexual Selection.- 4.3.3.1 Biological Circumstances.- 4.3.3.2 Theory on Intersexual Selection.- 4.3.3.3 Mate Choice and Sexual Dimorphism in Plants.- 4.4 Conclusions.- References.- 5 Sexual Dimorphism in Flowers and Inflorescences.- 5.1 Introduction.- 5.2 Patterns.- 5.2.1 Perianth Size.- 5.2.2 Perianth Shape.- 5.2.3 Nectar.- 5.2.4 Vestigial Characters.- 5.2.5 Other Flower Characters.- 5.2.6 Multi-Flower Characters.- 5.2.7 Questions.- 5.3 Evolutionary Hypotheses.- 5.3.1 Sexual Selection and Character Exaggeration.- 5.3.2 Specific Tests, Hypotheses, and Uncertainties.- 5.3.2.1 Perianth Size.- 5.3.2.2 Perianth Shape.- 5.3.2.3 Nectar.- 5.3.2.4 Vestigial Character.- 5.3.2.5 Other Flower Characters: Longevity.- 5.3.2.6 Multi-Flower Characters.- 5.4 Conclusions.- 5.4.1 Towards Quantitative Understanding.- 5.4.2 Size-Number Trade-Offs.- 5.4.3 Costs of Exaggeration.- 5.4.4 Variation in Costs and Benefits.- 5.4.5 Macro evolution.- References.- 6 Sexual Dimorphism in Live History.- 6.1 Introduction.- 6.2 Predictions Based on Sex-Differential Reproductive Investment.- 6.3 Patterns of Sexual Dimorphism in Life-History Traits.- 6.3.1 Response to Stress.- 6.3.2 Case Studies of Two Species in which the Cost of Reproduction Is Higher for Females.- 6.4 Factors Offsetting Between-Sex Differences in the Cost of Reproduction.- 6.4.1 Sexual Dimorphism in the Timing of Investment in Reproduction Versus Growth Within a Season.- 6.4.2 Sexual Dimorphism in the Timing of Flowering Within a Season.- 6.4.3 Sexual Dimorphism in the Frequency of Flowering.- 6.4.4 Sexual Dimorphism in Age of Maturation.- 6.4.5 Sexual Dimorphism in Physiological Traits.- 6.4.6 Sex-Differential Herbivory.- 6.5 The Contrary Case of Silene latifolia.- 6.6 Conclusions.- References.- 7 Dimorphism in Physiology and Morphology.- 7.1 Introduction.- 7.1.1 Causes of Sexual Dimorphism in Physiology and Vegetative Morphology.- 7.1.2 Physiological and Morphological Responses to Natural Selection.- 7.1.3 Physiological and Morphological Responses to Sexual Selection.- 7.1.4 Functional Significance of Dimorphism in Physiology and Morphology.- 7.2 History of Studies on Sexual Dimorphism in Plants.- 7.3 Sexual Dimorphism in Plant Form and Function in Species with SSS.- 7.3.1 Salix (Willow Salicaceae).- 7.3.2 Acer negundo (Boxelder Aceraceae).- 7.3.3 Simmondsia chinensis (Jojoba/Goat Nut Buxaceae).- 7.3.4 Phoradendron juniperinum (Mistletoe Viscaceae).- 7.3.5 Other Species.- 7.4 Sexual Dimorphism in Plant Form and Function in Species Without SSS.- 7.4.1 Silene latifolia (White Campion Caryophyllaceae).- 7.4.2 Leucadendron (Proteaceae).- 7.4.3 Other Species.- 7.4.3.1 Agricultural and Weedy Species.- 7.4.3.2 Populus (Aspen Salicaceae).- 7.5 Conclusions and Future Directions.- References.- 8 Sexual Dimorphism and Biotic Interactions.- 8.1 Introduction.- 8.1.1 Reproductive Allocation and Biotic Interactions.- 8.2 Sexual Differences in Competitive Ability.- 8.3 Sexual Differences in Herbivory.- 8.3.1 Herbivore Preference.- 8.3.2 Correlates of Sexual Differences in Herbivore Damage.- 8.3.3 Herbivore Performance on Male and Female Hosts.- 8.3.4 Sexual Differences in Response to Herbivory.- 8.4 Sexual Differences in Parasitism.- 8.4.1 Foliar Pathogens.- 8.4.2 Flower-Infecting Pathogens.- 8.4.3 Nonfungal Parasites.- 8.5 General Discussion.- 8.5.1 Biotic Interactions and Biased Sex Ratios.- 8.5.2 Evolution of Sexual Differences in Herbivory.- 8.5.3 Future Studies.- References.- 9 Genetics of Gender Dimorphism in Higher Plants.- 9.1 Introduction.- 9.2 Monoecious Plants.- 9.2.1 Gender Dimorphism in Cucumber.- 9.2.2 Molecular Biology of Gender Dimorphism in Maize.- 9.2.2.1 Tasselseed2.- 9.2.2.2 Gibberellin and gender dimorphism in maize.- 9.2.2.3 The Anther ear1 gene.- 9.3 Multigenic gender determination systems in dioecious plants.- 9.3.1 Mercurialis annua.- 9.3.2 A single gender determination locus.- 9.3.3 Sex chromosomes.- 9.3.3.1 Morphologically distinct sex chromosomes.- 9.3.3.2 Structure of sex chromosomes in plants.- 9.3.3.3 X/autosome balance can regulate gender dimorphism.- 9.3.3.4 X/autosome balance in Drosophila melanogaster.- 9.3.4 Comparison of Active Y Sex Chromosomes in Plants and Animals.- 9.3.4.1 The active-Y gender determination of white campion.- 9.3.4.2 The mammalian active-Y gender determination mechanism.- 9.3.4.3 Does dosage compensation occur in white campion?.- 9.3.5 Evolution of the active-Y chromosome: Male sterility.- 9.3.5.1 Cytoplasmic male sterility.- 9.3.5.2 Suppression of carpel or pistil development.- 9.4 Expression of MADS-box genes in unisexual flowers.- 9.5 Conclusions.- References.- 10 Quantitative Genetics of Sexual Dimorphism.- 10.1 Introduction.- 10.2 Quantitative Genetic Models of Sexual Dimorphism.- 10.3 Integration of Quantitative Genetics with Sexual Selection.- 10.4 Correlated Evolution and Divergence of Male and Female Traits in Dioecious Plants.- 10.5 Correlated Evolution and Divergence of Male and Female Function in Hermaphroditic Plants.- 10.6 Conclusions.- 10.7 References.- Taxonomic Index.

Gender‐Specific Physiology, Carbon Isotope Discrimination, and Habitat Distribution in Boxelder, Acer Negundo

In the semiarid Intermountain West, boxelder, Acer negundo var. interior, a deciduous, dioecious tree, exhibits significant habitat-specific sex ratio biases. Although the overall sex ratio (male/female) does not deviate significantly from one, the sex ratio is significantly male biased (1.62) in drought-prone habitats, while it is significantly female biased (0.65) in moist, streamside habitats. The causes underlying gender-specific habitat associations in this species are not known. We hypothesized that spatial segregation of the sexes is maintained by differences in gender-specific photosynthetic behavior, water rela- tions characteristics, and both instantaneous and integrated water-use efficiency. Gender- specific physiological characteristics were measured and related to growth, reproduction, population age structure, and habitat distribution of male and female trees. Under both field and controlled-environment conditions, males and females differed significantly in a number of physiological traits. Males maintained lower stomatal con- ductance to water vapor (g), transpiration (E), net carbon assimilation (A), leaf internal CO2 concentration (ci), carbon isotope discrimination (AL; an index of time-integrated ci and water-use efficiency), and higher instantaneous (A/E) and long-term (A) water-use efficiency than females. Furthermore, male trees exhibited greater stomatal sensitivity to both declining soil water content and increasing leaf-to-air vapor pressure gradients, a measure of evaporative demand. Higher rates of carbon fixation in female trees were correlated with higher g, higher leaf nitrogen concentrations, and greater stomatal densities. For females growing in both wet and dry habitats, vegetative shoots had higher growth rates than reproductive shoots, while for males, growth rates of the two shoot types did not differ. In streamside habitats, female trees exhibited significantly greater vegetative shoot growth when compared to male trees. In contrast, males showed slightly greater vegetative and much greater reproductive shoot growth in non-streamside habitats. Re- gardless of habitat or growing conditions, females allocated proportionately more of their aboveground biomass to reproduction than did males. These results suggest that (1) gender-specific physiological traits can help explain the maintenance of habitat-specific sex ratio biases in A. negundo along a soil moisture gradient, and (2) that the combination of the gender-specific physiology, growth, and allocation differences contribute to differences in the size (=age) structure of male and female plants within the population. Gender-specific physiological differences may have evolved as a product of selection to meet significantly different costs associated with reproduction in male and female plants.

The Evolution of Plant Ecophysiological Traits: Recent Advances and Future Directions

lants exhibit enormous ecophysiological and functional diversity, which underlies variation in growth rates, productivity, population and community dynamics, and ecosystem function. The broad congruence of these variations with climatic and environmental conditions on local, regional, and global scales has fostered the concept that plant ecophysiological characteristics are well adapted to their local circumstances. For example, the repeated occurrence of plants with CAM (Crassulacean Acid Metabolism) photosynthesis and succulent leaves or stems in severely water-limited environments, and the independent evolution of these traits in numerous plant lineages, provides compelling evidence of the physiological evolution of these water-conserving traits under the influence of natural selection (Ehleringer and Monson 1993). Similarly, studies of the evolution of heavy metal tolerance confirm that natural selection may cause rapid ecophysiological evolution in just a few generations, leading to local adaptation in populations just a few meters apart (Antonovics et al. 1971). Many ecophysiological traits—considered here as all aspects of resource uptake and utilization, including biochemistry, metabolism, gas exchange, leaf structure and function, nutrient and biomass allocation, canopy structure, and growth—are likely to influence fitness and undergo adaptive evolution. Traits affecting the assimilation and use of resources such as carbon, water, and nutrients directly influence plant growth. Patterns of resource allocation to growth, reproduction, defense, and stress tolerance are also likely to be under strong selection. Phenotypic plasticity, the expression of different phenotypes by

Seasonal water uptake and movement in root systems of Australian phraeatophytic plants of dimorphic root morphology: a stable isotope investigation

A natural abundance hydrogen stable isotope technique was used to study seasonal changes in source water utilization and water movement in the xylem of dimorphic root systems and stem bases of several woody shrubs or trees in mediterranean-type ecosystems of south Western Australia. Samples collected from the native treeBanksia prionotes over 18 months indicated that shallow lateral roots and deeply penetrating tap (sinker) roots obtained water of different origins over the course of a winter-wet/summer-dry annual cycle. During the wet season lateral roots acquired water mostly by uptake of recent precipitation (rain water) contained within the upper soil layers, and tap roots derived water from the underlying water table. The shoot obtained a mixture of these two water sources. As the dry season approached dependence on recent rain water decreased while that on ground water increased. In high summer, shallow lateral roots remained well-hydrated and shoots well supplied with ground water taken up by the tap root. This enabled plants to continue transpiration and carbon assimilation and thus complete their seasonal extension growth during the long (4–6 month) dry season. Parallel studies of other native species and two plantation-grown species ofEucalyptus all demonstrated behavior similar to that ofB. prionotes. ForB. prionotes, there was a strong negative correlation between the percentage of water in the stem base of a plant which was derived from the tap root (ground water) and the amount of precipitation which fell at the site. These data suggested that during the dry season plants derive the majority of the water they use from deeper sources while in the wet season most of the water they use is derived from shallower sources supplied by lateral roots in the upper soil layers. The data collected in this study supported the notion that the dimorphic rooting habit can be advantageous for large woody species of floristically-rich, open, woodlands and heathlands where the acquisition of seasonally limited water is at a premium.

Modeling root water uptake in hydrological and climate models

Abstract From 30 September to 2 October 1999 a workshop was held in Gif–sur–Yvette, France, with the central objective to develop a research strategy for the next 3–5 years, aiming at a systematic description of root functioning, rooting depth, and root distribution for modeling root water uptake from local and regional to global scales. The goal was to link more closely the weather prediction and climate and hydrological models with ecological and plant physiological information in order to improve the understanding of the impact that root functioning has on the hydrological cycle at various scales. The major outcome of the workshop was a number of recommendations, detailed at the end of this paper, on root water uptake parameterization and modeling and on collection of root and soil hydraulic data.