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Forests cover approximately one third of the terrestrial land surfaces (Hansen et al. 2000) and are arguably the most important biome type on Earth for acquiring, transforming, and recycling major and limiting biogeochemical resources such as water, carbon, and many mineral elements such as nitrogen or phosphorous; well-documented drivers of global biogeochemical cycles. Studying the processes that govern the manner and magnitude of biogeochemical cycling through forest systems with their enormous stature, age, complexity, diversity, and spatiotemporal heterogeneity and that are rarely ever in steady-state possess real and mind-boggling challenges for forest scientists. These challenges may cause some to abandon efforts to understand the complex nature of forests biogeochemical systems altogether. However, in recent years, the application of stable isotope methods, at both natural abundance levels and through targeted experiments using enriched isotopes have revolutionized our capacity to explore a wide range of biogeochemical processes in forests. In this regard, stable isotopes are now known to provide new and important insights from tracing the origin and movements of key elements and substances through the Earth–plant–atmosphere continuum (Gat 1996; Dawson et al. 1998, 2002; Fry 2006; Sharp 2007), to indicating the presence and the magnitude of key Earth system processes (Holton et al. 2006; Bowling et al. 2008), to integrating various biogeochemical processes in both space and time (Bowen et al. 2009; Craine et al. 2009; West et al. 2010b), to also recording biological responses to the Earth’s changing environmental conditions (McCarroll and Loader 2004; Augusti et al. 2006; Dawson and Siegwolf 2007; Sternberg 2009).

Trees are spectacular organisms that can accumulate a large amount of biomass, live for millennia, grow in stressful environments, and have global importance to organisms and human society as a result of their roles in contributing to diverse ecosystem services. They survive in an enormous range of environments with disturbances of various frequencies and severities, and by their very nature of being long-lived and sessile, they must possess the ability to change. This chapter provides a context for the chapters that follow, by describing a history of the investigations and of several important technological innovations that have enabled research to progress, and some of the foundations of water relations which underpin many of the later chapters. The chapter then discusses seven recurrent themes brought out by the diverse chapter authors: microclimate and other abiotic forces that change with tree and stand age; the changing structure and function employed as trees grow; what we can learn from scaling —which scaling laws pertain, when, and what is the biology or physics embedded in these laws; the tradeoffs that occur as structures and functions change through growth and development; the causes and correlates of the decline in productivity of trees and stands with age; the extent to which changes are plastic; and the meaning of whether changes occur relative to age versus size. The chapter concludes with a discussion on the emerging issues and questions raised in the chapters of this book. The overall aim is to give insights into the whole tree, whole lifespan perspective of the structural and functional strategies trees employ to persist in the face of internal and external factors that change as trees grow and age. This knowledge can be used to develop management approaches to manipulate trees and stands to provide a wide range of ecosystem goods and services.

The stable hydrogen isotopic ratio (δD) of stem water is shown to deviate from source water values in young, nonsuberized woody stems, reflecting an isotopic enrichment associated with cuticular water loss and/or transpiration. The extent of this enrichment was greater in trees which produce a new set of leaves annually (e.g., deciduous-leaved species) than in trees which retain their leaves for two or more years (e.g., coniferous or evergreen-leaved species). Stem-water enrichment was also greater in rapidly growing stem tissues than in mature, fully suberized or lignified stems which had ceased growing and showed little or no isotopic enrichment. These results are discussed in terms of their implications for studies which use xylem-sap δD values to evaluate plant water sources and uptake. We also discuss our findings in the context of long-term investigations that use cellulose or cellulose nitrate δD values for climatic reconstruction or to infer past water-use or distributional patterns in woody vegetation.

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.

We investigated the role of photosynthesis by reproductive organs in meeting the carbon costs of sexual reproduction in the snow-buttercup, Ranunculus adoneus. The exposed green carpels of snow-buttercup flowers have 1-2 stomata each. Net carbon assimilation rates of flowers are negative during bud expansion, but rise to zero at maturity, and become positive during early fruit growth. Experimental removal of separate whorls of flower parts demonstrated that the showy, nectary-housing petals account for most of the respiration cost of flower presentation. Conversely, photosynthesis by female organs contributes to a flower's carbon balance. Dipteran pollinators of R. adoneus are most active in sunny mid-morning to mid-afternoon intervals. At this time of day, rates of carpel photosynthesis (Amax) meet respiratory costs of pollinator attraction in fully expanded flowers. Achenes remain photosynthetically active until dispersal, and positive net carbon assimilation rates characterize infructescences throughout fruit maturation. Photosynthetic rates of achenes are positively correlated with infructescence growth rates. We tested the causal basis of this relationship by experimentally shading developing infructescences. Mature achenes from shaded infructescences were 16-18% smaller than those from unshaded controls. Leaf photosynthetic rates did not differ between plants bearing shaded and unshaded seed heads. Since female reproductive organs are only 8% more costly in terms of caloric investment than male ones and contribute to their own carbon balance, it is plausible that the energy cost of male function equals or exceeds that of female function in this hermaphroditic species.

[1] Fine root (

Stem photosynthesis can contribute significantly to woody plant carbon balance, particularly in times when leaves are absent or in 'open' crowns with sufficient light penetration. We explored the significance of woody tissue (stem) photosynthesis for the carbon income in three California native plant species via measurements of chlorophyll concentrations, radial stem growth, bud biomass and stable carbon isotope composition of sugars in different plant organs. Young plants of Prunus ilicifolia, Umbellularia californica and Arctostaphylos manzanita were measured and subjected to manipulations at two levels: trunk light exclusion (100 and 50%) and complete defoliation. We found that long-term light exclusion resulted in a reduction in chlorophyll concentration and radial growth, demonstrating that trunk assimilates contributed to trunk carbon income. In addition, bud biomass was lower in covered plants compared to uncovered plants. Excluding 100% of the ambient light from trunks on defoliated plants led to an enrichment in ¹³C of trunk phloem sugars. We attributed this effect to a reduction in photosynthetic carbon isotope discrimination against ¹³C that in turn resulted in an enrichment in ¹³C of bud sugars. Taken together our results reveal that stem photosynthesis contributes to the total carbon income of all species including the buds in defoliated plants.

The use of isotope ratio infrared spectroscopy (IRIS) for the stable hydrogen and oxygen isotope analysis of water is increasing. While IRIS has many advantages over traditional isotope ratio mass spectrometry (IRMS), it may also be prone to errors that do not impact upon IRMS analyses. Of particular concern is the potential for contaminants in the water sample to interfere with the spectroscopy, thus leading to erroneous stable isotope data. Water extracted from plant and soil samples may often contain organic contaminants. The extent to which contaminants may interfere with IRIS and thus impact upon data quality is presently unknown. We tested the performance of IRIS relative to IRMS for water extracted from 11 plant species and one organic soil horizon. IRIS deviated considerably from IRMS for over half of the samples tested, with deviations as large as 46 per thousand (delta(2)H) and 15.4 per thousand (delta(18)O) being measured. This effect was reduced somewhat by using activated charcoal to remove organics from the water; however, deviations as large as 35 per thousand (delta(2)H) and 11.8 per thousand (delta(18)O) were still measured for these cleaned samples. Interestingly, the use of activated charcoal to clean water samples had less effect than previously thought for IRMS analyses. Our data show that extreme caution is required when using IRIS to analyse water samples that may contain organic contaminants. We suggest that the development of new cleaning techniques for removing organic contaminants together with instrument-based software to flag potentially problematic samples are necessary to ensure accurate plant and soil water analyses using IRIS.

Stable isotope studies of hydrogen and oxygen stable isotope ratios of water within plants are providing new information on water sources, competitive interactions and water use patterns under natural conditions. Variation in the utilization of summer rain by aridland species and limited use of stream water by mature riparian trees are two examples of how stable isotope studies have modified our understanding of plant water relations. Analyses of xylem sap and tree rings have the potential of providing both short-term and long-term information on plant water use patterns.

“Isoscapes” have recently emerged as a coherent framework for the analysis, visualization, distribution, and application of environmental isotope data and have already created new disciplinary and interdisciplinary research opportunities. The chapters in this volume document the diverse and important roles that spatial monitoring and data analysis play in the application of isotope chemistry within contemporary science. The authors of this volume demonstrate how isoscapes serve as a tool for basic research, a template for data analysis and visualization, and a common platform and language for establishing a dialogue among specialists and non-specialists. The diversity and depth of the work demonstrates the relevance of isoscapes-based research to the needs of science and society in the twenty-first century, especially where problems are recognized to involve complex systems with widely distributed impacts, necessarily studied at large spatial scales (also see Bowen et al. 2009). Many chapters also highlight the significant challenges and gaps in understanding that must be overcome if the potential of these methods are to be realized. These are clear targets for future research that promise to expand discovery through improved understanding and application of spatio-temporal isotope variability.

Evaluations of plant water use in ecosystems around the world reveal a shared capacity by many dif- ferent species to absorb rain, dew, or fog water directly into their leaves or plant crowns. This mode of water uptake provides an important water subsidy that relieves foliar water stress. Our study provides the first comparative evaluation of foliar uptake capacity among the dominant plant taxa from the coast redwood ecosystem of California where crown-wetting events by summertime fog frequently occur during an otherwise drought-prone season. Previous research demonstrated that the dominant overstory tree species, Sequoia sempervirens, takes up fog water by both its roots (via drip from the crown to the soil) and directly through its leaf surfaces. The present study adds to these early findings and shows that 80% of the dominant species from the redwood forest exhibit this foliar uptake water acquisition strategy. The plants studied include canopy trees, understory ferns, and shrubs. Our results also show that foliar uptake provides direct hydration to leaves, increasing leaf water content by 2-11%. In addition, 60% of redwood forest species investigated demonstrate noc- turnal stomatal conductance to water vapor. Such findings indicate that even species unable to absorb water directly into their foliage may still receive indirect benefits from nocturnal leaf wetting through suppressed transpiration. For these species, leaf-wetting events enhance the efficacy of nighttime re-equilibration with available soil water and therefore also increase pre-dawn leaf water potentials.

We examined changes in branch hydraulic, leaf structure and gas exchange properties in coast redwood (Sequoia sempervirens) and giant sequoia (Sequoiadendron giganteum) trees of different sizes. Leaf-specific hydraulic conductivity (k(L)) increased with height in S. sempervirens but not in S. giganteum, while xylem cavitation resistance increased with height in both species. Despite hydraulic adjustments, leaf mass per unit area (LMA) and leaf carbon isotope ratios (delta(13)C) increased, and maximum mass-based stomatal conductance (g(mass)) and photosynthesis (A(mass)) decreased with height in both species. As a result, both A(mass) and g(mass) were negatively correlated with branch hydraulic properties in S. sempervirens and uncorrelated in S. giganteum. In addition, A(mass) and g(mass) were negatively correlated with LMA in both species, which we attributed to the effects of decreasing leaf internal CO(2) conductance (g(i)). Species-level differences in wood density, LMA and area-based gas exchange capacity constrained other structural and physiological properties, with S. sempervirens exhibiting increased branch water transport efficiency and S. giganteum exhibiting increased leaf-level water-use efficiency with increasing height. Our results reveal different adaptive strategies for the two redwoods that help them compensate for constraints associated with growing taller, and reflect contrasting environmental conditions each species faces in its native habitat.

13 páginas, 3 tablas, 7 figuras., 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 (δ15N), foliar N concentrations, mycorrhizal type and climate for over 11 000 plants worldwide. Arbuscular mycorrhizal, ectomycorrhizal, and ericoid mycorrhizal plants were depleted in foliar δ15N by 2‰, 3.2‰, 5.9‰, respectively, relative to nonmycorrhizal plants. Foliar δ15N increased with decreasing mean annual precipitation and with increasing mean annual temperature (MAT) across sites with MAT ≥ −0.5°C, but was invariant with MAT across sites with MAT < −0.5°C. In independent landscape-level to regional-level studies, foliar δ15N increased with increasing N availability; at the global scale, foliar δ15N 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 δ15N and ultimately global patterns in N cycling., J.M.C. was supported by the Andrew W. Mellon Foundation and A.E. was supported by the Henry Luce Foundation.

We investigated several ecological correlates of seed mass variation in the hemiparasitic, xylemtapping mistletoe, Phoradendron juniperinum. Mean seed mass varied two-fold among plants between the ages of 4 and 14 years old and was positively correlated with parental plant age. Both the standard deviation and the coefficient of variation in mean seed mass decreased with increasing plant age demonstrating that, on average, younger plants produced seed with more variable mass. Nitrogen concentrations (mg nitrogen per gram of seed) of both the seed and "fruit" (pericarp) were not correlated with mass or the age of the parent plant from which the seed was taken. However, the nitrogen content per seed (mg nitrogen per seed) was positively correlated with the mean seed dry mass and the age of the seed parent, suggesting that the carbon to nitrogen ratio of individual seeds remained relatively constant as seed mass increased and plants grew older. Seed germination ranged between 20% and 86% and was positively correlated with mass and parental plant age. Heavier seeds (seeds from older plants) also had the highest root radicle growth rates. Furthermore, the final root radicle length after 76 d of growth was positively correlated with seed dry mass. When grown on a medium containing an extract prepared from the host plant foliage, all seeds showed lower germination, grew more slowly and had shorter overall root radicles, but had significantly greater development of the haustorial "disks" (the holdfast which forms the host-parasite junction in Phoradendron) than seeds grown on a control medium. Our results suggest that, on average, seeds of greater mass produced by older plants have a greater total resource pool per propagule (fruit + seed). This resource pool may be important in conferring a greater potential for dispersal (fruit), survival, colonization, and establishment.

Grassland communities of arid western North America are often characterized by a seasonal increase in ambient temperature and evaporative demand and a corresponding decline in soil moisture availability. As the environment changes, particular species could respond differently, which should be reflected in a number of physiological processes. Carbon isotope discrimination varies during photosynthetic activity as a function of both stomatal aperture and the biochemistry of the fixation process, and provides an integrated measure of plant response to seasonal changes in the environment. We measured the seasonal course of carbon isotope discrimination in 42 grassland species to evaluate changes in gas exchange processes in response to these varying environmental factors. The seasonal courses were then used to identify community-wide patterns associated with life form, with phenology and with differences between grasses and forbs. Significant differences were detected in the following comparisons: (1) Carbon isotope discrimination decreased throughout the growing season; (2) perennial species discriminated less than annual species; (3) grasses discriminated less than forbs; and (4) early flowering species discriminated more than the later flowering ones. These comparisons suggested that (1) species active only during the initial, less stressful months of the growing season used water less efficiently, and (2) that physiological responses increasing the ratio of carbon fixed to water lost were common in these grassland species, and were correlated with the increase in evaporative demand and the decrease in soil moisture.

Non-photosynthetic, or heterotrophic, tissues in C

We present evidence of genetic variation in and covariation between leaf-level gas exchange properties and leaf size among family lines of Polygonum arenastrum. This self-fertilizing annual had previously been shown to vary genetically in developmental phenology and in morphology (size of leaves, internodes, flowers and seeds) (Geber 1990). Significant family differences were found in photosynthetic carbon assimilation rate (A), lcaf conductance to water vapor (g), instantaneous water-use efficiency (WUE), and leaf carbon isotope discrimination (Δ). A strong positive genetic correlation between A and g suggested that there was stomatal limitation on A. In addition, higher g led to relatively greater increases in transpiration, E, than in assimilation, A, so that families with high rates of gas exchange had lower instantaneous WUE and/or higher carbon isotope discrimination values. Leaf size and gas exchange were genetically correlated. In earlier studies leaf size was found to be genetically correlated with developmental phenology (Geber 1990). The pattern that emerges is one in which small-leaved families (which also have small internodes, flowers, and seeds) tend to have high gas exchange rates, low WUE, rapid development to flowering and high early fecundity, but reduced life span and maximum (vegetative and reproductive) yield compared to large-leaved families. We suggest that this pattern may have arisen from selection for contrasting suites of characters adapted to environments differing in season length.

We report a nondestructive index of age based on the branching structure of the mistletoe Phoradendron juniperinum and correlate it with an independent determination of age based on anatomical studies

We addressed the following questions : what is the sex ratio in this population of P. juniperinum? Do males and females allocate biomass equally to reproduction? Do the sexes exhibit similar age distribution patterns? Are the sexes evenly distributed across the same locations on the bost? and Do female mistletoes predominate in locations expected to be most favorable to their carbon balance?

Combined delta(13)C and delta(18)O analyses of leaf material were used to infer changes in photosynthetic capacity (A(max)) and stomatal conductance (g(l)) in Fagus sylvatica and Picea abies trees growing under natural and controlled conditions. Correlation between g(l) and delta(18)O in leaf cellulose (delta(18)O(cel)) allowed us to apply a semi-quantitative model to infer g(l) from delta(18)O(cel) and also interpret variation in delta(13)C as reflecting variation in A(max). Extraction of leaf cellulose was necessary, because delta(18)O from leaf organic matter (delta(18)O(LOM)) and delta(18)O(cel) was not reliably correlated. In juvenile trees, the model predicted elevated carbon dioxide (CO(2)) to reduce A(max) in both species, whereas ozone (O(3)) only affected beech by reducing CO(2) uptake via lowered g(l). In adult trees, A(max) declined with decreasing light level as g(l) was unchanged. O(3) did not significantly affect isotopic signatures in leaves of adult trees, reflecting the higher O(3) susceptibility of juvenile trees under controlled conditions. The isotopic analysis compared favourably to the performance of leaf gas exchange, underlining that the semi-quantitative model approach provides a robust way to gather time-integrated information on photosynthetic performance of trees under multi-faced ecological scenarios, in particular when information needed for quantitative modelling is only scarcely available.

Little is known about water transfer via mycorrhizal hyphae to plants, despite its potential importance in seedling establishment and plant community development, especially in arid environments. Therefore, this process was investigated in the study reported in this paper in laboratory-based tripartite mesocosms containing the shrub Arctostaphylos viscida (manzanita) and young seedlings of sugar pine (Pinus lambertiana) and Douglas-fir (Pseudotsuga menziesii). The objectives were to determine whether water could be transported through mycorrhizal symbionts shared by establishing conifers and A. viscida and to compare the results obtained using two tracers: the stable isotope deuterium and the dye lucifer yellow carbohydrazide. Water containing the tracers was added to the central compartment containing single manzanita shrubs. The fungal hyphae were then collected as well as plant roots from coniferous seedlings in the other two compartments to determine whether water was transferred via fungal hyphae. In addition, the length of the hyphae and degree of mycorrhizal colonisation were determined. Internal transcribed spacer–restriction fragment length polymorphism (ITS-RFLP) analysis was used to identify the fungal species involved in dye (water) transfer. Results of the stable isotope analysis showed that water is transferred via mycorrhizal hyphae, but isotopically labelled water was only detected in Douglas-fir roots, not in sugar pine roots. In contrast, the fluorescent dye was transported via mycorrhizal hyphae to both Douglas-fir and sugar pine seedlings. Only 1 of 15 fungal morphotypes (identified as Atheliaceae) growing in the mesocosms transferred the dye. Differences were detected in the water transfer patterns indicated by the deuterium and fluorescent dye tracers, suggesting that the two labels are transported by different mechanisms in the same hyphae and/or that different fungal taxa transfer them via different routes to host plants. We conclude that both tracers can provide information on resource transfer between fungi and plants, but we cannot be sure that the dye transfer data provide accurate indications of water transfer rates and patterns. The isotopic tracer provides more direct indications of water movement and is therefore more suitable than the dye for studying water relations of plants and their associated mycorrhizal fungi., Financial support was provided by the Swedish Foundation for International Cooperation in Research and Higher Education, STINT (Dnr 99/666), the University of California, The A.W. Mellon Foundation and the National Science Foundation (DEB 9981548).

Previously we found that cloned cottonwood saplings (Populus deltoides) grew twice as large in New York, New York, USA, compared to surrounding rural environments and that soils, temperature, CO2, nutrient deposition, and microclimatic variables could not account for the greater urban plant biomass. Correlations between final season biomass and cumulative O3 exposures, combined with twofold growth reductions in an open-top chamber experiment provided strong evidence that higher cumulative O3 exposures in rural sites reduced growth in the country. Here, we assess the field gas exchange, growth and development, and allocation responses underlying the observed growth differences and compare them with isolated O3 responses documented in the open-top chamber experiment. Cottonwoods showed no visible foliar injury, reduced photosynthesis of recently expanded foliage, early leaf senescence, protective reduction in stomatal conductance, or compensatory allocation to shoot relative to root biomass for either the chamber or field experiment. Instead, O3-impacted chamber plants had significantly higher conductance and reduced photosynthesis of older foliage that led to reduced leaf area production and a twofold biomass reduction in the absence of visible injury. Rural-grown field plants showed the same pattern of significantly higher conductance in the absence of concomitant increases in photosynthesis that was indicative of a loss of stomatal control. Incremental changes in foliar production were also significantly inversely related to fluctuations in ambient O3 exposures. The similarity in biomass, gas exchange, phenological, and allocation responses between chamber and field experiments indicate that mechanisms accounting for reduced growth at rural sites were consistent with those in the open-top chamber O3 experiment. This study shows the limitation of visible symptoms as a sole diagnostic factor for documenting detrimental O3 impacts and points toward a new approach to show O3 impacts when visible injury is not present. Namely, O3-impacted vegetation showed an unusual inverse relationship of increased conductance with lower photosynthesis of older foliage that was indicative of a loss of stomatal control. This increased stomatal conductance of O3-impacted vegetation accentuates pollutant flux into affected foliage and has important implications for system water balance during warm, dry portions of the growing season when O3 concentrations are highest.

Publisher Summary This volume focuses on how stable isotope analyses have contributed to understanding ecological changes within atmospheric gasses and a vast array of materials collected from aquatic, terrestrial and marine ecosystems. It is noteworthy that biochemical, physiological and ecological processes, as well as climatological cycles, operate on different temporal and spatial scales. This requires that an isotopic proxy correctly match and record the changes on interest. One of the key challenges in using isotopes as recorders of ecological change is to ensure that activities of the isotope recorder properly match both the process, as well as the temporal and spatial scales of the process of interest. Considering the metabolism of an animal that gets recorded sequentially in hair; on shorter timescales, changes in dietary inputs are reflected in the carbon dioxide of breath and the isotopic of blood. Yet, because here the time constant of change is so short, the isotopic signal may be quite dynamic, responding to individual meals and making it challenging to interpret an ecological pattern. In this chapter, Cerling et al. address ways in which shorter-term dietary dynamics can be extracted from longer-term records (hair).

Publisher Summary The stable isotope investigations have grown steadily in the past 50 years or so and have become a cornerstone for many ecological investigations since the 1980s. The understanding of the many processes that lead to isotopic variation in natural environments is now often well understood and is improving each year. The challenge that is now being confronted with, is to extend what is known about these processes and the products that result from them, to see how changes have been ‘‘recorded’’ and how these changes can be interpreted. This requires a different approach that many past ecological investigations have not taken and one that forces ecologists to find materials such as carbonates, feathers, hard tissues, hair, tree-ring cellulose, and the many chemical compounds that compose them, so that they can be used to trace and therefore record change. By linking known processes to these changes, the growing field of ‘‘ecological forensics'’ that has emerged, is now documenting changes in many novel and powerful ways.Through innovative stable isotope methods and their novel application, the manner and magnitude of ecological transformations are now being documented across a wide range of natural systems. The environmental and ecological changes will continue and in many instances, will accelerate. Applying stable isotope analyses in these ways can lead to solutions that will help mitigate future ecosystem damage and environmental degradation.

The hydraulic limitation hypothesis of Ryan & Yoder (1997, Bioscience 47, 235‐242) suggests that water supply to leaves becomes increasingly difficult with increasing tree height. Within the bounds of this hypothesis, we conjectured that the vertical hydrostatic gradient which gravity generates on the water column in tall trees would cause a progressive increase in xylem ‘safety’ (increased resistance to embolism and implosion) and a concomitant decrease in xylem ‘efficiency’ (decreased hydraulic conductivity). We based this idea on the historically recognized concept of a safety‐efficiency trade-off in xylem function, and tested it by measuring xylem conductivity and vulnerability to embolism of Sequoia sempervirens branches collected at a range of heights. Measurements of resistance of branch xylem to embolism did indeed show an increase in ‘safety’ with height. However, the expected decrease in xylem ‘efficiency’ was not observed. Instead, sapwood-specific hydraulic conductivities ( K s ) of branches increased slightly, while leaf-specific hydraulic conductivities increased dramatically, with height. The latter could be largely explained by strong vertical gradients in specific leaf area. The increase in K s with height corresponded to a decrease in xylem wall fraction (a measure of wall thickness), an increase in percentage of earlywood and slight increases in conduit diameter. These changes are probably adaptive responses to the increased transport requirements of leaves growing in the upper canopy where evaporative demand is greater. The lack of a safety‐efficiency tradeoff may be explained by opposing height trends in the pit aperture and conduit diameter of tracheids and the major and semiindependent roles these play in determining xylem safety and efficiency, respectively.

Variation in flower size is an important aspect of a plant's life history, yet few studies have shown how flower size varies with environmental conditions and to what extent foliar responses to the environment are correlated with flower size. The objectives of this study were to (1) develop a theoretical framework for linking flower size and leaf size to their costs and benefits, as assessed using foliar stable carbon isotope ratio (delta(13)C) under varying degrees of water limitation, and then (2) examine how variation in flower size within and among species growing along a naturally occurring moisture availability gradient correlates with variation in delta(13)C and leaf size. Five plant species were examined at three sites in Oregon. Intra- and inter-specific patterns of flower size in relation to moisture availability were the same: the ratios of the area of flower display to total leaf area and of individual flower area to leaf area were greater at sites with more soil moisture compared to those sites with less soil moisture. The increase in flower area per unit increase in leaf area was greater at sites with more soil moisture than at sites where water deficit can occur. Values of delta(13)C, an index of water-use efficiency, were greater for plants with larger floral size. The patterns we observed generalize across species, irrespective of overall plant morphology or pollination system. These correlations between flower size, moisture availability, and delta(13)C suggest that water loss from flowers can influence leaf responses to the environment, which in turn may indirectly mediate an effect on flower size.

Hydraulic redistribution (HR), the nocturnal vertical transfer of soil water from moister to drier regions in the soil profile by roots, has now been observed in Amazonian trees. We have incorporated HR into an atmospheric general circulation model (the National Center for Atmospheric Research Community Atmospheric Model Version 2) to estimate its impact on climate over the Amazon and other parts of the globe where plants displaying HR occur. Model results show that photosynthesis and evapotranspiration increase significantly in the Amazon during the dry season when plants are allowed to redistribute soil water. Plants draw water up and deposit it into the surface layers, and this water subsidy sustains transpiration at rates that deep roots alone cannot accomplish. The water used for dry season transpiration is from the deep storage layers in the soil, recharged during the previous wet season. We estimate that HR increases dry season (July to November) transpiration by ≈40% over the Amazon. Our model also indicates that such an increase in transpiration over the Amazon and other drought-stressed regions affects the seasonal cycles of temperature through changes in latent heat, thereby establishing a direct link between plant root functioning and climate.

A number of operationally defined methods exist for pretreating plant tissues in order to measure C, N, and O isotopes. Because these isotope measurements are used to infer information about environmental conditions that existed at the time of tissue growth, it is important that these pretreatments remove compounds that may have exchanged isotopes or have been synthesized after the original formation of these tissues. In stable isotope studies, many pretreatment methods focus on isolating "cellulose" from the bulk tissue sample because cellulose does not exchange C and O isotopes after original synthesis. We investigated the efficacy of three commonly applied pretreatment methods, the Brendel method and two variants of the Brendel method, the Jayme-Wise method and successive acid/base/acid washes, for use on three tissue types (wood, leaves, roots). We then compared the effect of each method on C and O isotope composition (13C,14C,18O), C and N content, and chemical composition of the residue produced (using13C nuclear magnetic resonance (NMR)). Our results raised concerns over use of the Brendel method as published, as it both added C and N to the sample and left a residue that contains remnant lipids and waxes. Furthermore, this method resulted in18O values that are enriched relative to the other methods. Modifying the Brendel method by adding a NaOH step (wash) solved many of these problems. We also found that processed residues vary by tissue type. For wood and root tissues, the13C NMR spectra and the18O and13C data showed only small differences between residues for the Jayme-Wise and modified Brendel methods. However, for leaf tissue,13C NMR data showed that Jayme-Wise pretreatments produced residues that are more chemically similar to cellulose than the other methods. The acid/base/acid washing method generated13C NMR spectra with incomplete removal of lignin for all tissues tested and both isotopic, and13C NMR results confirmed that this method should not be used if purified cellulose is desired. © 2005 American Chemical Society.

Phenotypic plasticity is an important response mechanism of plants to environmental heterogeneity. Here, we explored the genetic basis of plastic responses of Arabidopsis thaliana to water deficit by experimentally mapping quantitative trait loci (QTL) in two recombinant inbred populations (Cvi x Ler and Ler x Col). We detected genetic variation and significant genotype-by-environment interactions for many traits related to water use. We also mapped 26 QTL, including six for carbon isotope composition (delta13C). Negative genetic correlations between fruit length and fruit production as well as between flowering time and branch production were corroborated by QTL colocalization, suggesting these correlations are due to pleiotropy or physical linkage. Water-limited plants were more apically dominant with greater root:shoot ratios and higher delta13C (higher water-use efficiency) when compared to well-watered plants. Many of the QTL effects for these traits interacted significantly with the irrigation treatment, suggesting that the observed phenotypic plasticity is genetically based. We specifically searched for epistatic (QTL-QTL) interactions using a two-dimensional genome scan, which allowed us to detect epistasis regardless of additive genetic effects. We found several significant QTL-QTL interactions including three that exhibited environmental dependence. These results provide preliminary evidence for proposed genetic mechanisms underlying phenotypic plasticity.

Recent studies have shown that natural abundance stable carbon isotopes can be used to partition measurements of net ecosystem CO 2 exchange into gross fluxes of photosynthesis and respiration. This chapter explores the related application of the natural abundance stable carbon isotope ratio of respired CO 2 to partitioning components of ecosystem respiration. Although, the exploitation of the natural variation in stable isotope abundance in respired CO 2 holds the most promise for widespread application among existing measurement networks (e.g., BASIN, FLUXNET, SIBAE) and for comparative studies focused on ecosystem C balance, the reliance on natural variation may also have limitations from the standpoint that such variation is typically small. Recent evidence suggests, however, that variation can be large and significant such that measurable differences among respiration signatures can allow partitioning of ecosystem respiration and therefore inform ecosystem C-balance studies.

Accurate estimates of sapwood properties (including radial depth of functional xylem and wood water content) are critical when using the heat pulse velocity (HPV) technique to estimate tree water use. Errors in estimating the volumetric water content (V(h)) of the sapwood, especially in tree species with a large proportion of sapwood, can cause significant errors in the calculations ofsap velocity and sap flow through tree boles. Scaling to the whole-stand level greatly inflates these errors. We determined the effects of season, tree size and radial wood depth on V(h) of wood cores removed from Acer saccharum Marsh. trees throughout 3 years in upstate New York. We also determined the effects of variation in V(h) on sap velocity and sap flow calculations based on HPV data collected from sap flow gauges inserted at four depths. In addition, we compared two modifications of Hatton's weighted average technique, the zero-step and zero-average methods, for determining sap velocity and sap flow at depths beyond those penetrated by the sap flow gauges. Parameter V(h) varied significantly with time of year (DOY), tree size (S), and radial wood depth (RD), and there were significant DOY x S and DOY x RD interactions. Use of a mean whole-tree V(h) value resulted in differences ranging from -6 to +47% for both sap velocity and sap flow for individual sapwood annuli compared with use of the V(h) value determined at the specific depth where a probe was placed. Whole-tree sap flow was 7% higher when calculated on the basis of the individual V(h) value compared with the mean whole-tree V(h) value. Calculated total sap flow for a tree with a DBH of 48.8 cm was 13 and 19% less using the zero-step and the zero-average velocity techniques, respectively, than the value obtained with Hatton's weighted average technique. Smaller differences among the three methods were observed for a tree with a DBH of 24.4 cm. We conclude that, for Acer saccharum: (1) mean V(h) changes significantly during the year and can range from nearly 50% during winter and early spring, to 20% during the growing season;(2) large trees have a significantly greater V(h) than small trees; (3) overall, V(h) decreases and then increases significantly with radial wood depth, suggesting that radial water movement and storage are highly dynamic; and (4) V(h) estimates can vary greatly and influence subsequent water use calculations depending on whether an average or an individual V(h) value for a wood core is used. For large diameter trees in which sapwood comprises a large fraction of total stem cross-sectional area (where sap flow gauges cannot be inserted across the entire cross-sectional area), the zero-average modification of Hatton's weighted average method reduces the potential for large errors in whole-tree and landscape water balance estimates based on the HPV method.

Understanding the responses of riparian trees to water availability is critical for predicting the effects of changes in precipitation on riparian ecosystems. Dioecious Acer negundo L. (box elder) is a common riparian tree that is highly sensitive to water stress. Earlier studies indicated that the genders of A. negundo respond differently to water availability, with males being more conservative in their water use than females. To assess the potential effects of changes in precipitation on the sex ratio of riparian trees, we extended earlier studies of A. negundo by analyzing responses of male and female genotypes to interannual differences in water availability in a common garden. We measured growth of tree rings and used stable carbon isotope analysis of tree ring alpha-cellulose to integrate physiological responses to annual water treatments. During dry years, male and female trees exhibited similar growth and physiological responses. However, during wet years, females exhibited higher growth rates and lower carbon isotope ratios (indicating less conservative water use) than did males. Furthermore, we found that male trees exhibited similar stomatal behavior (inferred from integrated carbon isotope ratios) whether years were wet or dry, whereas females did not exhibit a consistent response to changes in water availability. We predict that with increasing precipitation and soil water availability, the representation of females will be favored because of shifts in the competitive interactions of the genders. Such changes may affect the reproductive output of these riparian trees and may influence overall water flux from riparian ecosystems. In addition, this study demonstrates the utility of carbon isotope analysis for assessing long-term responses of tree populations to shifts in water availability.

Understanding the power of stable isotopes in research requires general knowledge of the fundamental principles of stable isotope chemistry as well as, more specifically, how the ratios of biologically relevant lighter stable isotopes of H, C, N, O, and S in the biosphere are measured using modern isotope ratio mass spectrometers (IRMS). Here, we provide a primer to these topics. The chapter serves as a springboard to the following chapters in this book. We make no attempt to be comprehensive. For more detailed reading on stable isotope chemistry or geochemistry we encourage readers to see treatments by Criss (1999), Hoefs (1997), Faure (1986) or Gat and Gonfiantini (1981). For in-depth discussions on particular topics we refer you to recent works by Kendall and McDonnell (1998; hydrology), Griffiths (1998; biology, ecology, atmospheric science, biogeochemistry), Clark and Fritz (1997; hydrogeology), Boutton and Yamasaki (1996; soils), Lajtha and Michener (1994; environmental sciences), Ehleringer et al. (1993; plant ecophysiology) and Rundel et al. (1989; ecology).

Virtually all elements of biological interest have multiple stable isotopic forms and the fractionation events associated with biological and physical processes help to create spatial and temporal variations in isotopic abundance that can be used to understand the dynamics of ecological systems. Stable isotope ratio analyses at natural abundance levels can provide integrated information on ecosystem functioning, such as variations in water-use activities by different elements within an ecosystem (Ehleringer et al. 1993; Dawson and Ehleringer 1998). Stable isotope ratio analyses do not provide information on water flux rates through the ecosystem, but instead they help constrain the analysis of flux data, such as through identifying those specific soil layers that are the source of current moisture use by the vegetation or the ratio of carbon dioxide-to-water (CO2-to-H2O) flux.

Sugar cane cropping for biofuel production reduces water discharge from a northern Indian basin and threatens downstream communities. Regulators want to partition blame between climate change—induced declines in mountain snowpack and excessive evaporation from poorly managed fields. In the same basin, a tiger is found shot. Is it the nuisance animal that has been tormenting local communities, or is it a different animal poached from the upland forests?

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.

Sexually dimorphic organisms provide elegant comparative systems in which to study adaptation (Darwin 1859, 1877; Shine 1989; Chap. 4). Their elegance derives from the fact that the sexes of dioecious species are similar in many aspects of their phenotype and biology, even when they differ in some secondary sex characters and in reproductive ecology. It is therefore possible to study the functional significance of dimorphism in morphology, physiology, and behavior while holding much of the rest of the phenotype and ecology constant. By contrast, comparative studies of interspecific differences in form and function may be hampered by confounding differences among species in geography, history (phylogeny), and ecology (Dawson and Bliss 1989; Shine 1989). Even for dioecious taxa, however, the functional significance of dimorphic traits may be difficult to interpret without a comprehensive understanding of gender differences in life history and reproductive biology (Shine 1989).

Publisher Summary In order to understand the role of plants in catchment-level hydrologic processes, one must be able to accurately quantify the components of the catchment balance. Understanding what influences a hydrologic cycle has and will continue to be important for ecological, economic, and social reasons. This chapter explains that there is much to be gained by applying stable isotope techniques toward elucidating the role that plants play in the hydrologic cycle; such a research agenda should be part of all future hydro-logic research. However, for this goal to be realized, it will be important for plant biologists and hydrologists who are employing isotope tools and tracers in their research to collaborate. The chapter illustrates how the application of stable isotopes can provide valuable information on the short-term dynamics of water uptake and water use by plants from a variety of different ecosystems. It discusses the role of plants and vegetated surfaces in water movement patterns, and their abilities to influence these patterns and rates of water movement at the catchment level. The analysis of stable isotopes of hydrogen and oxygen in plant- and source-waters has, in recent years, substantially improved the ability to better understand the soil-plant and plant-atmosphere interfaces.

Net photosynthesis and stomatal conductance were measured in ten red spruce (Picea rubens Sarg.) saplings, growing near Ithaca, New York, throughout the early spring and late-fall growing periods. Gas exchange and daily minimum and maximum soil and air temperatures were also measured. Linear regression analysis showed that rates of net photosynthesis were positively correlated with both minimum daily soil and air temperatures but that minimum soil temperature was a better predictor of net photosynthesis. Moreover, net photosynthesis was more sensitive to changes in soil temperature than to changes in air temperature, and photosynthesis was approximately twice as sensitive to temperature changes during the fall than during the spring.

Different requirements for reproduction between sexes in dioecious plants can lead to gender-specific physiological responses to environmental constraints. Females generally endure higher reproductive investment and costs than males, displaying lower water and nutrient use efficiency and growth. We studied the ecophysiology of the dioecious shrub Corema album L. (D.) Don. to assess how drought impacts physiological responses and gender dimorphism along an aridity gradient within the species’ range. Leaf gas exchange, photochemical efficiency (Fv/Fm), water potential (Ψ), vegetative growth (VG) and leaf C isotopic discrimination and nitrogen content were measured in three populations under markedly different climatic conditions. Physiological responses and growth indicated higher level of stress in the drier environments, leading to increased WUE (Δ) and lower VG. Physiological stress increases among plants were related to incremental changes in the degree of gender dimorphism, with significant effects on Ψ and Fv/Fm (P

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.