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PubMed2005

Effects of senescence-induced alteration in cytokinin metabolism on source-sink relationships and ontogenic and stress-induced transitions in tobacco.

Senescence and reserve mobilization are integral components of plant development, are basic strategies in stress mitigation, and regulated at least in part by cytokinin. In the present study the effect of altered cytokinin metabolism caused by senescence-specific autoregulated expression of the Agrobacterium tumefaciens IPT gene under control of the P(SAG12) promoter (P(SAG12)-IPT) on seed germination and the response to a water-deficit stress was studied in tobacco (Nicotiana tabacum L.). Cytokinin levels, sugar content and composition of the leaf strata within the canopy of wild-type and P(SAG12)-IPT plants confirmed the reported altered source-sink relations. No measurable difference in sugar and pigment content of discs harvested from apical and basal leaves was evident 72 h after incubation with (+)-ABA or in darkness, indicating that expression of the transgene was not restricted to senescing leaves. No difference in quantum efficiency, photosynthetic activity, accumulation of ABA, and stomatal conductance was apparent in apical, middle and basal leaves of either wild-type or P(SAG12)-IPT plants after imposition of a mild water stress. However, compared to wild-type plants, P(SAG12)-IPT plants were slower to adjust biomass allocation. A stress-induced increase in root:shoot ratio and specific leaf area (SLA) occurred more rapidly in wild-type than in P(SAG12)-IPT plants reflecting delayed remobilization of leaf reserves to sink organs in the transformant. P(SAG12)-IPT seeds germinated more slowly even though abscisic acid (ABA) content was 50% that of the wild-type seeds confirming cytokinin-induced alterations in reserve remobilization. Thus, senescence is integral to plant growth and development and an increased endogenous cytokinin content impacts source-sink relations to delay ontogenic transitions wherein senescence in a necessary process.

Abscisic Acid↗
PubMed2026

From stress signaling to yield stability: physiological and molecular mechanisms of wheat resilience to heat and drought stress.

Wheat resilience depends on coordinated signaling, reproductive protection, and source-sink regulation, providing a framework to breed robust trait combinations that stabilize yield under combined heat and drought. Climate change is increasing the frequency and severity of heat and drought events, posing a major threat to wheat productivity, yield stability, and food security. Because these stresses often coincide in the field, their combined effects can impair growth, reproductive development, grain filling, and final yield more severely than either stress alone. Wheat resilience under such conditions depends on coordinated physiological adjustment and molecular regulation that sustain cellular homeostasis, protect reproductive tissues, and preserve yield-related traits. This review synthesizes current knowledge on the physiological and molecular bases of wheat resilience to heat and drought, with emphasis on their combined effects. We discuss major physiological responses, including photosynthetic adjustment, stomatal regulation, canopy cooling, osmotic balance, antioxidant defense, membrane stability, and source-sink coordination. We also examine key regulatory pathways involved in stress perception and adaptation, including calcium and reactive oxygen species signaling, mitogen-activated protein kinase cascades, phytohormonal crosstalk, transcriptional regulation, heat shock proteins, late embryogenesis abundant proteins, and osmoprotective and redox-associated pathways. In addition, we highlight the growing contribution of transcriptomics, proteomics, metabolomics, and phenomics to the identification of candidate genes, biomarkers, and adaptive traits. Finally, we consider how mechanistic insights can be translated into wheat improvement through molecular markers, genomic selection, gene editing, and climate-realistic phenotyping. An integrated understanding of stress signaling and adaptive trait deployment will be essential for developing wheat cultivars with improved resilience and yield stability under future climates.

Triticum↗
PubMed2002

Controls of fine root dynamics across a gradient of gap sizes in a pine woodland.

Controls of fine dynamics were investigated in a Pinus palustris Mill. (longleaf pine) woodland subjected to two understory vegetation treatments (control versus removed) and four overstory treatments (no gap control, and canopy gaps of three sizes with constant total gap area per stand). Fine root (<2 mm diameter) dynamics were measured over 11 months using ingrowth cores (all treatments) and minirhizotrons (understory removed in no gap control and large gap treatments only). At the fine (microsite) spatial scale, pine and non-pine root mass production responded negatively to each other (P=0.033). Each life form was significantly (P< or =0.028) related to nearby overstory density, and pine root production compensated for reductions in non-pine roots if understory vegetation was removed. Soil moisture and NO(3) mineralization rate were negatively related to pine root mass production (ingrowth cores; P<0.001 and P=0.052) and positively related to pine root length production, mortality and turnover (minirhizotrons; P from <0.001 to 0.078). Temperature variance was negatively related to pine root lifespan P<0.001) and positively related to pine root turnover (P=0.003). At the ecosystem scale, pattern of overstory disturbance (gap size and number) had no significant effect on non-pine, pine, or total root production. However, the presence of gaps (versus the no-gap control) increased non-pine root mass production (ANOVA, P=0.055) in natural understory conditions, and reduced pine root mass production (P=0.035) where the understory was removed. Ecosystem-wide pine root length production, mortality and turnover were positively related to weekly soil temperature (P< or =0.02). In natural systems, fine root dynamics are highly variable and strongly affected by biotic factors. Roots quickly close belowground gaps because one life form (pine or non-pine) compensates for the absence of the other. When understory vegetation is removed, however, pine roots respond to the local abiotic environment, particularly moisture and NO(3).

Biomass↗
PubMed2003

Effects of a simple plant morphological mutation on the arthropod community and the impacts of predators on a principal insect herbivore.

Plant features that enhance predator effectiveness can be considered extrinsic-resistance factors because they result in reduced insect herbivory. In this paper we test the hypothesis that reduced epicuticular wax (EW) in Pisum sativum L. is an extrinsic-resistance factor contributing to field resistance to Acyrthosiphon pisum (Harris). We monitored pea aphid populations in the field on reduced EW and normal EW near isolines of peas for two seasons and confirmed that aphid populations are lower on reduced EW peas than on normal EW peas. We also monitored predators within the canopies of the two pea lines to discover community level patterns in response to differences in EW. We found that while predator numbers were similar between the two lines, there were more syrphids on the normal EW peas, and a trend towards more coccinellids on reduced EW peas. We tested the impact of predators on pea aphids on the two EW lines by monitoring their population levels in cages that excluded predators, and in cages that allowed predators to enter. We found that pea aphid populations were similar on the two EW lines when predators were excluded. When predators were allowed access to the plants, pea aphid populations were reduced more on reduced EW peas than on normal EW peas. We also examined the intrinsic resistance to aphids in reduced EW peas with laboratory dual-choice tests comparing aphid response to reduced EW and normal EW peas, and found that walking, apterous aphids displayed no preference for one pea line over the other. Bioassays to measure growth and fecundity of the pea aphid on the two EW types in the greenhouse and in the field showed that intrinsic rate of increase, and other life table parameters, were not different for aphids on the two lines. Together these results support the hypothesis that reduced EW in peas is a predator-dependent extrinsic resistance factor. Genetically reducing EW bloom in peas and other waxy crop plants might improve the effectiveness of arthropod natural enemies of insect pests. More generally, the results show that a subtle change in plant morphology can substantially influence the impact of predators on insect herbivore populations. The benefit of extrinsic resistance to herbivory conferred by reduced EW may balance any benefits of a prominent EW bloom, thereby sustaining EW polymorphisms in some natural plant populations.

Adaptation, Physiological↗
PubMed2003

Forest mediated light regime linked to amphibian distribution and performance.

The vegetation in and around the basins of ephemeral wetlands can greatly affect light environments for aquatic species such as amphibians. We used hemispherical photographs to quantify the light environment in terms of the global site factor (GSF), the proportion of available solar radiation that actually strikes the wetland. We compared GSF to the distribution and performance of two amphibian species (Pseudacris crucifer and Rana sylvatica) within 17 ephemeral wetlands in northeastern Connecticut, USA. We found that P. crucifer is restricted to lighter wetlands (GSF >0.34) and that its abundance is proportional to GSF. By contrast, R. sylvatica is found across the light gradient and its abundance is unrelated to GSF. For both species, GSF is a strong predictor of larval developmental rate. In addition, P. crucifer growth rate is higher in lighter wetlands. Through thermal effects, changes in resources, or other influences, light appears to be an important predictor of the distribution and performance of amphibians. Because the structure of canopies can change rapidly, and because amphibians can be strongly impacted by these changes, vegetation mediated effects on wetland light environments may be critical to understanding the dynamics of amphibian populations within forested biomes.

Animals↗
PubMed2003

Factors limiting the intertidal distribution of the mangrove species Xylocarpus granatum.

The tree species Xylocarpus granatum is commonly described as occurring in the upper intertidal zone of mangrove forests, but mature trees are occasionally found at lower elevations. In the Utwe River basin, on the Pacific island of Kosrae, we investigated the relative importance of several biotic and abiotic factors that may control the intertidal distribution of X. granatum. Factors we evaluated included differential seed predation across the lower, mid, and upper intertidal zones and seedling responses to salinity, tidal flooding, and shade. Seed predation was 22.4% over the first 34 days and varied little among zones or in gaps versus under the forest canopy. By day 161, there were still no differences in seed mortality, but a significant difference was found in seedling establishment, with much greater establishment in the upper intertidal plots. X. granatum seedlings in a greenhouse experiment exhibited greater growth in freshwater than seedlings in 23 ppt salinity, which is typical of salinity levels found in the mid intertidal zone in our field study sites in Micronesia, where mature X. granatum trees are generally absent. Seedlings grown in 23 ppt salinity, however, exhibited few visible signs of stress associated with patterns in growth. Seedlings grown in a simulated tidal flooding treatment (with 23 ppt salinity) also showed few signs of stress. Growth declined dramatically under 80% shade cloths, but there were few interactions of shading with either 23 ppt salinity or simulated tidal flooding. Differential seed predation is not likely to be the primary factor responsible for the intertidal distribution of X. granatum on Kosrae. However, seedling tolerance of flooding or salinity may be more important, especially relative to a potential contribution to secondary stress mortality. Other factors may ultimately prove to be more critical, such as physiological effects of salinity on seed germination, effects of tides on seed dispersal and rooting, or differential herbivory on seedlings.

Adaptation, Physiological↗
PubMed2003

The natural abundance of (15)N in mat-forming lichens.

Natural abundance of (15)N and [N] was studied in thalli of mat-forming lichens collected from tundra and heathland sites in the northern and southern hemispheres. The study includes samples of British Cladonia portentosa from sites in regions of high and low N-loading and in heathland growing both directly on peat and independently of the soil substratum, in a canopy of prostrate gorse ( Ulex minor). In the mat-forming lichens examined, a non-random pattern in [N] and delta(15)N was characterised by a minimum in delta(15)N, which occurred most frequently at 20-40 mm below the thallus apex. Nitrogen concentration increased above this point, towards the apex, though remained invariably low towards the thallus base. We discuss the significance of the pattern in [N] and delta(15)N for current theories describing the uptake and recycling of nitrogen by mat-forming lichens in oligotrophic habitats. Our data are incompatible with the suggested uptake of soil organic-N depleted in (15)N, though are consistent with possible internal recycling and the development of a structural necromass. The study emphasises the internal fractionation of nitrogen isotopes and provides a caveat against the assumption that values of delta(15)N provide an unequivocal indicator of source-sink relationships in nitrogen cycling.

Chemical Fractionation↗
PubMed2003

The leaf size-twig size spectrum and its relationship to other important spectra of variation among species.

There is a spectrum from species with narrow, frequently branched twigs carrying small leaves and other appendages, to species with thick twigs carrying large leaves and appendages. Here we investigate the allometry of this spectrum and its relationship to two other important spectra of ecological variation between species, the seed mass-seed output spectrum and the specific leaf area-leaf lifespan spectrum. Our main dataset covered 33 woody dicotyledonous species in sclerophyll fire-prone vegetation on low nutrient soil at 1,200 mm annual rainfall near Sydney, Australia. These were phylogenetically selected to contribute 32 evolutionary divergences. Two smaller datasets, from 390 mm annual rainfall, were also examined to assess generality of cross-species patterns. There was two to three orders of magnitude variation in twig cross-sectional area, individual leaf size and total leaf area supported on a twig across the study species. As expected, species with thicker twigs had larger leaves and branched less often than species with thin twigs. Total leaf area supported on a twig was mainly driven by leaf size rather than by the number of leaves. Total leaf area was strongly correlated with twig cross-section area, both across present-day species and across evolutionary divergences. The common log-log slope of 1.45 was significantly steeper than 1. Thus on average, species with tenfold larger leaves supported about threefold more leaf area per twig cross-section, which must have considerable implications for other aspects of water relations. Species at the low rainfall site on loamy sand supported about half as much leaf area, at a given twig cross-section, as species at the low rainfall site on light clay, or at the high rainfall site. Within sites, leaf and twig size were positively correlated with seed mass, and negatively correlated with specific leaf area. Identifying and understanding leading spectra of ecological variation among species is an important challenge for plant ecology. The seed mass-seed output and specific leaf area-leaf lifespan spectra are each underpinned by a single, comprehensible trade-off and their consequences are fairly well understood. The leaf-size-twig-size spectrum has obvious consequences for the texture of canopies, but we are only just beginning to understand the costs and benefits of large versus small leaf and twig size.

Analysis of Variance↗
PubMed2003

Productivity responses to altered rainfall patterns in a C4-dominated grassland.

Rainfall variability is a key driver of ecosystem structure and function in grasslands worldwide. Changes in rainfall patterns predicted by global climate models for the central United States are expected to cause lower and increasingly variable soil water availability, which may impact net primary production and plant species composition in native Great Plains grasslands. We experimentally altered the timing and quantity of growing season rainfall inputs by lengthening inter-rainfall dry intervals by 50%, reducing rainfall quantities by 30%, or both, compared to the ambient rainfall regime in a native tallgrass prairie ecosystem in northeastern Kansas. Over three growing seasons, increased rainfall variability caused by altered rainfall timing with no change in total rainfall quantity led to lower and more variable soil water content (0-30 cm depth), an approximately 10% reduction in aboveground net primary productivity (ANPP), increased root to shoot ratios, and greater canopy photon flux density at 30 cm above the soil surface. Lower total ANPP primarily resulted from reduced growth, biomass and flowering of subdominant warm-season C4 grasses while productivity of the dominant C4 grass Andropogon gerardii was relatively unresponsive. In general, vegetation responses to increased soil water content variability were at least equal to those caused by imposing a 30% reduction in rainfall quantity without altering the timing of rainfall inputs. Reduced ANPP most likely resulted from direct effects of soil moisture deficits on root activity, plant water status, and photosynthesis. Altered rainfall regimes are likely to be an important element of climate change scenarios in this grassland, and the nature of interactions with other climate change elements remains a significant challenge for predicting ecosystem responses to climate change.

Andropogon↗
PubMed2003

Functional correlates of leaf demographic response to gap release in saplings of a shade-tolerant tree, Elateriospermum tapos.

For a shade-tolerant SE Asian tropical tree, Elateriospermum tapos (Euphorbiaceae), we studied field-established saplings in gaps and the shaded understory to test the hypothesis that differences in leaf demography and leaf life span under contrasting light regimes should be functionally correlated with architecture, self-shading and nitrogen distribution within the sapling crown. Rates of leaf production and net leaf gain were greater for saplings in gaps than those in the understory. Median leaf life span was approximately 26 months in the gap saplings, while it was estimated to be greater than 38 months in the understory saplings. Consequently, gap saplings had a greater standing leaf number and experienced greater degrees of self-shading than understory saplings. Light availability at individual leaves, estimated by a combination of canopy photos and a three-dimensional architecture model, were negatively correlated with leaf age in gap saplings but not so in understory saplings. Leaf nitrogen content per unit area (Narea) was influenced more by light availability than by leaf age in the gap saplings. In contrast, in understory saplings, Narea was neither correlated with light availability nor with leaf age, and did not decrease significantly before 38 months in leaf age. We conclude that saplings of this shade-tolerant species apparently prolong their leaf life span in the shaded understory through slower rates of leaf production, lower standing number of leaves and lower degrees of self shading than in gap, and that the rate of decline of Narea with leaf age depends on architecture and self-shading regimes that respond to changes in light regimes.

Adaptation, Physiological↗
PubMed2003

Disentangling a rainforest food web using stable isotopes: dietary diversity in a species-rich ant community.

For diverse communities of omnivorous insects such as ants, the extent of direct consumption of plant-derived resources vs. predation is largely unknown. However, determination of the extent of "herbivory" among ants may be crucial to understand the hyper-dominance of ants in tropical tree crowns, where prey organisms tend to occur scarcely and unpredictably. We therefore examined N and C stable isotope ratios (delta(15)N and delta(13)C) in 50 ant species and associated insects and plants from a tropical rainforest in North Queensland, Australia. Variation between ant species was pronounced (range of species means: 7.1 per thousand in delta(15)N and 6.8 per thousand in delta(13)C). Isotope signatures of the entire ant community overlapped with those of several herbivorous as well as predacious arthropods. Variability in delta(15)N between ants was not correlated with plant delta(15)N from which they were collected. Ant species spread out in a continuum between largely herbivorous and purely predacious taxa, with a high degree of omnivory. Ant species' delta(15)N were consistent with the trophic level predicted by natural feeding observations, but not their delta(13)C. Low delta(15)N levels were recorded for ant species that commonly forage for nectar on understorey or canopy plants, intermediate levels for species with large colonies that were highly abundant on nectar and honeydew sources and were predacious, and the highest levels for predominantly predatory ground-foraging species. Colonies of the dominant weaver-ants (Oecophylla smaragdina) had significantly lower delta(15)N in mature forests (where preferred honeydew and nectar sources are abundant) than in open secondary vegetation. N concentration of ant dry mass showed only very limited variability across species and no correlation with trophic levels. This study demonstrates that stable isotopes provide a powerful tool for quantitative analyses of trophic niche partitioning and plasticity in complex and diverse tropical omnivore communities.

Animals↗
PubMed2003

Elevated CO2 reduces sap flux in mature deciduous forest trees.

We enriched in CO2 the canopy of 14 broad-leaved trees in a species-rich, ca. 30-m-tall forest in NW Switzerland to test whether elevated CO2 reduces water use in mature forest trees. Measurements of sap flux density (JS) were made prior to CO2 enrichment (summer 2000) and throughout the first whole growing season of CO2 exposure (2001) using the constant heat-flow technique. The short-term responses of sap flux to brief (1.5-3 h) interruptions of CO2 enrichment were also examined. There were no significant a priori differences in morphological and physiological traits between trees which were later exposed to elevated CO2 (n=14) and trees later used as controls (n=19). Over the entire growing season, CO2 enrichment resulted in an average 10.7% reduction in mean daily JS across all species compared to control trees. Responses were most pronounced in Carpinus, Acer, Prunus and Tilia, smaller in Quercus and close to zero in Fagus trees. The JS of treated trees significantly increased by 7% upon transient exposure to ambient CO2 concentrations at noon. Hence, responses of the different species were, in the short term, similar in magnitude to those observed over the whole season (though opposite because of the reversed treatment). The reductions in mean JS of CO2-enriched trees were high (22%) under conditions of low evaporative demand (vapour pressure deficit, VPD <5 hPa) and small (2%) when mean daily VPD was greater than 10 hPa. During a relatively dry period, the effect of elevated CO2 on JS even appeared to be reversed. These results suggest that daily water savings by CO2-enriched trees may have accumulated to a significantly improved water status by the time when control trees were short of soil moisture. Our data indicate that the magnitude of CO2 effects on stand transpiration will depend on rainfall regimes and the relative abundance of the different species, being more pronounced under humid conditions and in stands dominated by species such as Carpinus and negligible in mono-specific Fagus forests.

Betulaceae↗
PubMed2003

Water resource partitioning, stem xylem hydraulic properties, and plant water use strategies in a seasonally dry riparian tropical rainforest.

This study investigated seasonal variation in the origin of water used by plants in a riparian tropical rainforest community and explored linkages between plant water source, plant xylem hydraulic conductivity and response to the onset of dry conditions. The study focused on five co-dominant canopy species, comprising three tree species ( Doryphora aromatica, Argyrodendron trifoliolatum, Castanospora alphandii) and two climbing palms ( Calamus australis and Calamus caryotoides). Stable isotope ratios of oxygen in water (delta(18)O) from soil, groundwater, stream water and plant xylem measured in the wet season and the subsequent dry season revealed water resource partitioning between species in the dry season. Measurement of stem-area-specific hydraulic conductivity ( K(S)) in the wet season and subsequent dry season showed a significant dry-season loss of K(S) in three of the five species ( Castanospora alphandii, Calamus australis and C. caryotoides) and a decrease in mean K(S) for all species. This loss of hydraulic conductivity was positively correlated with the difference between wet-season and dry-season midday leaf water potentials and with leaf carbon isotope discrimination, indicating that plants that were less susceptible to loss of conductivity had greater control over transpiration rate and were more water-use efficient.

Adaptation, Physiological↗
PubMed2003

Carbon balance of a tropical savanna of northern Australia.

Through estimations of above- and below-ground standing biomass, annual biomass increment, fine root production and turnover, litterfall, canopy respiration and total soil CO(2) efflux, a carbon balance on seasonal and yearly time-scales is developed for a Eucalypt open-forest savanna in northern Australia. This carbon balance is compared to estimates of carbon fluxes derived from eddy covariance measurements conducted at the same site. The total carbon (C) stock of the savanna was 204+/-53 ton C ha(-1), with approximately 84% below-ground and 16% above-ground. Soil organic carbon content (0-1 m) was 151+/-33 ton C ha(-1), accounting for about 74% of the total carbon content in the ecosystem. Vegetation biomass was 53+/-20 ton C ha(-1), 39% of which was found in the root component and 61% in above-ground components (trees, shrubs, grasses). Annual gross primary production was 20.8 ton C ha(-1), of which 27% occurred in above-ground components and 73% below-ground components. Net primary production was 11 ton C ha(-1) year(-1), of which 8.0 ton C ha(-1) (73%) was contributed by below-ground net primary production and 3.0 ton C ha(-1) (27%) by above-ground net primary production. Annual soil carbon efflux was 14.3 ton C ha(-1) year(-1). Approximately three-quarters of the carbon flux (above-ground, below-ground and total ecosystem) occur during the 5-6 months of the wet season. This savanna site is a carbon sink during the wet season, but becomes a weak source during the dry season. Annual net ecosystem production was 3.8 ton C ha(-1) year(-1).

Australia↗
PubMed2003

Carbon dioxide and water vapor exchange in a warm temperate grassland.

Grasslands cover about 40% of the ice-free global terrestrial surface, but their contribution to local and regional water and carbon fluxes and sensitivity to climatic perturbations such as drought remains uncertain. Here, we assess the direction and magnitude of net ecosystem carbon exchange (NEE) and its components, ecosystem carbon assimilation ( A(c)) and ecosystem respiration ( R(E)), in a southeastern United States grassland ecosystem subject to periodic drought and harvest using a combination of eddy-covariance measurements and model calculations. We modeled A(c) and evapotranspiration (ET) using a big-leaf canopy scheme in conjunction with ecophysiological and radiative transfer principles, and applied the model to assess the sensitivity of NEE and ET to soil moisture dynamics and rapid excursions in leaf area index (LAI) following grass harvesting. Model results closely match eddy-covariance flux estimations on daily, and longer, time steps. Both model calculations and eddy-covariance estimates suggest that the grassland became a net source of carbon to the atmosphere immediately following the harvest, but a rapid recovery in LAI maintained a marginal carbon sink during summer. However, when integrated over the year, this grassland ecosystem was a net C source (97 g C m(-2) a(-1)) due to a minor imbalance between large A(c) (-1,202 g C m(-2) a(-1)) and R(E) (1,299 g C m(-2) a(-1)) fluxes. Mild drought conditions during the measurement period resulted in many instances of low soil moisture (theta<0.2 m(3)m(-3)), which influenced A(c) and thereby NEE by decreasing stomatal conductance. For this experiment, low theta had minor impact on R(E). Thus, stomatal limitations to A(c) were the primary reason that this grassland was a net C source. In the absence of soil moisture limitations, model calculations suggest a net C sink of -65 g C m(-2) a(-1) assuming the LAI dynamics and physiological properties are unaltered. These results, and the results of other studies, suggest that perturbations to the hydrologic cycle are key determinants of C cycling in grassland ecosystems.

Carbon Dioxide↗
PubMed2003

Response of net ecosystem gas exchange to a simulated precipitation pulse in a semi-arid grassland: the role of native versus non-native grasses and soil texture.

Physiological activity and structural dynamics in arid and semi-arid ecosystems are driven by discrete inputs or "pulses" of growing season precipitation. Here we describe the short-term dynamics of ecosystem physiology in experimental stands of native (Heteropogon contortus) and invasive (Eragrostis lehmanniana) grasses to an irrigation pulse across two geomorphic surfaces with distinctly different soils: a Pleistocene-aged surface with high clay content in a strongly horizonated soil, and a Holocene-aged surface with low clay content in homogenously structured soils. We evaluated whole-ecosystem and leaf-level CO2 and H2O exchange, soil CO2 efflux, along with plant and soil water status to understand potential constraints on whole-ecosystem carbon exchange during the initiation of the summer monsoon season. Prior to the irrigation pulse, both invasive and native grasses had less negative pre-dawn water potentials (Psipd), greater leaf photosynthetic rates (Anet) and stomatal conductance (gs), and greater rates of net ecosystem carbon exchange (NEE) on the Pleistocene surface than on the Holocene. Twenty-four hours following the experimental application of a 39 mm irrigation pulse, soil CO2 efflux increased leading to all plots losing CO2 to the atmosphere over the course of a day. Invasive species stands had greater evapotranspiration rates (ET) immediately following the precipitation pulse than did native stands, while maximum instantaneous NEE increased for both species and surfaces at roughly the same rate. The differential ET patterns through time were correlated with an earlier decline in NEE in the invasive species as compared to the native species plots. Plots with invasive species accumulated between 5% and 33% of the carbon that plots with the native species accumulated over the 15-day pulse period. Taken together, these results indicate that system CO2 efflux (both the physical displacement of soil CO2 by water along with plant and microbial respiration) strongly controls whole-ecosystem carbon exchange during precipitation pulses. Since CO2 and H2O loss to the atmosphere was partially driven by species effects on soil microclimate, understanding the mechanistic relationships between the soil characteristics, plant ecophysiological responses, and canopy structural dynamics will be important for understanding the effects of shifting precipitation and vegetation patterns in semi-arid environments.

Analysis of Variance↗
PubMed2003

Carbon dioxide effects on stomatal responses to the environment and water use by crops under field conditions.

Reductions in leaf stomatal conductance with rising atmospheric carbon dioxide concentration ([CO2]) could reduce water use by vegetation and potentially alter climate. Crop plants have among the largest reductions in stomatal conductance at elevated [CO2]. The relative reduction in stomatal conductance caused by a given increase in [CO2] is often not constant within a day nor between days, but may vary considerably with light, temperature and humidity. Species also differ in response, with a doubling of [CO2] reducing mean midday conductances by <15% in some crop species to >50% in others. Elevated [CO2] increases leaf area index throughout the growing season in some species. Simulations, and measurements in free air carbon dioxide enrichment systems both indicate that the relatively large reductions in stomatal conductance in crops would translate into reductions of <10% in evapotranspiration, partly because of increases in temperature and decreases in humidity in the air around crop leaves. The reduction in evapotranspiration in crops is similar to that in other types of vegetation which have smaller relative reductions in stomatal conductance, because of the poorer aerodynamic coupling of the canopy to the atmosphere in crops. The small decreases in evapotranspiration at elevated [CO2] may themselves be important to crop production in dry environments, but changes in climate and microclimate caused by reduced stomatal conductance could also be important to crop production.

Air↗
PubMed2003

Multiple disturbances accelerate invasion of reed canary grass (Phalaris arundinaceaL.) in a mesocosm study.

Disturbances that intensify with agriculture and/or urban development are thought to promote the spread of invasive plants, such as the clonal perennial reed canary grass (Phalaris arundinaceaL). To test this relationship and interactions among disturbances, we subjected wet prairie assemblages within 1.1 m(2) mesocosms to invasion by Phalaris and addition of nutrients, sediments, and flooding. Species richness decreased with the application of sediments and/or flooding of 4 consecutive weeks or longer. Losses of up to six dominant and subdominant species in these treatments increased light transmission through the plant canopy by as much as 400% over the control. Light availability in July and September was a strong predictor of end-of-season aboveground biomass of Phalaris. Phalaris was also 35% and 195% more productive when nutrients were added at low and high levels, respectively. Multiple factors in combination were usually additive in their effects on invasion, but sediments and nutrients interacted with flood regime to synergistically increase invasion in some cases. A separate experiment likewise revealed a synergistic interaction between added nutrients and simulated grazing. We suggest that multiple factors be mitigated simultaneously to reduce invasion of Phalaris.

Agriculture↗