Reduced Plant Transpiration Research Articles

Integrated above and below-ground responses of the gypsum specialist Helianthemum squamatum (L.). to drought

Gypsum endemics (i.e. gypsophiles) have adapted to live in gypsum-rich soils where nutrient unbalance and drought can be extreme. Despite their relevance as plants adapted to extreme conditions, a complete analysis of the physiological responses of gypsophiles to drought is still lacking. Helianthemum squamatum (L.) Dum. Cours. is a conspicuous Iberian gypsophile that has been reported to use gypsum crystallization water during the driest period, but the mechanisms behind this process are unknown. To characterize gypsophile responses to drought and unravel the mechanisms underlying gypsum crystalline water use, H. squamatum plants were grown in pots with natural gypsum soil and gypsum soil with deuterium-labelled crystalline water. After three years, a drought experiment was carried out and whole-plant responses were investigated. Unexpectedly, the labelling treatment affected soil physicochemical characteristics and reduced microbial biomass and organic matter content, decreasing plant aerial biomass. H. squamatum plants did not use gypsum crystallization water during simulated drought neither in the labelled soil, nor in the natural one. Drought reduced plant transpiration, stomatal conductance, water use, photosynthetic rate and the foliar concentration of most elements except P and N, which were higher in the drought stressed plants. We detected increased root exudation of choline, an osmoprotector, by drought stressed plants. The drought treatment also affected the structure of microbial communities but did not reduce the relative abundance of functional microbial groups, highly adapted to the natural drought pulses. Our results highlight an integrated water-saving strategy of H. squamatum plants in the short-term, where responses at the leaf level are combined with belowground processes, like altered root exudation. Our findings also underline the remarkable resistance to drought of microbial communities present in gypsum soils.

CO2 emission and physiological aspects of sugarcane ratoon as interactive functions of nitrogen and silicon applications

Sugarcane is the primary renewable energy source in Brazil, necessitating efficient and sustainable production practices. This study assessed the impact of nitrogen sources and their combination of silicon as foliar application on carbon dioxide (CO2) emissions and physiological characteristics of sugarcane cultivated in tropical environment. The experiment was conducted using the second sugarcane ratoon (variety CTC-04), as 2x5 factorial design, with two sources of N-fertilizer (urea and calcium ammonium nitrate - CAN, association with DMPSA − 3,4-dimetilprirazol succínico, nitrification inhibitor) and different silicon concentrations such as 0, 150, 300, 450, and 600 g ha−1. Photosynthetic leaf gas exchange, water use efficiency, pigments, CO2 emissions (ECO2), ammonium and nitrate levels in the soil were observed. The results were subjected to analysis of variance using the F test along with polynomial regression and mean comparison tests. Using urea as a source of nitrogen, there was an increase in internal CO2 concentration of the plants by 19% in relation to treatment with calcium ammonium nitrate association with DMPSA (CAN + DMPSA). Nevertheless, urea increased ECO2 emissions into the atmosphere by up to 15.3% higher compared to CAN + DMPSA application. Foliar application of Si (300 g ha−1) reduced plant transpiration rate (21%), irrespective the source of N-fertilizer utilized. Thus, it is concluded that the use of calcium ammonium nitrate associated with a nitrification inhibitor is an interesting strategy for reducing CO2 emissions, as well as Si is capable of reducing the plant’s transpiration rates, making it efficient in the use of water.

Influence of Mixed Conifer Forest Thinning and Prescribed Fire on Soil Temperature and Moisture Dynamics in Proximity to Forest Logs: A Case Study in New Mexico, USA

Forest management activities often include fuels reduction through mechanized thinning followed by prescribed fire to remove slash. Management prescriptions may include the retention of logs for wildlife habitat and microsites for enhanced tree regeneration. We examined aboveground microclimate and belowground soil temperature and volumetric water content (VWC) dynamics beneath and adjacent to logs at 10, 20, and 30 cm depths in a mixed conifer forest. We assessed the soil variables over 7 years during pre-treatment, post-thinning, and post-fire using a Before–After/Control–Impact experimental design. We found that thinning and burning caused large increases in solar radiation and mean and maximum wind speeds, but only small changes in air temperature and humidity. The treatments increased the soil temperatures beneath the logs by up to 2.7 °C during spring, summer, and fall; the soil VWC increased from 0.05 to 0.08 m3/m3 year-round at 20 and 30 cm depths. Microsites 1–2 m away from the logs also showed soil temperature increases of up to 3.6 °C in spring, summer, and fall, while the measurements of the soil VWC produced variable results (moderate increases and decreases). The increased VWC in late winter/spring likely resulted from reduced plant transpiration and greater snow amounts reaching the ground without being intercepted by the forest canopy. Log retention on thinned and burned sites provided microsites with increased soil temperature and moisture in the top 30 cm, which can enhance soil ecosystem processes and provide refugia for invertebrate and vertebrate wildlife.

Multi-trait selection for mean performance and stability of maize hybrids in mega-environments delineated using envirotyping techniques.

Under global climate changes, understanding climate variables that are most associated with environmental kinships can contribute to improving the success of hybrid selection, mainly in environments with high climate variations. The main goal of this study is to integrate envirotyping techniques and multi-trait selection for mean performance and the stability of maize genotypes growing in the Huanghuaihai plain in China. A panel of 26 maize hybrids growing in 10 locations in two crop seasons was evaluated for 9 traits. Considering 20 years of climate information and 19 environmental covariables, we identified four mega-environments (ME) in the Huanghuaihai plain which grouped locations that share similar long-term weather patterns. All the studied traits were significantly affected by the genotype × mega-environment × year interaction, suggesting that evaluating maize stability using single-year, multi-environment trials may provide misleading recommendations. Counterintuitively, the highest yields were not observed in the locations with higher accumulated rainfall, leading to the hypothesis that lower vapor pressure deficit, minimum temperatures, and high relative humidity are climate variables that -under no water restriction- reduce plant transpiration and consequently the yield. Utilizing the multi-trait mean performance and stability index (MTMPS) prominent hybrids with satisfactory mean performance and stability across cultivation years were identified. G23 and G25 were selected within three out of the four mega-environments, being considered the most stable and widely adapted hybrids from the panel. The G5 showed satisfactory yield and stability across contrasting years in the drier, warmer, and with higher vapor pressure deficit mega-environment, which included locations in the Hubei province. Overall, this study opens the door to a more systematic and dynamic characterization of the environment to better understand the genotype-by-environment interaction in multi-environment trials.

Genetic basis of sorghum leaf width and its potential as a surrogate for transpiration efficiency

Key messageLeaf width was correlated with plant-level transpiration efficiency and associated with 19 QTL in sorghum, suggesting it could be a surrogate for transpiration efficiency in large breeding program.Enhancing plant transpiration efficiency (TE) by reducing transpiration without compromising photosynthesis and yield is a desirable selection target in crop improvement programs. While narrow individual leaf width has been correlated with greater intrinsic water use efficiency in C4 species, the extent to which this translates to greater plant TE has not been investigated. The aims of this study were to evaluate the correlation of leaf width with TE at the whole-plant scale and investigate the genetic control of leaf width in sorghum. Two lysimetry experiments using 16 genotypes varying for stomatal conductance and three field trials using a large sorghum diversity panel (n = 701 lines) were conducted. Negative associations of leaf width with plant TE were found in the lysimetry experiments, suggesting narrow leaves may result in reduced plant transpiration without trade-offs in biomass accumulation. A wide range in width of the largest leaf was found in the sorghum diversity panel with consistent ranking among sorghum races, suggesting that environmental adaptation may have a role in modifying leaf width. Nineteen QTL were identified by genome-wide association studies on leaf width adjusted for flowering time. The QTL identified showed high levels of correspondence with those in maize and rice, suggesting similarities in the genetic control of leaf width across cereals. Three a priori candidate genes for leaf width, previously found to regulate dorsoventrality, were identified based on a 1-cM threshold. This study provides useful physiological and genetic insights for potential manipulation of leaf width to improve plant adaptation to diverse environments.

CO2 Elevation and Nitrogen Supply Alter the Growth and Physiological Responses of Tomato and Barley Plants to Drought Stress

Global climate change will modify plants in terms of growth and physiology. To better understand the consequences of this effect, the responses of the leaf water relations and nitrogen (N) use efficiency of barley and tomato plants to elevated CO2 (e[CO2], 800 ppm) combined with progressive drought stress at two levels of N supply (N1, 0.5 g N pot−1 and N2, 1.0 g N pot−1) were studied. The plants were grown in two separate phytotrons at ambient CO2 (a[CO2], 400 ppm) and e[CO2], respectively. The leaf physiological parameters as well as carbon (C) and N concentrations were determined; plant growth, water and N use efficiencies were evaluated. The results showed that e[CO2] increased photosynthesis and water use efficiency (WUE) while decreased specific leaf area (SLA) in both species, whereas N supply level differentially influenced WUE in barley and tomato plants. The abscisic acid (ABA)-induced stomatal closure during progressive soil drying varied between the two species where the stomatal conductance (gs) of barley plants was more sensitive to leaf ABA than tomato plants, though CO2 environment did not affect the response in both species. Compared to a[CO2], e[CO2] reduced plant transpiration rate (Tplant) in barley but not in tomato. e[CO2] increased the leaf C:N ratio ([C:N]leaf) in plants by enhancing leaf C concentration ([C]leaf) in barley and by dilution of leaf N concentration ([N]leaf) in tomato, respectively, but N2 substantially decreased [C:N]leaf, and thus, N treatment was the dominant factor controlling [C:N]leaf. Collectively, appropriate N supply may modulate the acclimation of plants to e[CO2] and soil water deficits. This study provides some novel insights into N management of different plant species for adapting to future drier and CO2-enriched environment.

Machine Learning vs. Physics-Based Modeling for Real-Time Irrigation Management

Real-time monitoring of soil matric potential has now become a common practice for precision irrigation management. Some crops, such as cranberries, are susceptible to both water and anoxic stresses. Excessive variations in soil matric potential in the root zone may reduce plant transpiration, due to either saturated or dry soil conditions, thereby reducing productivity. A timely supply of the right amount of water is, therefore, fundamental for efficient irrigation management. In this paper, we compare the capabilities of a machine learning-based model and a physics-based model to predict soil matric potential in the root zone. The machine learning model is a random forest algorithm, while the physics-based model is a two-dimensional solver of Richards equation (HYDRUS 2D). After training and calibration on a dataset collected in a cranberry field located in Quebec (Canada), the performance of the two models is evaluated for 30 different time frames of 72-h soil matric potential forecasts. The results highlight that both models can accurately forecast the soil matric potential in the root zone. The machine learning-based model can achieve better performance when compared to the physics-based model, but forecasting accuracy decreases rapidly toward the end of the 72-h lead time, while the error for the Richards equation-based model does not increase with time and remain small compared to the typical measurement error.

Modulation of K+ translocation by AKT1 and AtHAK5 in Arabidopsis plants.

Root cells take up K+ from the soil solution, and a fraction of the absorbed K+ is translocated to the shoot after being loaded into xylem vessels. K+ uptake and translocation are spatially separated processes. K+ uptake occurs in the cortex and epidermis whereas K+ translocation starts at the stele. Both uptake and translocation processes are expected to be linked, but the connection between them is not well characterized. Here, we studied K+ uptake and translocation using Rb+ as a tracer in wild-type Arabidopsis thaliana and in T-DNA insertion mutants in the K+ uptake or translocation systems. The relative amount of translocated Rb+ to the shoot was positively correlated with net Rb+ uptake rates, and the akt1 athak5 T-DNA mutant plants were more efficient in their allocation of Rb+ to shoots. Moreover, a mutation of SKOR and a reduced plant transpiration prevented the full upregulation of AtHAK5 gene expression and Rb+ uptake in K+ -starved plants. Lastly, Rb+ was found to be retrieved from root xylem vessels, with AKT1 playing a significant role in K+ -sufficient plants. Overall, our results suggest that K+ uptake and translocation are tightly coordinated via signals that regulate the expression of K+ transport systems.

Coupled effects of atmospheric CO2 concentration and nutrients on plant-induced soil suction

Some studies have shown that an increasing atmospheric CO2 concentration reduces plant transpiration while others have demonstrated that it interacts with nutrients in soil to enhance plant transpiration and plant growth including an increase in leaf area. However, none of these studies has focused on plant-induced suction in vegetated soils. The objective of this study is to quantify transpiration induced soil suction by Schefflera heptaphylla in NPK (Nitrogen, Phosphorus, Potassium) nutrient supplied, heavily compacted silty sand under two different atmospheric CO2 concentrations (400 ppm and 1000 ppm). Three replicates of the plant were grown in NPK nutrient supplied silty sand and their plant characteristics and soil matric suction were measured under 400 and 1000 ppm atmospheric CO2 for three months. Due to the supply of nitrogen-rich nutrient in silty sand, leaf area index (LAI) of plant increased by 22% under 1000 ppm CO2 compared to 400 ppm CO2. Thus, the larger LAI induced higher soil suction substantially since no significant difference in soil suction was found between current and elevated atmospheric CO2. LAI of Schefflera heptaphylla could be a reliable parameter to understand and predict soil suction as verified by strong correlation coefficient (R2 = 0.85–0.98; P value < 0.1) of peak induced suction and atmospheric CO2 concentration. This study reveals that the use of vegetation in soil structures needs proper management with additional supply of nutrients to thrive under future atmospheric condition and to induce suction hence improve shear strength in vegetated soil structures.

Elevated CO2 and warming effects on grassland plant mortality are determined by the timing of rainfall.

Global warming is expected to increase the mortality rate of established plants in water-limited systems because of its effect on evapotranspiration. The rising CO 2 concentration ([CO 2 ]), however, should have the opposite effect because it reduces plant transpiration, delaying the onset of drought. This potential for elevated [CO 2 ] (eCO 2 ) to modify the warming effect on mortality should be related to prevailing moisture conditions. This study aimed to determine the impacts of warming by 2 °C and eCO 2 (550 μmol mol -1 ) on plant mortality in an Australian temperate grassland over a 6-year period and to test how interannual variation in rainfall influenced treatment effects. Analyses were based on results from a field experiment, TasFACE, in which grassland plots were exposed to a combination of eCO 2 by free air CO 2 enrichment (FACE) and warming by infrared heaters. Using an annual census of established plants and detailed estimates of recruitment, annual mortality of all established plants was calculated. The influence of rainfall amount and timing on the relative impact of treatments on mortality in each year was analysed using multiple regression techniques. Warming and eCO 2 effects had an interactive influence on mortality which varied strongly from year to year and this variation was determined by temporal rainfall patterns. Warming tended to increase density-adjusted mortality and eCO 2 moderated that effect, but to a greater extent in years with fewer dry periods. These results show that eCO 2 reduced the negative effect of warming but this influence varied strongly with rainfall timing. Importantly, indices involving the amount of rainfall were not required to explain interannual variation in mortality or treatment effects on mortality. Therefore, predictions of global warming effects on plant mortality will be reliant not only on other climate change factors, but also on the temporal distribution of rainfall.

Hydroclimate changes across the Amazon lowlands over the past 45,000 years.

Reconstructing the history of tropical hydroclimates has been difficult, particularly for the Amazon basin-one of Earth's major centres of deep atmospheric convection. For example, whether the Amazon basin was substantially drier or remained wet during glacial times has been controversial, largely because most study sites have been located on the periphery of the basin, and because interpretations can be complicated by sediment preservation, uncertainties in chronology, and topographical setting. Here we show that rainfall in the basin responds closely to changes in glacial boundary conditions in terms of temperature and atmospheric concentrations of carbon dioxide. Our results are based on a decadally resolved, uranium/thorium-dated, oxygen isotopic record for much of the past 45,000 years, obtained using speleothems from Paraíso Cave in eastern Amazonia; we interpret the record as being broadly related to precipitation. Relative to modern levels, precipitation in the region was about 58% during the Last Glacial Maximum (around 21,000 years ago) and 142% during the mid-Holocene epoch (about 6,000 years ago). We find that, as compared with cave records from the western edge of the lowlands, the Amazon was widely drier during the last glacial period, with much less recycling of water and probably reduced plant transpiration, although the rainforest persisted throughout this time.

Simulating Carbon Dioxide Effects on Range Plant Growth and Water Use with GPFARM-Range Model

Steadily rising carbon dioxide (CO2) in the Earth’s atmosphere has the potential to increase plant biomass production and reduce plant transpiration in semiarid rangelands. Incorporating results from field CO2-enrichment experiments into process-based simulation models enhances our ability to project climate change impacts on these rangelands. In this study, we added algorithms for computing changes in plant biomass growth and stomatal resistance under elevated [CO2] to the GPFARM-Range (Great Plains Framework for Agricultural Resource Management in Rangelands) model, a newly developed stand-alone software package for rangeland management. The GPFARM-Range model was tested against 5 yr (1997–2001) of soil water and plant biomass data from CO2-enrichment (720 ppm) field experiments conducted in shortgrass steppe in northern Colorado. Simulated results for both peak standing crop biomass and soil water for both ambient and elevated [CO2] treatments had a percent bias within ± 10%, Nash-Sutcliffe efficiency ≥ 0.5, and index of agreement > 0.70. The model also captured the observed trend of increased C3 grass biomass and reduced plant transpiration under elevated [CO2]. The model was used to evaluate the separate effectiveness of elevated [CO2] on plant growth rate (C3 grasses only) and stomatal resistance (both C3 and C4 grasses). Two separate simulations showed that increased growth rate and stomatal resistance due to elevated [CO2] enhanced total plant biomass gain (C3 + C4) by 22% and 17%, respectively. The results indicate the algorithms used to simulate the impacts of elevated [CO2] on range plant growth and water use are reliable and can be used to evaluate rangeland production for predicted increases in [CO2]. However, further studies are necessary because the reduction in plant transpiration under elevated [CO2] was underestimated, and increase in nitrogen use efficiency due to elevated [CO2] is not included.

Stem girdling evidences a trade-off between cambial activity and sprouting and dramatically reduces plant transpiration due to feedback inhibition of photosynthesis and hormone signaling.

The photosynthesis source–sink relationship in young Pinus canariensis seedlings was modified by stem girdling to investigate sprouting and cambial activity, feedback inhibition of photosynthesis, and stem and root hydraulic capacity. Removal of bark tissue showed a trade-off between sprouting and diameter growth. Above the girdle, growth was accelerated but the number of sprouts was almost negligible, whereas below the girdle the response was reversed. Girdling resulted in a sharp decrease in whole plant transpiration and root hydraulic conductance. The reduction of leaf area after girdling was strengthened by the high levels of abscisic acid found in buds which pointed to stronger bud dormancy, preventing a new needle flush. Accumulation of sugars in leaves led to a coordinated reduction in net photosynthesis (AN) and stomatal conductance (gS) in the short term, but later (gS below 0.07 mol m-2 s-1) AN decreased faster. The decrease in maximal efficiency of photosystem II (FV/FM) and the operating quantum efficiency of photosystem II (ΦPSII) in girdled plants could suggest photoprotection of leaves, as shown by the vigorous recovery of AN and ΦPSII after reconnection of the phloem. Stem girdling did not affect xylem embolism but increased stem hydraulic conductance above the girdle. This study shows that stem girdling affects not only the carbon balance, but also the water status of the plant.

THE EFFECTS OF ANITRANSPIRANT DI-1-P-MENTHENE ON SOME PHYSIOLOGICAL TRAITS OF STRAWBERRY

Strawberry is a species sensitive to water shortages, especially during fruit growth and ripening. One method of limiting water loss in plant production involves the use of antitranspirants, which reduce plant transpiration. One method of limiting water loss in plant production involves the use of antitranspirants, which reduce plant transpiration. One of substances used for this purpose is, amongst others, natural terpene polymer di-1-p-menthene (pinolene). Research on the influence of a pinolene-containing antitranspirant (with the commercial name of Vapor Gard) on gas exchange parameters (intensity of net CO2 assimilation, intensity of transpiration, stomatal conductance for water, substomatal CO2 concentration), the water balance and the content of assimilation pigments (chlorophyll “a”, “b”, total chlorophyll, carotenoids) in the leaves of the cv. ‘Salsa’ strawberry was conducted in the years 2009–2010. The antitranspirant was used once at a concentration of 0.75% before flowering. The measurements were performed four times: before flowering (1st measurement date), when the plants were in full flowering (2nd measurement date), in the middle of the harvest season (3rd measurement date) and after the end of the harvest season (4th measurement date). Foliar application of the Vapor Gard antitranspirant decreases the intensity of strawberry transpiration without changing the CO2 assimilation activity. Plants sprayed with the tested preparation were characterised by a higher relative water content (RWC) in leaves and a higher value of the photosynthetic index of water use efficiency (WUE). The antitranspirant did not influence the content of assimilation pigments in strawberry leaves. The values of the determined physiological features depended on the measurement date (developmental stage) of the tested strawberry cultivar.

Incorporating the effects of increased atmospheric CO2 in watershed model projections of climate change impacts

Summary Simulation models such as the Hydrologic Simulation Program – FORTRAN (HSPF) and Soil–Water Assessment Tool (SWAT) are frequently used to project the responses of watershed processes to climate change, but do not always represent the effects of changes in atmospheric CO 2 concentrations on plant growth. Projected increases in atmospheric CO 2 concentrations may decrease the need for plants to maintain stomatal conductance to achieve sufficient CO 2 inputs, thereby also reducing the transpiration of water with potentially important effects on watershed water balance. We first compare the SWAT model, which provides an option to explicitly represent the effects of increased CO 2 to implementations of the SWAT model without this option and to the HSPF model, which does not include a representation of CO 2 response. Both models are capable of representing watershed responses to current climatic conditions. For analysis of response to future conditions, the SWAT model with integrated plant growth response to increased CO 2 predicts an increase in streamflows relative to models without the CO 2 response, consistent with previous research. We then develop methods to incorporate CO 2 impacts on evapotranspiration into a physically based modeling framework, such as HSPF, that does not explicitly model plant growth. With these modifications, HSPF also projects an increase in future runoff relative to simulations without accounting for the CO 2 effect, although smaller than the increase predicted by SWAT with identical assumptions for stomatal conductance. The results suggest that, while the effect of reduced plant transpiration due to increased atmospheric CO 2 is important, it is likely to be over-estimated by both the current formulation of the SWAT model and modified versions that reduce the stomatal conductance response for woody plants. A general approach to modifying watershed models to simulate response of plant transpiration to increased atmospheric CO 2 under climate change is also proposed.

Climate response to changes in atmospheric carbon dioxide and solar irradiance on the time scale of days to weeks

Recent studies show that fast climate response on time scales of less than a month can have important implications for long-term climate change. In this study, we investigate climate response on the time scale of days to weeks to a step-function quadrupling of atmospheric CO2 and contrast this with the response to a 4% increase in solar irradiance. Our simulations show that significant climate effects occur within days of a stepwise increase in both atmospheric CO2 content and solar irradiance. Over ocean, increased atmospheric CO2 warms the lower troposphere more than the surface, increasing atmospheric stability, moistening the boundary layer, and suppressing evaporation and precipitation. In contrast, over ocean, increased solar irradiance warms the lower troposphere to a much lesser extent, causing a much smaller change in evaporation and precipitation. Over land, both increased CO2 and increased solar irradiance cause rapid surface warming that tends to increase both evaporation and precipitation. However, the physiological effect of increased atmospheric CO2 on plant stomata reduces plant transpiration, drying the boundary layer and decreasing precipitation. This effect does not occur with increased solar irradiance. Therefore, differences in climatic effects from CO2 versus solar forcing are manifested within days after the forcing is imposed.

Water table response to an experimental alley farming trial: dissecting the spatial and temporal structure of the data

Clearing vegetation for traditional agriculture diminishes native habitat and reduces plant transpiration, leading to increased groundwater recharge and onset of dryland salinization due to rising groundwater and mobilization of salt stores in the soil profile. This change in hydrology and salinity can also negatively affect biodiversity in many semiarid regions. Alternating native perennial tree belts with mono-species agriculture within the tree belt alleys is one possible system that can provide recharge control and recover some of the ecosystem services of degraded agricultural landscapes. To assess the effect of this agroforestry technique on groundwater levels, an alley farming trial was established in 1995, incorporating different combinations of belt width, alley width, and revegetation density. Transects of piezometers within each design have been monitored from October 1995 to January 2008. The data set consisted of 70 piezometers monitored on 39 dates. Two trends were observed within the raw data: An increase in water table depth with time and an increase in the range of depths monitored at the site were clearly discernible. However, simple hydrograph analysis of the data has proved unsuccessful at distinguishing the effect of the tree belts on the water table morphology. The statistical techniques employed in this paper to show the effect of the experiment on the water table were variation partitioning, principal coordinates of neighbor matrices (PCNM), and canonical redundancy analysis (RDA). The environmental variables (alley farming design, distance of piezometer from the tree belt, and percentage vegetation cover including edge effect) explained 20-30% of the variation of the transformed and detrended data for the entire site. The spatial PCNM variables explained a further 20-30% of the variation. Partitioning of the site into a northern and southern block increased the proportion of explained variation for the plots in the northern block. The spatial PCNM variables and vegetation cover remained the most significant variables. The PCNM analysis revealed no spatial pattern that could be attributed to the trial. The high proportion of unexplained variation may be due to site variables that have not been considered in this study.

Importance of carbon dioxide physiological forcing to future climate change

An increase in atmospheric carbon dioxide (CO(2)) concentration influences climate both directly through its radiative effect (i.e., trapping longwave radiation) and indirectly through its physiological effect (i.e., reducing transpiration of land plants). Here we compare the climate response to radiative and physiological effects of increased CO(2) using the National Center for Atmospheric Research (NCAR) coupled Community Land and Community Atmosphere Model. In response to a doubling of CO(2), the radiative effect of CO(2) causes mean surface air temperature over land to increase by 2.86 +/- 0.02 K (+/- 1 standard error), whereas the physiological effects of CO(2) on land plants alone causes air temperature over land to increase by 0.42 +/- 0.02 K. Combined, these two effects cause a land surface warming of 3.33 +/- 0.03 K. The radiative effect of doubling CO(2) increases global runoff by 5.2 +/- 0.6%, primarily by increasing precipitation over the continents. The physiological effect increases runoff by 8.4 +/- 0.6%, primarily by diminishing evapotranspiration from the continents. Combined, these two effects cause a 14.9 +/- 0.7% increase in runoff. Relative humidity remains roughly constant in response to CO(2)-radiative forcing, whereas relative humidity over land decreases in response to CO(2)-physiological forcing as a result of reduced plant transpiration. Our study points to an emerging consensus that the physiological effects of increasing atmospheric CO(2) on land plants will increase global warming beyond that caused by the radiative effects of CO(2).

Effects of Hydrogel Amendment to Different Soils on Plant Available Water and Survival of Trees under Drought Conditions

AbstractThe effect of super absorbent polyacrylate (SAP) hydrogel amendment to different soil types on plant available water (PAW), evapotranspiration and survival of Eucalyptus grandis, Eucalyptus citriodora, Pinus caribaea, Araucaria cunninghamii, Melia volkensii, Grevillea robusta, Azadirachta indica, Maesopsis eminii and Terminalia superba was investigated. The seedlings were potted in 3 kg size polythene bags filled with sand, loam, silt loam, sandy loam and clay soils, amended at 0 (control), 0.2 and 0.4% w/w hydrogel. The tree seedlings were allowed to grow normally with routine uniform watering in a glass house set up for a period of eight weeks, after which they were subjected to drought conditions by not watering any further. The 0.4% hydrogel amendment significantly (p &lt; 0.05) increased the PAW by a factor of about three in sand, two fold in silt loam and one fold in sandy loam, loam and clay soils compared to the control. Similarly, the addition of either 0.2 or 0.4% hydrogel to the five soil types resulted in prolonged tree survival compared to the controls. Araucaria cunninghammi survived longest at 153 days, while Maesopsis eminii survived least (95 days) in sand amended at 0.4% after subjection to desiccation. Evapotranspiration was reduced in eight of the nine tree species grown in sandy loam, loam, silt loam and clay soils amended at 0.4% hydrogel. It is probable that soil amendment with SAP decreased the hydraulic soil conductivity that might reduce plant transpiration and soil evaporation.

Cultivation and grazing altered evapotranspiration and dynamics in Inner Mongolia steppes

To examine the effects of cultivation and grazing on evapotranspiration (ET), continuous measurements of ET were conducted over almost two years, from December 2005 to September 2007, using the eddy-covariance technique in two paired ecosystems: a steppe and a cropland in Duolun and a fenced and a degraded steppe in Xilinhot, Inner Mongolia, China. The ET of the four ecosystems approached or exceeded precipitation in both years. During the growing season (May–September), cultivation reduced the ecosystem ET in Duolun by 15% in the wet year (2006) and 7% in the dry year (2007). Grazing reduced the ET of the steppe in Xilinhot by 13% during the growing season of 2006, while there was similar ET between the degraded and fenced steppes in 2007. The low soil moisture in the cropland and the degraded steppe compared with the steppe at each area was the reason for the decrease in ET of the steppe ecosystems. In addition, a shorter growing period during the growing season, due to the changes in type and phenology characteristics of the vegetation associated with cultivation, was suspected for the reduction in ET in Duolun. The low soil moisture, due to the low precipitation in the dry year, limited vegetation growth and decreased canopy surface conductance ( g c), resulting in reduced plant transpiration. In addition, cultivation and grazing increased the sensitivity of ET to soil moisture in the dry year, suggesting that future changes in precipitation would not only affect ET by changing soil moisture directly, but would also influence the relationship between ET and soil moisture.