Changes In Water Use Efficiency Research Articles

The impact of monetary policy instruments on sustainable development

In this research, the effect of monetary policy on five sustainable development indicators prepared by the United Nations was investigated in Iraq. For this purpose, variables of interest rate, exchange rate and volume of money in circulation have been used as monetary policy tools and the indicators have been used such as Proportion of the population living below the international poverty line (1.1.1), Proportion of population using safely managed drinking water (6.1.1.), Change in water-use efficiency (6.4.1), Renewable energy (7.2.1), share in the total final energy consumption, and Annual growth rate of real GDP per capita (8.1.1.). For this purpose, five regressions were estimated using the Two Stage Least Squares (2SLS) method. The results show that the exchange rate has a positive effect on indicators of poverty, efficient management of water resources, and economic growth per capita, and a negative effect on access to safe drinking water. Among other monetary policy instruments, only currency in circulationhas a positive effect on poverty reduction, and in other cases it has no significant effect.

Changes in Water-Use Efficiency of Eucalyptus Plantations and Its Driving Factors in a Small County in South China

Ecosystem water-use efficiency (WUE) has been central in revealing the variability in terrestrial carbon and water cycles. Short-rotation plantations such as Eucalyptus plantations can simultaneously impact net primary production (NPP) and actual evapotranspiration (ETa), components of WUE, resulting in changes in terrestrial carbon and water cycles. However, there are few detailed studies on the changes in the WUE of Eucalyptus plantations at the catchment scale with high spatial remote sensing imagery. Here, we present the changes in the WUE of Eucalyptus plantations and its driving factors (i.e., NPP and ETa) using satellite-based models combined with 5-m spatial resolution RapidEye imagery in a small county in South China. The increases in ETa of Eucalyptus plantations are primarily the result of climate warming and result in low WUE of Eucalyptus plantations. The management practice used (short rotation in this study) can enhance the effect of climate warming on WUE by varying the NPP of Eucalyptus plantations. A high value of NPP leads to a high WUE of Eucalyptus plantations at the end of a short rotation, while a low value of NPP results in a low WUE at the beginning of another short rotation. Changes in the WUE of Eucalyptus plantations indicated large spatial and temporal variability, associated with climate warming and short-rotation practices.

Climate Change Impacts on Cacao: Genotypic Variation in Responses of Mature Cacao to Elevated CO2 and Water Deficit

Climate change poses a significant threat to agricultural production in the tropics, yet relatively little research has been carried out to understand its impact on mature tropical tree crops. This research aims to understand the genotypic variation in growth and photosynthesis in mature cacao trees in response to elevated CO2 and water deficit. Six genotypes were grown under greenhouse conditions at ambient (ca. 437 ppm) and elevated CO2 (ca. 724 ppm) and under well-watered and water deficit conditions for 23 months. Leaf- and canopy-level photosynthesis, water-use efficiency, and vegetative growth increased significantly in response to elevated CO2. Water deficit had a significant negative effect on many photosynthetic parameters and significantly reduced biomass production. The negative effect of water deficit on quantum efficiency was alleviated by elevated CO2. Genotypic variation was observed in several parameters including stomatal conductance, stomatal density and index, quantum efficiency, and biomass production, indicating the potential to develop more climate-change-resilient genotypes that can cope with predicted future climate change conditions. Elevated CO2 reduced some of the negative effects of water deficit through changes in water-use efficiency and light utilisation and reduced the negative impact of water deficit on biomass accumulation, but this was genotype-specific.

Spatiotemporal variations of agricultural water use efficiency and its response to the Grain to Green Program during 1982–2015 in the Chinese Loess Plateau

The interaction between agricultural water use efficiency (WUEc) and change in vegetation dynamics (mainly crops coverage) was analyzed to assess the ecological effect of the implementation of China's Grain to Green Program (GGP) and to further analyze the impact of the policy on agriculture on a long-term scale from an ecosystem perspective. Remote sensing data (e.g., gross primary productivity, GPP; evapotranspiration, ET; and the normalized difference vegetation index, NDVI) were used to investigate the spatiotemporal characteristics of WUEc in the Loess Plateau during three periods: 1982–2015, 1982–2001, and 2001–2015. The results indicated that, first, the average and the trend of WUEc showed strong spatial heterogeneity over 1982–2015, a period that shows a strip distribution. The average value increased from the northwest to the southeast while the multiyear trend value decreased from the northwest to the southeast, and a positive slope prevailed (p < 0.05). Second, statistically significant positive trends (p < 0.001) in the annual mean WUEc for 1982–2015, with a value of 7.11 × 10−3 g C kg−1 H2O yr−1, indicate that the rate of carbon sequestration in the agricultural ecosystem was faster than that of water loss. Finally, the GGP was conducive to the improvement of WUEc in this arid and semiarid area, with interannual trend values of WUEc from 1982 to 2001 of 4.26 × 10−3 g C kg−1 H2O, and 9.88 × 10−3 g C kg−1 H2O (p < 0.05) after the GGP. Our analyses explored how the changes in WUEc after the GGP were more sensitive to the NDVIc (the subscript c stands for cropland) trends, which means that NDVIc was an important controller influencing GPPc and ETc tendency and thus further influencing WUEc in response to the GGP. This study contributes to a better understanding of the variations of agricultural carbon-water coupling and provides references and theories for the development of water-saving agriculture in an ecological restoration area.

Shifting biomass allocation determines community water use efficiency under climate warming

Understanding how net primary production (NPP) and its allocation respond to climate warming is of fundamental importance in predicting ecosystem carbon (C) cycle and C-climate feedback. Especially, the optimal partitioning theory suggests that plants preferentially allocate photosynthates toward the above- or below-ground parts to acquire the limiting resources to maximize their growth rate and optimize resource use under environmental change. However, it remains elusive on how NPP allocation changes and regulates community water-use efficiency (WUEc) under climate warming. In this study, we conducted a manipulative warming experiment with three levels of warming treatments (control, + 1.5 °C and + 2.5 °C) to explore the response of NPP allocation and its regulation on WUEc in an alpine meadow. Results showed that above-ground NPP (ANPP) and below-ground NPP (BNPP) responded differently to warming. On average, W1.5 (+ 1.5 °C) and W2.5 (+ 2.5 °C) treatments increased BNPP by 28.98% and 33.28% and increased NPP by 20.05% and 38.70%, respectively, across 4 years. Whereas no consistent warming effect on ANPP was observed across years. The fraction of BNPP to total NPP (fBNPP) responded positively to warming under low ambient temperature and community biomass while it responded negatively under high ambient temperature and community biomass. Notably warming-induced changes in fBNPP negatively correlated with warming-induced changes in WUEc. These results suggested that warming effect on NPP allocation was largely contextually dependent and implied important biological regulation on WUEc. The emerging trade-offs between NPP allocation and WUEc reflect adaptation strategy of plant community under climate change.

Gas exchange and water‐use efficiency in plant canopies

In this review, I first address the basics of gas exchange, water-use efficiency and carbon isotope discrimination in C3 plant canopies. I then present a case study of water-use efficiency in northern Australian tree species. In general, C3 plants face a trade-off whereby increasing stomatal conductance for a given set of conditions will result in a higher CO2 assimilation rate, but a lower photosynthetic water-use efficiency. A common garden experiment suggested that tree species which are able to establish and grow in drier parts of northern Australia have a capacity to use water rapidly when it is available through high stomatal conductance, but that they do so at the expense of low water-use efficiency. This may explain why community-level carbon isotope discrimination does not decrease as steeply with decreasing rainfall on the North Australian Tropical Transect as has been observed on some other precipitation gradients. Next, I discuss changes in water-use efficiency that take place during leaf expansion in C3 plant leaves. Leaf phenology has recently been recognised as a significant driver of canopy gas exchange in evergreen forest canopies, and leaf expansion involves changes in both photosynthetic capacity and water-use efficiency. Following this, I discuss the role of woody tissue respiration in canopy gas exchange and how photosynthetic refixation of respired CO2 can increase whole-plant water-use efficiency. Finally, I discuss the role of water-use efficiency in driving terrestrial plant responses to global change, especially the rising concentration of atmospheric CO2 . In coming decades, increases in plant water-use efficiency caused by rising CO2 are likely to partially mitigate impacts on plants of drought stress caused by global warming.

石羊河流域综合治理灌区水效率变化研究

石羊河流域综合治理灌区水效率变化研究

Growth dynamics, climate sensitivity and water use efficiency in pure vs. mixed pine and beech stands in Trentino (Italy)

Abstract Understanding to what extent species mixtures modify the growth of trees and their responses to climate, in comparison with pure stands, is important to support forest adaptation and mitigation strategies. In this sense, information stored in tree rings can be useful to evaluate whether the positive relationship between species diversity and tree productivity holds true under disturbance (e.g., drought). This paper aimed at assessing (i) how radial growth of trees responded to local variation in climate patterns (Standardised Precipitation-Evapotranspiration Index; SPEI), and (ii) whether there was a relationship with intrinsic water use efficiency (WUEi) and tree-ring δ18O in two important tree species, occurring in pure and mixed forest stands. Three sites with similar topographic and pedo-climatic conditions were identified in a single location in the Italian Alps. The first two are characterized by pure stands, respectively dominated by European beech (Fagus sylvatica L.) and Scots pine (Pinus sylvestris L.). The third site refers to a mixed stand of both previous species. In particular, in order to assess the annual changes in WUEi, we analysed δ13C in tree rings. The influence of the stomatal conductance was also investigated through δ18O. Our results indicated that: (i) Basal Area Increments (BAI) consistently increased in all stands except for the pure Scots pine stand, in the 1994–2003 period; (ii) SPEI highlighted a drought period between 1991 and 2007 (62.2% of the average precipitation); and (iii) the WUEi values were generally higher in pure than in mixed stands, especially for European beech. The divergence between BAI and SPEI values in the 1990s and early 2000s could be a consequence of moderate thinning. We conclude that past forest management (namely thinning) can be more influential on tree growth than current climatic oscillations.

Effects of associating Quercus robur L. and Alnus cordata Loisel. on plantation productivity and water use efficiency

Mixed-species plantations containing non-nitrogen (N)-fixing species and N-fixing species have the potential to increase the productivity and the ecophysiological performance of the target tree species in comparison to monoculture. However, there are few and conflicting data on the effects of fixed N transfer on species water use efficiency in the short, medium and long term. In this paper, we evaluate the productivity and intrinsic water use efficiency (WUEi), i.e., the ratio of photosynthesis (A) to stomatal conductance (gs), in Quercus robur L. (pedunculate oak) stands growing in monoculture and in combination with Alnus cordata Loisel. (Italian alder) in a ratio of 25% (stand F25%) and 50% (stand F50%). We analyzed δ13C in tree rings as a proxy of the inter-annual changes in WUEi, while the influence of the stomatal conductance was explored through δ18O data. Our results indicate a considerable increase in cumulative basal area (BAI) and in WUEi in the two mixed stands during the period 1990–2002, largely resulting from an increase in N availability, due to the N-fixing species that induced an enhancement of A. After 2002, the differences between the three stands in terms of N% in tree rings, BAI and WUEi leveled off, when natural mortality or management practices decreased the competitive ability of Italian alder. Our study shows the importance of intercropping with a suitable N-fixing species to ensure high levels of productivity and water use efficiency of the target tree species in a Mediterranean-type agroforestry system, but also the need to understand how species interaction changes in time and space with a view to setting the management criteria to maximize tree performance.

不同强度净风频繁吹袭对樟子松(Pinus sylvestris var.mongolica)幼苗光合蒸腾特征的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 不同强度净风频繁吹袭对樟子松(Pinus sylvestris var.mongolica)幼苗光合蒸腾特征的影响 DOI: 10.5846/stxb201510142070 作者: 作者单位: 中国科学院寒区旱区环境与工程研究所,中国科学院寒区旱区环境与工程研究所,鲁东大学生命科学学院,中国科学院寒区旱区环境与工程研究所,中国科学院寒区旱区环境与工程研究所,中国科学院寒区旱区环境与工程研究所 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金面上项目（31270752，30972422） Effects of wind frequency on the rates of photosynthesis and transpiration in Pinus sylvestris var. mongolica seedlings Author: Affiliation: Cold and Arid Regions Environment and Engineering Institute,CAS,Lanzhou,Cold and Arid Regions Environment and Engineering Institute,CAS,Lanzhou,,Cold and Arid Regions Environment and Engineering Institute,CAS,Lanzhou,Cold and Arid Regions Environment and Engineering Institute,CAS,Lanzhou,Cold and Arid Regions Environment and Engineering Institute,CAS,Lanzhou Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:为了解不同强度净风频繁吹袭对樟子松幼苗光合蒸腾特性的影响，2013年春季在内蒙古科尔沁沙地研究了0（对照）、6、9、12、15、18 m/s等6个风速处理（分别相当于0、4、5、6、7、8级风）4次吹袭下樟子松幼苗光合速率、蒸腾速率、水分利用效率等指标的变化。结果表明，净风频繁吹袭没有改变樟子松幼苗的光合速率和蒸腾速率的日变化规律，但可使其光合蒸腾的“午休”时间加长、“休眠”程度加深；随着风吹强度的增加，其日均光合能力和蒸腾速率显著降低，其中18 m/s处理较对照分别下降27.6%和22.3%；随着风吹强度增加，气孔导度、胞间CO2浓度均先下降后回升，除18 m/s处理胞间CO2浓度显著高于CK外，其他处理均显著低于CK；随着风吹强度增加，水分利用效率和光能利用效率均先增加后下降，其中除18 m/s处理的水分利用效率显著低于CK，6 m/s处理的光能利用效率高于CK外，其他处理的水分利用效率均高于CK，光能利用效率均低于CK；日均光合蒸腾速率的下降主要源于气孔导度的降低，而水分利用效率和光能利用效率的变化均受制于光合速率和蒸腾速率的变化。 Abstract:To understand the effects of frequent gusts of wind on the rates of photosynthesis and transpiration in Pinus sylvestris var. mongolica seedlings, a field wind-blowing experiment using a wind speed gradient of 0 (CK), 6, 9, 12, 15, and 18 m/s and four episodes of wind blowing was conducted in the spring of 2013 in the Horqin Sand Land of Inner Mongolia. The results showed that:1) Frequent wind blowing did not change the diurnal variation in the rates of photosynthesis and transpiration in seedlings, but resulted in in an extension of midday depression and a higher degree of dormancy in photosynthesis and transpiration. 2) Increasing the wind strength led to a significant decrease in the average daily rates of photosynthesis and transpiration, with a decrease of 27.6% and 22.3% in the 18 m/s-treated seedlings compared to the CK, respectively. With an increase in wind strength, stomatal conductance and intercellular CO2 concentrations first increased and then decreased, and were significant lower in all treatment groups as compared with the CK group, with the exception of intercellular CO2 concentration, which was higher in the 18 m/s group. 3) Increasing the wind-sand flow strength led to a significant increase in water-use efficiency, except for a significant decrease in the 18 m/s treatment group as compared with the CK group. In addition, the efficiency of sunlight energy use tended to decrease except for a significant increase in the 6 m/s-treatment group as compared with the CK group. 4) Decreases in the average daily rates of photosynthesis and transpiration resulted from a decrease in stomatal conductance, and changes in water-use efficiency and sunlight energy-use efficiency resulted from changes in the rates of photosynthesis and transpiration. 参考文献 相似文献 引证文献

Increasing Water Use Efficiency Comes at a Cost for Norway Spruce

Intrinsic water use efficiency (WUEi) in trees is an indication of the ratio of carbon assimilation to the rate of transpiration. It is generally assumed that it is a response to water availability. In agricultural research, the question of drought tolerance by increased WUEi has been well studied. In general, the increase is a trade-off for productivity and is therefore not desired. For forest trees, this question is less clearly understood. Using stable carbon isotopes derived from tree rings combined with productivity as the product of the annual growth increment and annual density measurements, we compared the change in WUEi over a 15 year period. While WUEi increased over this period, the productivity decreased, causing an opposing trend. The gradient of the correlation between WUEi and productivity varies between provenances and sites. Counterintuitively, the populations at the drier site showed low WUEi values at the beginning of the investigation. Slopes vary with the provenance from Poland showing the least decline in productivity. In general, we found that a decline in productivity aligned with an increase in WUEi.

Tree-ring carbon and oxygen isotopes indicate different water use strategies in three Mediterranean shrubs at Capo Caccia (Sardinia, Italy)

Key message Variations in stable carbon and oxygen isotope compositions of co-occurring plant species reflect their different water use strategies and indicate the importance of screening species' WUEi to plan cli- mate change adaptation strategies. Abstract The different abilities of plant species to cope with drought have been associated with structural and ecophysiological constraints. In this paper, we evaluate interspecific differences in intrinsic water use efficiency (WUEi) and the ratio of photosynthesis (A) to stomatal conductance (gs) in three co-occurring Mediterranean shrubs: two broad-leaved evergreen (Pistacia lentiscus and Phillyrea angustifolia) and one needle-like-leaved ever- green (Juniperus phoenicea). We used d 13 C in rings to assess inter-annual changes in WUEi while the influence of the stomatal conductance was explored through d 18 O. Our results indicate consistent differences in WUEi in the three species, largely determined by leaf traits and differences in stomatal conductance control. Juniperus phoenicea could be the most threatened by the current trend of increasing temperature and summers drought. Phillyrea angustifolia and P. lentiscus seem to be less affected by drought stress because of their tighter stomatal control and high survival rate under field conditions. Our study shows that shrubs with different leaf traits employ different plant ecophysi- ological strategies under drought stress.

Change in terrestrial ecosystem water-use efficiency over the last three decades.

Defined as the ratio between gross primary productivity (GPP) and evapotranspiration (ET), ecosystem-scale water-use efficiency (EWUE) is an indicator of the adjustment of vegetation photosynthesis to water loss. The processes controlling EWUE are complex and reflect both a slow evolution of plants and plant communities as well as fast adjustments of ecosystem functioning to changes of limiting resources. In this study, we investigated EWUE trends from 1982 to 2008 using data-driven models derived from satellite observations and process-oriented carbon cycle models. Our findings suggest positive EWUE trends of 0.0056, 0.0007 and 0.0001g Cm(-2) mm(-1) yr(-1) under the single effect of rising CO2 ('CO2 '), climate change ('CLIM') and nitrogen deposition ('NDEP'), respectively. Global patterns of EWUE trends under different scenarios suggest that (i) EWUE-CO2 shows global increases, (ii) EWUE-CLIM increases in mainly high latitudes and decreases at middle and low latitudes, (iii) EWUE-NDEP displays slight increasing trends except in west Siberia, eastern Europe, parts of North America and central Amazonia. The data-driven MTE model, however, shows a slight decline of EWUE during the same period (-0.0005g Cm(-2) mm(-1) yr(-1) ), which differs from process-model (0.0064g Cm(-2) mm(-1) yr(-1) ) simulations with all drivers taken into account. We attribute this discrepancy to the fact that the nonmodeled physiological effects of elevated CO2 reducing stomatal conductance and transpiration (TR) in the MTE model. Partial correlation analysis between EWUE and climate drivers shows similar responses to climatic variables with the data-driven model and the process-oriented models across different ecosystems. Change in water-use efficiency defined from transpiration-based WUEt (GPP/TR) and inherent water-use efficiency (IWUEt , GPP×VPD/TR) in response to rising CO2 , climate change, and nitrogen deposition are also discussed. Our analyses will facilitate mechanistic understanding of the carbon-water interactions over terrestrial ecosystems under global change.

Water-use efficiency of four native trees under CO2enrichment and N addition in subtropical model forest ecosystems

We aimed to evaluate the changes in water-use efficiency (WUE) in native tree species in forests of subtropical China, and determine how coexisting species would be responding to increases in atmospheric carbon dioxide (CO2) concentrations and nitrogen (N) deposition. We used model forest ecosystems in open-top chambers to study the effects of elevated CO2 (ca. 700 µmol mol−1) alone and together with N addition (NH4NO3 applied at 100kg N ha−1year−1) on WUE of four native tree species (Schima superba, Ormosia pinnata, Castanopsis hystrix and Acmena acuminatissima) from 2006 to 2010. Our result indicated that all species increased their WUE when they were exposed to elevated CO2. Although higher WUE was shown in faster-growing species (S. superba and O. pinnata) than that of slower-growing species (C. hystrix and Acmena acuminatissima), the increased extent of WUE induced by elevated CO2 was higher in the slower-growing species than that of the faster-growing species (P < 0.01). The N treatment decreased WUE of S. superba, while the effects on other species were not significant. The interactions between elevated CO2 and N addition increased intrinsic WUE of S. superba significantly (P < 0.001), however, it did not affect WUE of the other tree species significantly. We conclude that the responses of native tree species to elevated CO2 and N addition are different in subtropical China. The species-specific effects of elevated CO2 and N addition on WUE would have important implications on species composition in China’s subtropics in response to global change.

Carbon isotope discrimination, radial growth, and NDVI share spatiotemporal responses to precipitation in Aleppo pine

A common pattern in Aleppo pine Δ 13 C responses to both spatial and temporal variability in precipitation was observed, with a general agreement between NDVI, Δ 13 C and growth that confirms precipitation as key environmental driver. The aim of this study was to assess the spatiotemporal variability of carbon isotope discrimination (Δ13C) records and its relationship with radial growth (RG) and Normalized Difference Vegetation Index (NDVI) data using a tree-ring network of Aleppo pine (Pinus halepensis Mill.) in the eastern part of the Iberian Peninsula. For this purpose, we collected a biennial time series of Δ13C (1949–1998), together with mean annual precipitation, tree-ring width, and remote sensing (NDVI) data for seven locations along a precipitation gradient. We evaluated how intra-site correlations between variables changed across locations, and how inter-site (or spatial) correlations changed across years. We found that correlations between Δ13C and precipitation were higher in dry than in wet sites, in agreement with previous studies. Mean RG and NDVI were good indicators of site-specific Δ13C sensitivity to precipitation. The strongest spatial associations between Δ13C and precipitation were also found during the driest biennia. However, spatial correlations were strongly affected by carryover effects of extreme events. Overall, we found a good agreement between Δ13C, NDVI, and RG, although they showed different response patterns to precipitation. We suggest that the combination of these proxies may be useful for monitoring changes in water-use efficiency and productivity at the regional level.

Elevated CO2 increases tree‐level intrinsic water use efficiency: insights from carbon and oxygen isotope analyses in tree rings across three forest FACE sites

Elevated CO₂ increases intrinsic water use efficiency (WUE(i) ) of forests, but the magnitude of this effect and its interaction with climate is still poorly understood. We combined tree ring analysis with isotope measurements at three Free Air CO₂ Enrichment (FACE, POP-EUROFACE, in Italy; Duke FACE in North Carolina and ORNL in Tennessee, USA) sites, to cover the entire life of the trees. We used δ¹³C to assess carbon isotope discrimination and changes in water-use efficiency, while direct CO₂ effects on stomatal conductance were explored using δ¹⁸O as a proxy. Across all the sites, elevated CO₂ increased ¹³C-derived water-use efficiency on average by 73% for Liquidambar styraciflua, 77% for Pinus taeda and 75% for Populus sp., but through different ecophysiological mechanisms. Our findings provide a robust means of predicting water-use efficiency responses from a variety of tree species exposed to variable environmental conditions over time, and species-specific relationships that can help modelling elevated CO₂ and climate impacts on forest productivity, carbon and water balances.

Ontogenetic changes in water-use efficiency ( δ13C) and leaf traits differ among tree species growing in a semiarid region of the Loess Plateau, China

On the Loess Plateau, China, several planted tree species such as Populus hopeinsis and Robinia pseudoacacia suffer occasional diebacks in the top shoots, reducing growth rates after maturation. However, this does not usually occur in other species, e.g., Ulmus pumila and Zizyphus jujuba. We compared stable carbon isotope ratios ( δ 13C) as indicators of leaf water-use efficiency (WUE), leaf mass per unit area (LMA), and leaf area- and mass-based nitrogen content (N area and N mass) by tree height (1, 5, and 10 m) in P. hopeinsis, R. pseudoacacia, U. pumila, and Z. jujuba. In P. hopeinsis and R. pseudoacacia, leaf δ 13C and N area were significantly lower in 10-m trees than in 1-m saplings, indicating that leaf WUE and photosynthetic rates of both species decreased with tree height. In contrast, δ 13C in Z. jujuba varied little with tree height. The δ 13C of 10-m-tall U. pumila trees was significantly higher than that of 1-m plants, demonstrating an increase in WUE with tree height. Decreasing WUE, leaf N area, and N mass with height increases in P. hopeinsis and R. pseudoacacia may be related to water and nutrient limitations for these species in semiarid regions. In contrast, stable or increasing δ 13C with tree height in U. pumila and Z. jujuba may account (to some extent) for successful production of these species under identical environmental conditions. Diameter growth rate also decreased with maturity in P. hopeinsis and R. pseudoacacia, but increased or was stable for U. pumila and Z. jujuba. The differences in leaf WUE and LMA among species with tree maturity may be related to species’ growth patterns and susceptibility to drought stress, and are likely to be important new criteria for plantation species selection on the Loess Plateau.

Correction of tree ring stable carbon isotope chronologies for changes in the carbon dioxide content of the atmosphere

Tree-ring stable carbon isotope ratios ( δ 13C) often display a decline over the industrial period (post-AD1850) that is only partly explained by changes in the isotopic ratio of carbon dioxide (CO 2) and may represent a response to increased atmospheric concentrations of CO 2 ( ca). If this is not addressed, reconstructions using long tree-ring stable isotope chronologies calibrated using the modern period, for which meteorological records are available, may be compromised. We propose a correction procedure that attempts to calculate the δ 13C values that would have been obtained under pre-industrial conditions. The correction procedure uses nonlinear (loess) regression but the magnitude of the adjustment made is restricted by two logical constraints based on the physiological response of trees: first, that a unit increase in ca cannot result in more than the same unit increase in the internal concentration of CO 2 ( ci), and second, that increases in water-use efficiency as a result of an increase in ca are limited to maintaining a constant ci/ ca ratio. The first constraint allows retention of a falling trend in δ 13C, which exceeds that which could logically be attributed to a passive response to rising ca. The second constraint ensures that any increase in δ 13C, reflecting a change in water-use efficiency beyond maintenance of a constant ci/ ca, is not removed. The procedure is tested using ‘pseudoproxies’, to demonstrate the effect of the correction on time-series with different shapes, and data from three sites in Finland and Norway. Two of the time-series retain a significant trend after correction, and in all three cases the correction improves the correlation with local meteorological measurements.

Variation in the δ13C of foliage of Pinus sylvestris L. in relation to climate and additions of nitrogen: analysis of a 32‐year chronology

AbstractWe report an analysis of both the long‐ and short‐term drivers of the carbon (C) isotope composition (δ13C) values of current year needles of Pinus sylvestris L. linked to changing atmospheric carbon dioxide (CO2) concentrations (ca) and climate using data from a uniquely long‐term nitrogen (N) fertilization experiment in the north of Sweden (consisting of three N dosage levels and a control treatment) from 1970 until 2002. N loading produced trees with less negative δ13C of foliage, by around 0.45‰ on average, with the difference in δ13C between control and N treatments not dependant upon N dosage. The average δ13C values decreased at a rate of around 0.03‰ yr−1, even after accounting for the Suess effect (the decrease in the atmospheric CO2δ13C due to anthropogenic emissions of isotopically light CO2). This decrease is large enough to cause a significant, progressive change in the δ13C down through a soil profile. Modelled values of plant intrinsic water use efficiency (WUEi) and the ratio of leaf internal to external [CO2] (ci/ca) showed that this was the result of ci increasing in parallel with ca (while ci/ca increased), thus causing little change in WUEi over the 32 years of study. The residuals from the relationships between year and δ13C were used to examine the impact of climate on the interannual variation of C isotope composition of needles. This included the use of a fire hazard index (FHI) model, which integrates climatic factors known to influence plant stomatal conductance and hence δ13C. The FHI produced the best fit with δ13C values when climate data were averaged over the whole growth season (for control plots) and for July for all the N treatments, explaining ca. 60% of the total interannual variation in δ13C. Further, trees from the N treatments appeared more susceptible to air‐humidity‐based climate parameters, as seen from higher correlation coefficients, than were control trees. Thus, our data suggest the possibility of increased susceptibility to drought conditions in ecosystems with moderate to high N deposition rates. Also, there is the possibility that, because there was no apparent change in WUEi of P. sylvestris in this ecosystem over the last 32 years, the rate of sequestration of C into boreal ecosystems may not increase with ca, as has been predicted.

The changes in water-use efficiency and stoma density of Leymus chinensis along Northeast China Transect

Leymus chinensis is a keystone species in the temperate zone grassland of China. Along the NECT (Northeast China Transect) in 2001, water-use efficiency of L. chinensis was analyzed with δ 13C, and changes in the stoma density of its leaves were observed and computed under a microscope. Results showed that the ecological plasticity modulation of the stoma density of L. chinensis and its water-use efficiency were two important mechanisms for its broad ecological adaptability. From east to west along the NECT, the δ 13C of the species varied from -27.49‰ to -23.57‰, consisting with the reduction of annual precipitation, soil water and annual average temperature, but increased with the increase of the elevation of sampling sites. The stoma density increased from 96.9/mm 2 to 169.5/mm 2, indicating that the water-use efficiency for the species was improved along the gradient. The linear coefficient between the two parameters was significant ( R 2 = 0.7338). The results of a stepwise regression analysis showed that the soil water was the first marked factor for determining the stoma density, and the next was the annual precipitation, which suggested that the water factors were the primary ecological factors influencing the stoma density of L. chinensis. The findings in this study showed that the responses of the stoma density and the water-use efficiency for L. chinensis to environmental changes were very complicated. They may be the outcome operated synthetically by all environmental factors in the long-term adaptation to different ecological environments, including human activity, for L. chinensis.