Response of plants' water use efficiency to increasing atmospheric CO2 concentration

Addressing the Relationship between Leaf Nitrogen and Carbon Isotope Discrimination from the Three Levels of Community, Population and Individual

The carbon, nitrogen and water cycles of terrestrial ecosystems are important biogeochemical cycles. Addressing the relationship of leaf nitrogen (N) and carbon isotope discrimination (Δ) will enhance the understanding of the links between these three cycles in plant leaves because Δ can reflect time-integrated leaf-level water-use efficiency (WUE) over the period when the leaf material is produced. Previous studies have paid considerable attention to the relationship. However, these studies have not effectively eliminated the interference of environmental factors, inter-species, and inter-individual differences in this relationship, so new research is necessary. To minimize these interferences, the present work explored the relationship at the three levels of community, population, and plant individual. Three patterns of positive, negative and no relationship were observed across communities, populations, and individuals, which is dependent on environmental conditions, species, and plant individuals. The results strongly suggested that there is no general pattern for the relationship between leaf N and Δ. Furthermore, the results indicated that there is often no coupling between leaf-level long-term WUE and leaf N in the metabolic process of carbon, N and water in leaves. The main reason for the lack of this relationship is that most plants do not invest large amounts of nitrogen into photosynthesis. In addition, the present study also observed that, for most plant species, leaf N was not related to photosynthetic rate, and that variations in photosynthetic rates are mainly driven by stomatal conductance.

Analysis of coniferous tree growth gradients in relation to regional pollution and climate change in the Miyun Reservoir Basin, China

Forests play a crucial role in regulating regional climate and mitigating local air pollution, but little is known about their responding to such changes. This study aimed to examine the potential responses of Pinus tabuliformis, the major coniferous tree species in the Miyun Reservoir Basin (MRB), along an air pollution gradient in Beijing. Tree rings were collected along a transect, and ring width (basal area increment, BAI) and chemical characteristics were determined and related to long-term climatic and environmental records. The results showed that Pinus tabuliformis showed an overall increase in intrinsic water-use efficiency (iWUE) at all sites, but the relationships between iWUE and BAI differed among the sites. The contribution of atmospheric CO₂ concentration (c_a) to tree growth was significant at the remote sites (> 90%). The study found that air pollution at these sites might have caused further stomatal closure, as evidenced by the higher δ¹³C levels (0.5 to 1‰ higher) during heavy pollution periods. The analysis of tree ring δ¹⁵N also revealed the potential of using δ¹⁵N to fingerprint major nitrogen (N) deposition, as shown in the increasing tree ring δ¹⁵N, and major nitrogen losses due to denitrification and leaching, as shown in the higher δ¹⁵N in tree rings during heavy rainfall events. Overall, the gradient analysis indicated the contributions of increasing c_a, increasing water deficit and elevated air pollution to tree growth and forest development. The different BAI trajectories suggested that Pinus tabuliformis has the ability to adapt to the harsh environment in the MRB.

Robust Response of Terrestrial Plants to Rising CO2

Human-caused CO₂ emissions over the past century have caused the climate of the Earth to warm and have directly impacted on the functioning of terrestrial plants. We examine the global response of terrestrial gross primary production (GPP) to the historic change in atmospheric CO₂. The GPP of the terrestrial biosphere has increased steadily, keeping pace remarkably in proportion to the rise in atmospheric CO₂. Water-use efficiency, namely the ratio of CO₂ uptake by photosynthesis to water loss by transpiration, has increased as a direct leaf-level effect of rising CO₂. This has allowed an increase in global leaf area, which has conspired with stimulation of photosynthesis per unit leaf area to produce a maximal response of the terrestrial biosphere to rising atmospheric CO₂ and contemporary climate change.

Effects of nutrient supply on carbon and water economies of C4 grasses

C3 plants can increase nutrient uptake by increasing transpiration, which promotes the flow of water with dissolved nutrients towards the roots. However, it is not clear if this mechanism of nutrient acquisition, termed 'mass flow', also operates in C4 plants. This is an important question, as differences in mass flow capacity may affect competitive interactions between C3 and C4 species. To test if mass flow can be induced in C4 species, we conducted an experiment in a semiarid seasonal savanna in South Africa. We grew six C4 grasses in nutrient-poor sand and supplied no nutrients, nutrients to the roots or nutrients spatially separated from the roots. We measured the rates of photosynthesis and transpiration, water-use efficiency (WUE), nitrogen gain and biomass. For all species biomass, N gain, photosynthesis and transpiration were lowest in the treatment without any nutrient additions. Responses to different nutrient positioning varied among species from no effect on N gain to a 50% reduction when nutrients were spatially separated. The ability to access spatially separated nutrients showed a nonsignificant positive relationship with both the response of transpiration and the response of WUE to spatial nutrient separation. This indicates that nutrient acquisition is not regulated by decreasing WUE in C4 grasses. Overall, our study suggests that under elevated CO2, when evaporative demand is lower, C4 species may be at a competitive disadvantage to C3 species when it comes to nutrient acquisition.

Foliar δ13C Showed No Altitudinal Trend in an Arid Region and Atmospheric Pressure Exerted a Negative Effect on Plant δ13C

Previous studies have suggested foliar δ13C generally increases with altitude. However, some observations reported no changes or even decreased trends in foliar δ13C. We noted that all the studies in which δ13C increased with elevation were conducted in the human regions, whereas those investigations in which δ13C did not vary or decreased were conducted in areas with water stress. Thus, we proposed that the pattern of increasing δ13C with elevation is not a general one, and that δ13C may remain unchanged or decrease in plants grown in arid environments. To test the hypothesis, we sampled plants along altitude gradients on the shady and sunny slopes of Mount Tianshan characterized by arid and semiarid climates. The measurements of foliar δ13C showed no altitudinal trends for the plants grown on either of the slopes. Therefore, this study supported our hypothesis. In addition, the present study addressed the effect of atmospheric pressure on plant δ13C by accounting for the effects of temperature and precipitation on δ13C. This study found that the residual foliar δ13C increased with increasing altitude, suggesting that atmospheric pressure played a negative role in foliar δ13C.

A Regional Stable Carbon Isotope Dendro-Climatology from the South African Summer Rainfall Area

Carbon isotope analysis of four baobab (Adansonia digitata L.) trees from the Pafuri region of South Africa yielded a 1000-year proxy rainfall record. The Pafuri record age model was based on 17 radiocarbon dates, cross correlation of the climate record, and ring structures that were presumed to be annual for two of the trees. Here we present the analysis of five additional baobabs from the Mapungubwe region, approximately 200km west of Pafuri. The Mapungubwe chronology demonstrates that ring structures are not necessarily annually formed, and accordingly the Pafuri chronology is revised. Changes in intrinsic water-use efficiency indicate an active response by the trees to elevated atmospheric CO2, but this has little effect on the environmental signal. The revised Pafuri record, and the new Mapungubwe record correlate significantly with local rainfall. Both records confirm that the Medieval Warm Period was substantially wetter than present, and the Little Ice Age was the driest period in the last 1000 years. Although Mapungubwe is generally drier than Pafuri, both regions experience elevated rainfall peaking between AD 1570 and AD 1620 after which dry conditions persist in the Mapungubwe area until about AD 1840. Differences between the two records correlate with Agulhas Current sea-surface temperature variations suggesting east/west displacement of the temperate tropical trough system as an underlying mechanism. The Pafuri and Mapungubwe records are combined to provide a regional climate proxy record for the northern summer rainfall area of southern Africa.

A Regional Stable Carbon Isotope Dendro-Climatology from the South African Summer Rainfall Area

Carbon isotope analysis of four baobab (Adansonia digitata L.) trees from the Pafuri region of South Africa yielded a 1000-year proxy rainfall record. The Pafuri record age model was based on 17 radiocarbon dates, cross correlation of the climate record, and ring structures that were presumed to be annual for two of the trees. Here we present the analysis of five additional baobabs from the Mapungubwe region, approximately 200km west of Pafuri. The Mapungubwe chronology demonstrates that ring structures are not necessarily annually formed, and accordingly the Pafuri chronology is revised. Changes in intrinsic water-use efficiency indicate an active response by the trees to elevated atmospheric CO₂, but this has little effect on the environmental signal. The revised Pafuri record, and the new Mapungubwe record correlate significantly with local rainfall. Both records confirm that the Medieval Warm Period was substantially wetter than present, and the Little Ice Age was the driest period in the last 1000 years. Although Mapungubwe is generally drier than Pafuri, both regions experience elevated rainfall peaking between AD 1570 and AD 1620 after which dry conditions persist in the Mapungubwe area until about AD 1840. Differences between the two records correlate with Agulhas Current sea-surface temperature variations suggesting east/west displacement of the temperate tropical trough system as an underlying mechanism. The Pafuri and Mapungubwe records are combined to provide a regional climate proxy record for the northern summer rainfall area of southern Africa.

The key factor limiting plant growth in cold and humid alpine areas also plays a dominant role in plant carbon isotope discrimination

Many environmental factors affect carbon isotope discrimination in plants, yet the predominant factor influencing this process is generally assumed to be the key growth-limiting factor. However, to our knowledge this hypothesis has not been confirmed. We therefore determined the carbon isotope composition (δ(13)C) of plants growing in two cold and humid mountain regions where temperature is considered to be the key growth-limiting factor. Mean annual temperature (MAT) showed a significant impact on variation in carbon isotope discrimination value (Δ) irrespective of study area or plant functional type with either partial correlation or regression analysis, but the correlation between Δ and soil water content (SWC) was usually not significant. In multiple stepwise regression analysis, MAT was either the first or the only variable selected into the prediction model of Δ against MAT and SWC, indicating that the effect of temperature on carbon isotope discrimination was predominant. The results therefore provide evidence that the key growth-limiting factor is also crucial for plant carbon isotope discrimination. Changes in leaf morphology, water viscosity and carboxylation efficiency with temperature may be responsible for the observed positive correlation between Δ and temperature.

Warming-induced northwestward migration of the East Asian monsoon rain belt from the Last Glacial Maximum to the mid-Holocene

Glacial-interglacial changes in the distribution of C3/C4 vegetation on the Chinese Loess Plateau have been related to East Asian summer monsoon intensity and position, and could provide insights into future changes caused by global warming. Here, we present δ(13)C records of bulk organic matter since the Last Glacial Maximum (LGM) from 21 loess sections across the Loess Plateau. The δ(13)C values (range: -25‰ to -16‰) increased gradually both from the LGM to the mid-Holocene in each section and from northwest to southeast in each time interval. During the LGM, C4 biomass increased from <5% in the northwest to 10-20% in the southeast, while during the mid-Holocene C4 vegetation increased throughout the Plateau, with estimated biomass increasing from 10% to 20% in the northwest to >40% in the southeast. The spatial pattern of C4 biomass in both the LGM and the mid-Holocene closely resembles that of modern warm-season precipitation, and thus can serve as a robust analog for the contemporary East Asian summer monsoon rain belt. Using the 10-20% isolines for C4 biomass in the cold LGM as a reference, we derived a minimum 300-km northwestward migration of the monsoon rain belt for the warm Holocene. Our results strongly support the prediction that Earth's thermal equator will move northward in a warmer world. The southward displacement of the monsoon rain belt and the drying trend observed during the last few decades in northern China will soon reverse as global warming continues.

Tree growth and intrinsic water-use efficiency of inland riparian forests in northwestern China: evaluation via δ13C and δ18O analysis of tree rings

The rising atmospheric CO2 concentration (Ca) has increased tree growth and intrinsic water-use efficiency (iWUE). However, the magnitude of this effect on long-term iWUE and whether this increase could stimulate the growth of riparian forests in extremely arid regions remain poorly understood. We investigated the relationship between growth [ring width; basal area increment (BAI)] and iWUE in a riparian Populus euphratica Oliv. forest to test whether growth was enhanced by increasing CO2 and whether this compensated for environmental stresses in the lower reaches of the inland Heihe River, northwestern China. We accomplished this using dendrochronological methods and carbon (δ(13)C) and oxygen (δ(18)O) isotopic analysis. We found an increase in BAI before 1958, followed by a decrease from 1958 to 1977 and an increase to a peak around 2000. Tree-ring carbon discrimination (Δ) and δ(18)O indicated significant negative overall trends from 1920 to 2012. However, the relationship shifted in strength and direction around 1977 from significantly negative to a weak connection. The seasonal minimum temperature in April to July showed strong influence on Δ, and δ(18)O was controlled by relative humidity (negatively correlated) and temperature (positively correlated) in June and July. The patterns of internal to atmospheric CO2 (Ci/Ca) suggest a specific adaptation of tree physiology to increasing CO2. Intrinsic water-use efficiency increased significantly (by 36.4%) during the study period. The increased iWUE explained 19.8 and 39.1% of the observed yearly and high-frequency (first-order difference) variations in BAI, respectively, after 1977. Our results suggest significant CO2 stimulation of riparian tree growth, which compensated for the negative influences of reductions in river streamflow and a drying climate during the study period.

Growth and carbon isotopes of Mediterranean trees reveal contrasting responses to increased carbon dioxide and drought

Forest dynamics will depend upon the physiological performance of individual tree species under more stressful conditions caused by climate change. In order to compare the idiosyncratic responses of Mediterranean tree species (Quercus faginea, Pinus nigra, Juniperus thurifera) coexisting in forests of central Spain, we evaluated the temporal changes in secondary growth (basal area increment; BAI) and intrinsic water-use efficiency (iWUE) during the last four decades, determined how coexisting species are responding to increases in atmospheric CO2 concentrations (C(a)) and drought stress, and assessed the relationship among iWUE and growth during climatically contrasting years. All species increased their iWUE (ca. +15 to +21%) between the 1970s and the 2000s. This increase was positively related to C(a) for J. thurifera and to higher C(a) and drought for Q. faginea and P. nigra. During climatically favourable years the study species either increased or maintained their growth at rising iWUE, suggesting a higher CO2 uptake. However, during unfavourable climatic years Q. faginea and especially P. nigra showed sharp declines in growth at enhanced iWUE, likely caused by a reduced stomatal conductance to save water under stressful dry conditions. In contrast, J. thurifera showed enhanced growth also during unfavourable years at increased iWUE, denoting a beneficial effect of C(a) even under climatically harsh conditions. Our results reveal significant inter-specific differences in growth driven by alternative physiological responses to increasing drought stress. Thus, forest composition in the Mediterranean region might be altered due to contrasting capacities of coexisting tree species to withstand increasingly stressful conditions.