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Terrestrial paleoclimate records are rare in the South Atlantic, limiting opportunities to provide a prehistoric context for current global changes. The tussock grass, Poa flabellata , grows abundantly along the coasts of the Falkland Islands and other subantarctic islands. It forms extensive peat records, providing a promising opportunity to reconstruct high-resolution regional climate records. The isotopic composition of leaf and root tissues deposited in these peats has the potential to record variation in precipitation, temperature, and relative humidity over time, but these relationships are unknown for P. flabellata. Here, we measured the isotopic composition of P. flabellata and precipitation and explore relationships with seasonal temperature and humidity variations across four study locations in the Falkland Islands. We reveal that inter-seasonal differences in carbon and oxygen stable isotopes of leaf α -cellulose of living P. flabellata correlated with monthly mean temperature and relative humidity. The carbon isotope composition of leaf α -cellulose (δ13C leaf) records the balance of CO2 supply through stomata and the demand by photosynthesis. The positive correlation between δ13C leaf and temperature and negative correlation between δ13 C leaf and relative humidity suggest that photosynthetic demand for CO2 relative to stomatal supply is enhanced when conditions are warm and dry. Further, the positive correlation between δ13 C leaf and δ18O leaf (r=0.88 ; p<0.001 ; n=24) indicates that stomatal closure during warm dry periods explains seasonal variation in δ13C leaf. We observed significant differences between winter and summer seasons for both δ18 O leaf and δ13C leaf and among study locations for δ18O leaf but not δ13C leaf. δ18 O values of monthly composite precipitation were similar between seasons and among study locations, yet characteristic of the latitudinal origin of storm tracks and seasonal winds. The weak correlation between δ18O in monthly composite precipitation and δ18 O leaf further suggests that relative humidity is the main driver of the δ18O leaf. The oxygen isotopes in root α -cellulose did not reflect, or only partially reflected (at one study location), the δ18O in precipitation. Overall, this study supports the use of peat records formed by P. flabellata to fill a significant gap in our knowledge of the long-term trends in Southern Hemisphere climate dynamics. [ABSTRACT FROM AUTHOR]

The plant family Cactaceae is considered among the most threatened groups of organisms on the planet. The threatened status of the cacti family has created a renewed interest in the highly evolved physiological and morphological traits that underpin their persistence in some of the harshest subtropical environments in the Americas. Among the most important anatomical features of cacti is the modification of leaves into spines, and previous work has shown that the stable isotope chemistry of cacti spines records potential variations in stem water balance, stress, and Crassulacean acid metabolism (CAM). We review the opportunities, challenges, and pitfalls in measuring δ 13C, δ 2H, and δ 18O ratios captured in spine tissues that potentially reflect temporal and spatial patterns of stomatal conductance, internal to atmospheric CO2 partial pressures, and subsequent patterns of photosynthetic gas exchange. We then evaluate the challenges in stable isotope analysis in spine tissues related to variation in CAM expression, stem water compartmentalization, and spine whole-tissue composition among other factors. Finally, we describe how the analysis of all three isotopes can be used in combination to provide potentially robust analysis of photosynthetic function in cacti, and other succulent-stemmed taxa across broad spatio-temporal environmental gradients. [ABSTRACT FROM AUTHOR]

Aims: Our goal was to better understand tree growth and photosynthetic responses to variations in plant available water and elucidate the role of the inverse texture effect in snow dominated montane forests. Methods: We measured tree ring carbon isotope composition and annual growth over a 31-year record for lodgepole pine (Pinus contorta Douglas ex Loudon) growing on three different-aged glacial till surfaces in Wyoming, USA. Results: Soils of different ages developed on till surfaces from three separate glaciation events were significantly different in clay content and distribution with depth; maximum clay content at depth ranged from 19 to 20% on the two youngest till surfaces, but was as high as 36% on the oldest till surface. Ring growth was lowest at the youngest till sites, and only on these coarse-textured soils was growth positively correlated with annual maximum snow water equivalent (SWEmax). Δ13C was highest for trees at these young till sites, suggesting that hydraulic conductivity and stomatal conductance is comparatively high during growth periods on these coarse-textured soils. Conclusion: Taken together, we found that age of glacial till and related soil texture differences strongly influenced tree-ring growth and Δ13C response of lodgepole pine to interannual variation in precipitation and drought severity, but responses did not support an inverse texture effect in these semi-arid forest systems. [ABSTRACT FROM AUTHOR]

We review the relevance and use of stable isotopes for the study of plant community succession. Stable isotope measurements provide information on the origin of resources acquired by plants, the processes governing resource uptake and transformation, and the physiological and environmental conditions of plant growth. When combined with measurements of the stable isotope ratio values of soil microbial biomass, soil organic matter and plant litter, isotope measurements of plants can indicate effects of successional changes on ecosystem processes. However, their application to questions of plant succession and ecosystem change is limited by the degree to which the underlying assumptions are met in each study, and complementary measures may be required, depending upon the question of interest. First, we discuss the changes that occur in the stable isotope composition of plants and ecosystems with ontogeny and species replacements, as well as their potential evolutionary significance. Second, we discuss the imprints of plant competition and facilitation on leaf and wood tissue, as well as how stable isotopes can provide novel insights on the mechanisms underlying plant interactions. Finally, we discuss the capacity for stable isotope measurements to serve as a proxy record for past disturbances such as fire, logging and cyclones. [ABSTRACT FROM PUBLISHER]

Nitrogen (N) oligotrophication is increasing globally across terrestrial ecosystems and manifested in decreasing nitrogen concentration ([N]) and changes in the stable nitrogen isotope composition (δ 15N) of foliage. Heterogeneity in plant nitrogen sources makes it challenging to detect the effects of N oligotrophication even at a small catchment scale with complex topography. Understanding the spatial and temporal variation of foliar δ 15N and [N] at such a scale is required to develop useful ecological indicators and monitoring methods to support catchment management with a potential N oligotrophication problem. This study examined spatial and high-resolution temporal variation of foliar δ 15N and [N] and their influencing factors in ten trees grouped by Eucalyptus and Acacia in a native forest vegetation catchment. Over 16 sampling campaigns within a 12-month period, foliar δ 15N and [N] increased in Eucalyptus but were constant in the N 2 -fixing Acacia. The higher foliar [N] and δ 15N in Acacia reflected its N 2 -fixation ability. Topographic flow accumulation area (NDVI) explained 46% (77%) of spatial variation in dry-season Eucalyptus foliar δ 15N ([N]). For Eucalyptus , foliar δ 15N was higher at the downslope than the upslope locations, but no hillslope location differences were observed for foliar [N]. These results suggest that in the non-N 2 -fixing Eucalyptus , seasonal water stress related nitrogen availability may be reflected in foliar δ 15N rather than foliar [N]. As such, foliar δ 15N of non-N 2 -fixing plants potentially is a more sensitive indicator of seasonal or topographical N availability than foliar [N]. [Display omitted] • Foliar δ 15N and [N] of Eucalyptus varied in a year as those of Acacia kept constant. • Foliar δ 15N and [N] are positively correlated for both Eucalyptus and Acacia. • Eucalyptus foliar δ 15N was higher at the downslope than the upslope locations. • Eucalyptus foliar [N] was independent of slope locations. • Drainage area explained 46% spatial variation of Eucalyptus dry-season foliar δ 15N. [ABSTRACT FROM AUTHOR]

Stable isotope measurements are employed extensively in plant–water relations research to investigate physiological and hydrological processes from whole plant to ecosystem scales. Stable isotopes of hydrogen and oxygen are routinely measured to identify plant source water. This application relies on the assumption that no fractionation of oxygen and hydrogen isotopes in water occurs during uptake by roots. However, a large fraction of the water taken up through roots in halophytic and xerophytic plants transverses cell membranes in the endodermis before entering the root xylem. Passage of water through this symplastic pathway has been hypothesized to cause fractionation leading to a decrease in 2H of root xylem water relative to that in the surrounding soil medium. We examined 16 woody halophytic and xerophytic plant species in controlled conditions for evidence of hydrogen isotope fractionation during uptake at the root–soil interface. Isotopic separation (Δ2H = δ2Hsoil water − δ2Hxylem water) ranging from 3‰ to 9‰ was observed in 12 species. A significant positive correlation between salinity tolerance and the magnitude of Δ2H was observed. Water in whole stem segments, sapwood, and roots had significantly lower δ2H values relative to soil water in Prosopis velutina Woot., the species expressing the greatest Δ2H values among the 16 species examined. Pressurized water flow through intact root systems of Artemisia tridentata Nutt. and Atriplex canescens (Pursh) Nutt. caused the δ2H values to decrease as flow rate increased. This relationship was not observed in P. velutina. Destroying the plasma membranes of root cells by excessive heat from boiling did not significantly alter the relationship between δ2H of expressed water and flow rate. In light of these results, care should be taken when using the stable isotope method to examine source-water use in halophytic and xerophytic species. [ABSTRACT FROM AUTHOR]

The relative contributions of overstory and understory plant transpiration and soil evaporation to total evapotranspiration (ET) in a semiarid savanna woodland were determined from stable isotope measurements of atmospheric water vapor. The savanna overstory was dominated by the deeply rooted, woody legume Prosopis velutina (“mesquite”), and the understory was dominated by a perennial C4 grass, Sporobolus wrightii. “Keeling plots” (turbulent mixing relationships) were generated from isotope ratios (δD and δ18O) of atmospheric water vapor collected within the tree (3–14 m) and understory (0.1–1 m) canopies during peak (July) and post-monsoon (September) periods of 2001. The unique regression intercepts from upper and lower profiles were used to partition the ET flux from the understory layer separately from that of the whole ecosystem. Although ET partitioning was problematic during the first sampling period in July, our results in September provided support to the validity of this method for measuring and understanding the dynamic behavior of water balance components in this semiarid savanna woodland.During the post-monsoon period (22nd September), transpiration accounted for 85% of ecosystem ET. Transpiration by the grass layer accounted for 50% of the understory ET over the same period. The total ecosystem ET estimated by eddy covariance (EC) on 22nd September was 3.5 mm per day. Based on partitioning by the isotope method, 2.5 mm per day (70%) was from tree transpiration and 0.5 mm per day (15%) was from transpiration by the grass layer. Independent estimates of overstory and understory ET partitioning from distributed understory EC measurements were remarkably consistent with our isotope approach. [Copyright &y& Elsevier]

[Display omitted] • The δ15N of biomonitors reveal N dep gradients and emission sources. • The δ15N of lichen and moss biomonitors integrate both source and isotope effects. • Interpretation of δ15N of biomonitors requires consideration of multiple factors. • δ15N of biomonitors often exhibits a negative correlation with ratio of NH x :NO x in N dep. • Lichen/moss from herbaria show anthropogenic shifts in N dep δ15N over the past century. Anthropogenic nitrogen (N) emissions and subsequent deposition can cause harm to ecosystem function and processes. Identifying emission sources of atmospheric N deposition (N dep) is critical for land management and policy development to understand what N species are impacting an area and where action can be taken to reduce loading. Measuring and monitoring deposition is time intensive and expensive, but biomonitors, such as mosses and lichens, can be used to evaluate finer-scale deposition patterns and source attribution using stable isotope analysis. The N stable isotope composition (δ15N) of N dep provides unique insight into emission sources and chemistry at different temporal and spatial scales because it is determined in part by the δ15N of contributing N emission sources that vary at a local, regional, and global level. The δ15N of biomonitor organisms can be used to monitor N dep at these diverse scales capturing the spatial and temporal heterogeneity of N dep rates and source chemistries. However, the δ15N of biomonitors integrates not only source effects, but also isotopic effects associated with canopy dynamics and organism biology, thus interpretation of biomonitor δ15N needs to consider multiple factors including emission source, atmospheric transformations, and canopy and physiological effects. Despite local scale and organism specific responses, lichens and mosses from herbaria collections document coherent anthropogenically driven shifts in N dep δ15N signals over the last century consistent with observations from lake sediments and ice cores. [ABSTRACT FROM AUTHOR]

Compound-specific δ2H values of leaf wax n -alkanes are increasingly being used to infer past hydroclimates. However, differences in n -alkane production and apparent fractionation factors (ε app) among different plant groups complicate the relationships between n -alkane δ2H values and those of environmental water. Mid- and long-chain n -alkanes in sedimentary archives (i.e., n -C 23 and n -C 29) are thought to derive from aquatic and terrestrial plants, respectively, and track the isotopic composition of either lake water or precipitation. Yet, the relationship between n -C 23 δ2H values and lake water δ2H values is not well constrained. Moreover, recent studies show that n -alkane production is greater in terrestrial plants than in aquatic plants, which has the potential to obscure n -alkane aquatic inputs to sedimentary archives. Here, we investigated n -alkane contributions to sedimentary archives from both aquatic and terrestrial plants by analyzing n -alkane δ2H values in plants and lake sediments at 29 sites across mid-latitude North America. We find that both aquatic and terrestrial plants synthesize n -C 23 and that sedimentary n -C 23 δ2H values parallel those of terrestrial plants and differ from those of aquatic plants. Our results indicate that across mid-latitude North America, both mid- and long-chain n -alkanes in lake sediments commonly derive from terrestrial higher plants challenging the assumption that submerged aquatic plants produce the n -C 23 -alkane preserved in lake sediments. Moreover, angiosperm and gymnosperm plants exhibit similar ε app values between n -C 29 and mean annual precipitation (MAP) δ2H values across North America. Therefore, vegetation shifts between angiosperm and gymnosperm plants do not strongly affect ε app values between n -C 29 and MAP. Our results show that both mid- and long-chain n -alkanes track the isotopic composition of MAP in temperate North America. [ABSTRACT FROM AUTHOR]

Abstract: Seasonal and annual variations of rainfall and humidity are recorded in the carbon and oxygen stable isotope ratios of sequentially grown spines found on the columnar cactus, Carnegiea gigantea. A 26-year long composite δ 18O and δ 13C isotope record from the spines of five saguaro cacti was created using bomb radiocarbon and semi-annual variations in δ 13C. Once dating errors in the composite record are corrected, mean annual spine δ 18O is negatively correlated (P <0.001) with total annual precipitation (TAP) from November through October and positively correlated (P <0.01) with mean annual nighttime vapor pressure deficit (VPD). Year-to-year decreases (>2‰) in the maximum annual spine δ 18O are positively correlated (P <0.01) with the Southern Oscillation Index (SOI). We attribute these decreases to enhanced winter rainfall associated with the El Niño phase of the El Niño-Southern Oscillation. Minimum annual δ 13C is negatively correlated with TAP (P <0.05) and mean nighttime VPD (P <0.05). These results bolster proposed mechanistic models of isotopic variation in the spines of columnar cactus and demonstrate how isotopic spine series may be used as climate proxies in regions of the Americas where trees suitable for traditional or isotopic dendrochonology are absent. [Copyright &y& Elsevier]

• Event water dominates snowmelt, while pre-event water dominates during rain events. • Specific conductance as a tracer had less uncertainty than stable water isotopes. • lc-excess was correlated to diurnal snowmelt contributions during the rising limb. • Isotope composition was positively correlated to elevation during low flow periods. The utility of a particular tracer to perform hydrograph separations depends on the dominant watershed properties combined with meteorological patterns; therefore, drawing conclusions from one tracer can be misleading. Combining information from multiple tracers can reveal complimentary insights that advance our knowledge of runoff generating processes. We performed hydrograph separations during spring snowmelt and for one summer rain event at two seasonally arid, montane watersheds in southeastern Wyoming using two independent tracer systems: Stable water isotopes (18O and 2H; 1–4 h resolution) and specific conductance (SC; 15 min resolution). Event water dominated streamflow generation during snowmelt using both tracers, but much lower uncertainty in hydrograph separations were achieved during this period with SC than with stable isotope tracers; the stable isotopic composition of pre-event and event water were too similar which led to large degrees of uncertainty. Stable isotopes of stream water did not vary as much as SC throughout the year and indicated the dominance of stream water derived from snowmelt. During the main snowmelt period, high frequency stream water isotopic composition was remarkably consistent, despite considerable variability measured in snowpack and snowmelt samples. Stable isotope measurements were useful for partitioning streamflow during a summer rain event when source waters were more isotopically distinct and suggested the dominance of pre-event water at generating streamflow. Despite the inability to partition event and pre-event streamflow during snowmelt, relationships between isotopic composition, SC, and watershed properties helped to understand mechanisms for streamflow generation. For instance, line conditioned excess (lc-excess) on the rising limb of the main snowmelt period was significantly positively correlated to the amount of rapid diurnal snowmelt contributions, the fastest moving compartment of event flow, supporting evidence that freshly melted snow, with relatively higher lc-excess, preferentially contributed to rapid diurnal snowmelt contributions. During fall low flow stream conditions, the isotopic value of streamflow was significantly positively correlated to elevation, where the highest-elevation watersheds had the highest delta values. This relationship was contradictory to what was expected (i.e. "elevation effect") and may be explained by extensive snowpack fractionation occurring later in the season in deeper snowpacks located at higher elevations, or by a larger amount rainfall, with enriched isotopic composition relative to snowmelt, occurring at higher elevations. Through synthesizing patterns in two different natural tracers in conjunction with watershed properties, insights into the dominant controls on streamflow generation were gained from seasonally arid, snowmelt-dominated, headwater watersheds. [ABSTRACT FROM AUTHOR]