John S. Roden

A mechanistic model for interpretation of hydrogen and oxygen isotope ratios in tree-ring cellulose

A mechanistic model is presented to quantify both the physical and biochemical fractionation events associated with hydrogen and oxygen isotope ratios in tree-ring cellulose. The model predicts the isotope ratios of tree-rings, incorporating both humidity and source water environmental information. Components of the model include (1) hydrogen and oxygen isotope effects associated with leaf water enrichment; (2) incorporation of leaf water isotope ratio values into photosynthetic carbohydrates along with the biochemical fractionation associated with autotrophic synthesis; (3) transport of exported carbohydrates (such as sucrose) from leaves to developing xylem in shoots and stems where cellulose is formed; (4) a partial exchange of oxygen and hydrogen isotopes in carbohydrates with xylem sap water during conversion into cellulose; and (5) a biochemical fractionation associated with cellulose synthesis. A modified version of the Craig-Gordon model for evaporative enrichment adequately described leaf water dD and d 18 O values. The leaf water model was robust over a wide range of leaf waters for both controlled experiments and field studies, far exceeding the range of values to be expected under natural conditions. The isotopic composition of cellulose was modeled using heterotrophic and autotrophic fractionation factors from the literature as well as the experimentally derived proportions of H and O that undergo exchange with xylem water during cellulose synthesis in xylem cells of tree-rings. The fraction of H and O from carbohydrates that exchange with xylem sap water was estimated to be 0.36 and 0.42, respectively. The proportions were based on controlled, long-term greenhouse experiments and field studies where the variations in the dD and d 18 O of tree-ring cellulose were measured under different source water isotopic compositions. The model prediction that tree-ring cellulose contains information on environmental water source and atmospheric vapor pressure deficit (related to relative humidity) was tested under both field and greenhouse conditions. This model was compared to existing models to explain cellulose isotope ratios under a wide range of source water and humidity conditions. Predictions from our model were consistent with observations, whereas other models showed large discrepancies as soon as the isotope ratios of source water and atmospheric water deviated from each other. Our model resolves the apparently conflicting and disparate interpretations of several previous cellulose stable isotope ratio studies. Copyright

Expressing leaf water and cellulose oxygen isotope ratios as enrichment above source water reveals evidence of a Péclet effect.

There is an increasing ecological interest in understanding the gradients in H218O enrichment in leaf water (i.e. a Péclet effect), because an appreciation of the significance of the Péclet effect is important for improving our understanding of the mechanistic processes affecting the 18O composition of leaf water and plant organic material. In data sets where both source water and leaf water 18O data are available, we can evaluate the potential contribution of a Péclet effect. As an example, we recalculate data published earlier by Roden and Ehleringer (1999, Oecologia 121:467–477) as enrichments in leaf water (ΔL) and cellulose (Δcell) above source water. Based on these recalculations, we present support for the relevance of a Péclet effect in leaves. Further, we demonstrate that the subtle variations in ΔL and Δcell caused by a Péclet effect may be masked in experimental systems in which variation in the source water oxygen isotope ratio is considerable.

Assessing Ecosystem-Level Water Relations Through Stable Isotope Ratio Analyses

Virtually all elements of biological interest have multiple stable isotopic forms and the fractionation events associated with biological and physical processes help to create spatial and temporal variations in isotopic abundance that can be used to understand the dynamics of ecological systems. Stable isotope ratio analyses at natural abundance levels can provide integrated information on ecosystem functioning, such as variations in water-use activities by different elements within an ecosystem (Ehleringer et al. 1993; Dawson and Ehleringer 1998). Stable isotope ratio analyses do not provide information on water flux rates through the ecosystem, but instead they help constrain the analysis of flux data, such as through identifying those specific soil layers that are the source of current moisture use by the vegetation or the ratio of carbon dioxide-to-water (CO2-to-H2O) flux.

Hydrogen and oxygen isotope ratios of tree-ring cellulose for riparian trees grown long-term under hydroponically controlled environments

Abstract Saplings of three riparian tree species (alder, birch and cottonwood) were grown for over 5 months in a hydroponics system that maintained the isotopic composition of source water in six treatments, ranging from –120 to +180‰δD and –15 to +10‰δ18O. The trees were grown in two greenhouses maintained at 25°C and at either 40 or 75% relative humidity, creating differences in transpiration rates and leaf water isotopic evaporative enrichment. The cellulose produced in the annual growth ring was linearly related to source water with differences in both slope and offset associated with greenhouse humidity. The slope of the isotopic composition of source water versus tree-ring cellulose was less than 1 for both δD and δ18O indicating incomplete isotopic exchange of carbohydrate substrate with xylem water during cellulose synthesis. Tests using the outer portion of the tree-ring and new roots were similar and showed that the tree-ring values were representative of the cellulose laid down under the imposed environmental conditions. The fraction of H and O in carbohydrate substrate that isotopically exchange with medium water was calculated to be 0.36 and 0.42 respectively, and biochemical mechanisms for these observed fractions are discussed. A mechanistic model of the biochemical fractionation events for both δD and δ18O leading to cellulose synthesis was robust over the wide range of cellulose stable isotope ratios. The experimental results indicate that both water source and humidity information are indeed recorded in tree-ring cellulose. These results help to resolve some of the disparate observations regarding the interpretation of stable isotope ratios in tree-rings found in the literature.

The Effect of Elevated [CO2] on Growth and Photosynthesis of Two Eucalyptus Species Exposed to High Temperatures and Water Deficits

Two species of eucalyptus (Eucalyptus macrorhyncha and Eucalyptus rossii) were grown for 8 weeks in either ambient (350 [mu]L L-1) or elevated (700 [mu]L L-1) CO2 concentrations, either well watered or without water additions, and subjected to a daily, 3-h high-temperature (45[deg]C, maximum) and high-light (1250 [mu]mol photons m-2 s-1, maximum) stress period. Water-stressed seedlings of E. macrorhyncha had higher leaf water potentials when grown in elevated [CO2]. Growth analysis indicated that increased [CO2] may allow eucalyptus species to perform better during conditions of low soil moisture. A down-regulation of photosynthetic capacity was observed for seedlings grown in elevated [CO2] when well watered but not when water stressed. Well-watered seedlings grown in elevated [CO2] had lower quantum efficiencies as measured by chlorophyll fluorescence (the ratio of variable to maximal chlorophyll fluorescence [Fv/Fm]) than seedlings grown in ambient [CO2] during the high-temperature stress period. However, no significant differences in Fv/Fm were observed between CO2 treatments when water was withheld. The reductions in dark-adapted Fv/Fm for plants grown in elevated [CO2] were not well correlated with increased xanthophyll cycle photoprotection. However, reductions in the Fv/Fm were correlated with increased levels of nonstructural carbohydrates. The reduction in quantum efficiencies for plants grown in elevated [CO2] is discussed in the context of feedback inhibition of electron transport associated with starch accumulation and variation in sink strength.

Hydrogen and oxygen isotope ratios of tree ring cellulose for field-grown riparian trees

Abstract The isotopic composition of tree ring cellulose was obtained over a 2-year period from small-diameter riparian-zone trees at field sites that differed in source water isotopic composition and humidity. The sites were located in Utah (cool and low humidity), Oregon (cool and high humidity), and Arizona (warm and low humidity) with source water isotope ratio values of –125/–15‰ (δD/δ18O), –48/–6‰, and –67/–7‰, respectively. Monthly environmental measurements included temperature and humidity along with measurements of the isotope ratios in atmospheric water vapor, stream, stem, and leaf water. Small riparian trees used only stream water (both δD and δ18O of stem and stream water did not differ), but δ values of both atmospheric water vapor and leaf water varied substantially between months. Differences in ambient temperature and humidity conditions between sites contributed to substantial differences in leaf water evaporative enrichment. These leaf water differences resulted in differences in the δD and δ18O values of tree ring cellulose, indicating that humidity information was recorded in the annual rings of trees. These environmental and isotopic measurements were used to test a mechanistic model of the factors contributing to δD and δ18O values in tree ring cellulose. The model was tested in two parts: (a) a leaf water model using environmental information to predict leaf water evaporative enrichment and (b) a model describing biochemical fractionation events and isotopic exchange with medium water. The models adequately accounted for field observations of both leaf water and tree ring cellulose, indicating that the model parameterization from controlled experiments was robust even under uncontrolled and variable field conditions.

A controlled test of the dual-isotope approach for the interpretation of stable carbon and oxygen isotope ratio variation in tree rings

Seedlings of a conifer (Pinus radiata D. Don) and a broad leaf angiosperm (Eucalyptus globulus Labill.) were grown for 100 days in two growth cabinets at 45 or 65% relative humidity. The seedlings were exposed to treatments designed to modify carbon assimilation rates and capacities, stomatal conductance and transpiration to test conceptual models that attempt to clarify the interpretation of carbon isotope discrimination (Δ(13)C) by using oxygen isotope enrichment (Δ(18)O). Differences in relative humidity and within-cabinet treatments (including lower irradiance, lower nitrogen inputs, higher leaf temperature and lower moisture status than control seedlings) produced significant differences in assimilation rates, photosynthetic capacities, stomatal conductance, leaf transpiration rates and leaf evaporative enrichment. The dual-isotope approach accurately interpreted the cause of variation in wood cellulose Δ(13)C for some of the treatments, but not for others. We also tested whether we could use Δ(13)C variation to constrain the interpretation of δ(18)O variation. Carbon isotope discrimination appears to be influenced by transpiration (providing information on leaf evaporative enrichment), but the results did not provide a clear way to interpret such variation. The dual-isotope approach appears to be valid conceptually, but more work is needed to make it operational under different scenarios.

A tree-ring perspective on the terrestrial carbon cycle

Tree-ring records can provide valuable information to advance our understanding of contemporary terrestrial carbon cycling and to reconstruct key metrics in the decades preceding monitoring data. The growing use of tree rings in carbon-cycle research is being facilitated by increasing recognition of reciprocal benefits among research communities. Yet, basic questions persist regarding what tree rings represent at the ecosystem level, how to optimally integrate them with other data streams, and what related challenges need to be overcome. It is also apparent that considerable unexplored potential exists for tree rings to refine assessments of terrestrial carbon cycling across a range of temporal and spatial domains. Here, we summarize recent advances and highlight promising paths of investigation with respect to (1) growth phenology, (2) forest productivity trends and variability, (3) CO2 fertilization and water-use efficiency, (4) forest disturbances, and (5) comparisons between observational and computational forest productivity estimates. We encourage the integration of tree-ring data: with eddy-covariance measurements to investigate carbon allocation patterns and water-use efficiency; with remotely sensed observations to distinguish the timing of cambial growth and leaf phenology; and with forest inventories to develop continuous, annually-resolved and long-term carbon budgets. In addition, we note the potential of tree-ring records and derivatives thereof to help evaluate the performance of earth system models regarding the simulated magnitude and dynamics of forest carbon uptake, and inform these models about growth responses to (non-)climatic drivers. Such efforts are expected to improve our understanding of forest carbon cycling and place current developments into a long-term perspective.

Is the dual-isotope conceptual model fully operational?

The stable carbon isotopic composition (δ13C) of organic matter is a valuable measure of plant physiological processes. Unfortunately, the impacts of carbon assimilation (A, demand for CO2) and stomatal conductance (gs, supply of CO2) on Rubisco discrimination (through Ci/Ca) are difficult to separate. Thus, a conceptual model (Scheidegger et al. 2000) was developed to constrain the interpretation of δ13C variation by measuring oxygen isotopic composition (δ18O) on the same material (the ‘dual-isotope’ approach). The idea is that δ18O, as a proxy for evaporative flux, will be modified by gs but not A. Therefore, if changes in environmental conditions cause a long-term change in A and/or gs, this should be reflected in the carbon and oxygen isotope ratios of organic matter, respectively. This model is conceptually sound and a number of papers (Sidorova et al. 2009, Brooks and Mitchell 2011) have used this model for interpreting δ13C variation in tree rings. In the current issue of Tree Physiology, Barnard et al. have utilized the dual-isotope model to infer physiological responses of mature Pseudotsuga menziesii (Mirb) Franco trees to environmental variation associated with site differences and canopy position. They developed a novel approach to eliminate confounding factors by plotting relative changes (from the grand mean) in tree ring δ13C and δ18O values. They used the conceptual model to infer temporal changes in gs, spatial variation in A associated with leaf nitrogen content and different levels of physiological responsiveness to environmental forcing for trees from different canopy positions. This was an ambitious use of the dual-isotope model due to complex environmental variation associated with sites (slope and aspect differences), canopy position (three crown classes), seasonal separation (earlywood and latewood) and inter-annual climate (8 years). The dual-isotope model has not been tested under such complex conditions, and the highly variable results of Barnard et al. (2012) make model interpretation challenging. Our ability to take advantage of this conceptual model is limited by a number of model assumptions and constraints that are often overlooked. Barnard et al. (2012) recognized many of these assumptions and tried to quantify or limit confounding variation; however, they did not test whether model predictions regarding A and gs were realized in their system, which would have been a valuable confirmation of model applicability. They assumed that this conceptual model was operational for interpreting tree ring isotope variation in complex forest ecosystems. The purpose of this commentary is to highlight areas of caution that must be recognized before the dual-isotope model can be routinely utilized and encourage research that could help reduce model uncertainties. 1. The dual-isotope model interprets changes in δ18O as primarily influenced by gs. This requires that environmental influences on evaporative enrichment (Craig and Gordon 1965) including source water δ18O, atmospheric vapor δ18O, ambient humidity and leaf temperature must either be constant over time (for tree rings) or between treatments (common garden) or that all relevant fractionation events also influence gs (e.g., vapor pressure deficit [D] differences modify gs through a welldescribed relationship; thus, increasing D leads to enhanced evaporative enrichment in 18O, but it can also reduce gs leading to a concomitant increase in leaf water δ18O, amplifying the effect of D on organic matter δ18O). Documented inter-annual variation in the δ18O of precipitation, soil evaporative enrichment profiles and the fact that trees can tap different sources of water may reduce one’s confidence that constant source water δ18O is an appropriate assumption (Sarris et al. 2013). The paucity of data on δ18O variation in atmospheric vapor Commentary

Intra-annual variation in the stable oxygen and carbon isotope ratios of cellulose in tree rings of coast redwood (Sequoia sempervirens)

Fog is a seasonally variable hydrologic input to coast redwood (Sequoia sempervirens, D. Don) ecosystems with a stable isotopic composition (δ 18O, δ2H) that is distinct from rainfall. The intra-annual variation in tree ring cellulose δ18O of coast redwood was measured to determine what portion of the tree ring should be sampled to best target fog signals. Ten years of rings were subdivided into ten equal units for three trees at two sites in northern California. Intra-annual variation in cellulose δ18O was as high as 3‰ with a consistent pattern of enriched (in 18O) values from the annual ring boundary and depleted values from the central portion of the ring. These patterns show a strong coherence between replicate trees at the same site as well as significant correlations between individuals at sites over 250 km apart. Intra-annual variation in stable carbon isotope ratios (δ13C) was as high as 2‰ and produced a pattern with depleted (in 13C) values from the ring boundary and enriched values from the central portion of the ring. Although these δ13C patterns were not as strongly correlated between replicate trees as the patterns in δ18O variation, they did produce significant correlations with the variation in fog frequencies recorded at local airports. This study highlights the importance of quantifying intra-annual variation in tree ring stable isotope signals as a guide to further investigations on historic variability from long chronologies especially if the signal of interest is seasonally variable (eg, fog).