Leaf pigment content and spectral reflectance were examined in four conifer species from the Pacific Northwest and Canadian boreal forest. Our goal was to evaluate the causes of within-and between-stand variation in the Photochemical Reflectance Index (PRI), an indicator of xanthophyll cycle activity and carotenoid pigment content that often scales with photosynthetic light-use efficiency. Both the dark-state PRI values and the change in PRI upon dark-light transition (Delta PRI) were measured in situ in leaves from different canopy positions (top vs. bottom) having contrasting light histories (sun vs. shade). PRI varied with species, canopy position, and with the pool sizes of several photoprotective carotenoid pigments (relative to chlorophyll). Upper-canopy leaves had a greater Delta PRI than their shaded counterparts lower in the canopy, reflecting a higher investment of the photoprotective xanthophyll cycle pigments for sun-exposed top-canopy leaves. These results indicate that the relative concentration of different pigment groups and associated PRI responses varied with canopy position and light history over more than one time scale, and included rapidly changing (facultative) and slowly changing (constitutive) components. Most of the PRI variability among the forest trees sampled was due to constitutive pigment pool size variation associated with species and canopy position. We conclude that both facultative and constitutive pigment components should be considered when applying PRI to photosynthetic studies of forest stands with remote sensing. Leaf-level measurements of PRI and Delta PRI provide non-destructive probes of both facultative and constitutive pigment changes within plant canopies that could help interpret variation in PRI signal viewed from remote sensing platforms.

1077 I. 1078 II. 1079 III. 1080 IV. 1081 V. 1084 VI. 1087 VII. 1088 1089 References 1089 Summary The rate of CO2 assimilation by plants is directly influenced by the concentration of CO2 in the atmosphere, ca. As an environmental variable, ca also has a unique global and historic significance. Although relatively stable and uniform in the short term, global ca has varied substantially on the timescale of thousands to millions of years, and currently is increasing at seemingly an unprecedented rate. This may exert profound impacts on both climate and plant function. Here we utilise extensive datasets and models to develop an integrated, multi-scale assessment of the impact of changing ca on plant carbon dioxide uptake and water use. We find that, overall, the sensitivity of plants to rising or falling ca is qualitatively similar across all scales considered. It is characterised by an adaptive feedback response that tends to maintain 1ci/ca, the relative gradient for CO2 diffusion into the leaf, relatively constant. This is achieved through predictable adjustments to stomatal anatomy and chloroplast biochemistry. Importantly, the long-term response to changing ca can be described by simple equations rooted in the formulation of more commonly studied short-term responses.

Forest mortality constitutes a major uncertainty in projections of climate impacts on terrestrial ecosystems and carbon-cycle feedbacks. Recent drought-induced, widespread forest die-offs highlight that climate change could accelerate forest mortality with its diverse and potentially severe consequences for the global carbon cycle, ecosystem services, and biodiversity. How trees die during drought over multiple years remains largely unknown and precludes mechanistic modeling and prediction of forest die-off with climate change. Here, we examine the physiological basis of a recent multiyear widespread die-off of trembling aspen (Populus tremuloides) across much of western North America. Using observations from both native trees while they are dying and a rainfall exclusion experiment on mature trees, we measure hydraulic performance over multiple seasons and years and assess pathways of accumulated hydraulic damage. We test whether accumulated hydraulic damage can predict the probability of tree survival over 2years. We find that hydraulic damage persisted and increased in dying trees over multiple years and exhibited few signs of repair. This accumulated hydraulic deterioration is largely mediated by increased vulnerability to cavitation, a process known as cavitation fatigue. Furthermore, this hydraulic damage predicts the probability of interyear stem mortality. Contrary to the expectation that surviving trees have weathered severe drought, the hydraulic deterioration demonstrated here reveals that surviving regions of these forests are actually more vulnerable to future droughts due to accumulated xylem damage. As the most widespread tree species in North America, increasing vulnerability to drought in these forests has important ramifications for ecosystem stability, biodiversity, and ecosystem carbon balance. Our results provide a foundation for incorporating accumulated drought impacts into climatevegetation models. Finally, our findings highlight the critical role of drought stress accumulation and repair of stress-induced damage for avoiding plant mortality, presenting a dynamic and contingent framework for drought impacts on forest ecosystems.

Carbonyl sulfide (COS) is an atmospheric trace gas that participates in some key reactions of the carbon cycle and thus holds great promise for studies of carbon cycle processes. Global monitoring networks and atmospheric sampling programs provide concurrent data on COS and CO2 concentrations in the free troposphere and atmospheric boundary layer over vegetated areas. Here we present a modeling framework for interpreting these data and illustrate what COS measurements might tell us about carbon cycle processes. We implemented mechanistic and empirical descriptions of leaf and soil COS uptake into a global carbon cycle model (SiB 3) to obtain new estimates of the COS land flux. We then introduced these revised boundary conditions to an atmospheric transport model (Parameterized Chemical Transport Model) to simulate the variations in the concentration of COS and CO2 in the global atmosphere. To balance the threefold increase in the global vegetation sink relative to the previous baseline estimate, we propose a new ocean COS source. Using a simple inversion approach, we optimized the latitudinal distribution of this ocean source and found that it is concentrated in the tropics. The new model is capable of reproducing the seasonal variation in atmospheric concentration at most background atmospheric sites. The model also reproduces the observed large vertical gradients in COS between the boundary layer and free troposphere. Using a simulation experiment, we demonstrate that comparing drawdown of CO2 with COS could provide additional constraints on differential responses of photosynthesis and respiration to environmental forcing. The separation of these two distinct processes is essential to understand the carbon cycle components for improved prediction of future responses of the terrestrial biosphere to changing environmental conditions.

To test the capabilities of newly available instrumentation and to explore the dynamics of carbonyl sulfide (COS) as a proxy for the measurement of canopy-scale gross primary production (GPP), we conducted an experiment to measure the simultaneous net transfer of COS and CO2 between the atmosphere and a growing wheat canopy, senesced wheat, and the harvested field (located in the Southern Great Plains of the U.S.) using the eddy covariance technique. We found that during the growing season, there was a strong uptake of COS by the canopy (roughly between -10 and -40 pmol m(-2) s(-1)) with a strong diel signal that mirrored net CO2 fluxes. After senescence and over the harvested field, we observed a strong source of COS to the atmosphere (up to +40 pmol m(-2) s(-1)) that exhibited a weaker diel pattern, again similar to CO2. These results suggest that the dynamics of COS are more complicated than once thought, but that it may still be possible to independently derive canopy-scale GPP from direct COS measurements and to use them as model constraints on the atmospheric carbon cycle. To demonstrate this, we computed an average value of leaf relative uptake (LRU) (the scaling factor between GPP and ratios of the fluxes of COS and CO2 and ratios of the atmospheric concentrations of COS and CO2) that is in good agreement with laboratory results. (C) 2013 Elsevier B.V. All rights reserved.

Amazon forests exert a major influence on the global carbon cycle, but quantifying the impact is complicated by diverse landscapes and sparse data. Here we examine seasonal carbon balance in southern Amazonia using new measurements of column-averaged dry air mole fraction of CO2 (XCO2) and solar induced chlorophyll fluorescence (SIF) from the Greenhouse Gases Observing Satellite (GOSAT) from July 2009 to December 2010. SIF, which reflects gross primary production (GPP), is used to disentangle the photosynthetic component of land-atmosphere carbon exchange. We find that tropical transitional forests in southern Amazonia exhibit a pattern of low XCO2 during the wet season and high XCO2 in the dry season that is robust to retrieval methodology and with seasonal amplitude double that of cerrado ecosystems to the east (4ppm versus 2ppm), including enhanced dilution of 2.5ppm in the wet season. Concomitant measurements of SIF, which are inversely correlated with XCO2 in southern Amazonia (r=-0.53, p<0.001), indicate that the enhanced variability is driven by seasonal changes in GPP due to coupling of strong vertical mixing with seasonal changes in underlying carbon exchange. This finding is supported by forward simulations of the Goddard Chemistry Transport Model (GEOS-Chem) which show that local carbon uptake in the wet season and loss in the dry season due to emissions by ecosystem respiration and biomass burning produces best agreement with observed XCO2. We conclude that GOSAT provides critical measurements of carbon exchange in southern Amazonia, but more samples are needed to examine moist Amazon forests farther north

Globally documented widespread drought-induced forest mortality has important ramifications for plant community structure, ecosystem function, and the ecosystem services provided by forests. Yet the characteristics of drought seasonality, severity, and duration that trigger mortality events have received little attention despite evidence of changing precipitation regimes, shifting snow melt timing, and increasing temperature stress. This study draws upon stand level ecohydrology and statewide climate and spatial analysis to examine the drought characteristics implicated in the recent widespread mortality of trembling aspen (Populus tremuloides Michx.). We used isotopic observations of aspen xylem sap to determine water source use during natural and experimental drought in a region that experienced high tree mortality. We then drew upon multiple sources of climate data to characterize the drought that triggered aspen mortality. Finally, regression analysis was used to examine the drought characteristics most associated with the spatial patterns of aspen mortality across Colorado. Isotopic analysis indicated that aspens generally utilize shallow soil moisture with little plasticity during drought stress. Climate analysis showed that the mortality-inciting drought was unprecedented in the observational record, especially in 2002 growing season temperature and evaporative deficit, resulting in record low shallow soil moisture reserves. High 2002 summer temperature and low shallow soil moisture were most associated with the spatial patterns of aspen mortality. These results suggest that the 2002 drought subjected Colorado aspens to the most extreme growing season water stress of the past century by creating high atmospheric moisture demand and depleting the shallow soil moisture upon which aspens rely. Our findings highlight the important role of drought characteristics in mediating widespread aspen forest mortality, link this aspen die-off to regional climate change trends, and provide insight into future climate vulnerability of these forests.

It is unclear to what extent seasonal water stress impacts on plant productivity over Amazonia. Using new Greenhouse gases Observing SATellite (GOSAT) satellite measurements of sun-induced chlorophyll fluorescence, we show that midday fluorescence varies with water availability, both of which decrease in the dry season over Amazonian regions with substantial dry season length, suggesting a parallel decrease in gross primary production (GPP). Using additional SeaWinds Scatterometer onboard QuikSCAT satellite measurements of canopy water content, we found a concomitant decrease in daily storage of canopy water content within branches and leaves during the dry season, supporting our conclusion. A large part (r(2) = 0.75) of the variance in observed monthly midday fluorescence from GOSAT is explained by water stress over moderately stressed evergreen forests over Amazonia, which is reproduced by model simulations that include a full physiological representation of photosynthesis and fluorescence. The strong relationship between GOSAT and model fluorescence (r(2) = 0.79) was obtained using a fixed leaf area index, indicating that GPP changes are more related to environmental conditions than chlorophyll contents. When the dry season extended to drought in 2010 over Amazonia, midday basin-wide GPP was reduced by 15 per cent compared with 2009.

The distribution of nitrogen isotopes in the biosphere has the potential to offer insights into the past, present and future of the nitrogen cycle, but it is challenging to unravel the processes controlling patterns of mixing and fractionation. We present a mathematical model describing a previously overlooked process: nitrogen isotope fractionation during leaf-atmosphere NH3(g) exchange. The model predicts that when leaf-atmosphere exchange of NH3(g) occurs in a closed system, the atmospheric reservoir of NH3(g) equilibrates at a concentration equal to the ammonia compensation point and an isotopic composition 8.1 parts per thousand lighter than nitrogen in protein. In an open system, when atmospheric concentrations of NH3(g) fall below or rise above the compensation point, protein can be isotopically enriched by net efflux of NH3(g) or depleted by net uptake. Comparison of model output with existing measurements in the literature suggests that this process contributes to variation in the isotopic composition of nitrogen in plants as well as NH3(g) in the atmosphere, and should be considered in future analyses of nitrogen isotope circulation. The matrix-based modelling approach that is introduced may be useful for quantifying isotope dynamics in other complex systems that can be described by first-order kinetics.

Widespread drought-induced mortality of woody plants has recently occurred worldwide, is likely to be exacerbated by future climate change and holds large ecological consequences. Yet despite decades of research on plant water relations, the pathways through which drought causes plant mortality are poorly understood. Recent work on the physiology of tree mortality has begun to reveal how physiological dysfunction induced by water stress leads to plant death; however, we are still far from being able to predict tree mortality using easily observed or modeled meteorological variables. In this review, we contend that, in order to fully understand when and where plants will exceed mortality thresholds when drought occurs, we must understand the entire path by which precipitation deficit is translated into physiological dysfunction and lasting physiological damage. In temperate ecosystems with seasonal climate patterns, precipitation characteristics such as seasonality, timing, form (snow versus rain) and intensity interact with edaphic characteristics to determine when and how much water is actually available to plants as soil moisture. Plant and community characteristics then mediate how quickly water is used and seasonally varying plant physiology determines whether the resulting soil moisture deficit is physiologically damaging. Recent research suggests that drought seasonality and timing matter for how an ecosystem experiences drought. But, mortality studies that bridge the gaps between climatology, hydrology, plant ecology and plant physiology are rare. Drawing upon a broad hydrological and ecological perspective, we highlight key and underappreciated processes that may mediate drought-induced tree mortality and propose steps to better include these components in current research.