Do 2 H and 18 O in leaf water reflect environmental drivers differently?

We compiled hydrogen and oxygen stable isotope compositions (δ2 H and δ18 O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ2 H was more closely correlated with δ2 H of xylem water or atmospheric vapour, whereas leaf water δ18 O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ2 H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ18 O. We next expressed leaf water as isotopic enrichment above xylem water (Δ2 H and Δ18 O) to remove the impact of xylem water isotopic variation. For Δ2 H, leaf water still correlated with atmospheric vapour, whereas Δ18 O showed no such correlation. This was explained by covariance between air relative humidity and the Δ18 O of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that 2 H and 18 O in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water.

Multiple processes contribute to methane emission in a riparian cottonwood forest ecosystem.

Methane emission from trees may partially or completely offset the methane sink in upland soils, the only process that has been regularly included in methane budgets for forest ecosystems. Our objective was to analyze multiple biogeochemical processes that influence the production, oxidation and transport of methane in a riparian cottonwood ecosystem and its adjacent river. We combined chamber flux measurements on tree stems, forest soil and the river surface with eddy covariance measurements of methane net ecosystem exchange. In addition, we tested whether methanogens were present in cottonwood stems, shallow soil layers and alluvial groundwater. Average midday peak in net methane emission measured by eddy covariance was c. 12 nmol m-2  s-1 . The average uptake of methane by soils (0.87 nmol m-2  s-1 ) was largely offset by tree stem methane emission (0.75 nmol m-2  s-1 ). There was evidence of methanogens in tree stems but not in shallow soil. Growing season (May-September) cumulative net methane emission (17.4 mmol CH4  m-2 ) included methane produced in cottonwood stems and methane input to the nocturnal boundary layer from the forest and the adjacent river. The multiple processes contributing to methane emission illustrated the linked nature of these adjacent terrestrial and aquatic ecosystems.

Seasonal controls on ecosystem-scale CO2 and energy exchange in a Sonoran Desert characterized by the saguaro cactus (Carnegiea gigantea).

Episodic precipitation pulses are important for driving biological activity in desert ecosystems. The pattern of precipitation, including the size of rain events and the duration of time between events, can influence ecosystem net CO2 exchange (NEE) by shifting the balance between ecosystem photosynthesis and respiration. Our objective was to measure the response of NEE and its major components, to seasonal variation in precipitation and other environmental conditions. The study was conducted at a site, where 40-60% of annual precipitation comes from the North American Monsoon that typically brings rain in July-September, a time period when temperatures are near the seasonal peak. The results were compared to a model of the expected responses of NEE to seasonal changes in precipitation and temperature. We measured NEE using the eddy covariance technique during September 2015-August 2016. The ecosystem showed large (fivefold) seasonal variation in maximum photosynthesis and ecosystem respiration rate at 10 °C that corresponded to seasonal variation in precipitation and temperature. Ecosystem respiratory activity exceeded photosynthetic activity, so the ecosystem was a net source of CO2 to the atmosphere during June-October, a period that included monsoon rain inputs. Only during the winter months (November-March) did photosynthesis exceed respiration, resulting in net ecosystem carbon sequestration. The ecosystem recorded a net loss of 10 g C m-2 year-1, which was likely caused by below normal annual precipitation during the study. Our results illustrated the important interaction between seasonal variation in precipitation and temperature in controlling the ecosystem carbon budget.

The uncertain climate footprint of wetlands under human pressure.

Significant climate risks are associated with a positive carbon-temperature feedback in northern latitude carbon-rich ecosystems, making an accurate analysis of human impacts on the net greenhouse gas balance of wetlands a priority. Here, we provide a coherent assessment of the climate footprint of a network of wetland sites based on simultaneous and quasi-continuous ecosystem observations of CO2 and CH4 fluxes. Experimental areas are located both in natural and in managed wetlands and cover a wide range of climatic regions, ecosystem types, and management practices. Based on direct observations we predict that sustained CH4 emissions in natural ecosystems are in the long term (i.e., several centuries) typically offset by CO2 uptake, although with large spatiotemporal variability. Using a space-for-time analogy across ecological and climatic gradients, we represent the chronosequence from natural to managed conditions to quantify the "cost" of CH4 emissions for the benefit of net carbon sequestration. With a sustained pulse-response radiative forcing model, we found a significant increase in atmospheric forcing due to land management, in particular for wetland converted to cropland. Our results quantify the role of human activities on the climate footprint of northern wetlands and call for development of active mitigation strategies for managed wetlands and new guidelines of the Intergovernmental Panel on Climate Change (IPCC) accounting for both sustained CH4 emissions and cumulative CO2 exchange.

Joint control of terrestrial gross primary productivity by plant phenology and physiology.

Terrestrial gross primary productivity (GPP) varies greatly over time and space. A better understanding of this variability is necessary for more accurate predictions of the future climate-carbon cycle feedback. Recent studies have suggested that variability in GPP is driven by a broad range of biotic and abiotic factors operating mainly through changes in vegetation phenology and physiological processes. However, it is still unclear how plant phenology and physiology can be integrated to explain the spatiotemporal variability of terrestrial GPP. Based on analyses of eddy-covariance and satellite-derived data, we decomposed annual terrestrial GPP into the length of the CO2 uptake period (CUP) and the seasonal maximal capacity of CO2 uptake (GPPmax). The product of CUP and GPPmax explained >90% of the temporal GPP variability in most areas of North America during 2000-2010 and the spatial GPP variation among globally distributed eddy flux tower sites. It also explained GPP response to the European heatwave in 2003 (r(2) = 0.90) and GPP recovery after a fire disturbance in South Dakota (r(2) = 0.88). Additional analysis of the eddy-covariance flux data shows that the interbiome variation in annual GPP is better explained by that in GPPmax than CUP. These findings indicate that terrestrial GPP is jointly controlled by ecosystem-level plant phenology and photosynthetic capacity, and greater understanding of GPPmax and CUP responses to environmental and biological variations will, thus, improve predictions of GPP over time and space.

Variation in the carbon and oxygen isotope composition of plant biomass and its relationship to water-use efficiency at the leaf- and ecosystem-scales in a northern Great Plains grassland.

Measurements of the carbon (δ(13) Cm ) and oxygen (δ(18) Om ) isotope composition of C3 plant tissue provide important insights into controls on water-use efficiency. We investigated the causes of seasonal and inter-annual variability in water-use efficiency in a grassland near Lethbridge, Canada using stable isotope (leaf-scale) and eddy covariance measurements (ecosystem-scale). The positive relationship between δ(13) Cm and δ(18) Om values for samples collected during 1998-2001 indicated that variation in stomatal conductance and water stress-induced changes in the degree of stomatal limitation of net photosynthesis were the major controls on variation in δ(13) Cm and biomass production during this time. By comparison, the lack of a significant relationship between δ(13) Cm and δ(18) Om values during 2002, 2003 and 2006 demonstrated that water stress was not a significant limitation on photosynthesis and biomass production in these years. Water-use efficiency was higher in 2000 than 1999, consistent with expectations because of greater stomatal limitation of photosynthesis and lower leaf ci /ca during the drier conditions of 2000. Calculated values of leaf-scale water-use efficiency were 2-3 times higher than ecosystem-scale water-use efficiency, a difference that was likely due to carbon lost in root respiration and water lost during soil evaporation that was not accounted for by the stable isotope measurements.

Convergence of potential net ecosystem production among contrasting C3 grasslands.

Metabolic theory and body size constraints on biomass production and decomposition suggest that differences in the intrinsic potential net ecosystem production (NEPPOT ) should be small among contrasting C3 grasslands and therefore unable to explain the wide range in the annual apparent net ecosystem production (NEPAPP ) reported by previous studies. We estimated NEPPOT for nine C3 grasslands under contrasting climate and management regimes using multiyear eddy covariance data. NEPPOT converged within a narrow range, suggesting little difference in the net carbon dioxide uptake capacity among C3 grasslands. Our results indicate a unique feature of C3 grasslands compared with other terrestrial ecosystems and suggest a state of stability in NEPPOT due to tightly coupled production and respiration processes. Consequently, the annual NEPAPP of C3 grasslands is primarily a function of seasonal and short-term environmental and management constraints, and therefore especially susceptible to changes in future climate patterns and associated adaptation of management practices.

Heterotrophic aquatic metabolism and sustained carbon dioxide emissions in a mineral-soil wetland restored with treated effluent.

Wetlands are economically valuable ecosystems, in part because they purify wastewater by retaining and processing nutrients, organic matter (OM), and other pollutants. While natural wetlands are highly productive and sequester large pools of carbon (C), it is unclear whether the C cycle of restored treatment wetlands is functionally consistent with natural systems. This knowledge gap limits our appreciation for the role that wetland restoration can play as a natural solution to climate change. Here, we quantified metabolic and C cycling patterns of a restored, multi-basin wetland (Frank Lake, Alberta, Canada) receiving municipal and beef processing plant effluents rich in nutrients and OM. We conducted metabolic measurements in all three basins using dissolved oxygen sensors deployed under ice and in open water. Extreme production and respiration indicated that effluent was largely mineralized and replaced with wetland OM in transit. The heterotrophic status of all basins aligned with a published mass budget demonstrating the aquatic habitat of the wetland was an OM sink under current drought conditions that lengthen effluent processing time. Floating chamber measurements in open water zones confirmed that the wetland was a source of CO2 to the atmosphere. From input to outflow, sustained emissions led to declining pCO2 and a decline in the ratio of dissolved inorganic to organic C. Over 30 years post-restoration, the open water habitats in Frank Lake remain heterotrophic and a net source of CO2, suggesting that the trajectory of aquatic C cycling may be distinct from wetlands restored with non-effluent water sources.

Warmer and drier conditions stimulate respiration more than photosynthesis in a boreal peatland ecosystem: analysis of automatic chambers and eddy covariance measurements.

Continuous half-hourly net CO(2) exchange measurements were made using nine automatic chambers in a treed fen in northern Alberta, Canada from June-October in 2005 and from May-October in 2006. The 2006 growing season was warmer and drier than in 2005. The average chamber respiration rates normalized to 10 degrees C were much higher in 2006 than in 2005, while calculations of the temperature sensitivity (Q(10)) values were similar in the two years. Daytime average respiration values were lower than the corresponding, temperature-corrected respiration rates calculated from night-time chamber measurements. From June to September, the season-integrated estimates of chamber photosynthesis and respiration were 384 and 590 g C m(-2), respectively in 2006, an increase of 100 and 203 g C m(-2) over the corresponding values in 2005. The season-integrated photosynthesis and respiration rates obtained using the eddy covariance technique, which included trees and a tall shrub not present in the chambers, were 720 and 513 g C m(-2), respectively, in 2006, an increase of 50 and 125 g C m(-2) over the corresponding values in 2005. While both photosynthesis and respiration rates were higher in the warmer and drier conditions of 2006, the increase in respiration was more than twice the increase in photosynthesis.

Modelling environmental controls on ecosystem photosynthesis and the carbon isotope composition of ecosystem-respired CO2 in a coastal Douglas-fir forest.

We developed and applied an ecosystem-scale model that calculated leaf CO2 assimilation, stomatal conductance, chloroplast CO2 concentration and the carbon isotope composition of carbohydrate formed during photosynthesis separately for sunlit and shaded leaves within multiple canopy layers. The ecosystem photosynthesis model was validated by comparison to leaf-level gas exchange measurements and estimates of ecosystem-scale photosynthesis from eddy covariance measurements made in a coastal Douglas-fir forest on Vancouver Island. A good agreement was also observed between modelled and measured delta13C values of ecosystem-respired CO2 (deltaR). The modelled deltaR values showed strong responses to variation in photosynthetic photon flux density (PPFD), air temperature, vapour pressure deficit (VPD) and available soil moisture in a manner consistent with leaf-level studies of photosynthetic 13C discrimination. Sensitivity tests were conducted to evaluate the effect of (1) changes in the lag between the time of CO2 fixation and the conversion of organic matter back to CO2; (2) shifts in the proportion of autotrophic and heterotrophic respiration; (3) isotope fractionation during respiration; and (4) environmentally induced changes in mesophyll conductance, on modelled delta(R) values. Our results indicated that deltaR is a good proxy for canopy-level C(c)/C(a) and 13C discrimination during photosynthetic gas exchange, and therefore has several applications in ecosystem physiology.

Environmental controls on the carbon isotope composition of ecosystem-respired CO2 in contrasting forest ecosystems in Canada and the USA.

We compared the carbon isotope composition of ecosystem-respired CO2 (delta13C(R)) from 11 forest ecosystems in Canada and the USA and examined differences among forest delta13C(R) responses to seasonal variations in environmental conditions from May to October 2004. Our experimental approach was based on the assumption that variation in delta13C(R) is a good proxy for short-term changes in photosynthetic discrimination and associated shifts in the integrated ecosystem-level intercellular to ambient CO2 ratio (c(i)/c(a)). We compared delta13C(R) responses for three functional groups: deciduous, boreal and coastal forests. The delta13C(R) values were well predicted for each group and the highest R2 values determined for the coastal, deciduous and boreal groups were 0.81, 0.80 and 0.56, respectively. Consistent with previous studies, the highest correlations between delta13C(R) and changes in environmental conditions were achieved when the environmental variables were averaged for 2, 3 or 4 days before delta13C(R) sample collection. The relationships between delta13C(R) and environmental conditions were consistent with leaf-level responses, and were most apparent within functional groups, providing support for our approach. However, there were differences among groups in the strength or significance, or both, of the relationships between delta13C(R) and some environmental factors. For example, vapor pressure deficit (VPD) and soil temperature were significant determinants of variation in delta13C(R) in the boreal group, whereas photosynthetic photon flux (PPF) was not; however, in the coastal group, variation in delta13C(R) was strongly correlated with changes in PPF, and there was no significant relationship with VPD. At a single site, comparisons between our delta13C(R) measurements in 2004 and published values suggested the potential application of delta13C(R) measurements to assess year-to-year variation in ecosystem physiological responses to changing environmental conditions, but showed that, in such an analysis, all environmental factors influencing carbon isotope discrimination during photosynthetic gas exchange must be considered.

Influence of vegetation and soil CO2 exchange on the concentration and stable oxygen isotope ratio of atmospheric CO2 within a Pinus resinosa canopy.

Measurements were made of the concentration and stable oxygen isotopic ratio of carbon dioxide in air samples collected on a diurnal basis at two heights within a Pinus resinosa canopy. Large changes in CO2 concentration and isotopic composition were observed during diurnal time courses on all three symple dates. In addition, there was strong vertical stratification in the forest canopy, with higher CO2 concentrations and more negative δ18O values observed closer to the soil surface. The observed daily increases in δ18O values of forest CO2 were dependent on relative humidity consistent with the modelled predictions of isotopic fractionation during photosynthetic gas exchange. During photosynthetic gas exchange, a portion of the CO2 that enters the leaf and equilibrates with leaf water is not fixed and diffuses back out of the leaf with an altered oxygen isotopic ratio. The oxygen isotope ratio of CO2 diffusing out of a leaf depends primarily on the 18O content of leaf water which changes in response to relative humidity. In contrast, soil respiration caused a decline in the δ18O values of forest CO2 at night, because CO2 released from the soil has equilibrated with soil water which has a lower 18O content than leaf water. The observed relationship between diurnal changes in CO2 concentration and oxygen isotopic composition in the forest environment were consistent with a gas mixing model that considered the relative magnitudes of CO2 fluxes associated with photosynthesis, respiration and turbulent exchange between the forest and the bulk atmosphere.

Effect of changes in water content on photosynthesis, transpiration and discrimination against 13CO2 and C18O16O in Pleurozium and Sphagnum.

Photosynthetic gas exchange characteristics of two common boreal forest mosses, Sphagnum (section acutifolia) and Pleurozium schreberi, were measured continuously during the time required for the moss to dry out from full hydration. Similar patterns of change in CO2 assimilation with variation in water content occurred for both species. The maximum rates of CO2 assimilation for Sphagnum (approx. 7 µmol m-2 s-1) occurred at a water content of approximately 7 (fresh weight/dry weight) while for Pleurozium the maximum rate (approx. 2 µmol m-2 s-1) occurred at a water content of approximately 6 (fresh weight/dry weight). Above and below these water contents CO2 assimilation declined. In both species total conductance to water vapour (expressed as a percentage of the maximum rates) remained nearly constant at a water content above 9 (fresh weight/dry weight), but below this level declined in a strong linear manner. Short-term, "on-line" 13CO2 and C18O16O discrimination varied substantially with changes in moss water content and associated changes in the ratio of chloroplast CO2 to ambient CO2 partial pressure. At full hydration (maximum water content) both Sphagnum and Pleurozium had similar values of 13CO2 discrimination (approx. 15). Discrimination against 13CO2 increased continuously with reductions in water content to a maximum of 27 in Sphagnum and 22 in Pleurozium. In a similar manner C18C16O discrimination increased from approximately 30 at full hydration in both species to a maximum of 150 in Sphagnum and 90 in Pleurozium, at low water content. The observed changes in C18O16O were strongly correlated to predictions of a mechanistic model of discrimination processes. Field measurements of moss water content suggested that photosynthetic gas exchange by moss in the understory of a black spruce forest was regularly limited by low water content.

Flowering phenology, floral display and reproductive success in dioecious, Aralia nudicaulis L. (Araliaceae).

Aralia nudicaulis L. is a dioecious, perennial, herbaceous plant that is commonly found in the understory vegetation throughout the boreal forest of North America. Female remets have fewer flowers per inflorescence, initiate flowering earlier, and reach peak flowering before male ramets. The consequences of the asynchrony in flowering between the sexes on pollination and seed set were examined during a two-year study. In both years there was significant variation in seed set associated with the flowering times of individual female ramets. In 1983, seed production was highest in the middle of the flowering season. In 1984, seed production was greatest in the later stages of flowering. Variation in seed set was not attributed to lack of pollination in 1983. In 1984, pollination limited seed set per flower during peak flowering. However, seed production never reached the potential five seeds per flower, suggesting that resource limitation was the most important factor affecting fecundity in both years. The asynchronous pattern of flowering is suggested to be the result of the different inflorescence sizes between the sexes.

Stable oxygen and hydrogen isotope composition of leaf water in C3 and C4 plant species under field conditions.

In this paper we make comparisons between the observed oxygen and hydrogen stable isotope composition of leaf water and the predictions of the Craig-Gordon model of evaporative isotopic enrichment. Comparisons were made among two C3 species (Chenopodium album and Helianthus annuus) and two C4 species (Amaranthus retroflexus and Kochia scoparia), when plants were exposed to natural environmental conditions in the field. There were significant differences among the species for the hydrogen and oxygen isotopic composition of leaf water at mid-day. The Amaranthus and Helianthus plants had lower leaf water δD and δ18O values than did Kochia and Chenopodium. The observed leaf water δ values were significantly lower than those predicted by the evaporative enrichment model for all the species. The degree of discrepancy between the observed and modelled leaf water isotopic compositions differed among species. There was a strong linear relationship between the oxygen and hydrogen isotopic compositions of stem water, observed leaf water and the modelled leaf water for all species. The observed leaf water isotopic composition for the different species occurred at different points along the line connecting the stem water isotopic composition and the modelled leaf water isotopic composition in a plot of δD and δ18O. We interpret these linear relationships as mixing lines between the unfractionated source or stem water isotopic composition and the isotopic composition of water at the evaporation sites within leaves (as defined by the evaporative enrichment model).

Carbon isotope composition of boreal plants: functional grouping of life forms.

We tested the hypothesis that life forms (trees, shrubs, forbs, and mosses; deciduous or evergreen) can be used to group plants with similar physiological characteristics. Carbon isotope ratios (δ13C) and carbon isotope discrimination (Δ) were used as functional characteristics because δ13C and Δ integrate information about CO2 and water fluxes, and so are useful in global change and scaling studies. We examined δ13C values of the dominant species in three boreal forest ecosystems: wet Picea mariana stands, mesic Populus tremuloides stands, and dry Pinus banksiana stands. Life form groups explained a significant fraction of the variation in leaf carbon isotope composition; seven life-form categories explained 50% of the variation in δ13C and 42% of the variation in Δ and 52% of the variance not due to intraspecific genetic differences (n=335). The life forms were ranked in the following order based on their values: evergreen trees

Unusually low carbon isotope ratios in plants from hanging gardens in southern Utah.

Leaf carbon isotope ratios (δ13C) and photosynthetic gas exchange were measured on plants growing in hanging garden communities in southern Utah, USA. Hanging gardens are unusual, mesic cliff communities occurring where water seeps from the sandstone bedrock in an otherwise extremely arid region; there is very limited overlap in species distributions inside and outside these gardens. Solar exposure in hanging gardens varied with orientation and one of the gardens (Ribbon Garden) was shaded throughout the day. The leaf δ13C values of plants in hanging gardens were significantly more negative than for plants from either nearby ephemeral wash or riparian communities. In Ribbon Garden, the observed δ13C values were as low as -34.8‰, placing them among the most negative values reported for any terrestrial plant species growing in a natural environment. Hanging garden plants were exposed to normal atmospheric CO2 with an average δ13C value of -7.9‰ and so the low leaf δ13C values could not be attributed to exposure to a CO2 source with low 13C content. There was a seasonal change toward more negative leaf δ13C values at the end of the growing season. The observed leaf δ13C values were consistent with photosynthetic gas exchange measurements that indicated unusually high leaf intercellular CO2 concentrations associated with the relatively low light levels in hanging gardens. Thus, extremely negative leaf δ13C values would be expected if significant amounts of the seasonal carbon gain occur at light levels low enough to be near the light compensation point. Maximum observed photosynthetic rates varied with light levels at each of the gardens, with maximum rates averaging 20.3, 14.6, and 3.1 µmol m-2 s-1 at Double Garden, Lost Garden, and Ribbon Garden, respectively. Leaf nitrogen contents averaged 18.5 mg g-1 in species from the more shaded hanging gardens (Lost and Ribbon). When expressed on a leaf area basis, nitrogen contents averaged 117 mmol N m-2 at Lost Garden and 65 mmol N m-2 at Ribbon Garden (shadiest of the two gardens). Leaf nitrogen isotope ratios averaged -2.3‰ (range of -0.7 to -6.1‰), suggesting that most of the nitrogen was derived from a biological fixation source which is most likely the Nostoc growing on the sandstone walls at the seep. These values contrast with leaf nitrogen isotope ratios of 5-9‰ which have been previously reported for arid zone plants in nearby ecosystems.

Variation in water potential, hydraulic characteristics and water source use in montane Douglas-fir and lodgepole pine trees in southwestern Alberta and consequences for seasonal changes in photosynthetic capacity.

Tree species response to climate change-induced shifts in the hydrological cycle depends on many physiological traits, particularly variation in water relations characteristics. We evaluated differences in shoot water potential, vulnerability of branches to reductions in hydraulic conductivity, and water source use between Pinus contorta Dougl. ex Loud. var. latifolia Engelm. (lodgepole pine) and Pseudotsuga menziesii (Mirb.) Franco (interior Douglas-fir), and determined the consequences for seasonal changes in photosynthetic capacity. The Douglas-fir site had soil with greater depth, finer texture and higher organic matter content than soil at the lodgepole pine site, all factors that increased the storage of soil moisture. While the measured xylem vulnerability curves were quite similar for the two species, Douglas-fir had lower average midday shoot water potentials than did lodgepole pine. This implied that lodgepole pine exhibited stronger stomatal control of transpiration than Douglas-fir, which helped to reduce the magnitude of the water potential gradient required to access water from drying soil. Stable hydrogen isotope measurements indicated that Douglas-fir increased the use of groundwater during mid-summer when precipitation inputs were low, while lodgepole pine did not. There was a greater reduction of photosynthetic carbon gain in lodgepole pine compared with Douglas-fir when the two tree species were exposed to seasonal declines in soil water content. The contrasting patterns of seasonal variation in photosynthetic capacity observed for the two species were a combined result of differences in soil characteristics at the separate sites and the inherent physiological differences between the species.

Photosynthesis, chlorophyll fluorescence and spectral reflectance in Sphagnum moss at varying water contents.

Moss samples from the Fluxnet-Canada western peatland flux station in the Boreal Region of Alberta were measured in the laboratory to obtain the net photosynthesis rate and chlorophyll fluorescence of the moss under controlled environmental conditions, including the regulation of moss water content, simultaneously with measurements of moss spectral reflectance. One objective was to test whether the photochemical reflectance index (PRI) detected changes in moss photosynthetic light-use efficiency that were consistent with short-term (minutes to hours) changes in xanthophyll cycle pigments and associated changes in non-photochemical quenching (NPQ), as recorded by chlorophyll fluorescence. The rate of net photosynthesis was strongly inhibited by water content at values exceeding approximately 9 (fresh weight/dry weight) and declined as the water content fell below values of approximately 8. Chlorophyll fluorescence measurements of maximum photosystem II efficiency generally remained high until the water content was reduced from the maximum of about 20 to values of approximately 10-11, and then declined with further reductions in moss water content. A significant linear decline in NPQ was observed as moss water content was reduced from maximum to low water content values. There was a strong negative correlation between changes in NPQ and PRI. These data suggest that PRI measurements are a good proxy for short-term shifts in photosynthetic activity in Sphagnum moss. A second objective was to test how accurately the water band index (WBI, ratio of reflectance at 900 and 970 nm) recorded changes in moss water content during controlled laboratory studies. Strong linear relationships occurred between changes in moss water content and the WBI, although the slopes of the linear relationships were significantly different among sample replicates. Therefore, WBI appeared to be a useful tool to determine sample-specific water content without destructive measurements.

Molecular and carbon isotopic composition of leaf wax in vegetation and aerosols in a northern prairie ecosystem.

We measured the molecular and carbon isotopic composition of major leaf wax compound classes in northern mixed mesic prairie species (Agropyron smithii, Stipa viridula, Bouteloua gracilis, Tragopogon dubius) and in selected crops (Triticum aestivum, Brassica napus, Hordeum vulgare, Medicago sativa) of southern Alberta and also in aerosols collected 4 m above the prairie canopy. Our aims were to better constrain the wax biosynthetic carbon isotopic fractionation relative to the plant's carbon isotopic discrimination and to quantitatively assess the correspondence between wax composition in vegetation and in boundary layer aerosols. Wax molecular composition of the C(3)prairie species and bulked vegetation was characterized by high abundance of C(28) n-alkanol and C(31) n-alkane compounds whereas the C(4) species B. gracilis had several co-dominant n-alkanol and n-alkane compounds. Wax molecular composition of crop species differed significantly from that of prairie vegetation and was often dominated by a single compound. Results indicate that leaf wax isotopic composition is quantitatively related to the plant's carbon isotopic discrimination. Although species variations were evident, n-alcohol, n-acid and n-alkane wax compounds were on average depleted in (13)C by approximately 6.0+/-1 per thousand relative to total plant carbon. The magnitude of the depletion in wax delta(13)C was unaffected by environmental factors which altered photosynthetic carbon isotopic discrimination. No consistent difference in the magnitude of wax biosynthetic fractionation was observed between C(3) and C(4) species, indicating that photosynthetic pathway has little influence on the isotopic fractionation of wax during biosynthesis. The isotopic composition of ablated waxes in aerosols collected above the canopy was similar to that of the grassland vegetation but the molecular composition differed significantly and indicated that the source "footprint" of the ablated leaf wax particles we sampled in boundary layer air masses was of a regional or larger spatial scale.