B23A-0915
Eddy Covariance Measurements of CO2 and Energy Fluxes Above Mountain Grasslands in the Austrian Alps: Challenges and Results
Fluxes of CO2 and energy have been measured over several differently managed mountain grasslands in the Austrian Alps over the past seven years. Here we discuss the challenges associated with the application of the eddy covariance method in complex mountainous terrain and show how these flux measurements can be validated using a multiple-constraints approach. It is shown that defensible eddy covariance flux measurements over low-statured mountain grasslands are possible even in complex terrain as long as the measurement height is low enough to remain within the shallow equilibrium layer where the wind field exhibits characteristics akin to level terrain and rigorous quality control is exercised. Differences in ecosystem-atmosphere exchange of CO2 and energy between sites are related to gradients in land use and climate caused by differences in elevation. It is shown that elevation-mediated changes in climate, in particular the length of the vegetation period, and land use, in particular the cutting frequency, have an opposing influence on the carbon cycling of mountain grasslands, resulting in similar net carbon balances irrespective of elevation.
B23A-0916
Spatial and temporal variation in nighttime atmospheric carbon dioxide profiles in mountainous terrain.
On clear nights cold air sinks in mountainous terrain. This nocturnal cold air drainage contains high levels of respired CO2 that can be used to assess nighttime respiration rates and estimates of ecosystem water use efficiency. However, the depth and concentration of CO2 in this cold air pool likely varies with slope position, valley morphology, elevation, and time of night. We used a tethered helium balloon and attached tubing to investigate the CO2 concentration in the nocturnal cold air drainage flowing from two constrained forested watersheds in Northern Idaho. We monitored air temperature and CO2 concentration from 1m to 200m throughout the summer from dusk to dawn. At both sites, the inversion was deep, frequently reaching 100-150m, with observed lapse rates of 40-50 °C/km compared to more typical lapse rates of -6.5 °C/km in the absence of temperature inversions. At Mica Creek, the CO2 concentrations ranged from approx. 385 ppm at the top of the profile to 460 ppm at 1m, whereas, CO2 concentrations generally ranged from 375 ppm at the top of the profile to 500 ppm at 1m at Benton Creek. Temporal sampling indicated that CO2 concentrations were steady during the night at Mica Cr but they increased by about 50-60ppm at Benton Cr throughout the night. Interestingly, the increased CO2 concentrations, relative to ambient, exceeded the depth of the cold air pool at Benton Cr. These data indicate that spatial and temporal patterns in nocturnal CO2 concentrations varied depending if the cold air flowed like a river during the night or if a lake formed. CO2 concentrations remained steady all night in the river of cold air but steadily increased on the site where a lake formed. These results will be used to develop and test models describing cold air flow in complex terrain and to better assess the suitability of using cold air drainage to assess ecosystem processes.
B23A-0917
Diurnal Hysteresis Between Soil CO2 and Soil Temperature is Controlled by Soil Water Content
Soil temperature plays an important role in many model representations of soil CO2 production and transport. However, interactions among environmental variables such as temperature and soil moisture may introduce uncertainty into these models. Among the sources of uncertainty in models of soil CO2 production and transport is daily hysteresis between soil CO2 flux and soil temperature. We quantified the degree to which hysteresis between soil [CO2] and soil temperature is controlled by soil water content in a montane conifer forest, and how this nonlinearity impacts estimates of soil CO2 efflux. Based on chamber measurements at our site, a developed Q10 relationship overestimates CO2 flux by 42 g C m-2 (19%) for the entire growing season due to its inability to account for the daily cycle of soil [CO2], the variability of soil moisture, and moisture-dependent diffusive transport of CO2 through the soil column. Only under late- season dry conditions is the Q10 relationship able to predict CO2 flux. We found that at high levels of soil water content, hysteresis imposes organized, daily variability in the relationship between soil [CO2] and soil temperature, and at low levels of soil water content, hysteresis is minimized. Our results demonstrate that diurnal hysteresis between soil [CO2] and soil temperature is due mostly to the balance (or imbalance in wet soils) between production and diffusion. The seasonality in soil moisture controls the transition from an imbalanced system (where diurnal hysteresis is observed) to a balanced system (no diurnal hysteresis observed). The magnitude of hysteresis in the soil [CO2] – soil temperature relationship is an important indicator of the existence of concomitant, yet independent, autotrophic and heterotrophic soil [CO2] processes. As such, the role of soil water content in controlling the relationship between soil [CO2] and soil temperature should be considered when modeling the dynamics of carbon cycling in ecosystems with strong seasonality in soil water content.
B23A-0918
Effects of aspect and elevation on carbon storage in the Dry Creek Experimental Watershed
More understanding is needed of the role that semiarid lands play in the global storage of carbon and how changes of those lands will influence global warming. This study was conducted to identify the effect of aspect and elevation on total carbon and nitrogen storage in the Dry Creek Experimental Watershed. The 28-km2 watershed located is 16 km northeast of Boise, ID and ranges in elevation from 1000 to 2100m with an associated 37 cm to 100 cm precipitation gradient. The Samples were collected from mid slope sites throughout the watershed with data recorded for aspect, slope, elevation and vegetation at each of the 120 sampling points. Each site was sampled to a depth of 30 cm in 5 cm increments for 720 samples total. Soil analyses consisted of total carbon and nitrogen as well as clast-size distribution. Total carbon and nitrogen contents of the soil average 22 and 2.1 g kg-1, respectively, with ranges from 2.6 to 260 g kg-1 and 0.3 to 13 g kg-1. The ratio of nitrogen to carbon is inverse to accumulation with areas of high carbon accumulation exhibiting ratios as low as 1:22, while areas of low accumulation exhibiting ratios as high as 1:4. The lower and mid watershed locations (1000-1600m) show a strong influence of aspect on carbon and nitrogen accumulation with an average order of magnitude difference from southern to northern aspects. Elevation also influences carbon and nitrogen accumulation with lower sites of the same aspect showing 1/3 that of mid elevation sites. The upper watershed (1600-2000m) sites demonstrated similar results, but suggest a weaker link between carbon accumulation and aspect and elevation.
B23A-0919
Comparison of Soil CO2 Concentrations and Surface CO2 Efflux Across Riparian-Hillslope Transitions: Wet Versus Dry Growing Seasons
An outstanding gap in our understanding of carbon cycling is the role of climate variability on soil CO2 production and surface CO2 efflux. Few studies have contrasted soil respiration across wet and dry growing seasons. Inter-annual climate variability, specifically earlier snowmelt and decreased growing season precipitation, can impact soil water content and soil temperature, which partially control soil respiration. We investigated the spatial and temporal variability of soil CO2 concentrations and surface CO2 efflux across 4 topographically distinct riparian-hillslope transitions with strong gradients in water content and temperature. Our study sites were located in the 380 ha subalpine upper-Stringer Creek Watershed in the Tenderfoot Creek Experimental Forest, Montana. We present data from wet (2005) and dry (2006) growing seasons: June-August. Precipitation was 27 versus 16.5 cm and peak snowmelt occurred on June 6 and April 20, respectively. We collected measurements of soil temperature, soil water content, soil air CO2 concentrations (20 cm and 50 cm), and surface CO2 efflux at 32 locations across four transects and highlight results from one characteristic transect. Comparing wet:dry growing seasons, all locations had similar maximum soil water content, but the range was greater (minimum was lower) during the drier growing season. Median hillslope soil water content (17:10 %), and median soil CO2 concentrations at 20 cm (2200:1800 ppm) and surface CO2 efflux (0.48:0.43 g CO2 m-2 hr-1) were all lower during the dry growing season. In riparian zones, median soil water content was much lower in the dry growing season (52:43 %). However, median riparian soil CO2 concentrations (10,200:15,400 ppm) and surface CO2 efflux (0.48:0.61 g CO2 m-2 hr-1) were much higher during the dry growing season. In addition to differences in magnitude of CO2 and environmental variables, the timing of peak riparian soil CO2 concentrations and surface CO2 efflux shifted 12 and 6 weeks earlier from the wet to the dry growing season. Conversely, hillslope soil CO2 concentrations and surface CO2 efflux timing was consistent across both seasons. Our results suggest that inter-annual climate variability, specifically earlier snowmelt and decreased growing season precipitation, can impact growing season soil water content and temperature, leading to increased/decreased flux magnitudes and shifts in peak fluxes of weeks to months.
B23A-0920
Water-Use Efficiency and Stable Carbon Isotopes: Accounting for Photosynthetic Refixation
Three processes are performed by every green plant tissue: photosynthesis, respiration and refixation. Each of these affects the ratio of stable isotopes, 12C and 13C. Refixation allows plants to fix a portion of the CO2 produced via respiration prior to releasing the remaining CO2 back into the atmosphere. The process begins with a pool of CO2 already depleted in 13C and subsequently depletes it further, resulting in two simultaneous effects: enrichment of CO2 released into the atmosphere and depletion of biomass that is formed. Recently, considerable research has concentrated on identifying processes that determine the isotopic composition of a given plant tissue. A convincing explanation for the observed enrichment of stems versus leaves has still not been derived. We advocate that refixation can explain currently inexplicable patterns. We hypothesized that leaves re-fix carbon during their entire lifespan when light intensity is below the light compensation point and above total darkness. We grew Idaho hybrid poplars under controlled conditions in a growth chamber. Light intensity was regulated to create three different treatments: (1) Light (PAR=270 μmol/m2s), (2) Shade (PAR=89 μmol/m2s) and (3) Dark (PAR=0 μmol/m2s). For each treatment we modified respiration values by regulating the light environment between total darkness and the light compensation point. For the light treatment group, leaf respired CO2 was collected at 5% (PAR=14) and 22% (PAR=59) of the light growing environment. For the shade treatment group, leaf respired CO2 was collected at 22% (PAR=20) of the light growing environment. We estimated the amount of refixation as (Ddark- Dlight)/Ddark, where Ddark represents dark respiration (μmol/gs) and Dlight respiration during light periods (μmol/gs). Light treatments plants exhibited a maximum refixation level of 53% at PAR=59, with an associated enrichment of leaf respired C isotopic composition (δ13CLR) of 3.3‰. At PAR=14, refixation rate for Light plants decreased to 10% and the observed enrichment on δ13CLR was 1‰. Correspondingly, Shade plants showed a 37% level of refixation and 3.6‰ enrichment at PAR=20. Our findings support the hypothesis that leaves re-fix carbon under low-light intensity environments. The degree of refixation is proportional to light intensity, with higher refixation rates associated with higher light intensities and more depleted leaf biomass. The continuous refixation of the internal leaf carbon pool during leaf expansion together with diurnal refixation periods in mature leaves adds depleted biomass into the leaf that is likely to account for the patterns described in the literature (i.e. 2‰ depletion of leaf versus stem biomass). This can influence the interpretation of δ13C leaf biomass data of previous studies and can compromise the utility of δ13C from leaf tissue as a precise meter of water-use efficiency.
B23A-0921
Stable Carbon Isotopes As Indicators of Plant Water Use Efficiency
Stable carbon isotopes have been utilized to better understand how environmental variables influence the efficiency of photosynthesis, specifically what factors limit the uptake and absorption of CO2 during photosynthesis. An understanding of the controls over both gas exchange and stomatal conductance can provide an explanation for the possible environmental influences on plant WUE. The δ13C of extractive-free wood was used as an index of plant water use efficiency at Mica Creek Experimental Watershed, Shoshone County, ID. The δ13C values of tree rings were used to determine the effects of clear cut and partial cut harvesting practices, the effect of elevation, and species differences in intrinsic water use efficiency (WUE) among coniferous species including: Thuja plicata, Larix occidentalis, Picea engelmannii, Pseudotsuga menziesii, Abies lasiocarpa, and Abies grandis. We found significant effects of harvest treatments (p=0.0197), elevation (p= 0.0268), and species (p<0.001) on tree δ13C. The significantly more enriched isotopic signatures observed in Thuja plicata (δ13C = -23.37 ±0.17‰), indicate that it is a more water use efficient species compared to Larix occidentalis (δ13C = -25.66 ±0.43‰), and Abies grandis (δ13C = -25.83 ±0.15‰). There was also an overall trend of δ13C enrichment with elevation. The isotopic composition of tree rings has been estimated to increase by 0.003 ‰ per meter of elevation gain, which may be related to a decrease in soil moisture with elevation. Finally, the mean δ13C values observed on partial cut (δ13C = -24.73 ±0.10‰) and clear cut treatments (δ13C = -24.45 ±0.29‰) were significantly more enriched than the mean value for the control treatment (δ13C = -25.25 ±0.19‰). The more enriched isotopic signatures observed on the harvested treatments indicate that the trees are more water use efficient, which may be a result of increased photosynthetic capacity with an increase in the availability of water, foliar nitrogen, and light to individual trees on the harvested treatments. The reduction of stand density through harvesting may reduce the transpirational water losses on a stand level, thus increasing the water availability for individual trees.
B23A-0922
A 3PG-based Model to Simulate Delta-13C Content in Three Tree Species in The Mica Creek Experiment Watershed, Idaho
3PG (Physiological Principles in Predicting Growth), a process-based physiological model of forest productivity, has been widely used and well validated. Based on 3PG, a 3PG-δ13C model to simulate δ13C content in plant tissue is built in this research. 3PG calculates carbon assimilation from utilizable absorbed photosynthetically active radiation (PAR), and calculates stomatal conductance from maximum canopy conductance multiplied by physiological modifier which includes the effect of water vapor deficit and soil water. Then the equation of Farquhar and Sharkey (1982) was used to calculate δ13C content in plant. Five even-aged coniferous forest stands located near Clarkia, Idaho (47°15'N, 115°25'W) in Mica Creek Experimental Watershed, were chosen to test the model, (2 stands had been partial cut (50% canopy removal in 1990) and 3 were uncut). MCEW has been extensively investigated since 1990 and many necessary parameters needed for 3PG are readily available. Each of these sites is located near a UI Meteorological station, which recorded half-hourly climatic data since 2003. These site-specific climatic data were extend to 1991 by correlating with data from a nearby SNOTEL station (SNOwpack TELemetry, NRCS, 47°9' N, 116°16' W). Forest mensuration data were obtained form each stand using variable radius plots (VRP). Three tree species, which consist more than 95% of all trees, were parameterized for 3PG model, including: grand fir (Abies grandis Donn ex D. Don), western red cedar (Thuja plicat Donn ex D. Don a) and Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco). Because 4 out of 5 stands have mixed species, we also used parameters for mixed stands to run the model. To stabilize, the model was initially run under average climatic data for 20 years, and then run under the actual climatic data from 1991 to 2006. As 3PG runs in a monthly time step, monthly δ13C values were calculated first, and then yearly values were calculated by weighted averages. For testing the model, tree cores were collected from each stand and species. Ring-widths of tree cores were measured and cross-dated with a ring-width chronology obtained from MCEW. δ13C contents of tree- ring samples from known year were tested. Preliminary results indicate 3PG-δ13C simulated values are consistent with observed values in tree-rings. δ13C values of modeled species are different: western red cider has the highest delta13C values among the three species and western larch has the lowest.
B23A-0923
Interannual Variation of Carbon Fluxes From a Tropical, a Temperate, and a Boreal Evergreen Forest: the Role of Gap Dynamics and Climate
Interannual variation of carbon fluxes can be attributed to different biotic and abiotic controls that operate at different spatial and temporal scales. The type and frequency of disturbance, forest dynamics, and climate regimes are important sources of variability. Assessing the variability of carbon fluxes from these specific sources can enhance the interpretation of past and current observations. Being able to separate the variability caused by stand dynamics from that induced by climate will also give us the ability to determine if the current observed carbon fluxes are within an expected range or by contrast, other non-tested factors affect the annual variation in the overall carbon flux. We explored possible sources of variation in Net Ecosystem Carbon Balance (NECB) using the simulation model STANDCARB. With this model we identified key processes that introduce variation in annual carbon fluxes. Three contrasting ecosystems were used, a tropical forest, a temperate coniferous forest, and a boreal forest. We found that gap dynamics introduced a key source of variation to annual carbon fluxes, but its relative importance differed among the three ecosystems studied. In the tropical forest, gap dynamics and climate each contributed the same amount of variation to the annual carbon exchange. In the temperate and boreal sites, where many forest processes occur at a longer temporal scale than those at the tropical site, climate controlled more of the annual variation of carbon fluxes. These results suggest that the variability controlled by climate affects the internal rates of carbon exchange differently among sites. Our results also showed that the variation of annual carbon fluxes poses important challenges to determine the status of an ecosystem as source, sink or neutral at longer time scales. We found that for systems in dynamic equilibrium, there is a 5 to 10 % chance of incurring in Type I error when testing the hypothesis of neutrality, i.e., NECB = 0. Conversely, in simulations where climate change negatively affected ecosystem productivity, there was an 80 % chance of committing Type II error, even with 50 sequential years of data.
B23A-0924
Homogeneous biogeochemical functions in terrestrial ecosystems of the heterogeneous coastal temperate forest ecosystem
The coastal temperate rainforest ecosystem of southeast Alaska has a complex landscape morphology at both large and small scales. The drastic topographic variation of this mountainous post-glacial landscape combined with frequent storms has developed a mosaic of forests and wetlands. At the stand level, lateral variation in topography, caused by frequent disturbances, such as windthrow and tree mortality, has produced hummocks and hollows. This complex terrestrial system has posed a challenge to terrestrial ecologists and soil scientists who have attempted to map ecosystem types and determine biogeochemical functions within these terrestrial ecosystems. Initial hydropedological research in the region has revealed patterns associated with hydric soil formation along soil toposequences. We have used a hydropedological approach to determine common hydrologic and biogeochemical functions in several specific ecosystem types associated with the soil toposequence including uplands, forested wetlands and bogs. The terrestrial communities in southeast Alaska are influenced by hydrology, soil type, and geomorphology, which we term "Hydropedomorphic" to describe the communities and their associated biogeochemical functions. We have combined observations of soil hydrology, measurements of carbon, nitrogen and phosphorus concentrations in soil solution and tributary streams along with novel measurement techniques of dissolved organic matter components using PARAFAC analysis to establish homogeneous patterns among these diverse hydropedomorphic units. Distinct vertical gradients in soil saturation and redox potential lead to specific biogeochemical transformations among the hydropedomorphic units. The hydropedomorphic units contribute dissolved organic material to tributaries that varies in quality and quantity seasonally. The identification of DOM export associated with specific mappable landscape attributes provides a template for estimating watershed and regional cycles of carbon, nitrogen, and phosphorus dynamics for assessments of ecosystem development and climate change.
B23A-0925
Plant Foliar Response to Soil Nutrient Availability Across Contrasting Geologic Settings
Rock derived mineral nutrients such as P, Ca, Mg, Mn, and K play a significant, but poorly understood role in the structure and function of temperate forest ecosystems. Though these nutrients are not necessarily limiting to plant growth, they are essential to plant physiological functioning. In this study, we test the hypothesis that foliar nutrients are a proxy for soil nutrient availability across sites of different underlying geologies. Specifically, we focus on the plant nutrient-use strategies of rock derived nutrients (P and K) and how they relate to soil nutrient status. In order to assess the responses of plant species to nutrient availability, we monitored above ground net primary productivity (current annual increment + litterfall), plant chemistry, and soil nutrients for a period of 24 months. This research was completed in the San Juan Mountain region of southern Colorado, where there is a high local diversity of bedrock geochemistry. Within this region, two small sub-alpine basins were chosen; a sedimentary basin composed of Mesozoic cyclic limestone, sandstone & shale and a volcanic basin composed of Tertiary rhyolite. Across these basins, geology played a significant role in explaining the variability of rock derived nutrient availability. Initial results suggest that differences in bedrock geochemistry have little influence on the aboveground net primary production (ANPP) of plants or on the chemistry of foliar materials. This inflexibility of foliar chemistry to variations in nutrient availability suggests that genetic and physiologic controls play a strong role in determining the chemical content of plant materials. An alternative hypothesis is that deposition of eolian mineral dust into subalpine systems could play a role in offsetting the reliance of vegetation on deeper bedrock derived nutrient sources. An investigation is currently underway to assess the contribution of eolian dust derived nutrients to plant nutrition using Sr as a geochemical tracer.
B23A-0926
Extreme Environments in Tierra del Fuego, Argentina
The upper timberline of the Andes Cordillera on the island of Tierra del Fuego at the tip of South America is an environment subject to extreme conditions. In order to further understand this environment, ecosystem parameters were measured within two transects of the Andes at Glaciar Martial and Cerro Guanaco. The measurements included pH, soil temperature, soil moisture, nitrogen, sodium and potassium concentration, chlorophyll absorbance, and irradiance in the ultraviolet range (200-400 nm). These data comprise a survey that serves as a baseline for an intensive research program. Chlorophyll concentration and soil data were within the range of our observations at several other sites, from Lapataia Bay on the southwestern boundary with Chile, through the eastern end of Lake Fagnano. However, unusual levels of solar irradiance were found in the open sites of both transects while those in the forest exhibited lower UV values, suggesting strong absorption and/or reflection by the forest canopy. High levels of UV radiation damage important biomolecules and may be partially responsible for the presence of life forms such as the krummholz belt in the upper timberline. These UV values may be due to the effects of global ozone depletion and the ozone hole. The low temperatures, strong winds, snow and ice-covered soil and especially the exposure to UV radiation make this area an extreme environment for life.