B21D-01 INVITED
Complex Relationships Between the Spatial and Temporal Patterns of Climate in Mountainous Terrain
A complex interplay between the spatial and temporal patterns of climate is known to exist at the regional scale in the Pacific Northwest. For example, there is a surprising lack of relationship between maximum temperature in winter and minimum temperature in all seasons on mountain tops vs. those in valley bottoms. This lack of relationship exists at time scales ranging from sub-daily to monthly and longer. Recent analysis of temperature data from the H.J. Andrews Experimental Forest in Oregon's Cascade Mountains provides a case study from a small (10x12 km) watershed. Initial results suggest that: (1) Temporal variations in climate do not always occur in lock-step across the landscape - they are often asynchronous; (2) much of this asynchrony appears to be controlled by susceptibility to cold air flowing down drainages and pooling in valley bottoms; (3) the patterns and degrees of climatic asynchrony (i.e., cold air drainage) vary strongly with weather regime, season, time of day, topographic position, and other factors; and (4) effects of cold air drainage on the spatial patterns of snowpack persistence may produce positive feedbacks that can reinforce cold air pooling. This has great implications for long-term ecosystem monitoring and analysis in complex terrain. It suggests that terrestrial and aquatic ecosystems in very close proximity may be exposed to markedly different regimes of weather and climate trends and variability across scales ranging from seconds to months, and perhaps longer.
B21D-02
What Trees do at Night When no one is Looking: Impacts of Elevation on Hydraulic Redistribution in Mountainous Terrain
Changes in climate and terrain variables (such as vapor pressure deficit (VPD), temperature, soil and moisture), that occur along elevational gradients can result in physiological differences in steep mountainous terrain. This is particularly evident in the plant mediated portion of the hydrologic cycle such as the redistribution of soil moisture by tree roots or hydraulic redistribution (HR). In forests of the intermountain Northwest, HR typically occurs when the following conditions are met: 1) A sufficient water potential gradient exists within the soil profile, and 2) Nighttime VPD is negligible, whereby the lack of transpiration allows water transported in the roots from deeper moist soil horizons to "leak" out in the upper dry soil layers rather than being transpired by the tree. We hypothesized that the formation of nocturnal cold air pools in steep terrain increases the frequency and magnitude of water transported by HR and decreases nocturnal transpiration in cold low elevation sites relative to the warmer upper elevation sites. We measured VPD, nocturnal transpiration and soil moisture profiles along elevational gradients in two steep forested ecosystems in Northern Idaho. At Mica Creek, nighttime VPD averaged 0.60 kPa, 1.16 kPa and 0.94 kPa at the lower (1210m), mid (1250m) and upper (1390m) site respectively. We have confirmed that the combination of open stomata and high nocturnal atmospheric VPDs results in nocturnal transpiration and will present elevational trends of nocturnal transpiration and HR. At Benton Creek, average minimum daily VPDs increased from 0.28 kPa to 0.54 kPa as elevation increased from 900m to 1150m. Evidence of HR tended to occur later in the summer and moved less water as elevation increased. Shifts in the magnitude and timing of HR with elevation have potential hydrologic and ecologic ramifications. During the typical extended periods of summer drought in these ecosystems, HR may help to provide moisture to neighboring shallow rooted vegetation, maintain fine root viability and rhizosphere activity and allow for continued mineralization and access to nutrient stores in the upper soil layers.
B21D-03 INVITED
Using the δ13C of ecosystem respiration to monitor ecosystem metabolism of entire watersheds in complex terrain.
Complex terrain presents formidable challenges to ecosystem studies. Valleys, steep slopes, and windswept ridges impose wide variations in microclimate, soil properties, and plant communities; this variability greatly hinders strategies for systematic sampling and up-scaling. Our recent work in a deeply incised watershed (20 to 33° slopes) in Oregon's Cascade Mountains suggested the possibility of using the δ13CO2 of ecosystem respiration (δ13CER) in mountainous ecosystems as a tool to monitor seasonal and interannual variations in physiological processes in vegetation of entire watersheds. We demonstrated that nocturnal cold air drainage is persistent, occurs on greater than 80% of summer nights and is well mixed. Furthermore, nighttime air samples collected from the base of the watershed contain a representative sample of respired CO2 from most of the watershed. We also found that on most clear nights the range of CO2 concentrations over a single night is sufficient for using the Keeling Plot approach to determine the carbon isotopic composition of δ13CER. The goals of the current study were to determine if variations in δ13CER were correlated to environmental variables and could be used to predict expected variations in canopy-average stomatal conductance ( gs). As reported by other researchers, changes in δ13CER were significantly correlated to measured soil matric potential (ψm) and vapor pressure deficit (VPD) measured on the same day and six days earlier, respectively. Midday gs was estimated using a simple hydraulic model with only ψm and VPD as predictive variables. Midday gs from zero and five days earlier were correlated to δ13CER. To examine direct relationships between δ13CER and recent gs, we used models relating isotope discrimination to stomatal conductance and photosynthetic capacity at the leaf level to estimate values of stomatal conductance ("gs-I") that would be expected if respired CO2 were derived entirely from recent photosynthate. We compared these values with gs estimated from the hydraulic model. The magnitude, range and temporal variation in the two values were surprisingly similar. We conclude that δ13CER could potentially be used to directly monitor basin-average variations in gs in complex terrain. If our findings hold to closer scrutiny, this method may be used to monitor leaf physiological properties on the ecosystem scale in complex terrain.
B21D-04
Soil CO2 Efflux Variability in Complex Terrain: Towards Estimation of Watershed-Level Rates
Soil CO2 efflux is a primary component of ecosystem respiration and a key determinant of net ecosystem production (NEP). One obstacle to understanding/predicting the heterogeneity of soil CO2 efflux is the variability in patterns of soil physical and biogeochemical processes imposed by topography, particularly in complex terrain. Extrapolating from single- or multiple-point measurements to watershed-scale efflux estimates requires an understanding of the spatial variability of environmental variables (e.g. soil temperature, vegetation, substrate, soil physical properties). Additionally, soil CO2 efflux can vary at hourly, daily, and seasonal time scales as a result of the interaction among these variables, including the lateral redistribution of soil water. We examined the relationships between topographically-derived indices (e.g., upslope accumulated area, topographic indices, radiation indices) and the space/time variability of soil CO2 efflux to explore the concept of biogeochemically similar areas (BSAs) for estimating watershed-scale soil CO2 efflux. We suggest that characteristic dynamics of BSAs can be used to extrapolate from benchmark data collection locations to larger areas of the landscape and indicate watershed-level response to changes in soil temperature, soil water content, and precipitation. We use both discrete and continuous field-based observations of soil CO2 efflux from a 380-ha watershed in the Tenderfoot Creek Experimental Forest (TCEF), a montane conifer forest characteristic of sub-alpine ecosystems of the northern Rocky Mountains. These observations, in association with terrain analysis and process-based understanding, are used to characterize and quantify the spatial and temporal variability of soil CO2 efflux. Based on efflux measurements collected during two growing seasons (2005, 2006), there was moderate correlation between upslope accumulated area and rates of soil CO2 efflux across 18 diverse upland areas of the watershed (r2=0.37). However, this correlation improves significantly when analyzing efflux rates along single toposequences in moderately sloping SE aspects (r2=0.82) and steeper NW aspects (r2=0.96). Our results suggest that BSA analysis can facilitate estimation of watershed- level soil CO2 efflux rates and their integration with other measures of C flux (e.g. NEP). As such, BSAs offer potential to improve process understanding and quantitative assessment and modeling of watershed scale soil CO2 efflux in complex terrain. http://watershed.montana.edu/hydrology/Carbon_project.htm
B21D-05
A New Methodology For Estimating CO2 Advective Fluxes In Complex Terrain
A key problem in using the eddy correlation (EC) technique for estimating the carbon dioxide Net Ecosystem Exchange (NEE) of terrestrial ecosystems is the potential bias caused by advective fluxes of CO2. Advective fluxes are often not considered since they are difficult to identify and to quantify, especially in complex mountainous terrain with highly variable wind patterns and drainage flows. We propose a methodology to estimate these fluxes based on a full 3-Dimensional (3D) approach applied to the topographically complex alpine forest site of Renon (1736 m a.s.l.). This is an aerodynamic method based on the computation of advective fluxes across the aerial faces of a control volume including the plant ecosystem. Data used for the computation of CO2 advective fluxes were collected during an extensive field campaign performed in 2005 in the framework of CarboEurope-IP research project. Vertical profiles of wind, air temperature and CO2 concentration have been measured at five towers and a spatial interpolation was performed in order to get 3D fields of such variables. The frame of reference used was orthogonal and the vertical direction was parallel to the gravity. Each anemometer was aligned in this frame of reference and no rotations were applied to the wind velocity components. The analysis of the 3D fields of wind velocity, CO2 mixing ratio and air density highlighted the spatial heterogeneity of CO2 source/sink strength and the strong de-coupling between air flow below and above the canopy during stable nights. The total CO2 advection calculated using the proposed methodology exhibited prevailing positive values during the night-time period. Advective fluxes estimated during windy nights were of the same magnitude and sign of vertical turbulent flux measured above canopy by the EC technique. This observation suggests that the friction velocity correction routinely applied to night-time periods may not be efficient at the Renon site. During light windy nights high positive values of advective fluxes were observed, while EC fluxes were close to zero. Daytime advection showed prevailing negative values. The magnitude of daytime advection was lower than the magnitude of night-time advection, and they tend to partially cancel out each other because of the opposite signs. Ecosystem respiration estimated by a chamber-based method was in good agreement with night-time NEE corrected for storage and total advection. By taking into account day and night advection the carbon uptake at Renon was reduced more than 50%, and this corrected value was in good agreement with biometric estimation of Net Ecosystem Productivity (NEP).
B21D-06
Net Ecosystem Exchange in a Forested Montane Watershed: Trends and Trials in Complex Terrain.
Recent years have seen increased study of the land-atmosphere exchange of mass and energy as measured by the eddy covariance technique. Because these results have yielded significant promise, offering the opportunity to integrate small-scale heterogeneities at the ecosystem level, an increasing number of researchers have begun employing the method in montane areas typical of the Rocky Mountains. Problematically, these areas can exhibit complex terrain and tall canopies, introducing the need for a more complete treatment of the mass balance equation to account for advective flows and storage terms brought on by atmospheric stability. Promisingly, a variety of data filtering, modeling, and measurement techniques have shown potential in alleviating some these concerns. Because high altitude forests have shown considerable carbon sequestration potential and may be particularly susceptible to climate change scenarios affecting temperature, moisture, and snowpack accumulation, it is important that eddy covariance measurements continue in these non-ideal settings so that methodologies can be refined, and ecosystem-level mass and energy cycling dynamics can be evaluated. To this end, a 40-meter tower outfitted to FLUXNET specifications was erected over a lodgepole pine-dominated system in the Tenderfoot Creek Experimental Forest, Montana. Initial results indicate that on particularly stable nights (u×<0.1), subcanopy concentrations of CO2 became elevated to ~100ppm above levels measured on sufficiently turbulent nights (u×>0.4). Resultant turbulent flux, as shown by the unfiltered eddy covariance system, exhibit nocturnal carbon emissions that are much larger in times of high turbulence, and much smaller in times of low turbulence. The difference can be an order of magnitude. Presumably, this is due to understory-atmosphere decoupling in times of stability. In order to account for underestimation of nocturnal ecosystem respiration, data were conservatively filtered, excluding measurements under a friction velocity (u×) of 0.4 m/s. Resultant data gaps were filled with nocturnal respiration measurements obtained from soil and leaf chambers, and compared to nighttime eddy flux measurements obtained in sufficiently turbulent conditions. The results show that this forest is a substantial carbon sink at a rate of 350 g C/m2/yr, and indicate the promise of continuing these studies in complex terrain.
B21D-07
Preliminary Observations of Water and Carbon Dioxide Fluxes Across an Alpine Treeline.
Several studies have shown that alpine treeline (timberline) is especially sensitive to environmental changes, and is therefore a strong "early-warning" indicator of regional climate change. Many of these changes may be induced by factors including local land use changes, and understanding the difference and dynamics of water and carbon dioxide fluxes across the alpine treeline is important to help understand and assess regional climate change. Preliminary observations of water and carbon dioxide fluxes from a recently installed eddy covariance tower situated over alpine tundra are compared to fluxes measured over adjacent subalpine forest. During June-July 2007, the carbon dioxide, latent heat, and sensible heat half-hour fluxes over the tundra were 69%, 50%, and 44% less than over the forest, respectively. The evaporative fraction, however, was similar for both sites, and the cumulative carbon uptake was only 40% less at the tundra site compared to the forest site. Describing the differences and dynamics of fluxes across the alpine treeline is the first step towards understanding how changes in land use are/will affect the alpine environment. http://culter.colorado.edu/NWT/
B21D-08 INVITED
Measurement of Landscape-scale Fluxes in the Complex Terrain of a Rocky Mountain Subalpine Forest
For the past eight years, we have made observations using various approaches, including eddy covariance, mass-balance budgeting of advective fluxes, dispersion of SF6 tracer, and energy-budget closure to evaluate landscape-scale fluxes of CO2, H2O and energy in a complex mountain landscape in the Colorado Rocky Mountains. We discovered that daytime energy budget closure using eddy covariance fluxes balanced by available net radiation was better than 84% on a half-hourly basis in both winter and summer, and approached complete closure at surface friction velocities (u*) greater than 1 m s-1. Co-spectral analysis indicated that contributions from advective fluxes and low-frequency flux contributions were likely responsible for the lack of daytime energy budget closure at lower u*. Using multiple towers dispersed across a restricted (0.25 km2) footprint of the landscape we discovered that the highest nighttime CO2 concentrations occur at the lowest point (stream channel), due to the forces of gravitational drainage flows, that the downslope drainage of high-CO2 air is broken up gradually in the late morning when within canopy turbulence increases, and that there is likely photosynthetic assimilation of 'pooled' CO2 the next morning when upslope flows develop due to mountainside warming. Studies of nighttime dispersion of released SF6 tracer showed that the nighttime drainage flows are restricted to a relatively shallow layer of air beneath the canopy, with little vertical mixing. We used the multiple towers to estimate the contributions to the local CO2 budget by horizontal and vertical advective fluxes (driven by the nighttime drainage flows) and found that compared to traditional methods of correcting eddy covariance derived CO2 budgets on flat terrain, the complex terrain in at the Niwot Ridge site can induce an average 10% error in monthly cumulative net ecosystem exchange (NEE), which is amplified to a 65% error in cumulative NEE across six years. Thus, the errors to the local CO2 budget, when advective fluxes are ignored can be large. We developed the concept of a mountain 'carbonshed' to describe the effects of complex terrain on local CO2 budgets; the carbonshed is potentially a valuable concept that represents an organizing feature for studying landscape-scale fluxes in mountain-valley systems.