Research Description

Research at the Niwot Ridge AmeriFlux site is designed to be consistent with the National AmeriFlux Science Plan in following two fundamental efforts:

quantifying net carbon sequestration by a subalpine coniferous forest, as a contribution to current efforts to model the global carbon budget, and

developing predictive models of ecosystem-level photosynthesis and respiration.

In addition to these general aims, we are taking advantage of the challenging topography offered to us at the Niwot Ridge site to address the following issues:

modification of the traditional approaches to measuring ecosystem carbon and fluxes to accomodate the complex air flows that occur above mountainous terrain, and

utilization of the organizing features of complex terrain (e.g., convergent flow paths) to capture the integrated, landscape-level carbon fluxes that are required to balance the local carbon budget.

        These research efforts are meant to provide a foundation for understanding the role of high elevation forests in assimilating CO2 from the atmosphere and their possible sensitivity to perturbation by nearby urban activities and interannual-to-interdecadal climate variation. The studies will provide valuable insight for use in managing subalpine forests in particular, and ultimately the management of natural forest ecosystems in general, as human societies grapple with the problem of global CO2 increases and its influence on climate warming. It is important to keep in mind that forest ecosystems, such as those near Niwot Ridge provide recreational opportunities and serve an invaluable function in the generation and renewal of renewable resources in the western United States, and that they are increasingly being threatened by overuse and encroaching urban influences. The carbon budget of the forest is an important indicator of the health of the forest. Carbon assimilation by the forest is required to support tree growth, turn over soil organic matter and, by fostering the growth of a canopy, reatin snow cover and release melted snow water slowly in the spring.

Specific Research Objectives

Quantifying Forest Carbon Sequestration - The Niwot Ridge Ameriflux site includes several tall flux towers that are equipped with instrumentation capable of measuring net ecosystem CO2 flux by the eddy covariance method (as well as quantifying the energy budget and evapotranspiration rate of the forest). The complete array of instruments on each tower is described in the Instrumentation Table. Eddy covariance measurements of CO2 flux are combined with measurements of diurnal changes in the vertical CO2 concentration profile (which can be used to quantify CO2 storage beneath the canopy), to obtain net ecosystem exchange (NEE). These measurements are conducted continuously and collated into 30-minute averages, a form that is convenient for evaluation of the short- and long-term CO2 budgets for the site.

Measuring and Modeling Net Ecosystem CO2 Exchange - In order to understand the physical and physiological controls over ecosystem CO2 exchange, we are conducting a number of studies on trees and soil. The studies can be grouped into the following efforts:

(1) Studies on controls over soil respiration. We have initiated research into how the transport of photoassimilated carbon from the shoots of trees affects microbial processes in the rhizosphere, and thus influences soil respiration. In several forest plots, we have girdled trees (cut swaths of bark from their trunks) to prevent the downward transport of photoassimilated sugars. Our studies have revealed that approximately half the measured soil respiration in the summer is due to the influence of the trees (root respiration plus tree-supported microbial respiration). We are now characterizing the microbial communities that occupy girdled and non-girdled plots in an effort to determine which groups are most affected by the disappearance of the tree influence and how they affect soil respiration rates. We are using DNA fingerprinting techniques and studies of microbial growth under laboratory conditions to study these dynamics.

(2) Studies on the importance of winter biogeochemical processing of soil carbon. We have discovered that beneath the winter snow pack live thriving communities of soil microorganisms that oxidize soil organic matter and carbohydrate compounds that leak from plant roots. The combined respiratory activities of these organisms can cause the loss of over half the carbon assimilated by the trees the previous summer. Thus, contrary to past assumptions, the winter does not represent "down time" for the forest. The winter is a crucial period for biogeochemical cycling and these winter processes must be understood in order to correctly evaluate the annual forest carbon budget. We have made two important observations that help us to better understand winter processes. First, we observed that trees in the forest release a large pulse of sucrose into the soil during the winter. We know that the sucrose comes from the trees because girdled trees (see above) do not release the pulse. The trees of this forest are not photosynthetically active during the winter. We assume that the sucrose leaks from fine roots that were winter-hardened during the autumn, and then physically damaged during soil freeze-thaw events during the early winter. Second, we observed that the microbial community that occupies the winter soil is different in composition that the microbial community that occupies the summer soil. The winter microbes are capable of exponential growth and high rates of respiration during the winter.

(3) Studies on model-data fusion. In collaboration with Dave Schimel at the National Center for Atmospheric Research, we have initiated an effort to assimilate flux observations into ecosystem process models. We are using the multi-year eddy covariance data set from the Niwot Ridge Ameriflux site in order to obtain a high-density constraint on the Simple Photosynthesis and Evapotranspiration (SIPNET) model. By assimilating multiple years of data, and forcing the model to optimize its representation of ecosystem processes across the entire data set, we can obtain highly constrained estimates of model components and parameters. We have used this approach to partition the measured net ecosystem CO2 exchange (NEE) into its component processes of gross photosynthesis and ecosystem respiration. Recently, we have initiated a project with Dave Bowling of the University of Utah to measure nearly-continuous fluxes of the isotopic forms of CO2 (13CO2 and 12CO2) using a Tunable Diode Laser. This effort will allow us to get even better constraint on the modeling.

(4) Scaling carbon fluxes across the landscape and region. The ridges and valleys of a mountain landscape provide organizing features of the local air flows that can be used to integrate fluxes across large spatial scales. For example, at night cold air drains to the lowest points on the landscape and pools, resulting in an accumulation of CO2, which is then vented upward in the morning. In a joint effort with the National Center for Atmospheric Research, we have created a project called Carbon in the Mountains (CME), in which we use multiple-tower arrays, and aircraft sampling of the early-morning and midday atmosphere, to observe the spatial organization of CO2 across the landscape. Using these approaches, we will expand our understanding of forest CO2 fluxes at the Niwot Ridge site to the entire Front Range region of Colorado. This effort is a critical step toward gaining an accurate understanding of carbon cycle processes at the regional scale.

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