# Presentation: 698464

## Estimating nighttime respiration: how to get eddy covariance data in tune with chamber measurements in highly complex terrain

**Eva van Gorsel, Ray Leuning,**
**Leonardo Montagnani****1**** and Christian Feigenwinter****2**
**3/10/2007**
**1 ***Autonomous Province of Bolzano, Forest Services, Bolzano, Italy*
**2***Institute of Meteorology, Climatology and Remote Sensing, University of Basel*

## Background // Motivation

**Land surfaces play a key role in modulating the level of atmospheric CO2. Their sink strength varies strongly with time as compared to the oceans **
- Seasonal cycle
- Sensitivity to climatic drivers
**High variability:**
- Need of long-term high-resolution time series when interpreting ecosystem data.

- Estimating net ecosystem exchange in moderately complex topography

## Methods to measure NEE

**Eddy Covariance measurements **
- Meet need of long-term high-resolution time series
- Limitations high (fetch, topography)
**Chamber measurements**
- Few limitations
- Issues with representativeness
- Time resolution is low

- Estimating net ecosystem exchange in moderately complex topography

## Comparison between methods

**Comparisons between micrometeorological and chamber measurements almost consistently show:**
- Respiration is underestimated by eddy covariance measurements
- NEE overestimated
**The problem ****occurs due to advection (not usually measured)**

- Estimating net ecosystem exchange in moderately complex topography

## Method: Eddy Covariance

- NEE*=              +                     +  *

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
Mass balance for CO2 in a homogeneous forest (thus neglecting flux divergence) with

cc molar density of carbon
cd molar density of dry air

c is the mixing ratio of the molar density of carbon (cc) to dry air (cd) 

L length
hr reference height

Neglecting advection during day time is less of a problem than at night, when turbulent mixing is reduced.

## Turbulence conditions, CO2 concentrations and advection

- 0.5 m

- 70 m

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
time series of u∗ and CO2 concentrations measured at nine heights between 0.5 and 71.1 m on three nights in March 2005. 

On the first night, conditions were calm, with strong stable thermal stratification and the development of strong concentration gradients. Note that concentrations increased steadily only in the first few hours after dusk and then remained approximately steady until dawn. Advection of CO2 in the layer below hr explains these results because the reduction in the magnitude of term II in eq. (1) was not matched by an increase in the eddy flux through the top of the control volume. Changes in respiration rates do not explain the results because, there is expected to be only a small reduction in biologically controlled respiration as the soil and vegetation cooled slowly over night. Without advection, the concentrations would have continued to increase steadily throughout the night. 

The second, intermittent night, was characterized by well-mixed conditions before midnight and a period of high stability around midnight. Strong CO2 concentration gradients developed as soon as u∗ dropped below 0.25 m s−1, leading to significant changes in storage below hr. During the second part of the night, a considerable CO2-gradient was maintained despite the observed high-turbulence levels. This indicates that the u∗ values measured well above the canopy are not always representative of within-canopy processes. 

Continuous high-turbulence levels and near-neutral stability as observed throughout the third night ( u∗ >0.75ms−1), are needed to maintain a coupled flow and good mixing associated with small vertical CO2 concentration gradients and associated small changes in storage.

## The u* threshold

- Humphreys E.R. et al. (2006)

- Using u* threshold to select periods when eddy flux and storage, but not advection are important contributors to respiration.** **

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
Filtering data based on the friction velocity, u*, is the most commonly used approach to exclude periods when advection is likely to occur since high u* is associated with strong turbulence and advective effects are therefore likely to be small (Goulden et al., 1996). The drawback of this approach is that above-canopy measurements are used to represent within-canopy processes which is not justified when turbulence is intermittent or the flow decoupled (Goulden et al., 2006; van Gorsel et al., 2007). Another problem is that the u*-threshold is often determined subjectively (Papale et al., 2006), although Gu et al. (2005) did propose an objective method for its selection. Selection of the u*-threshold can be crucial, as shown by Miller et al. (2004) who found that relatively slight variations in the threshold led to a change in sign of the net ecosystem exchange of CO2 of a tropical forest. Furthermore there are sites where it is not possible to identify a unique u*-threshold (Wilson et al., 2002).

## u*-filtering: Is there always a threshold?

- u*-filtering: Is there always a threshold?

**Tumbarumba, 2001-2005**

- But there are sites where it is not possible to identify a unique *u**-threshold

- van Gorsel E. et al. (2007)

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
No threshold can be found at Tumbarumba -> need of an alternative method.

## Early evening maximum of R

**Tumbarumba **
**2001-2005**

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
Grey-ish blue: storage term
Blue:              Eddy flux
Green :          sum of above
Orange:         Soil temperature
Red:              Chamber
sum of max(Fc+Storage) in early evening is likely to be best approx of ‘true’ respiration

The Figure shows that eddy fluxes (blue) at 70.1 m are small during night. There is a maximum of appprox. 4 μmol CO2 m−2 s−1 in the sum of eddy flux and change in storage in the early evening which then decays to appprox. 1 μmol CO2 m−2 s−1 in the second part of the night.
The observed decrease exceeds the expected reduction in respiration due to decreasing soil and plant temperatures overnight, a conclusion supported by comparison with R derived from independent chamber measurements of respiration from soil leaves and wood (dashed red line). Clearly, micrometeorological measurements on a single tower underestimate NCE in the second part of the night, even though the Tumbarumba flux station is located in fairly moderate topography. The results provide strong evidence that a substantial fraction of the NCE is not captured because of horizontal and vertical advection of CO2 below the measurement height.

## Early evening maximum of R

- Aubinet M. et al. (2005)

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
That the change in storage term has max in early evening has been observed at several forested sites characterized by very different topographies, vegetation cover and wind regimes.

## Under stable conditions the canopy flow decouples from the flow above.
	Tumbarumba 06-13.03.2005

wind velocities u(z) normalized with u(6m) 
and wind direction. Slope wind direction is 

**Under stable conditions the canopy flow decouples from the flow above.**
**	**Tumbarumba 06-13.03.2005
- wind velocities *u(z)* normalized with *u(6m)*
- and wind direction. Slope wind direction is

- What do we know about the dynamics of advection?

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
During stable conditions within canopy flow is often decoupled from that above (Froelich and Schmid, 2006), and drainage flows can develop close to the ground even on shallow
slopes (Finnigan, 2007; Mahrt et al., 2001).

Figure: red colors indicate highest normalised wind velocities, blue colors lowest: During the day wind velocities decrease continuously with height (1h average example given at right hand side of slide) but from of  18:00 there is a secondary maximum 2 m height. At the same time wind direction turns downslope. 

## What do we know about the dynamics of advection?

**Once drainage flows commence, the down flowing air has to be replaced with air from above**. **Entrainment of CO****2**** poor air leads to development of horizontal CO****2**** gradients.**

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
Once drainage flows commence, the down flowing air has to be replaced with air from above. This air is characterized by lower CO2 concentration. As it is entrained into the canopy layer horizontal CO2 gradients develop, even if the vegetation cover is homogeneous. 

After the boundary layer becomes sufficiently stably stratified through radiative cooling of the canopy after sunset, we expect the sum of terms eddy flux and change in storage term to underestimate NEE because the advection term will then be significant as a result of the following postulated sequence of events:

Much of the respired CO2 is stored in the canopy, and CO2 concentrations close to the ground increase strongly. The cool layer within the canopy modifies buoyancy and hence the hydrostatic pressure gradient. Gravity flows commence once the air close to the surface has cooled to the extent that the hydrostatic pressure gradient exceeds the sum of hydrodynamic pressure gradient and foliage drag (Finnigan, 2007). Entrainment of air with a lower CO2 concentration at the hill crests leads to the development of horizontal CO2 gradients, and advection then drains CO2 out of the control volume (Leuning et al., 2007). 

However, van Gorsel et al. (2007) found that there is a time period of a few hours after sunset when advection is still small at Tumbarumba. This allows to use the maximum in the sum of eddy flux and change in storage term (Rmax) in the early evening to derive a temperature response function to parameterise respiration for the remaining hours of the night.

## Radiative cooling leads to stable stratification and collapse of turbulence. 

**Radiative cooling leads to stable stratification and collapse of turbulence. **

- What do we know about the dynamics of advection?

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
When radiative cooling starts vertical CO2 gradients get stronger. Much of the respired CO2 is stored in the canopy, and CO2 concentrations close to the ground increase strongly. The cool layer within the canopy modifies buoyancy and hence the hydrostatic pressure gradient. Gravity flows commence once the air close to the surface has cooled to the extent that the hydrostatic pressure gradient exceeds the sum of hydrodynamic pressure gradient and foliage drag (Finnigan, 2007). 

## What do we know about the dynamics of advection?

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
Entrainment of air with a lower CO2 concentration at the hill crests leads to the development of horizontal CO2 gradients, and advection then drains CO2 out of the control volume (Leuning et al., 2007). 

Hence there is a there is a time period of a few hours after sunset when advection is still small at Tumbarumba. This allows to use the maximum in the sum eddy flux and change in storage term (Rmax) in the early evening to derive a temperature response function to parameterise respiration for the remaining hours of the night.

## Comparison of different estimates of R

- 5.23 mol m-2 s-1

- 0.46 mol m-2 s-1

- 1.70 mol m-2 s-1

- 4.97 mol m-2 s-1

- Tumbarumba 6.-13.3. 2005

**Total Respiration**
- Chamber (Soil + Leaf + Wood)
- 5.18 mol m-2 s-1

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
Figure
Light green line: eddy flux and change in storage term 
Light blue line: eddy flux and change in storage term + horizontal advection (lowest 6 m). This is to show that Rmax is not affected by horizontal advection.
Numbers
eddy flux only (dark green) lower value: whole time period, higher value: windy night with pronounced turbulence
eddy flux and change in storage term (light green)
Light grey/blue maximum value early in the night 

-> confirms that Rmax provides an excellent estimate of NCE when compared to the chamber measurements. It also shows that turbulent fluxes alone, or in combination with the
change in storage term, significantly underestimate the NCE unless conditions of continuous very high turbulence are met

## Results from alternative approach:

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
Revised respiration (light green) compared to chamber measurements (stars) and values as derived from temperature response function calculated with a ustar-threshold of 0.25 ms-1 

## Multiple constraints on NEE

- y = 0.91 x + 0.08; R2 = 0.69

- NEE

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
CenW runs on a daily time step to simulate the fluxes of CO2, water and nitrogen through the stand and the soil. The model was parameterized by simultaneously comparing model outputs against a wide variety of observations obtained at the site. They included daytime micrometeorological measurements of CO2 and water vapour exchange, estimates of the various above- and below-ground biomass pools, growth rates measured through stem-diameter increments and measurement of specific ecosystem processes, such as litter fall and soil-respiration rates (see Kirschbaum et al, 2007, for more details). The comparison between the revised method of analyzing eddy covariance data and CenW simulation runs is therefore not totally independent because both methods were to some extent constrained by common data.

## Revised estimates for GPP

*P*GP,o= 1.007 *P*GP,m + 0.05 t C ha-1 month-1  R2 = 0.94

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
An even better agreement is obtained between GPP predicted by CenW and that estimated from the micrometeorological measurements. GPP is calculated as the difference between NEE
and daytime ecosystem respiration which is calculated using the temperature response function derived from Rmax and soil temperatures. Good agreement is expected because the daytime micrometeorological measurements were used to constrain CenW, although the two approaches use independent estimates of the nighttime respiration. 

## Testing on other sites:very complex topography

**Renon **
**07/2005**

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
Extract from Montagnani et al, submitted to JGR
Renon: site is located on the southern slope of the Alps, on a porfiric plateau, 12.2 km N-NE from the town of Bolzano, placed between the wide Isarco river valley and the narrow Sarentino river valley, upon the irregular slopes of the Cima del Lago Nero (2069 m), few meters East from aridge that keeps it separated from the steep slope of the Sarentino valley. It is 3.7 km away from the Corno di Renon (elevation 2259 m, direction N-NE) and 8.2 km from the Cima di Villandro (elevation 2509 m, direction N-NW).
The vegetation is a coniferous forest, with patches of pastures. The main forest species were spruce (Picea abies, (L.) Karst., 85% in number) followed by cembran pine (Pinus cembra L., 12%) and larch (Larix decidua Mill., 3%). The forest has natural origins and is used for wood production. The traditional cutting scheme is the uneven-aged one by groups, that gives rise to a forest comprising trees of an uneven age distribution, irregular in tree density and height.

-> Renon is a site which is complex both in terms of topography and homogeneity.

Greyish line: average of all days that have passed the quality control (see last, hidden slide) For July-Renon data these are 45%. Maximum is at 4.97 micromol/m2/s
Red line: median smoothed by 3 points
Red dot: average max value

1) Also in Renon a maximum in the sum of the turbulent flux and the change in storage of CO2 is observed in the early evening. 
2) Parameterised values derived from a temperature response function basing on early evening maxima [2] decay much slower through the night (light green line).
3) That this is more realistic is confirmed by soil chamber measurements (blue line).

## Total ecosystem respiration

** **

**Total ecosystem respiration is derived from soil respiration and factors from a forest of similar species composition and comparable climate. Data points in green assume no clouds, data points in black stand for overcast situations.**

- 1-25 July 2005

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
(after July 2005 pressure problems with chamber measurements due to high turbulence, pers.comm. Montagnani)

Extract from Montagnani et al, submitted to JGR
The availability of the long-term automated chamber LiCor 8100 enabled us to have values of CO2 efflux from soil in a single location during the entire study period, so the temporal variability of Soil Respiration (SR) was directly measured. In order to obtain values of total ecosystem respiration, in the first place a spatialization from the single location was done where the analyser was installed throughout the reference area, on the basis of the extensive campaign performed with the LiCor 6250 chambers.

Expansion factors from measured Soil Respiration to Total Ecosystem Respiration (TER) [...]. The expansion factor values were assumed from the data reported by Urban et al., 2007, which studied a forest similar in climate and species composition. They found with an array of chambers applied to soil, boles and leaves in a spruce forest, that the overall ecosystem respiration varied between 1.37 ± 0.23 and 1.51 ± 0.14 the soil efflux value.

## Conclusions I

**	****Gravity flows in even moderate topography can lead to very significant advection.**
**	****Data from time between decoupling of flow and onset of advection can be used to derive temperature response function. **
**	**
**	****Results reconcile chamber and EC measurements and agree well with ecosystem model.**
**   **
**   ****Results demonstrate:**
- Importance of multiple measurement techniques and modelling to mutually constrain NEE estimates.
** **
**	**

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
Testing of the limitations of this approach is ongoing but first results from a site in extreme topography are presented here:

## Conclusions II

**	****Gravity flows in even moderate topography can lead to very significant advection.**
**	****Data from time between decoupling of flow and onset of advection can be used to derive respiration temperature response function.**
** **
**  ****Results reconcile chamber and EC measurements and agree well with ecosystem model.**
** ****	**
**	****Results demonstrate:**
- Importance of multiple measurement techniques and modelling to mutually constrain NEE estimates.
**	****First results of testing this approach in complex topography are very promising.**

- Estimating net ecosystem exchange in moderately complex topography

**Notes:**
Testing of the limitations of this approach is ongoing but first results from a site in extreme topography are presented here:

## Outlook

**	****Testing alternative to u* filtering on 27 sites with varying topography, climatology and forest density.****	**

- Estimating net ecosystem exchange in moderately complex topography

## Thanks to Helen Cleugh, John Finnigan, Dale Hughes, Heather Keith, 

**CSIRO Marine and Atmospheric Research**
- Eva van Gorsel
- PhD
**Phone:** +61 2 6246 5611
**Email:** eva.vangorsel@csiro.au
**Web:** www.cmar.csiro.au

**Contact Us****Phone:** 1300 363 400 **or** +61 3 9545 2176**Email:** Enquiries@csiro.au  **Web:** www.csiro.au

- Thanks to Helen Cleugh, John Finnigan, Dale Hughes, Heather Keith,
- Miko Kirschbaum, Mark Kitchen, Tanja Suni and Steve Zegelin
- who all contributed to this work.