Measurements of CO2 Fluxes at Non-Ideal Eddy Covariance Sites

Summary

The presented protocol uses the eddy covariance method at non-typical locations, applicable to all types of short-canopy ecosystems with limited area, on a currently reforested windthrow site in Poland. Details of measuring site setup rules, flux calculations and quality control, and final result analysis, are described.

Abstract

This protocol is an example of utilizing the eddy covariance (EC) technique to investigate spatially and temporally averaged net CO₂ fluxes (net ecosystem production, NEP), in non-typical ecosystems, on a currently reforested windthrow area in Poland. After a tornado event, a relatively narrow “corridor” was created within surviving forest stands, which complicates such kind of experiments. The application of other measuring techniques, such as the chamber method, is even more difficult under these circumstances, because especially at the beginning, fallen trees and in general great heterogeneity of the site provide a challenging platform to perform flux measurements and then to properly upscale obtained results. In comparison with standard EC measurements carried out in untouched forests, the case of windthrow areas requires special consideration when it comes to the site location and data analysis in order to ensure their representativeness. Therefore, here we present a protocol of real-time, continuous CO₂ flux measurements at a dynamically changing, non-ideal EC site, which includes (1) site location and instrumentation setup, (2) flux computation, (3) rigorous data filtering and quality control, and (4) gap filling and net fluxes partitioning into CO₂ respiration and absorption. The main advantage of the described methodology is that it provides a detailed description of the experimental setup and measurement performance from scratch, which can be applied to other spatially limited ecosystems. It can also be viewed as a list of recommendations on how to deal with unconventional site operation, providing a description for non-specialists. Obtained quality-checked, gap filled, half-hour values of net CO₂, as well as absorption and respiration fluxes, can be finally aggregated into daily, monthly, seasonal or annual totals.

Introduction

Nowadays, the most commonly used technique in the atmosphere-land ecosystem carbon dioxide (CO₂) exchange studies is the eddy covariance (EC) technique¹. The EC method has been used for decades, and comprehensive descriptions of issues concerning all the methodological, technical and practical aspects have already been published^2,3,4. Compared with other techniques used for similar purposes, the EC method allows for obtaining the spatially and temporally averaged net CO₂ fluxes from automatic, point measurements that consider the ....

Protocol

1. Site location and instrumentation setup

1. Choose a measuring site location in relatively homogeneous and flat terrain to meet basic requirements of the EC method. Avoid places with complicated landforms (depressions, slopes) or located near aerodynamic obstacles (e.g., surviving tree stands), which can distort the air flow.
   1. Check species composition and plant cover. Choose a place with the most similar characteristics: age and height of the main vegetation type.
   2. If possible, conduct so.......

Discussion

This protocol presents the eddy covariance (EC) method to be used at non-ideal sites (here a reforested windthrow site): site location and measuring infrastructure setup, net CO₂ fluxes computation and post-processing, as well as some issues regarding gap filling and fluxes partitioning procedures.

Even though the EC technique is commonly used at many measuring sites around the world, most of them are non-disturbed ecosystems, where the design and the following data processing can b.......

Materials

Name	Company	Catalog Number	Comments
Adjustable mast with metal rails and electric engine (24 V)	maszty.net	-	Alternative basic construction. To be designed and made by professionals
EddyPro	LI-COR, Inc.	ver. 6.2.0.	Free commercial software for fluxes calculation. Available on a website: https://www.licor.com/env/products/eddy_covariance/software.html, on request
Enclosed-path infrared gas analyzer	LI-COR, Inc.	LI-7200	One of two instruments of the eddy covariance system (EC) used for CO2 fluxes measurements. Other types of fast analyzers (>10Hz sampling frequency) can be used
REddyProc	-	-	Free software for EC fluxes gap filling and partitioning. Available on Max Planck Institute for Biogeochmistry: https://www.bgc-jena.mpg.de/bgi/index.php/Services/REddyProcWeb. Both online tool and R package are provided.
Short aluminum tower base with concrete foundation	maszty.net	-	Alternative basic construction (pioneering solution). To be designed and made by professionals
Sonic anemometer	Gill Instruments	Gill Windmaster	One of two instruments of the eddy covariance system (EC) used for wind speed measurements. Other types of three-dimensional sonic anemometers can be used
Stainless-steel tripod	Campbel Scientific, Inc.	CM110 10 ft	The basic construction for eddy covariance (EC) system. Can be constructed by yourself- materials to be found in a hardware store
Sunshine sensor	Delta-T Devices Ltd.	BF5	One of the exemplary instruments for photosynthetic photon flux density measurements (PPFD). To be bought from several commercial companies. Remember to place it above the canopy, far from reflective surfaces.
Thermistors	Campbel Scientific, Inc.	T107	One of the exemplary instruments for soil temperature measurements. To be bought from several commercial companies. It is advisable to have a profile of soil temperature
Thermohygrometer	Vaisala Oyj	HMP155	One of the exemplary instruments for air temperature and humidity measurements. To be bought from several commercial companies. Remember to place it inside radiation shield at similar height as the EC system.