Progress and opportunities for monitoring greenhouse gases fluxes in Mexican ecosystems: the MexFlux network

R. VARGAS,^1,2 E. A. YÉPEZ,³ J. L. ANDRADE,⁴ G. ÁNGELES,⁵ T. ARREDONDO,⁶ A. E. CASTELLANOS,⁷ J. DELGADO-BALBUENA,⁶ J. GARATUZA-PAYÁN,³ E. GONZÁLEZ DEL CASTILLO,⁸ W. OECHEL,⁹ J. C. RODRÍGUEZ,⁷ A. SÁNCHEZ-AZOFEIFA,¹⁰ E. VELASCO,¹¹ E. R. VIVONI¹² and C. WATTS⁷

¹Centro de Investigación Científica y de Educación Superior de Ensenada, Carretera Ensenada-Tijuana 3918, Zona Playitas, 22860 Ensenada, Baja California, México. Corresponding author: R. Vargas; e-mail: rvargas@udel.edu

²Department of Plant and Soil Sciences, Delaware Environmental Institute, University of Delaware, 531 South College Ave., Newark, DE 19716, USA

³Instituto Tecnológico de Sonora, 5 de Febrero Sur 818, Centro, 85000 Ciudad Obregón, México

⁴Centro de Investigación Científica de Yucatán, Calle 43 núm. 130, Col. Chuburná de Hidalgo, 97200 Mérida, Yucatán, México

⁶Instituto Potosino de Investigación Científica y Tecnológica, Camino a la Presa de San José 2055, Lomas 4a. Sección, 78216 San Luis Potosí, México

⁷ Universidad de Sonora, Blvd. Luis Encinas y Rosales s/n, Col. Centro, 83000 Hermosillo, Sonora, México

⁸University of California-Davis, 1 Shields Ave, Davis, CA 95616, USA

⁹San Diego State University, Campanile Dr. San Diego, CA 92182, USA

¹⁰Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, T6G 2E3 Alberta, Canada

''CENSAM, Singapore-MIT Alliance for Research and Technology, 1 CREATE Way #09-03 CREATE Tower, Singapore 138602

¹²School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA

Received March 15, 2012; accepted September 4, 2012

RESUMEN

Para entender los procesos de los ecosistemas desde un punto de vista funcional es fundamental entender las relaciones entre la variabilidad climática, los ciclos biogeoquímicos y las interacciones superficie-atmósfera. En las últimas décadas se ha aplicado de manera creciente el método de covarianza de flujos turbulentos (EC, por sus siglas en inglés) en ecosistemas terrestres, marinos y urbanos para medir los flujos de gases de invernadero (p. ej., CO₂, H₂O ) y energía (p. ej., calor sensible y latente). En diversas regiones se han establecido redes de sistemas EC que han aportado información científica para el diseño de políticas ambientales y de adaptación. En este contexto, el presente trabajo delimita el marco conceptual y técnico para el establecimiento de una red regional de medición de flujos de gases de efecto invernadero en México, denominada MexFlux, cuyo objetivo principal es mejorar nuestra comprensión de la forma en que la variabilidad climática y la transformación ambiental influye en la dinámica de los ecosistemas mexicanos ante los factores de cambio ambiental global. En este documento se analiza primero la importancia del intercambio de CO₂ y vapor de agua entre los ecosistemas terrestres y la atmósfera. Después se describe brevemente la técnica de covarianza de flujos turbulentos para la medición de éstos, y se presentan ejemplos de mediciones en dos ecosistemas terrestres y uno urbano en México. Por último, se describen las bases conceptuales y operativas a corto, mediano y largo plazo para la continuidad de la red MexFlux.

ABSTRACT

Understanding ecosystem processes from a functional point of view is essential to study relationships among climate variability, biogeochemical cycles, and surface-atmosphere interactions. Increasingly during the last decades, the eddy covariance (EC) method has been applied in terrestrial, marine and urban ecosystems to quantify fluxes of greenhouse gases (e.g., CO₂, H₂O) and energy (e.g., sensible and latent heat). Networks of EC systems have been established in different regions and have provided scientific information that has been used for designing environmental and adaptation policies. In this context, this article outlines the conceptual and technical framework for the establishment of an EC regional network (i.e., MexFlux) to measure the surface-atmosphere exchange of heat and greenhouse gases in Mexico. The goal of the network is to improve our understanding of how climate variability and environmental change influence the dynamics of Mexican ecosystems. First, we discuss the relevance of CO₂ and water vapor exchange between terrestrial ecosystems and the atmosphere. Second, we briefly describe the EC basis and present examples of measurements in terrestrial and urban ecosystems of Mexico. Finally, we describe the conceptual and operational goals at short-, medium-, and long-term scales for continuity of the MexFlux network.

Keywords: Environmental networks, eddy covariance, FLUXNET, greenhouse gases, long-term measurements, surface-air exchange.

1. Introduction

Humankind faces new challenges to develop policies for the reduction, adaptation and mitigation of global environmental change. The scientific community has the responsibility of providing information to enable the development of such policies and strategies. This includes the generation of knowledge about the components, processes and mechanisms by which ecosystems respond to: (1) climate variability, and (2) the interaction and effects of greenhouse gases (e.g., CO₂, CH₄, N₂O) on global climate.

Through the processes of photosynthesis and respiration, ecosystems play a key role in the capture and emission of CO₂ (Fig. 1). Furthermore, the characteristics of vegetation cover also affect water vapor fluxes into the atmosphere through evapotranspiration (Fig. 1), and therefore the balance between sensible and latent heat fluxes that impact the atmosphere (Fisher et al., 2011). Additionally, the type and extent of vegetation defines the physical properties, such as surface albedo, emissivity and aerodynamic roughness that can affect air temperature, precipitation, and wind speed (Burba and Verma, 2005). In turn, climate is the main factor determining the presence and distribution of ecosystems around the world, and establishes complex feedbacks between the biosphere and global biogeochemical cycles (Bonan, 2008; Heimann and Reichstein, 2008).

Current knowledge on the interactions between climate and carbon and water cycles is still limited. This has been identified by the Intergovernmental Panel on Climate Change (IPCC) as a "key uncertainty" in our understanding of present and future climate (IPCC, 2007). The experimental evaluation of the interaction between weather and these cycles has made significant progress with the development of new methodologies to measure the mass (e.g., water vapor, CO₂) and energy exchange (e.g., sensible heat, solar radiation) at multiple spatial and temporal scales (Canadell et al., 2000). This development has required a multidisciplinary link between earth and atmospheric sciences, functional ecology, biogeochemistry, and mathematics, which has improved the application of model-data fusion (Vargas et al., 2011a). This scientific development, known as the third scientific paradigm, is complemented by the integration of knowledge from computer systems science, which is emerging as the fourth paradigm in scientific research (Hey et al., 2009).

During the last decades, several networks for monitoring ecosystem processes and attributes have been established (e.g., International Long Term Ecological Research Network, [ILTER], and Human and Biophysical Dimensions of Tropical Dry Forests in the Americas [Tropi-Dry], among others). These networks have standardized experimental protocols, which have allowed collecting a large amount of information and created regional synthesis reports (Greenland et al., 2003). Another example is FLUXNET—the global network for flux measurements of water vapor, CO₂, and energy in terrestrial ecosystems (Baldocchi et al., 2001). This network incorporates more than 500 study sites around the world, which have been used for the analysis of ecosystem primary productivity, evapotranspiration, and the response of ecosystems to disturbances (Baldocchi, 2008). FLUXNET is considered a network of networks that consists of several regional networks (e.g., AmeriFlux in the United States of America, CarboEurope in Europe). Despite its broad coverage, FLUXNET sites are mainly located in temperate ecosystems, with annual temperatures between 5 and 17 °C, annual rainfall between 600 and 1250 mm, and latitudes above 30 °N.

Similarly, in recent years new sites for monitoring CO₂ and energy fluxes to the atmosphere have been established at various urban locations (i.e., Urban Flux Network). The number of sites in this network is still much lower than those for FLUXNET with ~30 sites of which only 20 are active and only two correspond to sub-tropical cities. Importantly, more than 70% of CO₂ emissions of anthropogenic origin occurs in cities, despite occupying only 2% of the total land area (Le Quéré et al, 2009).

The time is ripe for the establishment of a Mexican eddy covariance (EC) network for several reasons. First, long-term impact of land use change and anthropogenic changes in biogeography have fragmented and fundamentally transformed Mexican landscapes. These transformations have created a highly heterogeneous vegetation cover forming complex landscape mosaics that differ in plant cover, age, plant species, and functional group composition over a wide range of spatial scales (Mittermeier and Goettsch, 1997). Second, arid/ semiarid and tropical ecosystems are underrepresented in FLUXNET (Baldocchi, 2008). This is of particular relevance for Mexico because over 60% of its territory is represented by arid and semiarid ecosystems located below the 30° N latitude (Challenger and Soberón, 2008). Furthermore, tropical ecosystems located in Mexico are highly productive and subject to natural disturbances (e.g., hurricanes) that substantially influence ecosystem fluxes magnitudes and variances (Vargas, 2012). Third, Mexico is experiencing large human migration from rural areas to urban centers, which will influence the long-term dynamics of natural and urban ecosystems. Thus, the establishment of new flux measurement sites in Mexico will broaden the representativeness of worldwide flux networks, and will increase our knowledge of ecosystem dynamics under seasonal warm-dry and warm-wet conditions.

The objective of this work is to outline a conceptual framework for the establishment of a regional network for monitoring greenhouse gases fluxes in Mexico: the MexFlux network. The principal goal of this network is to generate scientific knowledge to understand the dynamics and role of Mexican ecosystems within the context of global environmental change (Tables I and II). This network will provide the scientific basis for public policy to reduce emissions, adaptation/mitigation to environmental change and early detection of environmental hazards (e.g., loss of habitat/biodiversity, extreme weather events). This manuscript addresses the following issues: (1) description of CO₂ and water vapor fluxes from terrestrial ecosystems, (2) the EC technique for measuring these fluxes, and (3) the conceptual and operational goals at short-, medium- and long-term scales for permanent operation of the MexFlux network.

2. Main CO₂ and water vapor fluxes in terrestrial ecosystems

The main carbon fluxes consist essentially of gains and losses of CO₂ in ecosystems (Chapin et al., 2002) (Fig. 1). The total amount of carbon incorporated into the ecosystem through photosynthesis is known as gross primary production (GPP), and it is estimated that the global GPP average is about 123 PgCyr^-1 (Beer et al., 2010). A proportion of CO₂ captured by ecosystems returns to the atmosphere via ecosystem respiration (Reco). The balance between CO₂ captured via GPP and loss through Reco is the net ecosystem exchange (NEE), which ultimately determines if the ecosystem is a net carbon source or sink (Fig. 1). The NEE represents the total CO₂ exchanged vertically between land-surface and the atmosphere, and can be directly measured using the micrometeorological technique of EC (Aubinet et al., 2000; Baldocchi, 2003).

Evapotranspiration (ET) is the sum of water transferred from the Earth's surface to the atmosphere through evaporation (Ev) and plant transpiration (Tr, Fig. 1) (Fisher et al., 2011). Evaporation comprises the movement of water into the atmosphere from soil, plant surfaces and water bodies. The transport of water from the surface to the atmosphere through evaporation and transpiration constitutes up to 75% of the total energy transfer on the planet (Jung et al., 2010). These processes are closely related to GPP, since the opening of stomata during photosynthesis is inevitably accompanied by water loss through transpiration, while the residence time of water in the soil largely controls the rate of decomposition of organic matter and consequently RS (Yepez and Williams, 2009). Therefore, it is critical to understand the coupling between the biophysical processes that regulate the magnitude of CO₂ and water vapor fluxes in time and space across ecosystems.

3. Eddy covariance (EC) technique

The energy and mass exchange between the biosphere and atmosphere can be studied through different methods depending on the characteristics of the ecosystem and the spatial and temporal scales of interest (Canadell et al., 2000). At the scale of experimental plots (i.e., square meters), gas exchange chambers provide information on microbial and plant ecophysiological processes that control the exchange of water vapor and CO₂ (Vargas et al., 2011a). However, the spatial sampling is limited, making it difficult to extrapolate results to larger spatial scales, and undesirable effects of confinement and manipulation may also be inherent to the methodology (Pumpanen et al., 2004). For scales between 100 to 1000 m² (which are most representative of an ecosystem) micrometeorological techniques are a useful approach to monitor ecosystem exchange processes of water vapor, CO₂, other biogenic gases, and aerosols. Among micrometeorological techniques, the EC has been the most widely used for estimating NEE by direct measurement of vertical fluxes of mass and energy across a horizontal plane above the canopy (Aubinet et al., 2000; Baldocchi, 2003). This technique is in situ and non-destructive, and can be applied to time scales ranging from fractions of an hour to years; it is therefore ideal to capture the flux dynamics across different weather conditions from diurnal cycles to long-term environmental changes (Baldocchi, 2003). The technique is also suitable for understanding the response of ecosystems to disturbance, management history and comparative studies between ecosystem functional types, or for different successional stages (Baldocchi, 2008). However, its implementation is complex because the study site should be in a relatively flat terrain and be homogeneous in terms of vegetation and soil. The installation and operation of instruments, as well as processing and data analysis require special attention for correct results representing the ecosystem (Papale et al., 2006).

Briefly, turbulent exchanges of momentum, heat, water vapor, CO₂, and in general any scalar, can be mathematically defined as the covariance between the instantaneous deviations or fluctuations of the scalar in question (e.g., temperature, concentration of water vapor or CO₂ mixing ratio) and vertical wind velocity over a specific time interval (e.g., 30 min).

For example, the vertical turbulent flux of CO₂ is defined as the covariance between the fluctuations of vertical wind speed and CO₂ mass density (w' c') multiplied by the average air density (Baldocchi, 2003). If the net flux is positive (i.e., towards the atmosphere) the ecosystem is a net source, but on the contrary if the flux is negative, then the ecosystem functions as a sink.

To monitor fluxes of greenhouse gases (GHGs) and energy using this technique, the installation of a micrometeorology tower with particular equipment located at a suitable height is required, to ensure the representativeness needed for the exchange surface in all directions. The height and position of the tower in turn depend on the height of the vegetation as well as the predominant wind direction and the average wind speed. A basic EC system to measure fluxes of CO₂ consists of a three-dimensional sonic anemometer and a CO₂ analytical sensor. Fast-response CO₂ analyzers are based on the principle of absorption of infrared radiation by trace gases, and often measure CO₂ and water vapor at the same time. Also an EC system includes basic equipment to measure hydrometeorological variables (e.g., precipitation, temperature, relative humidity, soil temperature and moisture), and the surface available energy (i.e., the net radiation, and ground heat flux). Several publications contain detailed descriptions of the EC method in terrestrial ecosystems (Goulden et al., 1996; Aubinet et al., 2000; Baldocchi, 2003) and cities (Velasco and Roth, 2010).

4. The conceptual framework and operational MexFlux

MexFlux is the network of flux measurements of GHGs and energy fluxes from terrestrial, urban, coastal and marine ecosystems in Mexico. Its main objective is to generate scientific knowledge about the role of the dynamics of Mexican ecosystems to global environmental change (Tables I and II). Mexico currently has 11 study sites using the EC technique and has over 30 site-years of data of water vapor and CO₂ fluxes (Vargas et al., 2012a). A number of sites are of recent installation (i.e., established in 2011), while others have been operating between six and nine years (Table III). The sites with the longest continuous measurements are located in semiarid ecosystems of Baja California Sur (established in 2002) and Sonora (established in 2004). With regard to distribution, the sites are located at: semiarid shru-bland in Baja California; oak woodland, subtropical, tropical dry forest and grassland in Sonora; tropical dry forest in Jalisco and Yucatán; shortgrass steppe in Jalisco; a managed pine forest in Hidalgo; and an urban residential/commercial site in Mexico City. Finally, a site was operated between 2008 and 2009 off the coast of Baja California measuring NEE over the ocean (Reimer et al., in review).

MexFlux brings together scientists interested in using the EC technique in ecosystems across Mexico. Up to two principal investigators represent each study site within MexFlux. These researchers are required to verify that the site follows the guidelines for measurement standardization to maintain proper functioning of the network and allow sites inter-comparisons. MexFlux members are working together towards standardization of data quality and a secure data management/analysis protocol. The network is intended to grow in coming years, as new sites will be established within strategic ecosystems to ensure the long-term operation of the network. The new sites, in addition to consistency with the assumptions for EC technique, should ideally be located at sites that represent the variability of Mexican ecosystems and/or at ecosystems and regions most vulnerable to environmental or land use change.

5. Water vapor and CO₂ fluxes in terrestrial ecosystems of Mexico

Mexican ecosystems provide rich opportunities to validate previous observations across ecosystems. Some examples of relationships that could be tested and validated are those observed between phenology and CO₂ fluxes (Richardson et al., 2010), control of GPP on RS (Vargas et al., 2011b), or the role of drought and water pulses (Thomey et al., 2011). What follows are examples from measurements at MexFlux sites to illustrate the diversity of patterns and magnitudes of CO₂ fluxes.

For example, the Ojuelos site located in a semi-arid grassland (Table III) is ideal for investigating the effect of precipitation pulses on NEE during the summer that result in higher CO₂ emissions. Data from this site show that before precipitation events, nocturnal NEE values (Reco by definition) were < 2 μmol CO₂ m² s^-1 while after precipitation, pulses increased to values > 5 umol CO₂ m² s^-1 (Fig. 2). The implications of these observations rely on climate change scenarios characterized by changes in frequency and intensity of precipitation and pulses.

These changes in precipitation patterns can substantially affect carbon sequestration in semiarid rangelands by reducing or increasing water stress within the ecosystem (Thomey et al., 2011, Vargas et al., 2012b).

In urban sites, CO₂ flux measurements have been used to evaluate emission inventories and develop mitigation strategies, while energy fluxes can be useful for urbanization planning through a better understanding of urban climatology (e.g., urban heat island). For example, Figure 4 compares the average diurnal CO₂ flux measured using eddy covariance during a short study over the Escandón district in Mexico City with emission profiles extracted from the local emissions inventory (Velasco et al., 2009, 2011). It is noteworthy that the CO₂ emission sources in urban ecosystems can be much more diverse than in natural ecosystems and their analysis requires a detailed interpretation of the measured fluxes (Fig. 4).

6. MexFlux short- and long-term goals

It is essential to strengthen collaborations among research groups for a successful coordination and operation of MexFlux. The network will use a strategy based on the coordination and combination of micrometeorological techniques, meteorological, remote sensing, biophysical and ecosystems-process modeling to obtain the best possible estimates of the GHG flux dynamics. In order to scale these measurements in time and space, it is essential to: (1) generate a standardized protocol for inter-calibration of measuring instruments and measurement methods; and (2) develop quality assurance and control (QA/ QC) procedures of raw data and post-processing of computed fluxes (i.e., gap-filling series, partition NEE into GPP and RS). This QA/QC must comply with international standards (Papale et al., 2006) to generate useful and consistent datasets for regional and global synthesis studies.

To facilitate synthesis studies and maximize data use, the data storage and standardized post-processing could be provided by existing cyberinfrastructure in different institutions of MexFlux members (e.g., University of Alberta, Canada with its Enviro-Net platform). Initially, a specific MexFlux database will contain the products of site-level measurements (i.e., raw data) along with the processed data obtained following standardized protocols (Papale et al., 2006). MexFlux has adopted a policy of submitting information from each study site to this database within a period of one year from its incorporation to the network. Only principal investigators will submit raw and processed data, and these will be freely available to the network members, under a fair use policy. Finally, MexFlux will promote training of human resources in the area of micrometeorological and biophysical measurements of energy and GHG fluxes.

The short-term challenges facing MexFlux are: (1) developing protocols and standardization among sites for reducing uncertainties in the flux measurements; (2) establishing rules for archiving and processing data within the MexFlux database; (3) establishing clear and fair use data policies for all members according to the funding source for each project (i.e., domestic agencies and international funding); and (4) the study sites must invest in measuring a common set of biological and physical parameters to describe the ecosystem (e.g., vegetation structure, phenology, soil properties, management history) to better interpret observed fluxes (Law et al., 2008).

We also suggest that long-term measurements within MexFlux should incorporate other GHGs, pollutants, aerosols and stable isotopes as technology becomes more accessible and more scientists join the network. This new information could be integrated into models of ecosystem processes to update and validate previous observations and parameters. MexFlux's long-term challenge is to seek the integration of researchers in fields of climate modeling, mathematics, biogeochemistry, and social science related to policy making. Finally, we believe that it is essential to develop multi-institutional training and financing mechanisms for the maintenance and operation of the study sites and secure the continuity of MexFlux. This will only be achieved if there is political will and evidence of the usefulness of value added products from the network for policy-making and scientific advancement.

Acknowledgements

The authors acknowledge funding from Conacyt (EcoRed-Conacyt) for meetings that have facilitated discussions between researchers and students. RV is grateful for funding from Conacyt (Ciencia Básica-152671). TA and CW thanks SEP-Conacyt. ASA and EGC thank the Inter-American Institute for Global Change Research, Tropi-Dry, supported by the National Science Foundation. EGC also acknowledges funding from a Conacyt-UC Mexus scholarship. CW gratefully acknowledges funding from Conacyt Ciencia Básica. EV acknowledges support from INE, MCE2, SMA-GDF and Singapore National Research Foundation trough SMART. GA and JLA acknowledge support from USAID and USFS.

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