Differences in plant cover and species composition of semiarid grassland communities of central Mexico and its effects on net ecosystem exchange

"Changes in land use across the semiarid grasslands of northern Mexico have driven a decline of plant cover and alteration of plant species composition. A number of different plant communities have resulted from these changes. Their implications, however, on the carbon (C) cycle and regional carbon balance are still poorly understood. Here, we examined the effects of plant cover loss and changes in species composition on net ecosystem CO2 exchange (NEE) and their biotic and abiotic controls. NEE was measured in five representative plant community types within a semiarid grassland by temporarily enclosing the entire aboveground ecosystem using a chamber method (i.e., geodesic dome). Sites included an oat crop (crop), a moderately grazed grassland (moderate grazing), a 28 yr-old grazing exclosure (exclosure), an overgrazed site with low perennial grass cover (overgrazed), and an overgrazed site presenting shrub encroachment (shrub encroachment). For natural vegetation, rates of standardized daytime NEE for sites with a high plant cover (exclosure and moderate grazing) were similar (P > 0.05) as compared to sites with low plant cover (overgrazed and shrub encroachment). However, yearly total nighttime NEE (carbon loss) was more than double (P < 0.05) for sites with high plant cover compared to sites with low cover, resulting to slight C sinks for the low plant cover sites, and neutral or sources for the high plant cover sites as accounted by daytime and nighttime NEE annual balance. Differences in plant cover and its associated biomass defined the sensitivity to environmental controls. Thus, daytime NEE in low plant cover sites reached light compensation points at lower photosynthetic photon flux density than those from high plant cover sites. Differences in species composition did not influence NEE rates even though there were transient or permanent changes in C3 vs. C4 functional groups. Our results allowed the detection of the large variability and contribution of different plant communities to regional C balance in patchy landscapes. Identification of the role of landscape patches in the regional C balance as either sinks or sources may provide tools allowing land use management strategies that could favor C uptake in patchy landscapes.

Net ecosystem exchange of carbon and water vapor among contrasting land-uses types in the semiarid short-grass steppe in Central México.

Tesis (Maestría en Ciencias Ambientales)"Ecosistemas áridos como los pastizales cubren el 47% de la superficie terrestre y almacenan el 15% del carbono (C) global del suelo. El cambio de uso de suelo (LUC) es la principal actividad humana que controla el intercambio de C, agua y energía entre el continuo suelo-atmósfera. Se requiere incrementar nuestro conocimiento sobre el impacto del LUC sobre el intercambio y almacenamiento de C por los ecosistemas para desarrollar herramientas de predicción y estrategias de mitigación del cambio ambiental global. En este estudio se determinó el intercambio neto de CO2 (NEE) y vapor de agua (ET) a escala diurna y estacional en cinco sitios de pastizal mediano abierto en los Llanos de Ojuelos, Jalisco: 1) una exclusión de ganado de 28 años (Exclusión), 2) un sitio con dominancia de especies de gramíneas generalmente subordinadas (Cambio de especies), 3) un sitio sobrepastoreado (sobrepastoreo), 4) un sitio sobrepastoreado invadido por arbustivas perennes y herbáceas exóticas (Invasión de arbustos), y 5) un sitio con agricultura de temporal (Agricultura). Se midió el índice de área foliar (LAI), la temperatura del aire (T), la densidad de flujo fotónico fotosintético (PPFD) y contenido volumétrico de agua en el suelo (SWC) como controles bióticos y abióticos de NEE y ET. Se midió el NEE y ET durante ocho fechas de octubre de 2008 a julio de 2009 usando el método del domo geodésico. Se establecieron 6 parcelas por sitio, sobre las que se hicieron 4 mediciones al día (08:00, 12:00, 16:00 y 20:00 h). Se usó un análisis de medidas repetidas (ANOVA, α=0.05) para comparar los flujos entre sitios. Se realizaron análisis de regresión lineares y no lineares entre los flujos y sus controles. No fue observado un patrón claro del NEE a través del gradiente de disturbio: el sitio Cambio de especies mostró mayores tasas de NEE diurnas y nocturnas con un balance positivo de captura de CO2 de 0.781 μmol m-2 s-1; el sitio sobrepastoreo mostró bajas tasas de intercambio, pero aún con captura de CO2 (0.236 μmol m-2 s-1); en contraste, la Exclusión mostró pérdida de CO2 (0.719 μmol m-2 s-1). Los sitios Cambio de especies y Exclusión mostraron las más altas tasas de ET (1.353 y 1.221 mmol m-2 s-1, respectivamente).""Dryland ecosystems such as grasslands cover up to 47% of terrestrial land and store around 15% of the global soil carbon. Land-use change (LUC) is the main human activity that controls the carbon, water and energy fluxes in the soilatmosphere continuum. International research agendas request to increase our understanding on the impact of LUC on the capacity of ecosystems to capture and store carbon to improve forecasting tools and mitigation strategies. In the region “Llanos de Ojuelos”, Jalisco, this study measured at diurnal and seasonal scales, net ecosystem exchange (NEE) and evapotranspiration (ET) of the short-grass steppe. Five different sites were included; 1) a 28 year exclosure (Exclosure), 2) a site dominated by commonly subordinated grasses (Species-shift), 3) an overgrazed site (Overgrazed), 4) an overgrazed site with shrub encroachment and exotic species invasion (Shrub encroachment) and 5) a site converted to rainfed oat crop (Oat crop). Biotic (leaf area index, LAI) and abiotic drivers (air temperature T, photosynthetic photon flux density PPFD, and volumetric soil water content SWC) were measured. NEE and ET were recorded on eight dates from October 2008 to July 2009 using the geodesic dome method. Six plots per site were established and gas exchange was measured on four times during the day (08:00, 12:00, 16:00 and 20:00 h). A repeated measures ANOVA (α=0.05) was performed to compare NEE and ET among sites. Linear and non-linear regression analyses between fluxes and drivers were performed. Results showed not clear trends for NEE following the gradient of disturbance; still, species-shift exhibited the highest diurnal and nocturnal NEE rates with a positive CO2 balance of 0.781 μmol m-2 s-2. The overgrazed site showed significantly lower rates but still a positive CO2 balance (0.236 μmol m-2 s-1). In contrast, the Exclosure site showed in this period a net CO2 efflux of 0.719 μmol m-2 s-1. Regarding ET rates, the Species shift and the Exclosure sites observed the largest rates (1.353 and 1.221 mmol m-2 s-1, respectively) that was attributable to a significant larger leaf area index (LAI). Diel time patterns of NEE and ET were driven by PPFD (R2=0.95 y 0.93, respectively) during the wet months (October, June and July); while in the dry and warm months (March-May) the main driver was air temperature.

Variability of carbon fluxes at different time-scales and their biotic and environmental controls on a short-grass steppe in Central Mexico

Tesis (Doctorado en Ciencias Ambientales)"Los ecosistemas terrestres controlan la dinámica del carbono (C) atmosférico sobre la tierra, removiendo hasta el 25% de las emisiones de C antropogénicas. Sin embargo, la alta variabilidad estacional e interanual de los flujos de C genera gran incertidumbre acerca de su capacidad de captura de C. El entendimiento de los factores que afectan las tasas de captura y liberación de C nos ayudará a mejorar las predicciones de los efectos del cambio climático sobre los procesos ecosistémicos, así como sus efectos de retroalimentación. El objetivo de este trabajo fue determinar los controles bióticos y ambientales sobre el intercambio neto de C a nivel ecosistema (NEE) y sus componentes para entender cuáles condiciones ambientales favorecen la captura de C en el pastizal semiárido, con énfasis en los efectos de “legado” de la precipitación (PPT) y los pulsos de C generados después de los eventos de PPT. Cuatro años continuos de mediciones del NEE con un sistema de covarianza de vórtices (EC) fueron analizados. Los flujos de C mostraron ciclos diarios y estacionales típicos con la temperatura del aire y la densidad de flujo fotónico fotosintético como controles diarios y la humedad del suelo y la dinámica de la vegetación (medida como EVI) controlando el NEE a escala estacional. El pastizal semiárido fue una fuente (16.37 and 93.83 g C m-2 a-1) y un sumidero de C ((-15.85 and -121.02g C m-2 a-1). Se identificaron efecto de legado de la PPT a escala estacional. La PPT invernal afectó positivamente la captura de C durante el verano; mientras que la PPT del verano no tuvo efectos sobre la productividad invernal. Cien mm de PPT invernal y 200 durante el verano fueron necesarios para convertir al pastizal en un sumidero neto de C. Por otra parte, la respiración del ecosistema (ER) respondió pocas horas después del evento de PPT, mientras que pasaron hasta 5 días para observar una respuesta de la productividad del ecosistema (GEE). Eventos de PPT tan bajos como 0.25mm incrementaron ER, pero PPT acumulada mayor a 40mm estimularon GEE. En general, los pulsos de C estuvieron relacionados con la magnitud del estímulo (ej. tamaño del evento de PPT) y las condiciones previas del ecosistema (ej. contenido de humedad del suelo previo al evento de PPT). Los resultados de este estudio demostraron la importancia de la PPT invernal y de los pulsos de C después de los eventos de PPT al balance de C anual en un pastizal semiárido con lluvias durante el verano.""Terrestrial ecosystems control the atmospheric carbon (C) dynamics on Earth, removing~25% of C emissions derived from anthropogenic activities. However, high seasonal and interannual variability generates large uncertainties about their C uptake capacity. Understanding environmental and biotic factors affecting C uptake and C release rates will help us to improve predictions of climate change impacts on ecosystem processes as well as its feedback effects. The aim of this study was to determine biotic and environmental drivers of the net ecosystem C exchange (NEE) and its components to understand which environmental conditions favor uptake or release of C in the semiarid grassland, with emphasis on precipitation (PPT) legacy effects and short term C fluxes following PPT events. Four years of continuous NEE measurements with an eddy covariance (EC) system were analyzed. Carbon fluxes showed typical diurnal and seasonal cycles with air temperature and photosynthetic photon flux density (PPFD) as the main diurnal drivers, whereas soil moisture and vegetation dynamics (the enhanced vegetation index, EVI) controlled NEE at seasonal scale. The semiarid grassland behaved as a source (16.37 and 93.83 g C m-2 y-1), and a sink of C (-15.85 and - 121.02g C m-2 y-1). Precipitation legacy effects on NEE and its components were identified at seasonal scale. Winter precipitation positively affected C uptake of summer, however, summer precipitation did not have effects on winter C fluxes. One hundred mm of winter PPT and 200 mm of summer PPT were needed for turning the ecosystem into a C sink. On the other hand, Ecosystem respiration (ER) responded within few hours following a PPT event whereas it took five days for gross ecosystem exchange (GEE) to respond. Precipitation events as low as 0.25 mm increased ER, but cumulative PPT > 40 mm that infiltrated deeper into the soil profile stimulated GEE. Overall, ER fluxes following PPT events were related with the size of the stimulus (e.g. PPT event size) and previous soil conditions (e.g. previous soil volumetric water content and inter PPT event period).

Landscape Controls on Water‐Energy‐Carbon Fluxes Across Different Ecosystems During the North American Monsoon

The dependence of arid and semiarid ecosystems on seasonal rainfall is not well understood when sites have access to groundwater. Gradients in terrain conditions in northwest México can help explore this dependence as different ecosystems experience rainfall during the North American monsoon (NAM), but can have variations in groundwater access as well as in soil and microclimatic conditions that depend on elevation. In this study, we analyze water-energy-carbon fluxes from eddy covariance (EC) systems deployed at three sites: a subtropical scrubland, a riparian mesquite woodland, and a mountain oak savanna to identify the relative roles of soil and microclimatic conditions and groundwater access. We place datasets during the NAM season of 2017 into a wider context using previous EC measurements, nearby rainfall data, and remotely-sensed products. We then characterize differences in soil, vegetation, and meteorological variables; latent and sensible heat fluxes; and carbon budget components. We find that lower elevation ecosystems exhibited an intense and short greening period leading to a net carbon release, while the high elevation ecosystem showed an extensive water use strategy with delayed greening of longer duration leading to net carbon uptake during the NAM. Access to groundwater appears to reduce the dependence of deep-rooted riparian trees at low elevation and mountain trees on seasonal rainfall, allowing for a lower water use efficiency as compared to subtropical scrublands sustained by water in shallow soils. Thus, a transition from intensive to extensive water use strategies can be expected where there is reliable access to groundwater

Landscape Controls on Water‐Energy‐Carbon Fluxes Across Different Ecosystems During the North American Monsoon

The dependence of arid and semiarid ecosystems on seasonal rainfall is not well understood when sites have access to groundwater. Gradients in terrain conditions in northwest México can help explore this dependence as different ecosystems experience rainfall during the North American monsoon (NAM), but can have variations in groundwater access as well as in soil and microclimatic conditions that depend on elevation. In this study, we analyze water-energy-carbon fluxes from eddy covariance (EC) systems deployed at three sites: a subtropical scrubland, a riparian mesquite woodland, and a mountain oak savanna to identify the relative roles of soil and microclimatic conditions and groundwater access. We place datasets during the NAM season of 2017 into a wider context using previous EC measurements, nearby rainfall data, and remotely-sensed products. We then characterize differences in soil, vegetation, and meteorological variables; latent and sensible heat fluxes; and carbon budget components. We find that lower elevation ecosystems exhibited an intense and short greening period leading to a net carbon release, while the high elevation ecosystem showed an extensive water use strategy with delayed greening of longer duration leading to net carbon uptake during the NAM. Access to groundwater appears to reduce the dependence of deep-rooted riparian trees at low elevation and mountain trees on seasonal rainfall, allowing for a lower water use efficiency as compared to subtropical scrublands sustained by water in shallow soils. Thus, a transition from intensive to extensive water use strategies can be expected where there is reliable access to groundwater

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

"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.""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.

Base de datos de flujos verticales de dioxido de carbono en ecosistemas terrestres y costeros en México

El dióxido de carbono (CO2) es uno de los principales gases de efecto invernadero (GEI) cuyo incremento en la atmósfera está asociado con el calentamiento global. Con el objetivo de promover estudios de síntesis que lleven a un mejor entendimiento de los procesos relacionados con el ciclo del carbono en ecosistemas terrestres y costeros de México, se construyeron bases de datos de flujos verticales de carbono. Se construyó una base de datos con flujos de CO2 a escala anual, para ocho sitios y 30 años por sitio, de la red MexFlux, cuya información se obtuvo de publicaciones en revistas científicas, memorias de resúmenes en extenso y documentos de tesis. Una segunda base se construyó a partir de datos a escala diaria, de los flujos de CO2 de 14 sitios de monitoreo y 53 años/ sitio, que fueron proporcionados directamente por los investigadores principales (PI) de cada sitio y denominada MexFlux_2019 V1. Esta última base de datos, a diferencia de la primera que es de libre acceso, está restringida. Las bases de datos incluyen información del intercambio neto de carbono a nivel ecosistema, la productividad primaria bruta, respiración del ecosistema y de variables meteorológicas y ambientales complementarias