We thank to Conacyt for funding the research project CB-2013-01: 221014,

Variations of net ecosystem production and evapotranspiration

across a successional gradient of tropical dry forest in

Northwestern Mexico Rojas-Robles, Nidia Elisa1; Enrico A. Yépez, Miguel A. Rivera1, Claudia Coronel2, José Manuel Madrigal2, Zulia

Sanchez-Mejía1, Jaime Garatuza-Payán1 , Rodrigo Vargas3

Corresponding author:nerojasrobles@gmail.com

1 Departamento de Ciencias del Agua y Medio Ambiente, Instituto Tecnológico de Sonora, Ciudad Obregón, Sonora, México. 2Centro de Investigación en Geografía y Geomatica, Ing. Jorge L. Tamayo, A.C. Ciudad de México, México

3 University of Delaware, 531 South College Avenue. Newark, DE, 19716, USA.

ABSTRACT

STUDY SITE

AKNOWLEGEMENTS

Alvarez –Yepiz et al. (2008), Forest Ecol Manag, 256,355-366. Perez-Ruiz et al. (2010), J Arid Environ, 74(5):556-5. Trejo and Dirzo (2000), Biol Conserv, 94(2000)132-142 Verduzco et al. (2015) , J. Geophys. Res. Biogeosci., 120, doi:10.1002/2015JG003119.

Reichstein, M., et al. (2005), Global Change Biol., 11, 1424–1439. Webb, E. K. and G. I. Pearman, and R. Leuning (1980), Q. J. R. Meteorol. Soc., 106, 85–100. Papale, D. et al. (2006). Biogeosciences, 3(4), 571-583.

CO2 AND WATER FLUXES MEASUREMENTS

REFERENCES

VARIATIONS OF NET ECOSYSTEM EXCHANGE AND ET

Fig 2. Ecosystem flux measurements at all sites were performed using Eddy covariance (EC) technique and complete meteorological arrangements. The high frequency (10Hz) water vapor and CO2 concentrations and air temperature measurements were used with the turbulent wind velocities to obtain latent heat flux (LE) or evapotranspiration (ET), sensible heat (H) and net ecosystem exchange (NEE) generally equal to net ecosystem production (-NEP). • The flux measurements were made using the Eddypro software. Data below a

threshold of a critical friction velocity were removed (table 1). • Data gaps were filled with the Max Planck on line tool Reddyproc (Reichstein et al.

2005)

Tropical dry forest (TDF) is a widely distributed land-cover type in America that has been impacted by human activities like agriculture and livestock. The subsequent abandonment of these areas results in various stages of succession. TDF is a seasonal ecosystem where precipitation occurs mainly in pulses causing wet-dry cycles which directly affect ecosystem processes. In Northwester of México, the TDF is under the influence of the North American monsoon which provides seasonal precipitation to this entire region staring from late Jun to September. However, the response of water and carbon fluxes to the climate variability in areas with a different structure in the vegetation and soil is poorly understood.

Fig 1. The successional gradient of TDF consist in: (a) An early-succession site (ES) 7-year abandonment, (b) A secondary forest (SF) ~ 35 to 40 years of recovery and (c) an old-growth forest (OG). All sites are within in a range of 2 km among each other and the mean annual precipitation is 710 mm and the mean annual temperature 24.3 °C

OBJECTIVE: Compare the response of the net ecosystem production and evapotranspiration to climate variability across of a

successional gradient of TDF

Instruments OG SE ES

IRGA LI-7500A LI-7500 LI-7500A

Anemometer Gill Csat 3 Gill

µ* 0.15 0.11 0.059

Table 1. Instruments to fluxes measurements

The study site is within the natural protected area “Reserva monte mojino”, within the protected area “Area natural protegida de flora y fauna sierra de Alamos-Río Cuchujaquí”

(b) (a) (c)

6.3 km

(SE) (ES) (OG)

Data quality OG SE ES

90% Footprint (m) 73 137 120

Energy Clousure (R2) 0.61 0.86 0.60

NEP= GPP - Reco

Site Tair (°C) Precipitation (mm)

ET (mm)

NEE (g C m-2)

Old-growth forest 29 706.1 527.1 -384.2

Secondary forest 27 717.8 509.9 -243.5

Early-succession 29.6 711.4 641.5 -372.3

HIGHLIGHTS

The onset of the monsoon in all sites started with an increase in NEE, indicating a large increase in respiration, which was followed by a decrease in NEE as vegetation uptake dominated, a pattern that is characteristic of seasonally dry ecosystems. The overall C exchange for the monsoon season of 2016 across sites was different. With the OG site showing the strongest C sink, and the secondary forest the weakest sink.

Fig 3. Time series of NEE (a) and ET (b) of 3 sites from May to November of 2016. With the monsoon onset the NEE of 3 sites show a marked period of respiration-dominance and after that, when the precipitation increased and thus the soil moisture, the NEE turns negative until October, indicating an assimilation dominance period. The 3 sites show the same seasonal pattern but the duration of

dominance periods and the flux magnitude were different at each site. Notably, ET was chronically higher at the early-succession site.

Table 3. Cumulative amounts of carbon and water fluxes, precipitation and the mean air temperature

Fig 5. Mean hourly values of NEE per month at the 3 sites. The months of larger assimilation for ES and SE were August and September for OG. The moths with less assimilation during the day were May and June. But in the case of the OG and the SE sites there were CO2 net gains of CO2 during the day.

(a)

(b)

Fig 4. Cumulative NEE and precipitation at the three successional sites. The carbon fluxes were higher in the OG, followed by the ES and finally the SE.

(ES) (OG) (SE)

ES

Fig 6. Wind rose and footprint at the three sites during the monsoon