Tropical dry forests (TDF) are one of the most widely distributed tropical land‐cover types in Mexico, but their regional‐to‐global contribution to the carbon and water cycles is still highly uncertain. We measured CO2 and water vapor fluxes between the ecosystem and atmosphere using the eddy covariance technique from 2016 to 2018 in an old‐growth TDF in northwestern Mexico. First, we investigated the magnitude and seasonality of evapotranspiration (ET), net ecosystem production (NEP) and its contributing fluxes, gross ecosystem production (GEP), and ecosystem respiration (Reco). Second, we explored the main environmental factors controlling carbon and water fluxes as well as tested if this ecosystem acted as a net carbon source or sink. During the study period, all precipitation entering the ecosystem went back to the atmosphere through ET (738.9 ±58.26 mm y−1). Ecosystem respiration (2203.16 ±244.2 g C m−2 y−1) was consistently larger than GEP (1975.32 ±295.52 g C m−2 y−1), determining an annual NEP (−227.6 ±59.4 g C m−2 y−1) that resulted in net annual carbon losses. This forest maintained its water use efficiency (WUE; GEP/ET) across years (2.53–3.24), but water availability constrained light use and maximum carbon assimilation rates. Our results render relevance to the feedback between soil water content and net radiation as the main environmental variables controlling ecosystem fluxes in this old‐growth TDF.

Soil respiration (RS) is an important component of the C cycle because it contributes significant CO2 emissions to the atmosphere that result from metabolism and respiration of its autotrophic and heterotrophic components. However, the relative importance of different biophysical controls that drive the variability of this flux and their influence along forest succession pathways is still unknown. We incorporate multiyear RS, ecosystem flux and meteorological measurements in old-growth (OG), mid-secondary (MS) and early-secondary (ES) tropical dry forests (TDFs) with the goal of assessing the temporal variation of RS and identifying the biophysical controls at each site by applying structural equation models (SEM). Along forest succession, RS followed the pattern of precipitation events; we identified by the end of the wet season that RS was sustained by a longer period at OG, while in MS and ES, RS decreased according to the soil moisture availability. According to SEM, soil moisture and soil temperature exert an effect on the variability of RS in all sites. However, we found that RS was also controlled by the vapor pressure deficit at MS and gross primary production at OG and ES. Our results suggest that seasonality has a different impact on RS along forest succession in TDFs found in northwestern Mexico and highlights the relevance of considering additional biophysical controls of RS for a better understanding this critical process of the C cycle.

No abstract

Los ecosistemas continuamente intercambian carbono y vapor de agua con la atmósfera a través de procesos ecosistémicos. En general, la producción neta del ecosistema refleja el balance entre los flujos de entrada de carbono por productividad y la liberación por la respiración ecosistémica. Evaluar esta producción a lo largo de los cambios producidos por la sucesión ecológica secundaria representa un desafío muy grande. En este contexto, la técnica de covarianza de vórtices está siendo ampliamente utilizada para cuantificar flujos ecosistémicos de manera continua. En el bosque tropical seco se han establecido varios sitios de monitoreo utilizando esta técnica para conocer la funcionalidad del ecosistema durante la sucesión ecológica. Esta información puede contribuir a la toma de decisiones en términos de conservación y administración de servicios ecosistémicos, así como para alimentar modelos para predecir la respuesta de este ecosistema estacional seco al cambio climático. Los ecosistemas tropicales secos son de gran relevancia para el ciclo del carbono debido a su considerable extensión y productividad. El reto de estudiar estos ecosistemas radica en que su dinámica de producción de carbono está acoplada principalmente a la disponibilidad estacional de agua. De manera que, el objetivo de este trabajo fue actualizar la información de las interacciones biosfera-atmósfera utilizando la técnica de covarianza de vórtices particularmente en bosques tropicales secos, con el fin de resaltar el efecto de la sucesión ecológica en la dinámica del carbono en estos bosques. Asimismo, se muestra y resalta la importancia local y global de este tipo de sistemas de monitoreo funcional de ecosistemas en México.