Environmental Controls on Carbon and Water Fluxes in an Old-Growth Tropical Dry Forest

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. Resumen El bosque tropical seco (BTS) es una de las coberturas terrestres mas ampliamente distribuidas en Mexico, pero su contribucion regional y global a los ciclos del carbono y del agua es aun muy incierta. Medimos los flujos de CO2 y vapor de agua entre el ecosistema y la atmosfera utilizando la tecnica de covarianza de vortices entre 2016 y 2018 en un BTS maduro en el noroeste de Mexico. Primero, investigamos la magnitud y la estacionalidad de la evapotranspiracion (ET), la produccion neta del ecosistema (NEP) y sus flujos contribuyentes, la produccion bruta del ecosistema (GEP) y la respiracion del ecosistema (Reco). En segundo lugar, exploramos los principales factores ambientales que controlan los flujos de carbono y de agua, y probamos si este ecosistema actuo como fuente o sumidero neto de carbono. Durante el periodo de estudio, la precipitacion que recibio el ecosistema regreso a la atmosfera a traves de ET (738.9 58.26 mm y(-)(1)). La respiracion del ecosistema (2203.16 +/- 244.2 g C m(-)(2) y(-)(1)) fue consistentemente mayor que la GEP (1,975.32 +/- 295.52 g C m(-)(2) y(-1)), determinando una NEP anual (-227.6 +/- 59.4 g C m(-)(2) y(-)(1)) que resulto en perdidas netas anuales de carbono. Este bosque mantuvo su eficiencia en el uso del agua (WUE; GEP / ET) a lo largo de los anos (2.53-3.24), pero la disponibilidad de agua restringio el uso de la luz y las tasas maximas de asimilacion de carbono. Nuestros resultados dan relevancia a la retroalimentacion entre el contenido de agua del suelo y la radiacion neta como las principales variables ambientales que controlan los flujos del ecosistema en este bosque tropical seco maduro. Plain Language Summary The contribution of tropical forests to the carbon and water cycles is still uncertain. This is especially true for tropical dry forests with high rainfall seasonality and light availability that may affect their carbon sequestration potential. In this study, we report the results of continuous measurements over a 3 year period of carbon and water fluxes between the atmosphere and a tropical dry forest ecosystem in northwestern Mexico using the eddy covariance technique. We found that the studied old-growth forest was a net carbon source to the atmosphere across years. This result can be explained by ecosystem respiration (the sum of plant and microbial respiration) since it was constantly larger than the gross ecosystem production (or whole-ecosystem photosynthesis). The main drivers of carbon and water fluxes were soil water content and net solar radiation, but the relative effect of net solar radiation was strongly dependent on soil moisture. In addition to demonstrating the importance of water and light for controlling whole-ecosystem processes in seasonally dry tropical forests, our results suggest old-growth tropical forests with little disturbance can act as carbon sources to the atmosphere. Therefore, further studies should investigate ecosystem fluxes in forests under different disturbance or successional stages. Key Points An old-growth tropical dry forest was a net carbon source regardless of precipitation variability Carbon and water fluxes were highly seasonal and varied interannually Main drivers of carbon and water fluxes were soil water content and net solar radiation