Due to their large extent and high primary productivity, tropical dry forests (TDF) are important contributors to atmospheric carbon exchanges in subtropical and tropical regions. In northwest Mexico, a bimodal precipitation regime that includes winter precipitation derived from Pacific storms and summer precipitation from the North American monsoon (NAM) couples water availability with ecosystem processes. We investigated the net ecosystem production of a TDF ecosystem using a 4.5 year record of water and carbon fluxes obtained from the eddy covariance method complemented with remotely sensed data. We identified a large CO2 efflux at the start of the summer season that is strongly related to the preceding winter precipitation and greenness. Since this CO2 efflux occurs prior to vegetation green‐up, we infer that respiration is mainly due to decomposition of soil organic matter accumulated from the prior growing season. Overall, ecosystem respiration has an important effect on the net ecosystem production but can be overwhelmed by the strength of the primary productivity during the NAM. Precipitation characteristics during NAM have significant controls on sustaining carbon fixation in the TDF into the fall season. We identified that a threshold of ~350 to 400 mm of monsoon precipitation leads to a switch in the annual carbon balance in the TDF ecosystem from a net source (+102 g C/m2/yr) to a net sink (−249 g C/m2/yr). This monsoonal precipitation threshold is typically exceeded one out of every 2 years. The close coupling of winter and summer periods with respect to carbon fluxes suggests that the annual carbon balance is dependent on precipitation amounts in both seasons in TDF ecosystems.

The sensitivity of semiarid ecosystems to climate change is not well understood due to competing effects of soil and plant‐mediated carbon fluxes. Limited observations of net ecosystem productivity (NEP) under rising air temperature and CO2 and altered precipitation regimes also hinder climate change assessments. A promising avenue for addressing this challenge is through the application of numerical models. In this work, we combine a mechanistic ecohydrological model and a soil carbon model to simulate soil and plant processes in a subtropical shrubland of northwest México. Due to the influence of the North American monsoon, the site exhibits net carbon losses early in the summer and net carbon gains during the photosynthetically active season. After building confidence in the simulations through comparisons with eddy covariance flux data, we conduct a series of climate change experiments for near‐future (2030–2045) scenarios that test the impact of meteorological changes and CO2 fertilization relative to historical conditions (1990–2005). Results indicate that reductions in NEP arising from warmer conditions are effectively offset by gains in NEP due to the impact of higher CO2 on water use efficiency. For cases with higher summer rainfall and CO2 fertilization, climate change impacts lead to an increase of ~25% in NEP relative to historical conditions (mean of 66 g C m−2). Net primary production and soil respiration derived from decomposition are shown to be important processes that interact to control NEP and, given the role of semiarid ecosystems in the global carbon budget, deserve attention in future simulation efforts of ecosystem fluxes.

According to the Intergovernmental Panel on Climate Change (IPCC), global temperatures have risen at an alarming pace since the early 20th century and this warming has been more pronounced since the 1970s. Temperature variations are significant because of their relation with thermal comfort and public health. In this study, we characterize the impacts of increasing maximum air temperatures in Sonora, Mexico. Heat days (HDs) and heat waves (HWs) were used as indicators to investigate historical trends in extreme heat. Furthermore, HDs were represented using a generalized linear regression model during the observed period (1966–2015) to generate future scenarios related to extreme heat and subsequently compared with six downscaled general circulation models (CNRM‐CM5, CSIRO Mk3.6.0, HadGEM2‐CC, HadGEM2‐ES, IPSL‐CM5A‐LR and IPSL‐CM5A‐MR) under low and high radiative scenarios (RCP4.5 and RCP8.5). Results of this work indicate that climate stations in Sonora have exhibited increases in the number of HDs and HWs in the historical record that can be associated to physical factors such as elevation, urban land cover and the percent of annual rainfall during the summer. Statistical and model‐based projections indicate that these trends will continue in the future up to 2060, with less moderate increases and high uncertainty noted for the difference scenarios of the downscaled models. These observed and projected trends in extreme heat are important for identifying adaptation strategies in the public and environmental health sectors in Sonora.