Climate-induced episodes of extensive tree mortality worldwide are leading to abrupt changes in forest carbon stocks. A severe frost in early February 2011 triggered widespread tree mortality in the lowland tropical dry forest (TDF) of northwestern Mexico. The studied landscape in southern Sonora is composed by a patchy matrix dominated by mature, secondary (originated in abandoned fields), and active agricultural fields. In this forest, we used allometric equations to assessed frost-induced changes in aboveground biomass (AGB) stocks in mature and secondary tropical dry forests. For AGB estimations we used 48 1-ha plots (24 plots per forest type) distributed within four distant subareas in our 83 230 ha study area. On each plot, we recorded all live/dead individuals, and a total of 11 205 woody plants were registered, of which 7 137 (with at least a stem DBH > 2.5 cm) were likely present before the frost, and the remaining smaller ones were considered as new recruits regenerated from seeds (4,068 individuals). From those plants present before the frost, 26 842 and 8 059 were live and dead stems, and 1 222 were dead individuals. All registered live and dead stems accounted for a total of 273.4 Mg of AGB in our study plots (4.8 ha). From this amount, 57.3 Mg was necromass (dead stems). Interestingly, only two out of a total of 74 registered species contributed with ca. 80% of this necromass. These highly sensitive species are the tree legumes Lysioma divaricatum and Acacia cochliacantha. On average, AGB in the studied mature and secondary TDF was 64.3 and 49.6 Mg ha-1, respectively. The corresponding necromass for these forests was 10.9 and 13 Mg ha, respectively. The 2011 frost induced a greater change from live biomass to necromass in secondary than mature forests, 26.2% and 16.9%, respectively, which can be explained by the higher abundance of individuals from sensitive species in secondary forests. Our results suggest that climate-induced shifts in carbon stocks are linked to previous land-use changes in tropical dry forests. Restoration plans of these degraded lands should consider the vulnerability of tropical dry forest species to climate extremes.

Soil moisture is an essential measurement to manage water and improve crop production. However, agricultural research in the Yaqui Valley (in northwestern Mexico) with extensive wheat fields (Triticum sp.) have focused on other monitoring schemes (e.g. remote sensing) with less attention to soil moisture. Most of this cultivated soil contains up to ~ 50% clay, which results in changes to soil properties from wet to dry conditions and challenges in the implementation of in-situ measurements of soil moisture. For this research, we selected a 1-ha wheat field in the Yaqui Valley representative of a typical flood irrigation system. We measured meteorological variables (ClimaVUE™50), and soil moisture for the winter crop-cycle from December 2019 to Abril 2020. Volumetric water content (VWC) was recorded from 5 to 50 cm using two TDR (SoilVUE™10), one located in the bottom of the furrow under bare conditions, and the other on the top under the vegetated condition for further integration and comparison. A Cosmic Ray Neutron Sensor (CRNS) was located alongside the meteorological sensor. The universal calibration equation was used to estimate VWC based on neutron counts. The comparison from the CRNS and the integrated TDR (5 to 50 cm) resulted in an RMSE of 0.02 m3m-3 and an r2 = 0.73. While both technologies respond to water inputs, the CRNS is a more reliable measurement during the dry-down periods when the high-clay soil cracks to the extent of 40 cm where soil is exposed to air. During this driest period, recorded VWC at 50 cm was, on average, 0.25 m3 m-3, while measurements with the CRNS was on average, 0.16 m3 m-3. Interestingly, both sensors peaked at 0.56 m3 m-3 during the flood irrigation event.