Extreme climatic events such as severe drought, heat and cold are predicted to increase in frequency and intensity. Extensive tree mortality related to extreme drought and heat waves has been reported worldwide. However, frost effects have focused mainly on croplands. We studied the consequences of a severe frost (February 2011) on a seasonally tropical dry forest (SDTF) in its northern range limit in America (Sonora, Mexico). SDTFs have been impacted by droughts and hurricanes but no die-off related to severe frosts has been documented before. As other STDFs, this region has a long history of cattle ranching and agriculture. Thus, the landscape is a complex mosaic of annual crops, mature and secondary forests up to 40 years old. After the extreme frost event, we established 120 study plots using a systematic field sampling designed to include mature and secondary forest fragments. In each plot, we identified and measured DBH and height of all live and dead woody stems (DBH > 1 cm) to estimate live and dead aboveground biomass (AGB). We used ANCOVA to relate the proportion of dead AGB to topography and forest type (mature and secondary forest).
We measured 12,193 stems from 2,732 individuals and 55 woody species. As expected, dead AGB was related to forest type and restricted to the lowlands. Dead AGB was higher in secondary forests and negatively related to elevation. The proportion of dead stems was 0.14 in mature forests and twofold in secondary forests. In secondary forests, dead AGB was on average 40% of the total AGB; some sites making up to 70% equivalent to 50 Mg/ha of standing necromass. Dead AGB in mature forests was much lower (25%) and showed less spatial variation. The two dominant secondary forest species contributed almost 90% to the total dead AGB in these forests, but only to 38% in the more diverse mature forests. Dead AGB in secondary and mature forests was considerably higher than the amount (8%) before the severe frost. The carbon pool currently stored as necromass could be as high as 25 Mg C/ha, especially in secondary forests severely affected by frost-induced mortality. Secondary forests, the main legacy of land-use change in STDF, are highly vulnerable to extreme climatic events with uncertain consequences for the regional carbon cycle and the successional pathway of these new forests.