Most of our knowledge about the effects of frosts on cacti comes from desert ecosystems where columnar and other cacti growth forms are prominent elements of the landscape. In tropical dry forests (TDF), some cacti exhibit arborescent growth, but their height rarely exceeds the forest canopy. Vegetation cover may protect cacti against extreme weather events such as severe frosts. An extreme freezing event in early February 2011 triggered widespread TDF tree mortality in northwestern Mexico. Temperatures dropped below freezing for at least three consecutive days. Our objective was to analyze the damage induced by this severe frost on arborescent and shrubby cacti species in a tropical dry forest in northwestern Mexico near its northernmost distribution in America. In our study area, the Sierra de Alamos-Rio Cuchujaqui Biosphere Reserve, we established 36 study sites to include most of the forest spatial heterogeneity in composition, structure and frost damage. Each site contained 10 plots of 50 x 2 m. Within each plot we identified and measured DBH and height of all individual stems (DBH > 2 cm, live and dead) of woody trees and arborescent and shrubby cacti.
Results/Conclusions
Dead individuals after the severe frost accounted for up to 20% of all woody trees in some TDF sites. Contrastingly, the most common damage to cacti species was dieback and not individual mortality. We found six species of arborescent (Pachycereus pecten-aboriginum, Stenocereus alamosensis, S. montanus, S thurberi) and shrubby (Cylindropuntia thurberi, Opuntia wilcoxii) cacti. Frost-induced mortality of cacti individuals was very low (<1%), occurring mainly in the taller species P. pecten-aboriginum. Dead individuals of this species were on average 3.1 m tall with 9.5 cm DBH whereas undamaged live individuals were 4.8 m tall with 14.1 cm DBH. Frost-induced dieback occurred particularly in the taller arborescent species (P. pecten-aboriginum and S. thurberi). Damaged individuals of S. thurberi were on average 6.3 m tall (14.4 cm DBH) and undamaged individuals were 4.8 m (13.3 cm DBH). Pachycereus pecten-aboriginum showed this same dieback pattern. Our analysis suggests that cacti individual size is related to frost resistance, and that short and tall individuals survived extreme frost through different mechanisms. Dieback was more frequent in the tallest individuals who responded to frost damage by resprouting.