Comparisons between laboratory and field results of frost tolerance of pines from the southern USA and Mesoamerica planted as exotics.
An artificial freezing study was conducted with 14 pine species and varieties from Mexico and Central America, and the southern and western USA. The pines chosen represented major commercial plantation species in the Southern Hemisphere such as Pinus caribaea var. hondurensis, P. taeda (multiple sources), P. patula and P. radiata, as well as promising species such as P. greggii, P. maximinoi and P. tecunumanii. Seedlings were grown in environmentally controlled growth chambers in the North Carolina State University Phytotron, and conditions were designed to mimic actual climatic conditions at Curitiba, Brazil, and Sabie, South Africa, located at approximately 25°S latitude. Early autumn conditions were simulated using shortened photoperiods and lower temperatures to harden the trees before the actual freeze testing. There were two freeze experiments: one containing tropical and subtropical material using four temperature treatments (-3, -7, -10, and -14°C), and one containing temperate and subtropical material using temperatures -7, -14, -21, and -28°C. Needle segments were frozen, and damage assessed using the electrolyte leakage technique. Rankings of species, varieties and sources corresponded well with field results and expectations based on climate of the source origins. The rankings of pure species and varieties should be useful to predict frost tolerance of pine hybrids, and the methodology shows promise for future experiments to quantify cold tolerance and genetic variation among hybrid progeny.