If the performance and survival of desert evergreens are limited by freezing at the high latitude edges of warm deserts, then individuals with higher freezing tolerance or greater acclimation potential could have better fitness at ecotones. Variation in cold tolerance and acclimation may be particularly important in warm desert evergreens, which exhibit weak dormancy in response to cold temperatures. The presence of heritable physiological variation in freezing tolerance has broad implications for plant distributions because it could provide the raw material for selection for increased freezing tolerance, facilitating the further expansion of desert shrubs into arid grasslands. Our goal was to construct support for the claim that high latitude Larrea tridentata and L. divaricata are locally adapted to freezing and to determine the potential for further freezing adaptation in both high and low latitude populations. We investigated responses to freezing in maternal families of 3 month old seedlings from two populations of L. divaricata from S. America, Bajada del Diablo (BDD, mean annual temperature, MAT 19oC), and Chamical (CHA, MAT 13oC), and one population of L. tridentata from N. America located at the Sevilleta LTER (SEV, MAT 13oC). We measured survival, chlorophyll fluorescence (FvFm) and projected plant area (PPA) before and one week after freezing. For L. divaricata we also made measurements of FvFm following 7d of cold acclimation.
High latitude SEV and BDD seedlings were more freezing tolerant than low latitude CHA seedlings, which had significantly lower survival following freezing. SEV seedlings which survived freezing had significantly higher FvFm and PPA than both BDD and CHA seedlings, but there were no significant differences within high latitude populations in post-freeze FvFm or PPA. There were significant positive correlations between pre-freeze and post-freeze performance, however, though BDD seedlings had lower FvFm and PPA than CHA seedlings before freezing, they had higher survival and performance following freezing. BDD seedlings also exhibited significantly lower FvFm following acclimation than CHA seedlings, indicating greater potential for low temperature acclimation in high latitude seedlings. Only CHA seedlings showed significant differences between maternal families in post-freezing survival, implying that further freezing adaptation could take place in this low latitude population. Higher freezing tolerance in the high latitude populations combined with a lack of differences between maternal families suggest that strong selection in the past may have reduced overall variation within these populations and thereby the potential for further selection for freezing tolerance.