Willows (Salix) are different than many other woody species because they do not demonstrate strong correlations between their cold-acclimated freezing tolerances and their latitudinal distributions. Although most willows, including tropical species, are able to survive sub-zero temperatures, there is evidence that species vary in their rate of cold-acclimation and their cues for senescence. The objective of this research was to examine whether twenty-seven North American willow species vary in their cold acclimation rates when grown in a common environment and whether this variation can explain their current range limits. Cuttings from twenty-seven species were collected from different latitudes across North America. All the plants were grown in a greenhouse which was maintained at 20C with a 12 hour photoperiod. Over one month in the winter, photoperiod was decreased from 12 to 9 hours and temperatures were reduced from 20 to 7C in the greenhouse. Chlorophyll fluorescence and growth were measured every two weeks and relative chlorophyll levels were monitored for two months to document senescence.

Results/Conclusions

This study found that species vary significantly in their growth rates and onset of cold-acclimation and senescence. Species from more southern latitudes demonstrated higher growth rates, and differences in species nonphotochemical quenching and photosynthetic efficiency corresponded with their latitude of origin. The relationship between species cold acclimation rates and their range limits can be explained by differences in their susceptibility to early freeze events. This study demonstrates that traits related to acclimation may be of paramount importance in determining species latitudinal distributions in a genus that exhibits limited variation in freezing tolerance.