Styrax americanus Lam. (American Snowbell) is a deciduous, woody plant distributed in patchy populations that are restricted to floodplains and swamp margins.  The species occurs primarily from Florida to southern Illinois, but we are studying a disjunct population of 15 or fewer individuals indigenous to a wetland in northeastern Illinois (latitude = 41º10’N), approximately 480 km north of the populations in southern Illinois.  Our objective is to determine how soil water supply impacts American snowbell physiologically and thereby represents a determinant of the species’ restriction to continuously or periodically saturated soils.  We assessed relationships between soil-water status, photosynthetic responses, and cold-hardiness of plants from the northern disjunct population and made comparisons to plants indigenous to two other populations, one in southern Illinois (latitude 37 º22’N), the other in northern Florida (latitude 30 º42’N).  Plants in pots were subjected to three hydration treatments in a greenhouse from June to November 2008.  Water was applied daily to drained root zones of plants in the container-capacity treatment, root zones of plants in the partial-flooding treatment were kept immersed in water maintained at 8 cm below the surface, and plants in the drought treatment received 100 mL water daily.

Results/Conclusions

Partial flooding increased net CO₂ exchange rate, stomatal conductance, and leaf area in all populations compared with plants in the drought treatment.  In September, before seasonal cessation of growth, stems of plants subjected to drought were ≈12% more cold-hardy than stems of partially flooded plants.  In contrast, stem cold-hardiness of plants under drought in November from northern Illinois, southern Illinois, and northern Florida was 7%, 23%, and 13% lower, respectively, than hardiness of plants in the container-capacity treatment.  Winter hardiness varied among populations in a manner suggesting increasing winter hardiness with latitude of origin.  Results indicate that both soil moisture and low temperature help to account for the distribution of S. americanus, and that the interplay of soil water supply and resistance of stems to low temperature varies during autumnal cold acclimation.