© 2004 Blackwell Publishing Ltd www.blackwellpublishing.com/geb 247 Global Ecology and Biogeography, (Global Ecol. Biogeogr.) (2004) 13, 247–258 RESEARCH PAPER Blackwell Publishing, Ltd. Use of a latitudinal gradient in bald cypress ( Taxodium distichum) production to examine physiological controls of biotic boundaries and potential responses to environmental change Beth A. Middleton and Karen L. McKee ABSTRACT Aim Predictions of vegetation change with global warming require models that accurately reflect physiological processes underlying growth limitations and species distributions. However, information about environmental controls on physiology and consequent effects on species boundaries and ecosystem functions such as production is limited, especially for forested wetlands that are potentially important carbon sinks. Location The bald cypress (Taxodium distichum) region of the south-eastern United States was studied to examine how production of an important forested wetland varies with latitude and temperature as well as local hydrology. Methods We used published data to analyse litter production across a latitudinal gradient from 26.2 to 37.8° N to determine how bald cypress swamps might respond to alternate climate conditions and what changes might occur throughout the distri- butional range. Results Litterfall rates followed a bell shaped curve, indicating that production was more limited at the distributional boundaries (c. 225 g /m 2 year -1 ) compared to the mid-range (795 – 1126 g /m 2 year -1 ). This pattern suggests that conditions are sub- optimal near both boundaries and that the absence of populations outside this latitu- dinal range may be largely due to physiological constraints on the carbon balance of dominant species. While dispersal limitations cannot be totally discounted, compe- tition with other wetland types at the extremes of the range does not seem likely to be important because the relative basal area of bald cypress does not decrease near the edges of the range. Impaired hydrology depressed production across the entire range, but more in the south than the north. Main conclusions Our findings suggest that (1) physiological limitations constrain biotic boundaries of bald cypress swamps; (2) future changes in global temperature would affect litter production in a nonlinear manner across the distributional range; (3) local changes in hydrology may interact with climate to fur- ther reduce litter production, particularly at lower latitudes; and (4) southernmost forests could be extirpated if environmental conditions compromise carbon balance and water-use efficiency of trees. Keywords Biotic boundary, curvilinear model, distribution, global climate change, impound- ment, litter production, Taxodium distichum, temperature. Correspondence: Beth A. Middleton, USGS, National Wetlands Research Center, 700 Cajundome Blvd., Lafayette, LA 70506 USA. E-mail: beth_middleton@usgs.gov USGS, National Wetlands Research Center, 700 Cajundome Blvd., Lafayette, LA 70506 USA. E-mail: beth_middleton@usgs.gov