Effect of Daily Minimum Temperature on Photosynthesis in Eastern Hemlock (Tsuga canadensis L.) in Autumn and Winter

Abstract

Most coniferous trees are capable of photosynthesis throughout the year, but low temperatures and frozen soil limit winter photosynthesis in many regions. In eastern hemlock (Tsuga canadensis L.) in central Massachusetts, U.S.A., midday light saturated photosynthesis (Pmax) in autumn was limited by subfreezing temperatures during the previous one to two nights. In autumn, minimum air temperature (Tmin) during the previous 24 h had a strong effect on midday Pmax only if Tmin was below −2°C. Pmax averaged about 5 μmol m−2 s−1 after Tmin near −2°C, but fell to about 1 μmol m−2 s−1 after a Tmin of −8°C. Maximum measured Pmax in winter was about 2.5 μmol m−2 s−1 after a Tmin of 7°C in mid-March, and Pmax in winter was more strongly related to average Tmin during the previous week than to the Tmin just prior to measurement. However, no major mid-winter thaws, with several successive above-freezing minimum temperatures, occurred during this study. A model of annual carbon exchange for the hemlock forest showed that daily ecosystem carbon exchange in December through March was very sensitive to variation between −5 and 2°C in average daily Tmin during the past 2 d. Fewer autumn frosts and prolonged winter thaws could increase annual photosynthesis and carbon storage in eastern hemlock forests in the northeastern United States.