© 2000 Heron Publishing—Victoria, Canada
Seasonal CO2 assimilation and stomatal limitations in a Pinus taeda canopy
David S. Ellsworth (1)
1. Environmental Biology and Instrumentation Division, Brookhaven National Laboratory, Upton, NY 11973-5000, USA (ellswort@bnl.gov) / Received April 26, 1999
Summary
Net CO2 assimilation (Anet) of canopy leaves is the principal process governing carbon storage from the atmosphere in forests, but it has rarely been
measured over multiple seasons and multiple years. I measured midday Anet in the upper canopy of maturing loblolly pine (Pinus taeda L.) trees in the piedmont region of the southeastern USA on 146 sunny days over 36 months. Concurrent data for leaf conductance
and photosynthetic CO2 response curves (Anet–Ci curves) were used to estimate the relative importance of stomatal limitations to CO2 assimilation in the field. In fully expanded current-year and 1-year-old needles, midday light-saturated Anet was constant over much of the growing season (5–6 μmol CO2 m–2 s–1), except during drought periods. During the winter season (November–March), midday Anet of overwintering needles varied in proportion to leaf temperature. Net CO2 assimilation at light saturation occurred when daytime air temperatures exceeded 5–6 °C, as happened on more than 90% of
the sunny winter days. In both age classes of foliage, winter carbon assimilation accounted for approximately 15% of the daily
carbon assimilation on sunny days throughout the year, and was relatively insensitive to year-to-year differences in temperature
during this season. However, strong stomatal limitations to Anet occurred as a result of water stress associated with freezing cycles in winter. During the growing season, drought-induced
water stress produced the largest year-to-year differences in seasonal CO2 assimilation on sunny days. Seasonal Anet was more drought sensitive in current-year needles than in 1-year-old needles. Relative stomatal limitations to daily integrated
Anet were approximately 40% over the growing season, and summer drought rather than high temperatures had the largest impact on
summer Anet and integrated annual CO2 uptake in the upper crown. Despite significant stomatal limitations, a long duration of near-peak Anet in the upper crown, particularly in 1-year-old needles, conferred high seasonal and annual carbon gain.
Keywords:
annual CO2 assimilation, canopy carbon gain, climate variability, drought, evergreen photosynthesis, stomatal conductance, temperature.
