SHOOT STRUCTURAL EFFECTS ON NEEDLE TEMPERATURES AND PHOTOSYNTHESIS IN CONIFERS

Greater needle packing (number of needles per unit stem length) among shoots of Picea engelmannii ( P a m ex. Engelm.), Abies lasiocarpa ([Hook] Nutt.), and Pznus contorta (Engelm.) collected at sun-exposed locations resulted in leaf temperatures (T,) that were well above air temperature (T,) and which were closer to optimum for maximum photosynthesis under field conditions, Maximum photosynthesis in these species occurred at leaf temperatures well above mean maximum T, during most of the summer growth period. An approximate doubling in needle packing on sun shoots of these species, however, led to an estimated temperature enhancement of daily photosynthetic carbon gain of 2 1 to 36% compared to estimations assuming T , = T, for the entire da),. Elevated T, due to needle packing may lead to substantial increases in summer carbon gain for conifers experiencing cold-limited, short growth seasons. NUMEROUS ADAPTATIONS in\-olving leaf size, shape, absorptance to solar irradiance, and orientation properties that have important influences on leaf temperature and photosynthesis are now recognized for a variety of broadleaved species (Gates, Alderfer, and Taylor, 1968; Taylor, 1978; Smith. 1978: Ehleringer and Mooney, 1978; Begg, 1980; Ehleringer, 1980). Comparatively little research of a similar nature has been reported for plants with narrow leaves or leaflets arranged in distinctive configurations on individual shoots, such as found commonly among needle-leaved coniferous tree species (Kramer and Clark, 1947; Levernz and Jarvis, 1979, 1980; Oker-Blom, Kellomaki, and Smolander. 1983; Carter and Smith, 1985). As an example, greater needle packing in conifer sun shoots substantially increased photosynthesis per unit stem length with only minimal decreases in photosynthesis per needle due to mutual shading and/or orientation effects (Carter and Smith, 198 5 ) . We report here on an additional influence of needle packing in conifers which resulted in an increase in needle temperatures to more optimum levels for photosynthesis throughout much of the summer growth period. Data are reported for the three dominant conifers ofthe central Rocky Mountains, USA (Engelmann spruce, Picea engelmannii Pamy ex. Engelm.; subalpine fir, Received for publication 2 March 1987; revision accepted 26 June 1987. We thank the NSF for partial financial support to W. K. Smith: J. L. Hadley and E. H. Delucia provided valuable discussion. * Current address: Earth Resources Laboratory, NASA, National Space Technology Laboratories, NSTL, Mississippi 39529. Abies lasiocarpa [Hook] Nutt.: lodgepole pine, Pinus contorta Engelm.). MATERIALS AND METHODSIn all of the following field studies (Medicine Bow Mountains, southeastern Wyoming, USA, 4 1'2 1 'N, 106" 1 0' W), photosynthetic measurements were taken using either an open-flow CO, exchange system with temperature-controlled leaf cuvette and an infrared gas analyzer (Anarad model 410) or a closed-flow portable photosynthesis system (model LI-6000, LI-COR, 442 1 Superior St., Lincoln, Nebraska). (See Carter and Smith, 1985, for more details.) The closed-flow system did not enable temperature regulation, but instead was used to sample shoots at various times of the day when shoot temperatures were different, but incident sunlight levels were above levels required to saturate photosynthesis. In all cases, mean leaf conductance values to water vapor (indicating degree of stomata1 opening) varied by no more than -L 18% and were always 22.5 mm s-'. Sun shoots were defined as those growing at locations receiving at least 70% of the total clear-day solar irradiance (horizontal sensor) compared to shade shoots that received less than 20% (LICOR 190s quantum sensors and Campbell Scientific CR7 data loggers). The temperature response for net photosynthetic C 0 2 uptake was measured for a total of 13 shoots from 6 individual spruce trees, 15 shoots from 6 trees of the fir, and 9 shoots from 4 trees of pine during the summers of 19821986 (all years combined). All measurements of net photosynthetic C02 uptake (pmol COz m-2 s' ) were determined either over continuous 2-6 hr periods using the controlled-en-