WWRC 88-28
Shoot Structural Effects on Needle Temperatures and Photosynthesis in Conifers
Abstract
Greater needle packing (number of needles per unit stem length) among shoots of Picea
engelmannii (Parry ex. Engelm.), Abies lasiocarpa ([Hook] Nutt.), and Pinus contorta (Engelm.)
collected at sun-exposed locations resulted in leaf temperatures (T1) that were well above ail
temperature (Ta) 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 Ta 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 21 to 36% compared to estimations assuming
T1 = Ta for the entire day. Elevated T1 due to needle packing may
lead to substantial increases in summer carbon gain for conifers experiencing cold-limited, short growth seasons.
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