Air and soil temperature limitations on photosynthesis in Engelmann spruce during summer

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Abstract
The relative importance of air and soil temperature as potential limitations to photosynthesis during summer was assessed in Engelmann spruce over its natural elevational range in southeastern Wyoming. Low photosynthetic rates measured at constant temperature, irradiance, and vapor pressure deficit were highly correlated with low minimum (night) air and soil temperature, but at different times during the early summer growth period. Substantial and irreversible reductions in photosynthesis occurred after exposure to night air temperatures of −4 to −5 °C that occurred through mid-June. After middle to late June, decreased photosynthetic rates were correlated with low soil temperature. Correlation analyses indicated that subfreezing air temperature followed by an extended period of low soil temperature were the primary limitations to photosynthesis in early summer. Additional laboratory and field experiments corroborate these field observations.

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