Air and soil temperature limitations on photosynthesis in Englemann spruce during summer.

Picea engelmannii was studied over its natural elevational range in SE 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 and -5oC that occurred through mid-June. After middle to late June, decreased photosynthetic rates were correlated with low soil temperature. Subfreezing air temperature followed by an extended period of low soil temperature were the primary limitations to photosynthesis in early summer. -from Authors