Sky exposure, crown architecture, and low-temperature photoinhibition in conifer seedlings at alpine treeline

In the alpine-treeline ecotone of the Snowy Range in Wyoming, USA, microsit e sky exposure of Englemann spruce (Picea englemannii) and subalpine fir (A bies lasiocarpa) seedlings (< 5 years) was associated with the avoidance of low-nocturnal temperatures and high insolation, factors which appeared to result in low-temperature photoinhibition, In a field experiment, light-sat urated photosynthesis (A(sat)) in current-year seedlings (newly germinated) of fir increased significantly (approximately seven-fold) in response to i ncreased long-wave irradiance at night (warming), solar shading (approximat ely five-fold), and the combination of the two treatments (approximately ei ght-fold). A(sat) in current-year spruce remained unchanged in response to all treatments, but was over four-times higher than fir in control plots. T hese results indicated substantial low-temperature photoinhibition, and wer e supported by similar A(sat) trends in natural seedlings, Increased needle inclination and clustering in more exposed microsites for both species imp licates the possible role of structural adaptations for decreased sky expos ure and warmer leaf temperatures at night.