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, microsite sky exposure of Englemann spruce (Picea englemannii) and subalpine fir (Abies 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-saturated photosynthesis (A(sat)) in current-year seedlings (newly germinated) of fir increased significantly (approximately seven-fold) in response to increased long-wave irradiance at night (warming), solar shading (approximately five-fold), and the combination of the two treatments (approximately eight-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. These results indicated substantial low-temperature photoinhibition, and were supported by similar A(sat) trends in natural seedlings, Increased needle inclination and clustering in more exposed microsites for both species implicates the possible role of structural adaptations for decreased sky exposure and warmer leaf temperatures at night.