Publication

This study examined the foliar response of putative hybrid Engelmann x white x Sitka spruce (Picea engelmannii Parry x Picea glauca (Moench) Voss x Picea sitchensis (Bong.) Carr) needles in relation to crown position and across three stages of development (15, 55, and 145 years). We focused on the morphological and anatomical response, and used physiological measures (photosynthesis and stomatal conductance) to emphasize the important relationship between structure and function. We found that needles from the upper outer crown position were adaptated to reduce stress from evapotranspiration. Trees from the 15- and 55-year-old stands generally responded the most, and trees from the 145-year-old stand responded the least. As they mature, these spruce trees may have reduced ability to adapt to their environment. Some of our results contradict what the literature considers "typical" for sun-shade dimorphism in temperate forests. For example, in all stands, sun needles were broader than shade needles and, in the two younger stands, sun needles were larger in area, not smaller, than shade needles. Also, in the youngest stand, stomatal pores were longer on sun needles than on shade needles. There were no definite patterns in stomatal spacing with regard to crown position. Our results are indicative of the strategies adopted to increase competitiveness in a resource-limited environment. We suggest that, in the 15-year-old stand, neither water nor light is limiting; in the 55-year-old stand, both water and light are highly limiting; and in the 145-year-old stand, water is most limiting.