Changes in needle morphology of shade-tolerant seedlings after partial overstory canopy removal

Recent studies with early-successional conifers have shown a correlation between changes in light regimes and changes in needle morphology. The objective of this study was to test the hypothesis that needles of late-successional conifers also respond to increased irradiance. We compared needle morphology of three co-occurring conifers, grand fir (Abies grandis (Dougl. ex D. Don) Lindl.), subalpine fir (Abies lasiocarpa (Hook.) Nutt.), and Engelmann spruce (Picea engelmannii Parry ex Engelm.). Morphological characteristics were examined for 4 years across three light environments: (1) open-canopy stands with 64% transmittance, (2) stands with 63% transmittance after a partial overstory removal treatment, and (3) closed-canopy stands with 29% transmittance. Needle width and length differed by year, but not by light environment. Mean stomatal density differed across years for all three species and increased with increasing irradiance for subalpine fir and Engelmann spruce. The number of mesophyll layers and the size of individual palisade mesophyll cells did not differ among the different light environments. The number of epidermis cell layers in each species was insensitive to changes in light environment and time. Our results indicate that these three conifers were unable to morphologically adapt their needles to an increase in light transmission caused by the partial removal of overstory canopies.