The effects of site type and shoot age on gas exchange and photosynthetic nitrogen use efficiency in pure and mixed <i>Picea abies</i> forests

Norway spruce ( Picea abies) is considered vulnerable to climate change in several parts of its range and growing in mixed stands is recommended as a mitigation solution. However, understanding spruce's physiological responses to site conditions depending on stand composition is still incomplete. We examined net photosynthesis (P n ), stomatal conductance, needle macronutrients content, and photosynthetic nitrogen use efficiency (PNUE) in different-aged shoots of spruce in response to site conditions and spruce proportions in the stands. The measurements were conducted in five typical spruce site types in Estonia ( Carex-Filipendula, Filipendula, Oxalis drained swamp, Oxalis, and Hepatica) ranging from waterlogged to moderately dry soils. The Hepatica site type had the lowest stomatal conductance, P n , and PNUE, while the distinction between moderately wet and temporarily waterlogged sites was unclear. Needle nitrogen concentration was higher in mixed stands in current-year shoots, whereas P n was higher in mixed than in pure stands in 1-year-old shoots suggesting acclimation to higher light availability in early growing season. However, the PNUE of current-year shoots was greater in pure stands. Our findings highlight the importance of stand composition and site conditions in shaping Norway spruce's photosynthetic traits, adaptive responses to environmental variations, and the advantage of mixed stands for enhancing resilience to climate change.