Needle traits of understory Silver fir, Norway spruce, and Scots pine in response to increased canopy openness in a birch-dominated stand

The study investigates the impact of varying environmental treatments on the needle morphology, chemistry, and photosynthetic performance of Silver fir (Abies alba), Norway spruce (Picea abies), and Scots pine (Pinus sylvestris) undergrowth in a birch (Betula pendula)-dominated young forest. The experiment analyzed the effects of different levels of birch canopy reduction. The treatments included a birch preparatory stand managed through positive tending of promising birch trees (PS), a restored small clearing (RC), and a former clearing overgrown by birch sprouts (OC), while the control treatment was left unmanaged. The aim was to evaluate the rate of adaptation to irradiation changes among these species. Fir and pine exhibited significant needle morphological changes within the first year after canopy opening, while spruce needles remained relatively stable. In the first year after release, pine and fir new needles in the RC treatment had reduced K (and Mg in fir), while spruce needles increased N and Ca concentrations. Chlorophyll a fluorescence measurements indicated that photosynthetic efficiency tended to slightly decrease across all species when exposed to higher irradiance. The results show high plasticity of the species in response to the changes in light conditions of the nurse stand. The means of the adaptation were specific for each species, but the new conditions did not limit the adaptation potential for any of them.