Effects of forest structure, soil, and macroclimate on specific leaf area of foundational Vaccinium plants

Bilberry (Vaccinium myrtillus) and cowberry (V. vitis-idaea) are foundational species in Northern Europe. Specific leaf area (SLA) is a key trait because it specifies the relative thickness of the leaves, indicating both the plant's response to environmental conditions and its effects on the environment. Identifying factors influencing SLA is therefore essential for understanding how these plants respond to environmental perturbations like climate change and forest management, as well as feedback effects on the plant's environment. Here, we test how the SLA of these two plant species responds to overstory density and tree species composition across an extensive climatic gradient. We focused on two forest types in Sweden: dominated by an overstory of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies); hereon spruce-pine, and an overstory of Norway spruce and birch (Betula sp.); hereon spruce-birch. Our results show that the SLA of both bilberry and cowberry increases (thinner leaves) significantly with higher forest density, whereas it declines with a greater proportion of pine or birch. Increasing soil moisture had a negative effect on SLA of both species in the spruce-birch context, whereas soil fertility never was a significant determinant. Additionally, we observed that the species-specific response in SLA was related to macroclimate and leaf age, with lower SLA in colder climate and older leaves. These findings highlight the importance of considering the effects of forest structure and composition on the SLA in forest management, as shifts in SLA may have implications for ecosystem functioning under present and future climates.