Effects of climate, soil, and forest structure on specific leaf area of understorey foundation plants

Forest overstorey's interaction with understorey vegetation is a crucial aspect that impacts the biodiversity and resilience of forest ecosystems. Particularly, the influence of the overstorey on the specific leaf area (SLA) in understorey plants remains poorly understood. This is due to limited research within particular climatic conditions and a narrow focus on limited biogeographical areas, which leaves significant knowledge gaps, especially in boreal regions. This study aims to address these gaps by exploring how the SLA of understorey plants responds to variations in forest structure, soil characteristics, and climate across diverse Swedish forests. To achieve this aim, this study embodied three distinct study designs, namely a large-scale study, a mixed forest study, and a transplant experiment. Cowberry and bilberry were chosen for the large-scale and mixed forest studies, while wild strawberry and goldenrod were selected for the transplant experiment. Leaf samples of these species were used for this study. Cowberry being evergreen, the leaf samples were collected from old shoots and new shoots, which are termed as “cowberry old growth” and “cowberry new growth”, respectively. The large-scale study consisted of three sites covering latitudes of 57° N to 64° N, each with 15-16 stands dominated by Scots pine or Norway spruce. The mixed forest study comprised a stand mainly with Norway spruce and birch where 40 plots were strategically placed across gradients of varying forest density and overstorey species composition. For the transplant experiment, understorey plants were planted in pairs with different climatic origins, at three sites in Sweden. Plants were planted in ten plots per site, established at diverse tree species composition and forest density values. Forest density had a significant positive effect on the SLA of understorey plants. The percentage of pine had a significant negative effect on SLA in the large-scale study. The percentage of broadleaves had a significant negative effect on SLA in mixed forest study and transplant experiment. Soil C-N ratio did not have significant effect on the SLA. Soil moisture availability had no significant impact on the SLA of the understorey plants except for the cowberry new growth in large-scale study and bilberry in mixed forest study. The interaction between forest density and the percentage of pine only significantly affected the cowberry new growth in the large-scale study. Furthermore, the SLA of wild strawberry and goldenrod was not affected by climatic origin, and the interaction between forest density and origin was also not significant. These findings help to predict how the SLA of understorey plants changes related to forest structure, as well as to changes induced by climate warming, while at the same time enriching our insight into plant-environment interactions