Interactive effects of elevated CO2 and neighbourhood competition on the radial growth of European beech (Fagus sylvatica L.) seedlings

This paper explores the effects of three types of admixture treatments: (1) monospecific (M: European beech surrounded by six European beech seedlings), (2) single-admixture (S: European beech surrounded by six Norway spruce seedlings), and (3) group-admixture (G: European beech surrounded by three Norway spruce and three European beech seedlings), and two different CO2 concentrations ([CO2]): ambient [AMBCO2, 385 μmol (CO2) mol−1] and elevated [ELECO2, 770 μmol (CO2) mol−1] on the stem diameter increment (DI), based on a 7-year monitoring of the diameter growth of European beech (Fagus sylvatica L.) seedlings. Furthermore, to improve our understanding of the influence of climatic variables, we assessed the effects of precipitation and temperature on radial growth. After 7 years’ experience (irrespective of admixture treatment), our data were not able to confirm the effect of elevated CO2 on the diameter growth (p value = 0.35), but the result was changed when the effect of elevated CO2 was considered under the different types of admixture. Elevated CO2 along with single-admixture increased the diameter increment significantly (p value = 0.001). In contrast, we characterized a significant effect of AMBCO2 × G interaction on DI (p value = 0.002). But DI was unaffected by elevated CO2 in M (p value = 0.77). In both [CO2], inter-specific competition with spruce resulted in the highest radial growth. The inter-specific interaction was most pronounced in the lower proportion with spruce (S) in ELECO2, while in ambient CO2, the higher proportion with spruce (G) had the greatest effect on the radial growth. With respect to the climatic factors, our observations showed that temperature had a stronger influence than precipitation in ELECO2, but in AMBCO2, precipitation had the strongest effect on the radial growth as indicated by the statistically significant correlation between climatic variables and radial growth. Overall, our result suggested that seedlings growth was more under the control of climate and edaphic factors than dependent on tree species diversity and CO2 concentrations.