LAUSR

Abstract The success of tree regeneration in forests strongly depends on light availability. In structured uneven-aged deciduous forests with continuous cover, radiation regimes in the understorey are typically characterized by low and heterogeneous light transmittance. The competitive ability of juvenile trees in these forests is therefore linked to their shade tolerance and expressed by different growth strategies. The related growth traits result in different morphological responses to light availability and shape recruitment dynamics. In the different light availability niches created by continuous-cover forestry, assessing tree growth patterns is crucially important towards silvicultural optimization, which aims to promote species diversity and the sustainable provision of forest ecosystem services. However, information on growth rates of small trees below the caliper limit of typical inventories or monitoring plots is relatively scarce. In an extensive field campaign, we sampled juvenile European beech (Fagus sylvatica L.) and sycamore maple (Acer pseudoplatanus L.) along light availability gradients and analyzed stem discs to quantify primary and secondary growth. We used linear mixed effects models to explain the annual increments of height, basal area and volume as well as allometric exponents. Beech growth rates were explained well by diffuse light availability and ontogeny. Models predicting sycamore growth were slightly less accurate, as indicated by a higher root mean square error. Compared to beech, light availability had a stronger influence on secondary growth of sycamore stems, and height increment decreased with tree age. Trajectories of height and diameter growth of the two species were different at low light availability, where sycamore invested more in height growth and thus was generally slender than beech. At light availability above 20%, similar growth trajectories but different allocation patterns were observed. Overall, we found markedly different growth strategies and different competitive abilities of beech and sycamore in small gaps in uneven-aged mixed deciduous forests. From a management perspective, sycamore should not be left suppressed in shaded conditions as it loses its height growth potential with increasing age. Tending measures may be needed for sycamore to maintain its stability and low height to diameter ratio, but not for juvenile beech, which benefits from positive effects of crowding on height growth.