Accurate guidelines for silvicultural management of exotic tree species in Germany are sparse. For example, northern red oak (Quercus rubra L.) is the most commonly planted exotic deciduous tree species in Germany, but its response to varying levels of competition intensity has not yet been adequately explored. Here, we used terrestrial laser scanning to non-destructively examine the responses of stem and crown characteristics of Quercus rubra to intraspecific competition. A total of 100 dominant red oak trees were investigated in ten pure red oak stands, located in five federal states of Germany. The external stem quality characteristics namely stem non-circularity and bark anomalies decreased with increasing tree competition. Also, the crown characteristics crown volume, crown surface area, maximum crown area, crown length, and branch length declined by the degree of individual tree competition. We conclude that individual tree properties can be controlled by competition intensity, resulting in improved timber quality as shown for other tree species.

In Europe, the non-native Northern red oak (Quercus rubra L.) is widely recommended for future cultivation. However, outside its natural range, Northern red oak has to date been insufficiently studied both in terms of silviculture and genetics. To clarify this, we studied the architecture of 92 (pre-) dominant Northern red oak trees in five German federal states using the non-destructive terrestrial laser scanning method (TLS). In addition, individual-based heterozygosity was calculated based on microsatellite data obtained by analyzing twelve potentially adaptive genic (EST derived markers) and eight putatively selectively neutral nuclear microsatellite markers. With these data the individual heterozygosity of the sample trees was calculated. Mean or median branch angles as well as branch angle ranges of first order branches decreased with individual heterozygosity calculated group-wise for all markers (H_oAll) and for EST-derived markers (H_oEST). Most other tree characteristics, including the number of bark anomalies and mean stem non-circularity and crown characteristics such as crown volume, crown surface area, or mean branch length of first order branches responded exclusively to competition. We conclude that competition, not genetics, is the main driver of Northern red oak stem and crown characteristics. Thus, stem quality and crown dimension can primarily be controlled by silvicultural interventions. The significant relationship between Northern red oak branch angle traits and individual tree heterozygosity was unexpected, and at this time we do not have any validated explanation for this. This issue needs to be further investigated.

Northern red oak (Quercus rubra L.) wood is valuable for a variety of applications not only in its natural distribution range in North America but also in Europe. Timber quality and stem diameter largely determine timber prices and respective uses. Silvicultural management is key to influencing tree growth and stem quality. In Germany, crop tree thinning is currently the standard treatment, while in Canada the shelterwood system is common practice. The objective of this study was to compare stem characteristics related to stem quality of northern red oaks from Canada with those from Germany to determine effects of different silvicultural treatments on stem quality. We examined stem characteristics from a total of 150 dominant northern red oaks in Canadian and German forest stands using the terrestrial laser scanning approach. Northern red oak stems in Canada (shelterwood system) were significantly straighter, whereas German stems (crop tree thinning approach) were significantly smoother on the surface and rounder on the upper parts of the stems (height 4–8 m). The number of bark anomalies decreased with increasing tree competition, indicating that competition is the main driver influencing external stem form and the occurrence or persistence of bark anomalies.