Abstract
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 (HoAll) and for EST-derived markers (HoEST). 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.
Highlights
In Europe, cultivation of Northern red oak (Quercus rubra L.), a non-native tree species, has become increasingly popular since it expands the portfolio of forest enterprises and is thought to minimize management risks (Spellmann et al 2011)
We investigated the relationship between competition (Hegyi-Index), heterozygosity measured by EST-simple sequence repeats (SSRs) markers “HOEST”, heterozygosity measured by all markers “HOAll”, and heterozygosity measured by nuclear simple sequence repeats (nSSR) markers “HONeutral” as independent variables, and morphological tree characteristics as dependent variables
Four crown characteristics were significantly related to heterozygosity measured by ESTSSR markers (HOEST), which are located in expressed genes, and three crown attributes to heterozygosity measured group-wise for “All” markers (HOAll)
Summary
In Europe, cultivation of Northern red oak (Quercus rubra L.), a non-native tree species, has become increasingly popular since it expands the portfolio of forest enterprises and is thought to minimize management risks (Spellmann et al 2011). The response of Northern red oak to silvicultural interventions has not yet been adequately investigated outside of its natural distribution range (Burkardt et al 2019). Questions such as how often Northern red oak stands are to be thinned and how intensively thinnings should be carried out are currently under debate (Nagel 2015). Since branchiness of trees increases when competition is reduced by crown thinning, trade-offs exist between stem quality and diameter growth (Sonderman and Rast 1988). Optimised approaches are required to achieve both good stem quality and high yields
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