Influence of competition and other factors on crown and stem characteristics of northern red oak
by Katharina Burkardt
Date of Examination:2021-07-22
Date of issue:2022-03-10
Advisor:Dr. Torsten Vor
Referee:Prof. Dr. Christian Ammer
Referee:Prof. Dr. Oliver Gailing
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Abstract
English
Native to northeastern America and southeastern Canada, northern red oak (Q. rubra L.) was introduced to Europe in 1691 and initially planted in parks and gardens. In the following centuries, three silvicultural cultivation periods in Europe demonstrated that northern red oak had both good growth and favorable silvicultural treatment options. The cultivation of native tree species, however, was always favored. Nowadays, the cultivation of non-native tree species is increasingly considered, since the consequences of climate change already range from losses of stand vitality to a dieback of entire forest areas, which causes the cul- tivation with native tree species to be subject to uncertainties. Northern red oak represents a tree species that is expected to withstand future climate conditions in Germany on suita- ble sites and reduce the risk of forestry operations. With an area share of 55000 ha, Q. rubra constitutes the most common non-native deciduous tree species. To date, there are few empirical studies on silvicultural treatment of northern red oak in Germany, and also, from the practitioners' point of view, there are various uncertainties regarding the intensity and frequency of interventions. Therefore, the aim of this study was to investigate the influence of competition — which can be controlled by the intensity and frequency of silvicultural interventions — on crown and stem characteristics of northern red oak. The following hypotheses were proposed: 1. increasing competition results in shorter and more slender tree crowns; 2. increasing com- petition leads to fewer external stem characteristics, which are negatively related to the commercial timber value of northern red oak; 3. competition effects on stem and crown characteristics are superimposed by genetic variation; 4. stem form and number of bark anomalies—both indicating stem quality of northern red oak trees—differ between Cana- dian and German stands due to different silvicultural practices; 5. increasing tree competi- tion is negatively related to external stem quality characteristics, and this relationship is constant under both management systems (shelterwood system vs. crop tree thinning). The results of the study show that competition is the main factor influencing the tree mor- phology of northern red oak. Specifically, it was observed that various crown characteristics associated with both the vertical and horizontal crown extent become smaller with increas- ing competition. Thus, the first hypothesis of this study can be confirmed. The results sug- gest that the crown size is a good predictor of the level of competition faced by a given tree. Light capturing depends on the crown extent, making a competition-induced reduction in crown size, such as crown radius, likely to affect growth performance and thus tree produc- tivity. The second hypothesis can also be verified, as high competitive strength in Q. rubra resulted in a decrease in external (i.e., undesirable) stem characteristics, particularly stem non-circularity and the number of bark anomalies. The measure of the number of bark anomalies includes both branchiness and irregularities on the stem surface (stem injuries such as bark seams). Since branch-related wood defects are considered a major cause of declining stem quality in many tree species, this study confirms that northern red oak re- quires, similar to many other native tree species, higher levels of competition during the early stages of stand development to produce high quality timber. A relationship was also found between abiotic factors and stem and crown characteristics. The third hypothesis can be partially confirmed, as a correlation between genetic diversity (heterozygosity "EST" and "All") and first-order branch angle measures (branch angle (mean; median) and branch an- gle range) was detected. With increasing heterozygosity, branch angles of Q. rubra became steeper. To verify the results, future studies may focus on identifying potential adaptive markers of northern red oak by modern high-throughput techniques such as RADseq (restriction site associated DNA sequencing) and on analyzing potential candidate genes for shaping crown morphology. A comparative assessment of external stem characteristics between the Ger- man and Canadian stands showed that values for the number of bark anomalies and stem non-circularity were significantly higher in the Canadian stands, whereas the stems in Ger- many were significantly more curved and crooked. We attribute the significant differences between countries to differences in silvicultural practices. Thus, the hypotheses 4 and 5 can also be confirmed. For silvicultural treatment in northern red oak, which aims at achieving high quality timber, our results indicate that stands should be kept mainly at high stand densities in the initial development stages until the desired branch-free stem length has reached the desired height. Subsequent thinning should be moderate to heavy and at regular intervals for the highly phototropic Q. rubra, to promote crown size and thus productivity. The respective thinning intensity and frequency should always be adapted to the respective site conditions.
Keywords: Northern red oak; Competition intensity; Terrestrial Laserscanning; Stem characteristics; Crown characteristics; Individual-based heterozygosity; Silvicultural treatment