Summer Branch Drop (SBD; synonyms are Sudden Branch/Limb Drop, Sudden Limb Shear) is the failure and collapse of individual, healthy branches of at least 5 cm in diameter and in full leaf. It usually occurs when there is no wind and after periods of prolonged drought and / or high temperatures [2]. 

In the currently available literature the phenomenon is characterised as follows:

The branch failure typically takes place on hot, calm summer afternoons [3; 4] or in calm weather after heavy summer rain [4].

First order structural branches of mature trees  are generally affected. Affected branches are often described as horizontally growing, widely spreading and located in the lower or middle crown area [1; 3; 5].

The fracture typically does not occur at the branch union, but often occurs at 1 – 4 m from the branch collar [3], leaving a stub. There is a lack of obvious tree risk features prior to branch failure. The fracture site shows no sign of decay or other internal defects [4].

Based on observations, it is assumed that some tree species are more susceptible to SBD, although the information varies depending on the literature cited: Clearly, hybrid poplars are particularly affected [1; 4; 5].
Furthermore, SBD has increasingly been observed in the genera of Acer (maple), Aesculus (horse chestnut), Fagus (beech), Fraxinus (ash), Quercus (oak) and Salix (willow). But it is also known for Cedrus (cedar), Eucalyptus, Liquidambar, Pinus (pine), Platanus (plane) and Ulmus (elm) and quite likely affects other species [3].

In the case of SBD, only large branches pose a risk for traffic safety. Therefore, a distinction must be made between branch tear out and SBD. Branch tear outs are commonly characterized by short leafy shoots tearing away from the branch collar. Some triggers that cause branch tear out are decreasing vitality or the influence of stressors such as a lack of water or root severance [1].

Literature:

[1] Dujesiefken, D. Jaskula, P. Kowohl, T. und Wohlers, A. (2023): Baumkontrolle unter Berücksichtigung der Baumart. Haymarket Media, Braunschweig. 320 S.

[2] FLL (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e. V.) (Hrsg.) (2020): Baumkontrollrichtlinien. Richtlinien für Baumkontrollen zur Überprüfung der Verkehrssicherheit. Selbstverlag, Bonn. 52 S. 

[3] Harris, R. W. (1983): Summer Branch Drop. In: Journal of Arboriculture 9(4). S. 111 – 113

[4] Sinn, G. (2003): Baumstatik. Stand- und Bruchsicherheit von Bäumen an Straßen, in Parks und der freien Landschaft. Thalacker Medien, Braunschweig. 184 S. 

[5] Weihs, U. (2014): Problem Grünastbrüche – Teil 1: Phänomen, Bewertung. AFZ-DerWald 16/2014, S. 34 – 37.