Qualitative and quantitative analyses of tree forking habit with single-image photogrammetry: a case study of old-growth temperate oaks in a mixed, deciduous forest remnant near Krakow, Poland

Tree forking is both ecologically and economically relevant, but remains much understudied. Here, thirty post-mature temperate oaks (Quercus robur or Q. petraea) forking habit was both qualitatively and quantitatively analysed with the single-image photogrammetry (SIP), in a north-exposed mixed, deciduous forest remnant (near Krakow; Poland). A new classification of mature oak architectures was proposed, based on the original Halle-Oldeman model, with modified locations of the main branches and presence or absence of bifurcation in the main stem. Two of the new classes were most clearly represented by the studied oaks. It was found that the trees tended to either keep branches at varying heights, with no forks, or to iterate forking, with no major (non-fork) branches below the first fork. The quantitative analysis confirmed the applicability of the branch to parent stem diameter ratio to define a fork. Branching ratio was positively correlated with both tree diameter and height of a branch above the ground, which is consistent with a previous study, based on much younger trees. It is concluded, that most probably the tree-level factors and phenomena, such as water supplies and posture control, played the key role in the studied oaks forking habit. The SIP method enabled valuable insights into the large oaks forking, both at the tree and branch levels, and may be further employed to study mature trees bifurcation patterns. Based on this study, some possible improvements to the methodology were discussed.