In situ quantification of experimental ice accretion on tree crowns using terrestrial laser scanning

Background/Question/Methods

In the eastern hardwood forests of North America ice storms are an important disturbance event. Ice storms strongly influence community dynamics as well as urban infrastructure via catastrophic branch failure; further, the severity and frequency of ice storms are likely to increase with climate change. However, despite a long-standing interest into the effects of freezing rain on forests, the process of ice accretion and thus ice loading on branches remains poorly understood. This is because a number of challenges have prevented in situ measurements of ice on branches, including: 1) accessing and measuring branches in tall canopies, 2) limitations to travel during and immediately after events, and 3) the unpredictability of ice storms. Here, utilizing a novel combination of outdoor experimental icing, manual measurements and terrestrial laser scanning, we perform the first in situ measurements of ice accretion on branches at differing heights in a tree crown and with increasing duration of exposure. 

Results/Conclusions

We found that terrestrial laser scanning can reproduce both branch and iced branch diameters with high fidelity, but some devices do not detect ice. Contrary to the expectations of ice accretion models, radial accretion varied sharply within tree crowns. Initially, radial ice accretion was similar throughout the crown, but by the second scanning interval radial ice accretion was much greater on upper branches than on lower (~factor of 3). The slope of the change in radial ice accretion along branches increased with duration of exposure and was significantly greater at the second scanning interval compared to the first. We conclude that outdoor icing experiments coupled with the use of terrestrial lidar scanning provide a robust basis for evaluation of models of ice accretion and breakage in tree crowns, facilitating estimation of the limiting breaking stress of branches by accurate measurements of ice loads.