Illustration of the method for measuring diameter of dowels and branches from point cloud data.

<p>The diameter of the sphere is determined by fitting cylinders to the point cloud using an algorithm and data is then output for the diameter at each point on the branch marked by a yellow sphere. Sphere diameter is then manually adjusted if visual inspection suggests a better fit is possible. The branch shown has 15 points of measurement at ∼30 cm intervals.</p

<i>In Situ</i> Quantification of Experimental Ice Accretion on Tree Crowns Using Terrestrial Laser Scanning

<div><p>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 <i>in situ</i> 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 (TLS), we perform the first <i>in situ</i> measurements of ice accretion on branches at differing heights in a tree crown and with increasing duration of exposure. We found that TLS can reproduce both branch and iced branch diameters with high fidelity, but some TLS instruments 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 after 6.5 hours of irrigation (second scanning) 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 compared to the first. We conclude that outdoor icing experiments coupled with the use of TLS 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.</p></div

Experimental design with the study tree in the center and scaffolding towers to elevate the sprinkler system (A) and progressive change in tree architecture with increasing ice accretion (B).

<p>Experimental design with the study tree in the center and scaffolding towers to elevate the sprinkler system (A) and progressive change in tree architecture with increasing ice accretion (B).</p

Summary of coefficients and 95% confidence intervals for a regression of caliper measurements of diameter (x) versus terrestrial laser scanning measurements of diameter (y) for a subset of measured tree branches and dowels.

*<p>Note: RSE is the residual standard error.</p

Change in radial ice thickness as measured by increasing distance from branch tips (A, B) or increasing diameter along branches (C, D) for upper (B, D) and lower (A, C) relative positions in the crown.

<p>Hollow circles indicate data collected at the first scanning and solid circles the second scanning (details in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064865#pone-0064865-t001" target="_blank">Table 1</a>). Fitted model coefficients presented in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064865#pone-0064865-t003" target="_blank">Table 3</a>.</p

Timing of terrestrial laser scanning scans of the tree during the experiment and average tip ice accretion at a given time.

*<p>Note: Radial ice accretion at branch tips taken from the predicted relationship (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064865#pone-0064865-t003" target="_blank">Table 3</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064865#pone-0064865-g005" target="_blank">Figure 5</a>).</p

Summary of coefficients for models describing the variation in radial ice accretion in a tree crown with crown position (P), location on branch (D)^* and time during the experimental exposure of the tree to freezing rain (T).

*<p>Note: D is described as both increasing distance from the tip (Dt) and with increasing branch diameter (Db). RSE is the residual standard error.</p

Comparison of the fidelity of terrestrial laser scanning estimates of the diameter of wooden dowels (A) and branches (B) with an ice coating.

<p>Branch and dowel diameter was measured using a caliper in the field and diameters for the same locations on branches were also extracted from terrestrial laser scanning point clouds. The solid line indicates a 1∶1 relationship and the dotted line the fitted relationship. Note: Model coefficients and summary statistics presented in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064865#pone-0064865-t002" target="_blank">Table 2</a>. Timing of terrestrial laser scanning scanning given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064865#pone-0064865-t001" target="_blank">Table 1</a>.</p

Relationship between equivalent radial ice thickness (R_eq) and caliper measurements of ice radial thickness.

<p>R_eq is estimated from the volume of ice (melt water) on branch samples with a given diameter and length. The solid line indicates a 1∶1 relationship and the dotted line the fitted relationship (y = −0.004+0.97x; F = 179.2, DF = 10, p<0.001, R² = 0.95).</p