Arboriculture & Urban Forestry

Trees are subjected to mechanical loading during their life span or face premature mortality. The strain resulting from loads intercepted by the canopy and transferred throughout the tree is of significant importance, not only for the survival of the tree, but for the safety and well-being of the human population found in close proximity. To test the function of tree orientation to an applied load, static load tests were conducted on 15 mature pin oak trees (Quercus palustris Muenchh.). We applied the static load tests to tilt the trees 0.1° from natural position. We used a digital image correlation system to map strain in the leeward, windward, and tangential roots in the root-stem transition zone. Results indicate that mean maximum strain magnitudes are similar in the leeward and windward orientations and lower on the tangential orientation. The leeward orientation experienced compressive strain, the windward orientation experienced tensile strain, and the tangential orientation had both tensile and compressive strain. This information provides the arboricultural and plant science sectors with a better understanding of how loading force moves through trees and will further enhance tree risk assessment and root zone management protocols. (A, U, M, T, L, Bs)

Trees are known to provide various ecosystem services and disservices to urban communities, which can be quantified using models based on field and environmental data. It is often uncertain how tree structure and environmental variables impact model output. Here we perform a sensitivity analysis (SA) of i-Tree Eco, a common urban forest model, to analyze the relative impact of different model inputs on three module outputs: biogenic volatile organic compound (BVOC)(isoprene and monoterpenes) emissions, carbon storage and sequestration, and dry deposition of nitrogen dioxide, sulfur dioxide, and ozone. The SA methods included novel applications of the Morris one-at-a-time method and a variance-based decomposition method, which integrates Monte Carlo simulation with Latin hypercube sampling and Iman Conover analysis. A case study was performed in New York City, New York, USA, with field plot data collected in 2013. Genus has the largest influence on BVOC emissions by determining base emission rates and its high interactions with other input factors, and BVOC emissions are sensitive to leaf biomass in a concave manner and temperature in a convex manner, while isoprene emissions show a strong linear relationship with photosynthetically active radiation (PAR). Diameter at breast height plays the most important role for both carbon storage and sequestration estimators; crown light exposure and tree condition are also important for carbon sequestration. Dry deposition velocity is sensitive to leaf area index and relative humidity in a nearly linear way, while sensitive to temperature and PAR in a concave manner. The results provide guidance to facilitate future field plot campaigns and model development. The knowledge revealed by the SA is also beneficial for model uncertainty reduction, which in turn facilitates more effective urban forest management and decision-making. (A, U, M, Bs, Bm)

Bark-included junctions are frequently encountered defects within the aerial structures of trees. The presence of included bark within a branch junction can substantially reduce the junction’s factor of safety. Recent research has found naturally occurring bracing to be a primary cause of the formation of included bark within branch junctions. This study tested the load-bearing capacity of branch junctions in hawthorn (Crataegus monogyna Jacq.) using rupture tests and compared the mechanical performance of “control” branch junctions, bark-included junctions with the natural bracing retained, and bark-included junctions where we had intentionally removed their natural braces by cutting them out. Substantial variability was observed in the failure kinematics of bark-included branch junctions when their natural braces were retained. The type of natural brace present affected the mode of failure of the branch junctions when pulled apart. A single specimen with fused branches presented the strongest form of natural brace in this study, followed by entwining branches, whereas crossing branches were found to provide the least mechanical resistance. This study provides initial evidence that the type of associated natural brace is an important consideration when an arborist is trying to assess the likely mechanical performance of a bark-included junction within a tree and its likelihood of failure. (A, M, T, Bs, Bp)

The objective of this study was to identify and quantify the hazards present during arboricultural operations. The Occupational Safety and Health Administration Fatality and Catastrophe incident database and other Bureau of Labor Statistic sources were analyzed for arboricultural operation incidents within the 17-year period from 2001 through 2017. There were 865 fatal and 441 nonfatal incidents reviewed from this period. The leading four fatal incidents, from the largest to the smallest number of fatalities, were climber falls, workers struck by a falling tree, workers making indirect contact with an electric current, and workers struck by a falling branch. Climber falls were also the leading incident for severe nonfatal injuries, followed by ground workers struck by a falling branch, workers struck by a chain saw, and falls by aerial device operators. The American National Standards Institute Z133 American National Standard for Arboricultural Operations—Safety Requirements establishes safety requirements and recommendations for arboricultural operations in the United States. It addresses common hazard sources and has guidelines to avoid, eliminate, or reduce them. Safety training programs should emphasize the most common hazard sources for fatal and nonfatal incidents and follow the ANSI Z. (A, U, M, T, L, Bp)

The incidence and severity of internal decay was assessed in 210 landscape American elms using sonic and electrical-resistance tomography. Sampled trees were partitioned into two fungicide injection groups: (1) regular injection; and (2) irregular injection or no known history of injection. The proportion of American elms with decay under regular injection (28/91; 31%) was nearly identical to non-injected elms (35/119; 29%). The results show that American elms undergoing regular fungicide injection do not experience a significantly higher frequency of internal decay compared to non-injected elms. (A, M, Bs, Bp)

In urban areas there is a limited amount of soil space available for tree root growth. However, many systems have been developed that provide rooting space below pavement while supporting the weight of vehicles and pedestrians. Two main approaches have emerged: 1) supported pavement, and 2) structural growing media. This research was composed of two controlled studies that compare variations of these two approaches. The first was a 10-year study using elm trees that compared gravel-based structural soil (GBSS), expanded slate structural soil (ESSS), expanded slate (ES) alone, a concrete supported pavement and a compacted control. The second study was a four-year study using Liriodendron trees that compared GBSS, sand-based structural soil (SBSS), Silva Cells™, Stratacells™, an open control, and a compacted control. The results of these two studies showed that the trees growing in the supported pavement treatments with low-density soil media resulted in significantly greater tree growth and a healthier appearance. The treatments with the highly compacted soil media had less root development and less top growth. However, soil media that were highly compacted experienced less subsidence. (A, M, Bs, Bm)

Cities contain dozens of street tree species, each with multiple arthropod pests. A survey was used to identify the most common and damaging pests of the most common trees in the southeastern U.S.A. Tree genera varied in number of common pests from Zelkova, which has few pests, to Quercus, which has many pests from many feeding guilds. Sucking pests, such as aphids and scale insects, were rated among the most important pests on several tree genera, including Acer, Liriodendron, and Lagerstroemia. Defoliators, primarily caterpillars, were ranked highest on Quercus spp. Knowledge of the most common and damaging pests will help focus integrated pest management development and educational activities. (A, M, Bs, Bm)

Limited water availability can cause significant stress to newly transplanted urban trees, necessitating supplemental watering during the tree establishment period. The authors compared three types of indirect watering devices in terms of their performance and their benefits to newly transplanted river birch (Betula nigra) trees in a controlled greenhouse environment. (A, M, T, Bm)

The topic of wire basket removal during planting remains an area of contention among nursery growers, landscapers, and arborists who work with balled-and-burlapped planting material. Those in favor of removal fear that the burlap and wire will impede root regrowth and lead to girdling roots, while those opposed to removal believe leaving materials intact improves transplant stability and establishment. (A, M, Bp)

Roadside trees provide benefits to drivers such as traffic calming, roadway definition, and driver stress reduction. However, trees are also one of several roadway infrastructure elements commonly involved in single-vehicle crashes. In this study, Florida Highway Safety and Motor Vehicle records were analyzed to: evaluate the relative frequency of tree-related crashes compared to other fixed-object crashes; assess the impact of roadway-, vehicle-, and driver-related factors on tree crash frequency; and compare the severity of tree crashes relative to other single-vehicle crashes. (A, M, Bm)

The urban forest is a valuable ecosystem service provider, yet cities are frequently degraded environments with a myriad of stressors and disturbances affecting trees. Vulnerability science is increasingly used to explore issues of sustainability in complex social-ecological systems, and can be a useful approach for assessing urban forests. The purpose of this study was to identify and explore drivers of urban forest vulnerability in a residential neighborhood. Understanding the causes of urban forest change and decline are essential for developing planning strategies to reduce long-term system vulnerability. (A, M, T, Bm)

Volunteerism is one way to handle limited resources for municipal inventories. This study follows the Forest Health Ambassador Program and how it used volunteers to track signs and symptoms of infestations by emerald ash borer, gypsy moth, and Asian longhorned beetle. (A, M, Bs)