Arboriculture & Urban Forestry

Unmanaged, foliar pathogens of urban trees can be detrimental to tree health and aesthetics. Overreliance on synthetic fungicides increasingly means alternative means of pathogen management are now required. The purpose of these studies was to investigate the efficacy of 3 commercially available agents, harpin protein, salicylic acid derivative, and liquid chitosan, which can initiate induced resistance (IR) in plants. IR agents were applied independently and in combination with a synthetic fungicide (boscalid + pyraclostrobin) against 2 foliar pathogens (Venturia pirina and Guignardia aesculi) under field conditions with Pyrus communis ‘Williams’ Bon Chrétien’ and horse chestnut (Aesculus hippocastanum) acting as tree hosts. These agents were tested over 3 consecutive years. In 4 of 5 field studies, the use of an IR agent alone reduced pathogen symptom severity, increased fruit/seed yield, and enhanced leaf chlorophyll content. In virtually all studies, application of boscalid + pyraclostrobin at 2/3 strength plus an IR agent provided the same degree of pathogen control as boscalid + pyraclostrobin at full strength. Application of boscalid + pyraclostrobin at 1/3 strength plus an IR agent provided a reasonable degree of foliar pathogen control. Results showed that a combined mix of an IR agent with a 1/3 reduced dose of boscalid + pyraclostrobin was as effective at reducing symptom severity of 2 foliar pathogens as boscalid + pyraclostrobin applied at full strength, provided at least 4 sprays were applied during a growing season. (A, U, M, Bs, Bp)

Municipally managed urban trees provide environmental, social, and economic benefits. Continued provision of these benefits depends on the health and sustainability of these trees, which depends in turn on tree managers having the type of information usually found in a tree inventory. The city of Ithaca, New York, USA possesses 7 inventories of its street and park trees dating back to 1902. This paper uses the data contained in these inventories to assess the health and sustainability of the city’s street and park tree populations. Attention is given to the structure of these populations with emphasis placed on species and genera diversity and DBH size class distributions. Prior to 1987, the city’s municipal tree population was dominated by a few species, such as Norway maple (Acer platanoides), and genera such as maples (Acer) and elms (Ulmus), and the DBH size class distribution was skewed unsustainably towards older trees. From 1987 onwards, new plantings have significantly increased species and genera diversity, and the DBH size class distribution suggests sufficient younger trees to account for tree mortality and removals. These changes did not occur quickly due to the persistent legacy effect of past planting preferences and practices, but required a consistent effort by municipal tree managers over many years. As a result, based on an analysis of the most recent tree inventory conducted in 2019, the city’s street and park trees and the benefits they provide look to be on a more sustainable footing, although challenges still remain. (A, U, M, Bm)

Reduction pruning of the main stem is commonly used during the maintenance of power lines to encourage the establishment and development of scaffold limbs away from wires. Understanding the physiology of epicormic branch initiation and growth as well as wound compartmentalization following reduction pruning are important for optimizing the pruning cycle and maintaining healthy and safe trees. In this study, the influence of both intensity and time of year of pruning on epicormic branch response and wound compartmentalization was investigated on 56 11-year-old Pennsylvania ash trees (Fraxinus pennsylvanica Marsh.) about 5 to 7 m in height within a controlled nursery environment. During the second growing season following reduction of the main stem, the number, height, and volume of epicormic branches, as well as tallest epicormic branches and the area of discolored wood, increased with pruning intensity. Pruning during the leaf-on season compared to the leaf-off season limited the establishment and development of epicormic branches without affecting wound-closure rate or the area of wood discoloration at the cutting point. Results are consistent with the known seasonal fluctuation of carbohydrates reserves. In the context of the electrical distribution network, where trees are subjected to pruning throughout the year, trees pruned in summer during a maintenance cycle could be pruned during the next cycle, in winter, and so on, to optimize the return interval of the pruning cycle. (A, U, M, T, L, Bp, Bm)

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)