Effect of topping on microclimate condition and human comfort

2016 | Francesco Ferrini, PhD, University of Florence, Italy and Co-Investigator Simone Orlandini, PhD, University of Florence, Italy

Urban trees create many benefits in terms of thermal comfort and Urban Heat Island (UHI) mitigation during the summer season. These benefits are strictly linked to tree canopy, but the management of the trees in the urban environment includes pruning activities.

The aim of this work is to evaluate the effects of topping on microclimate conditions in the area where trees are planted. We hypothesized that topping can affect temperature of air and soil and air relative humidity. Thus, we want to test the hypothesis that topping does not only depress tree health, but also directly reduces thermal comfort and human well being in cities. The experiment will be conducted using 96 15-year-old maple (Acer spp.) and linden (Tilia spp.) trees. Half of them will be topped in late winter, while the remaining half will be left unpruned, according to a randomized block statistical design with four replicates. Sensors for measuring air temperature and relative humidity during the summer season have been placed in early summer 2016 in the area of research. After topping, tree growth and physiology will be checked, and air and soil temperature and air relative humidity will be continuously monitored for two years, and the effect on human comfort will be calculated by applying biometeorological indices.

Project Final Outcomes:

Topping is often used to limit the conflict between trees and infrastructures, to decrease wind resistance of tree canopies and to remove structural defects for safety. Radical pruning, such as topping to cope with these issues is justified with the erroneous assumption that trees will recover in a short time. Current research is very little focused on the long-term effects of different pruning methods on microclimate conditions and, consequently, on human thermal comfort. Instead, most current efforts focus on short-term effects of pruning on growth response, timing of pruning, wood decay fungi, etc. Trees and vegetation have a natural cooling effect because of shading, potentially reducing surface temperature, from 5°-20°C (9-36 F). In addition, evapotranspiration from vegetation consumes a significant proportion of the available heat energy in the atmosphere. This energy is used to convert water in the leaves into water vapor, which is then transpired through the trees. When considering ground temperature, differences between shaded and unshaded areas can be more than 20°C (36 F) and this strongly affects human thermal comfort and well-being which are vital in urban areas. Thus, further studies on trees and human comfort are warranted.

In this experiment, funded by the Jack Kimmel International Grant Program in 2016, we investigated the effects of topping on tree health and microclimate conditions for two years. We found significantly lower thermal comfort in the plot with topped trees compared to those where the trees where not pruned. In the summer months (July/August) the temperature difference was measured and represented with two indices: ATI and Humidex. The Humidex index was developed in Canada in 1965 (revised in 1979) and is still used by the Canadian Meteo Service to estimate the perceived temperature in high temperature and humidity conditions. Apparent temperature index (ATI) was developed in 1979 and reviewed in 1994 and represents the temperature equivalent perceived by humans, caused by the combined effects of air temperature, relative humidity and wind speed. The difference between the plots with topped trees and control plots was up to 9°C (18 F) in the hottest part of the day, averaging 5°C (9 F) during the day.

In addition, topping had detrimental effects on tree physiology and morphology. Phenological phases were delayed in topped trees such that both budbreak and leaf fall were later than in unpruned trees. Shoot growth was much higher in topped trees (106.92 vs 29.6 cm in maple; 131.98 vs 20 cm in linden). Total leaf area was much lower in topped trees (12.01 vs 120.30 in maple and 26.45 vs 179.05 in linden), resulting in a much lower shading effect by trees. Physiological data confirm conclusions reached in previous research.  Topped trees had an altered physiology with a higher transpiration rate per unit of leaf area but overall lower on plant basis with a consequent reduced cooling effect on the atmosphere. If we hypothesize that these trees were shading a house, this would mean an estimated annual increased cost between $250-490  for additional air conditioning needed to lower the indoor temperature.

Year: 2016

Funding Duration: 1-3 years

Grant Program: Jack Kimmel International Grant

Grant Title:Effect of topping on microclimate condition and human comfort

Researcher: Francesco Ferrini

Presentations:

• Ferrini, Francesco. “Pruning to death: effect of topping on plant growth and physiology and on microclimate conditions.” Presented at Giornate Scientifiche della Società Italiana di Ortoflorofrutticoltura, Bologna, Italy, 19–21 June, 2018.
• Ferrini, Francesco. “Pruning to death: effect of topping on plant growth and physiology and on microclimate conditions.” Presented at 95th ISA Annual International Conference and Trade Show, Columbus, OH, 5–8 August, 2018.
• Ferrini, Francesco. “Effect of heavy pruning on tree growth and physiology and on microclimate conditions.” Presented at World Forum of Urban Forestry, Mantova, Italy, 28 November–1 December, 2018.

For more information on this project, contact the researcher via TREE Fund at treefund@treefund.org.