2 Literature review
Trees are widely acknowledged for their contributions not only to nature conservation but also to human well-being. These contributions, known as ecosystem services, manifest across various scales, encompassing local forest products, regional watershed services, and global advantages derived from mitigating global climate change through carbon storage and the sequestration of greenhouse gases (Birch et al., 2014). This literature review examines various methods including the regression analysis and the cost approach methods—in the context of tree valuation, exploring their applications and advantages.
In the realm of urban forests, the term “ecosystem services” pertains to the advantages bestowed upon humans through functional operations and interactions with the nearby environment and indigenous ecology (Livesley et al., 2016). Over the past few decades, extensive research has been conducted to unravel the multifaceted contributions of trees to human well-being and environmental sustainability. This literature review aims to synthesize existing knowledge on the ecosystem services provided by trees, examining their implications for both ecological integrity and human society.
Urban forests offer numerous benefits for human health, with one of the most significant being the interception and reduction of air pollution (McDonald et al., 2016; Nowak, Hirabayashi, Bodine, & Greenfield, 2014; Nowak, Hirabayashi, Doyle, McGovern, & Pasher, 2018). Air pollution, including ozone, carbon monoxide, and other pollutants, has been linked to various health problems such as bronchitis symptoms, cardiovascular issues, cognitive development problems in children, and mortality (Berhane et al., 2016; Di et al., 2017; Nwanaji-Enwerem et al., 2019; Peterson et al., 2015; Weichenthal, Hatzopoulou, & Goldberg, 2014; Zhong et al., 2016). Increasing the number of trees, especially mature species planted strategically, can effectively reduce particulate matter and other forms of air pollution, potentially lowering mortality and morbidity rates in urban centers.
Beyond pollution removal, trees offer additional direct and indirect health benefits, promoting overall well-being (Donovan, 2017). The presence of trees and green spaces has been strongly linked to reduced negative thoughts, symptoms of depression, improved moods, and increased life satisfaction (Bratman, Hamilton, Hahn, Daily, & Gross, 2015; Li et al., 2018; Taylor, Wheeler, White, Economou, & Osborne, 2015). While biophysical benefits are more easily understood and quantified, social benefits such as improved public health and crime reduction are less intuitive and more challenging to measure accurately (Donovan, H. 2017).
Trees in urban environments offer a multitude of ecosystem services, including reducing energy consumption and pollution, enhancing property values, and supporting local economies (Nowak & Greenfield, 2018; Nesbitt et al., 2017). The economic value of trees, estimated at $18.3 billion annually in the United States alone, highlights the importance of investing in tree planting and maintenance (Nowak & Greenfield, 2018). Additionally, urban trees provide resources such as food, contributing to food security and promoting well-being (Clark & Nicholas, 2013). Agroforestry practices utilizing urban orchards can provide nutrient-dense food to communities (Dawson et al., 2013). Overall, urban trees continue to provide valuable resources and benefits to communities even after they are cut down.
Climate change directly impacts human health, particularly through increases in heat-related deaths, diseases, and infectious diseases. This risk is exacerbated in urban areas due to the Urban Heat Island Effect, which intensifies heat waves (Ward et al., 2016). Trees are crucial in mitigating temperatures in built environments by providing shade and actively cooling the air through evapotranspiration (Hirons & Thomas, 2018). McDonald et al. conducted research that indicates that trees can significantly reduce temperatures in cities, with an average cooling effect of 1.9°C and the potential to lower temperatures by up to 9°C (McDonald et al., 2016), thus helping to alleviate the impacts of climate change on human health. Moreover, trees are essential for carbon sequestration, which mitigates climate change by storing carbon. Urban trees in the coterminous United States sequester 22.8 million tons of carbon annually and store a total of 700 million tons of carbon. (Turner-Skoff & Cavender 2019).
The study conducted by McPherson et al focuses on assessing the structure, function, and value of California’s street tree population, to quantify ecosystem services and prioritize management challenges by highlighting the significance of street trees in urban environments and their impact on urban life quality. The approach used for the assessment included climate zone subdivision, street tree inventories, i-Tree Streets modeling, and calculations of street tree structure, function, and value. The study utilized 49 municipal tree inventories encompassing 908,304 street trees, with data processed to standardize for comparison. The i-Tree Streets modeling tool was employed to quantify the structure, function, and value of ecosystem services provided by street trees. The findings highlight the contribution of street trees to various ecosystem services such as energy savings as the state’s 9.1 million street trees save energy worth a total of $101.15 million annually, air pollutant uptake with the average annual decreases per tree being 62.2 kilograms, with variations ranging from 50.6 kg to 93.3 kg. The corresponding monetary worth is $10.32 million ($1.67 million se) per year or $1.13 on average per tree, rainfall interception which has a $41.5 million monetary value, and property value benefits as street trees contribute to the sales prices of homes and provide other benefits valued at $838.94 million ($130.9 se) per year (McPherson et al., 2016)
Birch et al assessed ecosystem services of community trees using the Toolkit for Ecosystem Services Site-based Assessment (TESSA) to measure ecosystem services in a Himalayan Forest in Nepal. Various ecosystem services, including carbon storage, water provision, water quality, harvested wild goods, cultivated goods, and nature-based recreation, were assessed. For carbon storage, the study estimated a decrease of 64% in carbon stocks and 50% in annual sequestration under an alternative state without community forestry. Water-related services were also affected, with a slight decrease in overall water balance but a significant increase in soil erosion and sediment load, affecting water quality. Harvested wild goods, including fuelwood, fodder, and leaf litter, were estimated to decrease in value by 70% under the alternative state, although there would be a one-off benefit from wood products due to forest degradation. Cultivated goods, primarily crops, would provide an annual net benefit of $920,000 under the alternative state. Nature-based recreation at Phulchoki IBA generated a direct net income of $8000 annually for the community forest user groups (Birch et al., 2014).
Through an analysis of existing research, we have identified the multitude of biophysical and social benefits that trees offer, including energy savings, carbon sequestration, improved air quality, storm-water management, crime reduction, and enhanced public health.
Although it can be challenging to determine the monetary value of urban trees, several methods have been developed to estimate it (Grande-Ortiz et al. 2012). The most common way to determine the monetary value of urban trees is by using specific formulas. These valuation formulas, however, rely on different assumptions and approaches, leading to variations in their structure, application, and ultimately in the final monetary value assigned to the tree (Randrup 2005). Several studies have used regression analysis to enhance tree appraisal. For instance, Peper et al. (2001) and Tyrväinen (2001) applied regression models to determine how tree characteristics influence property values and public preferences for urban greenery. These studies found that larger trees and certain species have a significant positive impact on property values, highlighting the broader economic benefits of trees.
However, despite the extensive research conducted on urban trees and their ecosystem services, there remains a notable gap in the literature regarding the appraisal of community trees in Kamloops, Canada. While research by McPherson et al. (2016) has demonstrated that hybrid models incorporating cost-based values and predictive regression equations provide more balanced assessments, especially for large-scale tree inventories, there is limited research specific to Kamloops. This represents an opportunity for research to explore the applicability and effectiveness of the hybrid approach in assessing the ecosystem services provided by trees in Kamloops, thereby informing evidence-based urban forestry management practices in the region.
