Effects of urban trees on regional energy use and avoided carbon

Introduction

Urban forests can reduce atmospheric carbon dioxide in two ways. As long as trees are actively growing, their rate of CO₂ uptake through photosynthesis is greater than CO₂ release through respiration, resulting in net storage of carbon as biomass (referred to as sequestered CO₂ on an annual basis), hence net reduction of CO₂ in the atmosphere. Secondly, trees around buildings, through their moderating influence on solar gain, wind speed, and air temperature, can reduce demand for heating and air conditioning. This reduces associated emissions from fossil fuels associated with heating and production of electric power, primarily for cooling, referred to as avoided CO₂.

A number of factors suggest important regional differences in potential CO₂ reduction. Foremost among these is climate, which plays an important role in determining the magnitude of both sequestered and avoided CO₂. Regional climate differences influence tree growth rates, which are directly proportional to sequestration rates, and mature size. Similarly, avoided energy use is directly related to tree size and growth rate. Climate also determines relative importance of heating and cooling, which is particularly important since trees can increase as well as decrease heating loads. Other regional factors which may play a role are building construction practices (which are also influenced by climate), and fuel types used for space conditioning. Fuel type is important since it determines emissions per unit of energy produced.

Objectives

The purpose of this paper is to quantify the effects of regional differences in climate, trees, housing construction and fuel mix on space-conditioning energy use and avoided CO₂, and demonstrate some practical applications of the results for selecting and locating urban trees. With the goal of maximizing avoided carbon for each region, areas considered are: a. What are optimum tree placements, and which should be avoided? b. Which circumstances favor deciduous over evergreen trees? c. What generalizations can be made about tree selection and placement for heating-dominated versus cooling-dominated regions? d. What is the effect of different fuel types on atmospheric CO₂ reductions?

Results

Total avoided and sequestered CO₂ are given over the 40 year time span of the analysis for each of the 11 climate zones. Avoided emissions are further broken down to compare heating versus cooling, and net changes due to shade compared to those from lowered air temperatures and wind speed. General recommendations are presented regarding tree selection and placement for different regions of the United States.

The results presented here is preliminary due to the limited nature of some input data and model components. Current information available in the literature indicates that reductions predicted here are conservative. Until improved alternatives become available, these data provide a useful and widely applicable tool for selecting and locating urban trees based on their energy and carbon reduction potential.