Environmental Benefits	Shade Tree	Palm
	Live Oak, Quercus virginiana	Sabal Palm, Sabal palmetto
Diameter (DBH)	16"	16"
Carbon Dioxide (CO2) Sequestered (Absorbed)	510 punds/year	2.71 pounds/year
Rainfall Intercepted	725 gallons/year	81 gallons/year
Ozone removed from air	20 ounces/year	1.70 ounces/year
Carbon Dioxide stored	3,214 pounds over lifetime	26 pounds over lifetime
Energy Savings (A/C)	60 kWh	26 kWh
Energy Savings Value	$10.00	$4.60
Annual Value of Benefits	$31.00	$6.48
*Based on an analysis utilizing the USDA Forest Service's i-Tree MyTree benefits tool (www.itreetools.org) - v.2.4.16

 

Table 1: Comparison of the environmental benefits comparison between live oak and sabal palm. These species were selected because they are both native and in the top five of commonly used species.

 

Three of the main environmental benefits from a healthy urban tree canopy are reduction in urban heat island effect, improved air quality, and enhanced stormwater management.

 

1. Reduced Urban Heat Island Impact

According to the Environmental Protection Agency (EPA), a city with extensive areas of impervious surfaces can be 1-3 degrees Fahrenheit warmer than surrounding areas during the day, and up to 22 degrees Fahrenheit warmer at night. Impervious surfaces (e.g. roads, buildings and sidewalks) trap and retain heat, causing air temperatures to be hotter than nearby areas that are less built up. This is known as the urban heat island effect. With over 61% of the land area covered by impervious surfaces in Miami Beach, urban heat island impacts can be significant and may include:

 

• Respiratory difficulties (e.g. asthma), heat stroke, and heat-related mortality.

• Increased energy consumption needed for cooling homes and businesses, resulting in higher energy bills.

• Increased ground level ozone created by chemical reactions of atmospheric gases and compounds with sunlight and heat.

 

The coolest areas within Miami Beach, are those with the highest tree canopy coverage and least amount of impervious surfaces (Attachment B). The UFMP sets goals, targets, and recommendations to enhance tree canopy coverage in areas most impacted by our urban heat island.

 

2. Improved Air Quality

Trees and palms absorb atmospheric carbon and reduce greenhouse gas (GHG) emissions. Urban trees act as a sink of CO2 by storing excess carbon as biomass (e.g. trunk, branches, leaves, roots) during photosynthesis. The amount of CO2 stored is proportional to the biomass of the tree. Therefore, canopy trees provide seven to ten times more carbon sequestration capacity than that of a palm.

 

Trees and palms serve an important function in improving air quality, reducing air pollutants and helping ameliorate the public health effects of air pollution. Trees and palms intercept and filter particulate matter from the air, including dust, ash, pollen, and smoke. Canopy trees uptake more CO2 and pollutants than palms.  Increasing the number of canopy trees will assist improving air quality and reducing air pollutants.

 

3. Enhanced Stormwater Management

The Urban Land Institute evaluated the City’s resilience and stormwater management programs and encouraged the City to utilize green infrastructure as an integral component to stormwater management. A healthy urban forest is a main component in the City’s green infrastructure development. During storm events, trees intercept rainfall in their canopy acting as a mini reservoir. Intercepted rainfall evaporates from leaf surfaces or slowly soaks into the ground, reducing and slowing stormwater runoff, and lessening the impacts of rainfall on barren soils and impervious surfaces. This processes greatly reduces the flow and volume of stormwater runoff, reducing flooding and erosion and preventing sediments and pollutants from entering waterways.

 

Botany Overview of Trees & Palms

There are three main botany categories for trees and palms: deciduous trees, semi-deciduous trees, and evergreen tree and palms. Deciduous trees seasonally shed most of their leaves. Approximately 2% of Miami Beach’s tree canopy consists of deciduous trees, these include species such as royal poincianas and crepe myrtles. Semi-deciduous or semi-evergreen trees lose their foliage for a very short period, when old leaves fall off and new foliage growth begins. This is typical of sub-tropical woody species. Approximately 11% of the City’s canopy consists of semi-deciduous or semi-evergreen trees, such as mahoganies and live oaks. Evergreen trees, including palms have green foliage that remains green and functional through more than one growing season. However, evergreens do lose their leaves and fronds sporadically throughout the year to allow for new growth.  Approximately 30% of the City’s tree canopy are evergreen trees and 57% are palms. Evergreen trees include pitch apple, sea grapes, green buttonwood, and silver buttonwoods.

 

Deciduous	2%
Semi-deciduous	11%
Evergreen - Canopy	30%
Evergreen - Palms	57%

 

Grass clippings and leaf litter carry nutrients into the stormwater system when it rains. However, the largest new inputs of nitrogen and phosphorous are fertilizer and pet waste, respectively[3]. It should be noted, that due to soil conditions and pruning frequency, most palms require multiple applications of fertilizer throughout the year to sustain its nutritional needs, as opposed to canopy trees that do not require fertilizers once they are established.

 

The best management practices for managing nutrient loading from vegetative debris included enhanced municipal street sweeping operations, yard waste removal, street narrowing, or installation of pervious pavement[4].  The City’s Public Works Department has the most comprehensive maintenance program in Miami-Dade County to help reduce pollutant loading coming from vegetation. Unfortunately, fertilizers usage is a greater threat to water quality and nutrient loading into Biscayne Bay. Fertilizers carry phosphorous and nitrogen directly to the Bay when it rains and cannot easily be removed during maintenance activities.

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