Houston's Regional Forest - Tree Population Characteristics
Houston's Regional Forest
The Economic and Environmental Benefits of Urban Trees
Tree Population Characteristics
The total population of trees in the Houston area in 2002 is estimated to be 663 million trees or roughly 135 trees per person. Most (71 percent) are located in Forest areas. However, Urban areas contribute an important 84 million trees, roughly 13 percent of the region's tree population.

Table 3 - Most Common Tree Species

Chinese tallow (23 percent of all trees) and loblolly pine (19 percent) are the two most common tree species. Oak species account for 15 percent, these three representing fully 56 percent of the region's trees.

Size
Tree size provides important information on the structure, functions and values of the region's forest. Trees 5 inches or larger in diameter account for an estimated 191 million trees, with smaller trees accounting for the remaining 472 million. However, in terms of tree "volume" or "mass", the larger trees actually account for 85 percent of the forest. It is this relatively larger size that produces greater benefits.

Leaf area and leaf biomass are measures used to calculate a trees functions and values. Leaf surfaces slow rainwater runoff, remove pollutants from the air, and provide shade and cooling effects. Large trees do a better job. A single large tree may be the equivalent of hundred of seedlings or saplings.

Native and Non-Native Trees
Species native to the Houston region make up the majority of trees (76 percent). Besides the oaks and pines, other common native trees include cedar elm, sweetgum, sugarberry, American elm, baldcypress, and green ash. Native trees are adapted to the region's climate, geography and ecosystems, and generally require less maintenance, water, and artificial fertilizes. They are also critical components of native ecosystems that provide habitat and food for birds, butterflies, and mammals. Non-native trees, such as Chinese tallow, are a significant component of the region's tree population. Other trees that have been introduced into the area include Chinese elm, camphor-tree, crape myrtle, chinaberry and silver maple. Most of these species remain where they are planted, but some non-natives become invasive, overtaking disturbed areas such as abandoned agricultural and urban lands, coastal prairies, and forests, crowding out native plants.

Urban and Rural Trees
Different land cover types contain different tree populations. This study found that urban land cover types contained 84 million trees, compared to 579 million in rural areas. However, urban trees provide a greater contribution to key benefits such as carbon storage (20 percent) and replacement value (20 percent).

In urban and urbanizing areas, trees are often cleared from development sites to provide easier access and space for buildings and paved surfaces. Trees and other vegetation that are planted following construction are usually fewer in number and smaller that the trees they replace.

Table 4 - Comparison of Urban and Rural Trees
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