HOUSTON’S REGIONAL FOREST

STRUCTURE ■ FUNCTIONS ■ VALUES

SEPTEMBER 2005 For more information on this project, please contact:

PETER D. SMITH Texas Forest Service [email protected] 979-458-6650

MICHAEL MERRITT Texas Forest Service [email protected] 713-688-8931

DAVID NOWAK, Ph.D. U.S. Forest Service [email protected] 315-448-3212

DAVID HITCHCOCK Houston Advanced Research Center [email protected] 281-364-4007 HOUSTON’S REGIONAL FOREST

STRUCTURE ■ FUNCTIONS ■ VALUES

The population in and around Houston has grown rapidly over the past twenty years, now exceeding five million people. Studies of the area have noted that the loss of trees and changes to the forest makeup have gener- ally accompanied this growth. Trees and urban forestry practices can be Tused effectively to reduce many of the negative effects of urban growth and other changes occurring in the region. These include reducing urban heat island effects, mitigating negative health and environmental impacts, and improving the overall quality of life.

In 2001, the USDA Forest Service approved funding for a special project to build the tools and systems that will help state and local groups monitor and guide the development of the area’s green infrastructure. Houston’s Regional Forest project brought federal and state government researchers together with local planners, policy makers, and managers to analyze the region’s tree cover using field research and computer modeling. This report is one of the major outcomes of this project. It provides detailed informa- tion on the structure, functions, and values of Houston’s regional forest.

ACKNOWLEDGEMENTS Contributors to this project include:

David J. Nowak, PRINCIPAL INVESTIGATOR, USDA FOREST SERVICE, NORTHEASTERN RESEARCH STATION (NERS)

Peter D. Smith, TEXAS FOREST SERVICE Michael Merritt, TEXAS FOREST SERVICE John Giedraitis, TEXAS FOREST SERVICE Jeffrey T. Walton, USDA FOREST SERVICE, NERS Robert E. Hoehn, USDA FOREST SERVICE, NERS Jack C. Stevens, USDA FOREST SERVICE, NERS Daniel E. Crane, USDA FOREST SERVICE, NERS Mark Estes, TEXAS COMMISSION ON ENVIRONMENTAL QUALITY Stephen Stetson, GLOBAL ENVIRONMENTAL MANAGEMENT, INC. Charles Burditt, BURDITT NATURAL RESOURCE CONSULTING David Hitchcock, HOUSTON ADVANCED RESEARCH CENTER Wendee Holtcamp, HOUSTON ADVANCED RESEARCH CENTER This analysis quantifies some of Executive these functions including air pollu- tion removal, carbon storage, and Summary energy savings. The report does not include other important for- Whether viewed from the pine for- est functions such as quality of life ests to the northeast, the bottom- services to humans, habitat for wild- land hardwoods that line bayous life, storm water management, and and streams, or the trees that dot the flood prevention. Wprairie land to the west, tree cover is one of the defining features of the Forest values are the quantified eco- eight-county region surrounding nomic values of the forest functions the city of Houston. Houston’s Re- mentioned above—air pollution re- gional Forest: Structure • Functions moval, carbon storage, energy sav- • Values examines in detail some ings—plus the replacement value of of the crucial characteristics of this the forest. Other studies have mea- green infrastructure. sured values such as health benefits, property value increases, and flood- This new understanding of Hous- water retention. ton’s regional forest is based on satel- lite imagery, field data, and comput- er modeling using the Urban Forest MAJOR FINDINGS & CONCLUSIONS Effects Model (UFORE), which ana- Houston’s regional forest provides lyzes the structure, environmental impressive value to its citizens. The effects, and values of urban forests.1 replacement cost alone of the 663 Field surveys completed in 2001 and million trees found in the region 2002 provided input into this model is valued at over $205 billion. The and included data from 332 field value of the environmental benefits plots throughout the region located generated by trees each year is esti- within residential, commercial, for- mated at $456 million and forests ested, and agricultural areas. also store $721 million worth of A forest’s structure consists of vari- ous tree species, density, health, leaf area, biomass, species diversity, The and other elements that make up replacement the forest. This report summarizes the structure of Houston’s regional cost alone forest, providing a snapshot of the forest resources as well as a detailed of the 663 examination of where trees are lo- million cated by species. Using extensive trees found field surveys and satellite data, this analysis provides the most complete in the understanding of Houston’s forest region is structure to date. valued Forest functions include a wide range at over of environmental, ecosystem, and related services that trees and forests perform, some of which are highly $205 billion. valued by those living in the region. 2 Table 1 Summary of Findingsa

FEA T U R E MEASUR E Number of trees 663 million Trees under five inches in diameter 472 million (71% of trees) Trees in non-urban land cover classes 579 million (87% of trees) Most common native tree loblolly pine (19% of trees) Most common non-native tree chinese tallow (23% of trees) Oak species 15% of trees Replacement value $205.8 billion Carbon storage value $721 million Annual environmental value $456 million Forest land cover b 28.4% of total land area Forest land cover change: 1992-2000 -486 square miles (-17%) Primary forest threat land use change

a All data is year 2001 and 2002, unless indicated otherwise. bForest land cover type = tree canopies plus unforested areas in the immediate vicinity

total carbon. These benefits are dif- cent of the environmental benefits. ferent for rural and urban areas: re- Therefore, protecting the region’s placement and carbon storage values large trees is essential for producing are greater in rural areas due to the future benefits. large number of trees, while energy savings and environmental benefits Land use change, non-native tree are greater where the human popula- species, and insect pests pose sig- tion is concentrated. nificant threats to the future of the regional forest. Actions that sig- The forests of the Houston area are nificantly alter land surfaces have diverse, but also very different in reduced the number and density of the northern and southern parts of trees while providing opportunities the region. Area forests contain 67 different tree species. The highly di- The Houston metropolitan region includes verse North Forest covers a greater the City of Houston—the nation’s fourth largest city—and the area and has larger trees, with lob- eight surrounding counties: lolly pine the single most common Brazoria species. The South Forest includes Chambers Fort Bend forests dominated by cedar elm, Galveston sugarberry, and hawthorn as well as Harris Liberty remnant tracts of Columbia bottom- Montgomery land hardwood forests. Common to Waller all land cover types, Chinese tallow represents the single most common for invasive species and pest out- species, amounting to almost 23 breaks. Land use change continues percent of the region’s trees. to pose the biggest threat: between 1992 and 2000, forest land cover Large trees are disproportionately types in the region declined by an important in terms of forest ben- estimated 17 percent—a decrease of efits. The majority of the region’s 486 square miles, resulting in a net 663 million trees are small: over 70 loss of over 78 million trees. Out- percent are less than five inches in breaks of insect pests represent a diameter. Although trees five inch- potential threat to forests that would es and greater in diameter make up cause significant economic and en- less than 30 percent of the region’s vironmental losses. trees, they provide over 60 per- 3 Urban Forest Effects Model The Urban Forest Effects Model provides accurate estimates of ur- (UFORE) is a computer model ban forest effects based on credible that calculates the structure, envi- scientific research and analysis. ronmental effects and values of ur- Tban forests. It was developed in the A central computing engine houses 1990s by researchers at the United an integrated suite of computer pro- States Department of Agriculture grams that make scientifically sound (USDA) Forest Service, Northeast- estimates of urban forest effects. ern Research Station in Syracuse, These programs are based on peer- New York, and has been used exten- reviewed scientific equations to ac- sively in various places in the U.S. curately predict environmental and and abroad. Using land cover maps economic benefits. and field survey inputs, the model

The UFORE model is currently designed to provide estimates of:

� ■ Urban forest structure by land cover type. Forest structure data in- cludes such variables as species composition, number of trees, diameter, tree density, tree health, leaf area, and biomass of leaves and trees.

� ■ Hourly amount of pollution removed by the urban forest and associ- ated percent air quality improvement throughout the year. Pollution removal is calculated for ozone, sulfur dioxide, nitrogen dioxide, carbon monoxide and particulate matter 10 microns or less in size.

� ■ Hourly urban forest volatile organic compound (VOC) emissions and the relative impact of tree species on net ozone and carbon monoxide formation throughout the year.

� ■ Total carbon stored and net carbon annually sequestered.

� ■ Effects on building energy use and consequent effects on carbon diox- ide emissions from power plants.

� ■ Replacement value of the forest, as well as the dollar value of air pol- lution removal and carbon storage and sequestration.

4 Field Survey Data Items

PLOT LOCATION TREE DATA VARIABLES Field Plot • Plot Number • Tree direction 48.1' radius • Date • Tree distance 1/6 acre • Crew ID • Species or genus • Slope/Aspect • Diameter • Total height Microplot • Height to crown base PLOT COVER / LAND USE 13.6' radius • Percent impervious surface • Number of actual land uses 1/75 acre under tree • Actual land use • Percent shrub cover under tree • Percent of plot in each use • Street tree (Y/N) • Plot tree cover • Crown width • Plot shrub cover (two measurements) • Percent plantable space • Foliage absent Building • Ground cover percent • Dieback (each cover type) • Transparency • Number of shrub-genus types • Crown light exposure • Shrub layer height (each genus) • Building direction • Shrub layer percent of area • Building distance (each genus) • Condition (roots, trunk, • Shrub layer percent leaves branches, twigs, leaves) (each genus) • Utility conflict • Leaves condition Figure 1 • Utility conflict Sample Field Plot Dimensions Field Measurements An objective view of trees and forests plots in agriculture/rangeland areas. across a region as large as the eight- This selection process yielded a total county Houston region necessitates of 332 plots. the use of aerial photography, satel- Alite imagery, or other remote sens- In 2001 and 2002, field crews visited ing data. But satellites and airborne each of these selected grid points sensors only see the surface of the and recorded a series of measure- region’s tree canopy. Field measure- ments within a one-sixth acre plot ments are needed to provide details (7,260 square feet—similar in size that satellites miss—for example, the to a typical residential lot) (Figure size and species of individual trees 1). Plot measurements included or the location of trees with respect tree species, diameter, height, crown to buildings. This detailed informa- width, foliage parameters, ground 2 tion is also essential for the UFORE cover, and shrub cover. Crews also modeling analysis. identified a 1/75th acre microplot (approximately 581 square feet) for To collect the data, researchers se- measurements of trees between one lected sites throughout the region and five inches in diameter. Unlike using a sampling pattern similar to trees, data collected on shrubs em- one used in the Forest Inventory and phasized the leaf density and crown Analysis (FIA) program of the USDA volume that shrub layers occupied Forest Service. This grid pattern di- within the plot rather than the di- vides the region into 6,000-acre cells mensions of individual plants. from which field measurement plots were selected. To get a manageable sample size from the over 800 pos- sible plot locations, two out of every three plots in urban areas were cho- sen for study, one out of three plots in forested areas, and one out of four 5 As part of the Houston’s Regional The Forest project, Global Environmen- tal Management Inc. developed a Region’s Land comprehensive map of the region using 2000 LANDSAT satellite data, and Vegetation images of field plots, and other re- motely-sensed data. The map iden- A thorough understanding of the tified five major categories of land region’s trees begins with an analysis cover, including water bodies, forests of surface features that can be identi- and urban greenspace, agriculture/ Afied using satellite imagery and then range lands, residential areas, and separating the region into broad intensely urbanized areas (Figure 2). categories of vegetation, roads, and The Forest and Agriculture/Range zones of urban development. The categories were further subdivided Houston region has grown at a rela- into two geographic zones to bet- tively fast pace and the accompany- ter understand any differences in ing development has altered land species composition across the re- surfaces dramatically by clearing gion. These two zones are separated vegetation, constructing buildings, by Galveston Bay, Buffalo Bayou Dividing line through Houston, and U.S. Highway between Northern and Southern portions of the region

UFORE Land Cover Figure 2 Ag/Range Houston Regional Forest Land Cover Residential Urban – Built Water

and paving roads. Shifts in manage- 290 in Harris and Waller Counties. ment intensity in agriculture and Excluding water bodies, the total forestry also affect the region’s sur- land area studied was 7,581 square face characteristics. miles, divided into seven land cover classes (Table 2).

6 Table 2 Regional Land Cover Categories

AREA AREA TREES DENSITYa SPECIES per square mile, Sq. Miles % Millions Trees/Sq Mi Number LAND COVER TYPE the North For- North Forest 1,653 21.8 382.3 231,000 45 est has the highest South Forest 499 6.6 88.8 178,000 19 tree density of any of the land cover Forest subtotal: 2,152 28.4 471.1 219,000 51b types, almost three North Agriculture/Range 984 13.0 36.6 37,000 14 times the regional South Agriculture/Range 2,643 34.9 71.8 27,000 11 average. Agriculture/Range subtotal: 3,627 47.9 108.4 30,000 22 Residential 987 13.0 43.9 44,000 34 The South Forest Urban Built 402 5.3 5.9 15,000 14 contrasts with the heavier forest cov- Urban Green 413 5.4 33.9 82,000 26 er to the north by Urban subtotal: 1,802 23.7 83.7 46,000 46 having fewer trees c Total (excluding Water) 7,581 100.0 663.10 87,000 67 overall, lower spe- aAverage density, rounded to the nearest thousand. cies diversity, and bSubtotals for species in major cover classes represent species found in any one of the subcategories cTotal tree species identified in analysis different species. The South Forest Overall the region is roughly one- is 499 square miles in size and ex- fourth urban and three-fourths agri- tends from Houston to the Gulf of culture/rangeland and forests. While Mexico, increasing in coverage to- this non-urban land cover domi- ward the southwest. With 89 million nates the Houston region today, trees (178,000 trees per square mile) urban development will play an in- and 19 species total, roughly three- creasing role in changing these land quarters are represented by only cover percentages as the population four species—cedar elm, sugarber- of the region continues to grow. ry, hawthorn, and Chinese tallow. Other important species found here FORESTS include pecan, eastern redcedar, and American elm. The North Forest cover type occu- pies roughly 1,653 square miles and is located mostly in Montgomery, AGRICULTURE AND RANGELANDS Waller, and Liberty Counties. It con- South Agriculture/Range is by far tains 45 tree species, the greatest spe- the largest land cover type, occupying cies diversity of any land cover type. fully one-third of the region—2,643 Loblolly pine is most abundant, square miles. This cover type extends comprising 24 percent of the tree as far north as Waller County and population. Chinese tallow, the sec- south to the Gulf of Mexico. Its 72 ond most common tree, makes up million trees are spread throughout almost 15 percent of the tree popu- this large area, making tree densities lation. Combined as a genus, the ten among the lowest of the cover types oak species found here represent 16 at 27,000 trees per square mile. This percent of trees. The next one-fifth area also possesses the fewest num- of trees is fairly evenly split between ber of species (11) with Chinese tal- sweetgum, baldcypress, green ash, low comprising nearly 80 percent of and red maple. With a total of 382 all trees. Because of the inherent dis- million trees and nearly 231,000 trees turbances and abandonment of ag- ricultural fields and rangeland, the

7 South Agriculture/Range area pro- added over time by homeowners and vides ample opportunities for the developers. While the most com- expansion of this invasive species. mon tree is again Chinese tallow (30 The North Agriculture/Range cov- percent of Residential trees), there er type is intermixed with the North is a wide diversity of other species.

Forest and occupies roughly 984 Common trees within Residential areas include crape myrtle, loblolly square miles. It contains 37 million pine, and several species of oak. trees (37,000 trees per square mile), made up of 14 species. Loblolly pine is the most prevalent, at 62 percent of all trees, followed by sweetgum, which makes up about 12 percent While many of trees. Here, Chinese tallow repre- sents only five percent of the total, people consider distinguishing this area from the live oak to be South Agriculture/Range. one of the most URBAN LANDS common trees Urban land cover types include Ur- in the Houston ban Built, Residential, and Urban Green. The Urban Built cover type region, consists of areas dominated by im- pervious surfaces, such as high-den- it actually sity residential, commercial, and in- accounts for dustrial buildings, major roadways, and similar land uses. Only six mil- only lion total trees are found across the 402 square miles of Urban Built two percent land (15,000 trees per square mile). Of these, almost two-thirds are lob- of residential lolly pine and the next most com- trees. mon is American elm, which makes up 11 percent of trees in this cover type. Fourteen tree species were rep- resented in the sample. Urban Green is a cover type associ- ated with major parks, open space, Residential land cover includes low- and undeveloped tracts surrounded density development and its asso- by or in close proximity to urban ciated tree cover. It dominates the development, totaling 413 square central part of the Houston region, miles. It includes large open space emanating out from the central ur- areas such as Memorial Park, flood- ban core and covering 987 square water retention basins such as Ad- miles. Tree densities in Residential dicks Reservoir, and the vegetated areas vary widely, but on average corridors along bayous and streams. there are 44,000 trees per square mile Of the three urban land cover types, (44 million total trees). Residential Urban Green areas contain the high- areas contain the second-highest est density of trees, at over 82,000 number of tree species (34) of any trees per square mile (34 million cover type, many of which have been 8 trees total). Again the most common Table 3 species is Chinese tallow (43 percent Most Common Tree Species of trees), with hawthorn, water oak, (10 Million or More Trees) black willow, and green ash making Tree Species Number % of All up an additional 40 percent of trees. Trees A total of 26 tree species were found Chinese tallow 152,498,000 23.0 Loblolly pine 123,974,000 18.7 Cedar elm 45,546,000 6.9 Tree Water oak 35,608,000 5.4 Hawthorn 31,771,000 4.8 Sweetgum 30,699,000 4.6 Population Sugarberry 26,788,000 4.0 Green ash 24,388,000 3.7 Characteristics Baldcypress 21,628,000 3.3 The total population of trees in the Willow oak 20,993,000 3.2 Houston area in 2002 is estimated Red maple 17,162,000 2.6 to be 663 million trees. This aver- American hornbeam 11,809,000 1.8 ages roughly 87,000 trees per square Southern red oak 11,322,000 1.7 Tmile, or about 135 trees per person. Black tupelo 10,436,000 1.6 As might be expected, the majority is measured at “breast height”—or (71 percent) of trees are located in DBH, a point 4.5 feet from the areas classified as forests. However, ground—and can then be used to urban land cover types also contrib- calculate the cross-sectional area ute 84 million trees to the region’s of a tree trunk. Called basal area, total, amounting to 13 percent of this provides a better measure of all trees. total tree volume than tree num- Chinese tallow (23 percent of all bers alone. trees) and loblolly pine (19 percent) Houston’s tree population con- represent the two most common sists of an estimated 191 million tree species in this study (Table 3). trees five inches in diameter or When all oak species are combined, greater, while trees between one this genus accounts for 15 percent and five inches DBH make up 71 of the trees in the region, and taken percent of the population (472 together these three tree species/ million trees). Figure 3 shows that genera account for fully 56 percent although most of the region’s trees of the region’s trees. For a complete are less than five inches DBH, list of common and scientific names the majority of the basal area of for tree species either found on field Houston’s regional forest is found plots or discussed in this report, in larger trees (five inches DBH see the Index of Trees on page 22. and greater). This 29 percent of trees actually makes up 85 percent TREE SIZE of the regional forest’s volume. The relative size of trees provides Leaf area and leaf biomass are other important information on the struc- measures used to calculate a tree’s ture, functions and values of the functions and values. Leaf surfaces region’s forest. Tree size is assessed slow rainwater runoff, remove pol- using trunk diameter and its cor- lutants from the air, and provide responding cross-section. Diameter shade and cooling effects. Large trees 9 Figure 3 Number of Trees and Basal Area by Diameter 250,000,000 12,000,000

10,000,000 200,000,000

8,000,000 150,000,000

6,000,000

No. of Trees 100,000,000 4,000,000 Total Basal Area (square feet) Total

50,000,000 2,000,000

0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Diameter at Breast Height (DBH) in Inches

have more leaves, higher leaf surface tive ecosystems that provide habitat areas, and greater leaf biomass than and food for birds, butterflies, and small trees, thus providing substan- mammals. tially greater benefits. On average, a large tree (five inches DBH or However, non-native species repre- greater) removes more pollutants, sent a significant component of the provides more shade, and has much region’s trees (24 percent), domi- greater value than a small tree. A nated by a single species—Chinese single large tree may therefore be the tallow (23 percent). Other common equivalent of hundreds of seedlings trees that have been introduced to or saplings. For instance, urban ar- the region include Chinese elm, eas contain 28 million Chinese tal- camphor-tree, crape myrtle, chi- low trees compared to just two mil- naberry, and silver maple. Most of lion live oaks, yet these oaks possess these species remain where they 16 percent more leaf biomass, thus are planted, but certain non-native contributing greater total benefits. species become invasive, overtaking disturbed areas such as abandoned agricultural and urban lands, coastal NATIVE AND NON-NATIVE TREES prairies, and forests, crowding out Species native to the Houston re- native plants in areas where they gion make up the majority of trees were not intended. The South Ag- (76 percent) found in the study. Be- riculture/Range cover type pres- sides the oaks and pines mentioned ents a good example of this ten- previously, other common native dency, with Chinese tallow now trees include cedar elm, sweetgum, accounting for nearly 80 percent sugarberry, American elm, baldcy- of the tree population. press, and green ash. Native trees are adapted to the region’s climate, geog- URBAN AND RURAL TREES raphy and ecosystems and generally require less maintenance, water, and Different land cover types contain artificial fertilizers than non-native different tree populations. This species in landscape settings. They study found that urban land cover are also critical components of na- types contained 84 million trees (13 percent), compared to 579 mil- lion trees in rural cover types (87 10 Table 4 Comparison of Urban and Rural Trees FEA T U R E U R B A N R U R A L TOT AL species may also be NumberNumber ooff Trees (millions)(millions) 83.7 579.4 663.1 introduced as new % of Treesees 12.6% 87.4% 100.0% landscape features. CarbonCarbon SStoragetorage (in million tonstons)) 7.9 31.3 39.2 % of CarbonCarbon 20.1% 79.9% 100.0% Urbanization has ReplacementReplacement Value ($ billions)billions) $41.8 $164.0 $205.8 had different effects % ofof Replacement Valuealue 20.3% 79.7% 100.0% on the tree popula- tion across the re- percent) (Table 4). However, urban gion. For example, urban develop- trees provide a greater contribution ment in what was previously South to key benefits such as carbon stor- Agriculture/Range has added some age (20 percent) and replacement Texas native trees (live oak, loblolly value (20 percent). pine) to these landscapes, thus re- ducing the extent of non-native spe- In urban and urbanizing areas, cies such as Chinese tallow. Urban trees are often cleared from devel- development in North Forest areas opment sites to provide easier ac- has altered or replaced the mixed cess and space for buildings and pine/hardwood forests and bot- paved surfaces. Trees and other veg- tomland forests, in many places etation that are planted following providing opportunities for non- construction are usually fewer in native trees and invasive plants to number and smaller than the trees become established. they replace. Non-native and exotic Forest Functions and Values Trees and forests provide commu- to meet federal air quality standards. nity, economic, and ecosystem val- Houston’s regional forest helps im- ues that have been well documented prove air quality by reducing air in research literature. The UFORE temperatures, directly removing Tanalysis in this study calculated the pollutants from the air, and reducing extent and value of three major building energy use and consequent benefits of trees and forests in the pollutants from power plants. Houston region: pollution removal, The UFORE model calculated pol- carbon storage and sequestration, lution removal by Houston’s re- and energy savings. The study also gional forest using hourly pollution calculated the value of trees for what data and weather data (George Bush they are, not just what they do. This Intercontinental Airport weather replacement value corresponds to station) collected in 2000. The year the estimated worth of each tree as 2000 was used because the study re- it exists in the landscape. sults needed to match Texas Com- mission on Environmental Quality POLLUTION REMOVAL (TCEQ) air quality modeling proto- Poor air quality can impair human cols for that time period. health, damage crops and other veg- etation, and reduce visibility. Under Model results reveal that Houston’s the Clean Air Act, the eight-county regional forest removes 60,575 tons Houston region is legally mandated of criteria air pollutants (substances designated in the Clean Air Act) per year, which has an annual economic 11 Figure 4 Value of Air Pollution Removed $350

$300 $295.7

$250

$200

$150 $146.4 Millions of Dollars $100 $99.0

$ 50 $42.9 $1.1 $6.3 $ 0 Carbon Monoxide Nitrous Dioxide Ozone Particulate Matter Sulphur Dioxide TOTAL (10 microns or less) value to the region of nearly $300 CARBON STORAGE million dollars (Figure 4). These AND SEQUESTRATION estimates are based on the national median externality costs associated Trees play an important role with re- with air pollution.3 gard to the carbon cycle and associ- ated climate variability. Trees mod- However, the overall effect of trees erate the amount of carbon dioxide on air quality involves complex rela- in the atmosphere through the pro- tionships between the pollution trees cess of photosynthesis. Carbon that remove, biogenic emissions, and air remains locked up in trees from year temperature and moisture regimes. to year is referred to as carbon stor- While trees do emit volatile organic age. As trees grow larger each year, compounds (VOCs) that contribute they sequester additional carbon and to ground-level ozone formation, add it to the carbon already stored many studies have shown that in- in trunks, branches, and leaves. creasing tree cover over an area can Unlike deciduous trees, evergreens actually reduce these ozone levels.4 retain their leaves for more than Sophisticated modeling is required one season, thus adding to their to calculate the net effects of trees, stored carbon. which will be documented as a com- panion report on Air Quality and Houston’s Regional Forest. Figure 5 Value of Carbon Storage—Top Ten Species $120

$100

$ 80

$ 60

Millions of Dollars $ 40

$ 20

$ 0 Loblolly pine Chinese Water oak Live oak Shurnard oak Pecan White oak Sweetgum Willow oak Southern tallowtree red oak 12 Based on the UFORE model analy- sis, trees in Houston’s regional for- est store 39.2 million tons of car- bon, valued at $721 million.5 Of this amount, 15 percent is stored by one species alone—loblolly pine (Figure 5). In addition, the regional forest sequesters 1.6 million tons of carbon each year, at a value of $29 million. Trees vary significantly in their car- bon storage and sequestration rates based on species, size, health, and site conditions. Young, healthy trees in the prime of life have higher se- questration rates than older trees, but due to their stature large trees store more carbon. For example, live oak represents less than 0.5 percent Figure 6 of the region’s tree population but Value of Energy Savings $120 accounts for over nine percent of the total carbon stored. $111.8 $100 ENERGY SAVINGS Trees affect the amount of energy $180 that homes and buildings use by providing shade, by the evaporative $160 cooling effect of their leaves, and by blocking winter winds. Trees are Millions of Dollars particularly beneficial in areas like $140 Houston that rely on air condition- ing in summer months. In this anal- $120 ysis, trees located within sixty feet of $13.9 one- or two-story residential build- $5.4 ings were evaluated to calculate their $100 Cooling in Summer Heating in Winter Avoided Carbon effect on energy usage. Emissions savings in summer when trees shade The UFORE model calculated en- buildings and reduce the need for ergy savings from trees using tree air conditioning. During the win- size, distance and direction from ter, trees protect buildings from cold buildings, leaf type (deciduous or north winds, reducing the need for evergreen), and the percent cover of heating. By reducing energy con- buildings and trees as recorded on sumption, trees also reduce carbon field survey plots. Values are derived emissions from power plants that from the average regional costs for would have occurred had the trees electricity and fuel. not been present.6 The estimated Results show that the regional for- total value of energy savings and est saves energy in both the heating avoided carbon emissions for the and cooling seasons, with the largest Houston region amounts to $131 million annually (Figure 6). 13 REPLACEMENT VALUE Trees have an intrinsic value based from these procedures estimate on their presence and contribution the amount of money a tree owner to the landscape setting that can be should be compensated for the loss measured in dollars. This replace- of a particular tree. The value is ment value is calculated using pro- based on the market cost of planting cedures set forth by the Council of a replacement tree and four char- Tree and Landscape Appraisers7 and acteristics of the original tree: size, can be used to determine monetary species, condition, and location. settlements for damage or death of plants through litigation, insur- The UFORE model estimated a total ance claims, loss of property value replacement value of $205.8 billion for income tax deductions, and real for Houston’s regional forest. estate assessments. Values derived

Summary of Values

Houston’s regional forest has a total Table 5

estimated structural value of $206.5 Structural Value Amount billion. Structural value combines Replacement Value $205.8 billion Carbon storage $ 0.7 billion replacement values of trees and Total carbon storage values, and repre- Structural Value $206.5 billion sents the total value of the trees as Annual Functional Value Annual Amount they stand. But because trees live Carbon Sequestration $ 29.0 million and grow each year, the regional Air Pollution Removal $295.7 million Energy Savings $131.1 million forest provides annual functional Total Annual values (air pollution removal, car- Functional Value $455.8 million bon sequestration, and energy sav- ings) that total an additional $456 million. These values are summa- rized in Table 5.

14 Threats to the Region’s Forest Forests and trees are faced with many factors that can adversely affect tree populations and their associated values to people, the economy, and Fecosystems. These threats include extreme weather events, wildfire, in- sect pests, and land cover changes. This report provides an analysis of several of these factors.

PESTS Because the region’s forests are dom- inated by certain tree species—Chi- nese tallow, pine, oak, maple and ash—the structural and functional values of the forest as a whole are at risk from pests that target these potential hosts. The UFORE model estimated the effects of four insect pests of particular concern to the forests of the Houston area: gypsy moth, southern pine beetle, Asian longhorned beetle, and emerald ash borer. The total value of losses sustained during a serious outbreak of any one of these pests could be in the billions of dollars (Figure 7). These losses do not include costs that would be incurred to control such pests.

Figure 7 Percentage and Value of Trees Susceptible to Insect Pests $90

$80 $80.4 $70

$60

$50 $50.5 $40

Billions of Dollars $30

$20 24.7% $24.2 18.8% 15.7% $10 3.75% $5.1 $ 0 Southern Pine Beetle Gypsy moth Asian Longhorned Beetle Emerald Ash Borer 15 Gypsy moth8 represents the single ana, Ohio) and southern Ontario, greatest pest threat to Houston’s re- Canada. All ash species are suscepti- gional forest due to the large range ble to attack and multi-county quar- of suitable host species found here. antine zones within affected states This non-native insect has become have been established to limit the established in other parts of the U.S., notably the northeast and the south- ern Appalachians. It defoliates many species—particularly oaks—during Chinese tallow the growing season, causing wide- easily invades spread tree mortality if the outbreak is repeated several years in succes- areas sion. Should it become established in the Houston region, losses of ten disturbed percent of susceptible trees would by amount to approximately $8.0 bil- lion. human activity. Southern pine beetle9 is a native in- sect pest that has destroyed thou- sands of acres of pine forests in the movement of ash logs and firewood. southern U.S. in recent years. In 1995 Control measures currently involve alone, timber losses were estimated eliminating infected and potential at over $300 million. Infestations are host ash trees within three miles of cyclic, with outbreaks reaching epi- an outbreak, meaning that the dis- demic proportions about every six to covery of even one infested tree can ten years. Though the Houston area impact a much greater area of the has been spared such an outbreak region’s forest. Currently, 3.8 per- over the past decade, an infestation cent of Houston’s tree population is affecting ten percent of the region’s made up of ash trees. pine trees would cause more than $5 billion in structural value losses.

10 INVASIVE TREE SPECIES Asian longhorned beetle is another The movement of trees and plants pest from Southeast Asia that could across the globe for horticultural have serious impact on the region’s and agricultural purposes has dra- forest cover, since it bores into and matically affected ecosystems in the kills a wide range of hardwood spe- southern U.S. Some plants were cho- cies. Although not yet established in sen for their aesthetic qualities or to the region, it has been recorded at the perform some particular function, Port of Houston on shipping crates. while others are simply “stowaways” If an outbreak of this insect were to that arrived in shipments of other occur, the loss of just ten percent of plants or animals. A small number susceptible host trees would total of species introduced to the U.S. $2.4 billion. escape cultivation and become es- Emerald ash borer11 is a recently in- tablished in a variety of unplanned troduced pest that currently affects settings. With few natural pests or several U.S. states (Michigan, Indi- diseases in their new home, these escaped plants can become invasive

16 over the course of a few decades and This single species now accounts for begin to alter native ecosystems. over 152 million trees, almost one- fourth of all trees in the region. It Such is the case of Chinese tallow in is considered common in all of the the Houston region. Originally es- cover types sampled in this study, tablished in Texas in the early 1900s but the highest concentration occurs as a potential agricultural source in the South Agriculture/Range of seed oil to make soap, it quickly cover type where the species repre- escaped cultivation. It grows rap- sents nearly 80 percent of all trees. idly and matures at an early age; Some of these lands may ultimately birds spread its seeds widely; and represent new forests in areas where it favors the climate and soil condi- trees historically did not dominate tions over a vast portion of the lower the landscape, meaning that Chi- Texas coastal plain. It appears highly nese tallow is providing benefits in adapted to areas disturbed by hu- terms of new tree cover. However, it man activity, including agriculture, now makes up from 13 to 19 percent forestry, and land development. of trees in the Forest cover types, And once established, Chinese tal- which hints at the effect on native low tends to dominate areas by out- plant populations (Figure 8). competing native plants.

Figure 8 Chinese Tallow as Percentage of All Trees by Cover Type 80%

70%

60%

50%

40%

30%

20%

10%

0% South Urban - Built Urban - Green Residential Regional North Forest South Forest North Agriculture/Range Average Agriculture/Range

Horticulturists, urban foresters, and nursery and landscape professionals H all share a responsibility to consider the potential negative effects of introducing new species to the region. Future surveys of the regional forest will help monitor the populations of exotic species and their impact on the environment. 17 Several other non-native tree or such environmental degradation shrub species warrant careful con- in other parts of the U.S. that any sideration, either because they have proposed or accidental introduc- already escaped cultivation in the tion in the Houston region should Houston region, are being promoted be viewed with extreme caution. as landscape plants, or because their Though not discovered on field plots effects elsewhere have been highly in this study, experiences with trees negative. This first group includes such as Brazilian pepper in Florida, tree-of-heaven, chinaberry, Chinese saltcedar in the desert southwest, or privet, and mimosa. All of these spe- eucalyptus in California offer cau- cies can be found occasionally in na- tionary tales of allowing these plants tive or disturbed areas throughout to become established here—either the region, though not in numbers through accident or some well- comparable to Chinese tallow. meaning landscape design. The second group includes Chi- Horticulturists, urban foresters, and nese pistache, Chinese elm, Mexican nursery and landscape professionals white oak, and sawtooth oak. These all share a responsibility to consider species are now commonly found in the potential negative effects of in- nurseries throughout the region and troducing new species to the region. have been promoted as landscape Future surveys of the regional forest trees highly adapted to Houston’s will help monitor the populations of climate and soils. It is this very ad- exotic species and their impact aptation, however, that presents the on the environment. potential for escaping cultivation and becoming an invasive species. LAND COVER CHANGES— The third group is the most prob- 1992 TO 2000 lematic because it is impossible to Urban development and other hu- completely predict how a species man activities in the Houston re- will adapt to a new environment gion have had significant effects simply based on its habits elsewhere. on the regional forest’s extent and However, some species have caused composition. Previous studies have

Table 6 Land Cover Change 1992-2000a

1992 2000 CHANGE % CHANGE LAND COVER TYPE Sq. Miles Sq. Miles Sq. Miles

Forest 2,798 2,312 -485.6 -17.4% Agriculture/Rangeland 3,840 3,846 6.0 0.2% Residential 659 989 329.5 50.0% Urban Built 275 425 149.8 54.5% Total Land Cover 7,572 7,572 -00.3a 0.0%

a Changes in land cover should be considered approximate due to differences in methodologies used for the two study periods. Urban Green areas for 2000 redistributed across cover types to match 1992 cover classification system. Water areas not included.

18 indicated that urban expansion over increased by 55 percent during this the past thirty years has resulted in time period, occupying an addition- forest cover losses of approximately al 150 square miles. six percent per decade.12 The current study compared LANDSAT satellite These land cover changes had a dra- data for 2000 and U.S. Geological matic effect on the total number of Survey land cover data for 1992 to trees in the region. Assuming that the estimate land cover changes that oc- density of trees within a cover type curred over this more recent eight- would be similar from 1992 to 2000, year period (Table 6). the net effect of land cover change was a loss of over 78 million trees (al- The Forest land cover types declined most 10 million trees per year), with by a total of 17.4 percent over this a structural value of approximately eight-year period, an annual loss of $24 billion ($310 per tree). The en- 2.2 percent. Since the area occupied vironmental benefits from trees de- by Agriculture/Range lands stayed clined by an average of $56 million roughly the same, the primary per year, or $448 million. Unmitigat- source of change in land cover was ed losses at this rate will continue to the expansion of Residential areas erode the benefits that trees provide by 330 square miles, a growth rate to citizens in the region. of 6.3 percent per year. The associ- ated Urban Built land cover type

19 conservation subdivision design, The tree preservation ordinances, and a variety of educational opportunities Future for developers. Mortality of existing trees is not of generated by development pres- sures alone. Natural mortality rates Houston’s of forests have been studied exten- sively, but only limited urban for- Regional est mortality rates exist.13 A study of Baltimore’s trees estimated that Forest net losses averaged 4.2 percent an- Houston’s regional forest represents a vital part of the region’s identity and provides many benefits to Houston’s To ensure that Hpeople, environment, and economy. H the benefits of This project has quantified and il- lustrated some of these features. But trees will be these trees and forests don’t exist in a experienced by vacuum; they are affected by the cu- mulative daily activity of the region’s future genera- five million people. To ensure that tions, the benefits of trees will be experi- enced by future generations, local local and and regional leaders of today must begin to develop long-range poli- regional cies for maintaining and expanding leaders of tree cover. today must The greatest threat to the overall tree begin to population remains the land devel- develop opment process. Based on current practices, forest changes that are likely to occur include: decreased long-range poli- numbers, densities, and varieties of cies for main- native trees, combined with a poten- tial for an increased presence of pests taining and and non-native trees. This report expanding tree also demonstrates the dominant contribution larger trees make to cover. the overall value of the regional for- est, making it important for regional leaders to consider policies that ad- nually, which included the effects equately protect this key portion of of land use change. Using a compa- the resource. Examples of strategies rable rate for the Houston region, to preserve trees currently in use in net tree losses could total as many the region include: conservation as 28 million trees per year. Besides easements, parkland dedication, land purchases for parks or flood control, 20 affecting the rate or impact of land values to the region. For air quality use change in the region, one way to improvement it may be important to minimize these annual losses is to consider species that generate lower reduce tree mortality rates, which emissions of volatile organic com- could be accomplished through im- pounds (VOCs) as a key factor, while proved landscape care and mainte- planting programs for energy con- nance programs. Matching species servation or heat island mitigation to suitable locations is also critical might favor species that generate to survival of trees in planting pro- maximum shade over a longer lifes- grams. pan. Other factors include whether trees are to be planted on public or But ultimately, large scale tree plant- private property; whether the area is ing and other programs are needed urban or rural; whether a species is if any increase in tree cover is to be native or non-native; what the main- realized. And it will be important tenance requirements are; availabil- to match these programs with the ity for planting; and cost. benefits expected from trees, since different species provide different

Houston neighborhood with mature tree canopy coverage.

21 Index of Trees in UFORE Analysis

Common Name Scientific Name Native ? Found in Land Cover Types 1 American elm Ulmus americana Y NF, SF, SA, NA, R, UG, UB American holly Ilex opaca Y NF, R American hornbeam Carpinus caroliniana Y NF American sycamore Platanus occidentalis Y NF Baldcypress Taxodium distichum Y NF Black cherry Prunus serotina Y NF Black hickory Carya texana Y NF Black tupelo Nyssa sylvatica Y NF, NA Black walnut Juglans nigra Y NF Black willow Salix nigra Y NF, NA, UG, UB Boxelder Acer negundo Y UG Brazilian pepper Schinus terebinthifolius Discussed in report Bur oak Quercus macrocarpa Y R California palm Washingtonia filifera R Camphor-tree Cinnamomum camphora R Cedar elm Ulmus crassifolia Y NF, SF, SA, R, UG, UB Cherrybark oak Quercus falcata Y NF, UG var. pagodifolia Chinaberry Melia azedarach SF Chinese elm Ulmus parvifolia R Chinese pistache Pistacia chinensis Discussed in report Chinese privet Ligustrum sinense Discussed in report Chinese tallow Sapium sebiferum NF, SF, SA, NA, R, UG, UB Chittamwood Bumelia lanuginosa Y SF, SA, UG (Gum bumelia) Common fig Ficus carica UG Common persimmon Diospyros virginiana Y NF Crape myrtle Lagerstroemia indica R Eastern cottonwood Populus deltoides YR, UG Eastern hophornbeam Ostrya virginiana Y NF, R Eastern redcedar Juniperus virginiana Y NF, SF, R, UB Eucalyptus Eucalyptus spp. Discussed in report Flowering dogwood Cornus florida Y NF Green ash Fraxinus pennsylvanica Y NF, SF, SA, UG Hawthorn Crataegus spp. Y NF, SF, SA, R, UG Hercules-club Zanthoxylum clava-herculis Y NA Hickory Carya spp. Y NF Huisache Acacia farnesiana Y R, SA Laurel oak Quercus laurifolia Y NF, SF Live oak Quercus virginiana Y SF, SA, R, UG, UB Loblolly pine Pinus taeda Y NF, SA, NA, R, UG, UB Mexican white oak Quercus polymorpha Discussed in report Mimosa Albizia julibrissin Discussed in report Oriental arborvitae Thuja orientalis UB Osage-orange Maclura pomifera Y SF, R (Bois-d’Arc) Overcup oak Quercus lyrata Y NF Pecan Carya illinoensis Y NF, SF, SA, R, UG, UB 22 Common Name Scientific Name Native ? Found in Land Cover Types 1 Post oak Quercus stellata Y NF, SF, NA, R, UG Red maple Acer rubrum Y NF, NA Red mulberry Morus rubra Y NF, R, UG Redbay Persea borbonia Y NF Rusty blackhaw Viburnum rufidulum Y R Saltcedar (Tamarisk) Tamarix spp. Discussed in report Sawtooth oak Quercus accutissima Discussed in report Shortleaf pine Pinus echinata Y NF Shumard oak Quercus shumardii Y NF, R Silver maple Acer saccharinum R Slash pine Pinus elliottii R, UB Slippery elm Ulmus rubra Y NF, UG Southern catalpa Catalpa bignonioides R Southern magnolia Magnolia grandiflora Y NF, NA, R Southern red oak Quercus falcata Y NF, SF, NA, R, UG Sugarberry Celtis laevigata Y NF, SF, SA, R, UG, UB Swamp chestnut oak Quercus michauxii Y NF Sweetgum Liquidambar stryaciflua Y NF, NA, R, UG, UB Texas hawthorn Crataegus texana Y SF Tree-of-heaven Ailanthus altissima Discussed in report Unknown Spp. NF, UG Velvet (Arizona) ash Fraxinus velutina NF, R, UG Water-elm Planera aquatica Y NF Water hickory Carya aquatica Y NF, UG Water oak Quercus nigra Y NF, SF, NA, R, UG, UB Western soapberry Sapindus drummondii Y SF, R White ash Fraxinus americana Y NF White oak Quercus alba Y NF, NA, R Willow Salix spp. UB Willow oak Quercus phellos Y NF, R, UG Winged elm Ulmus alata Y NF, SF, NA, UG

1 Codes for land cover types: NF=North Forest, SF=South Forest, NA=North Ag/Range, SA=South Ag/Range, R=Residential, UG=Urban Green, UB=Urban Built.

Photography: David Hitchcock, Wendee Holtcamp, Alex MacLean, Michael Merritt, Quality of Life Coalition, John Warner

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TFS Communications/Urban & Community Forestry 9/05-5000 23 Footnotes/References

1 Nowak, D.J., and D.E. Crane. 2000. “The Urban For- 7 Based on Council of Tree and Landscape Appraisers est Effects (UFORE) Model: quantifying urban forests guidelines. For more information, see Nowak, D. J., D. structure and functions” in: Hansen, M. and T. Burk E. Crane, and J. F. Dwyer. 2002. Compensatory value of (Eds) Integrated Tools for Natural Resources Inventories urban trees in the United States. J. Arboric. 28 (4): 194- in the 21st Century. Proceedings of the IUFRO Confer- 199. ence. USDA Forest Service General Technical Report NC-212. North Central Research Station, St. Paul, 8 For more information, visit: http://na.fs.fed.us/spfo/ MN. Pp. 714 - 720. pubs/fidls/gypsymoth/gypsy.htm

2 For detailed methods, see: Nowak, D. J., D. E. Crane, J. 9 For more information, visit: http://www.barkbeetles. C. Stevens, and M. Ibarra. 2002. Brooklyn’s Urban For- org/spb/Index.HTML est. USDA Forest Service Gen. Tech. Rep. NE-290; and Urban Forest Health Monitoring Manual at: http:// 10 For more information, visit: http://na.fs.fed.us/fhp/ www.fs.fed.us/ne/syracuse/Tools/UFORE.htm alb/index.shtm

3 F. J. Murray, L. March, P.A. Bradford, New York State 11 For more information, visit: http://na.fs.fed.us/fhp/ Energy Plan (New York State energy Office, Albany, eab/index.shtm or http://www.emeraldashborer.info/ NY, 1994. Economic values to society are based on median monetized dollars per ton externality values 12 American Forests’ Urban Ecosystem Analysis For the used in energy decision making from various studies. Houston Gulf Coast Region. For more information, Values per ton in this study include: NO2 = $6,752, visit: http://americanforests.org/downloads/rea/AF_ PM10 = $4,508, SO2 = $1,653, and CO = $959. Houston.pdf

4 For more information, visit: http://www.fs.fed.us/ne/ 13 Nowak, D.J., M. Kuroda, and D.E. Crane. 2004. Tree syracuse/TREE%20Air%20Qual.pdf mortality rates and tree population projections in Bal- timore, Maryland, USA. Urban Forestry and Urban 5 Carbon values are multiplied by $20.3 per metric ton, Greening, 2:139-147. http://www.elsevier-deutschland. which is based on the estimated marginal social costs de/artikel/649602# of CO2 emission. Fanhauser, A., 1994. The social costs of greenhouse gas emissions: an expected value ap- proach. The Energy Journal 15(2), 157-184.

6 For more information, see: Abdollahi, K. K., Z. H. For further information on Urban Forest Health Moni- Ning, and A. Appeaning (Eds.) 2000. Global climate toring, UFORE, or urban forest effects, please visit: change and the urban forest. GCRCC and Franklin http://fhm.fs.fed.us/; http://www.fs.fed.us/ne/syracuse; Press, Baton Rouge, LA. or http://www.ufore.org HOUSTON’S REGIONAL FOREST

STRUCTURE ■ FUNCTIONS ■ VALUES HOUSTON’S REGIONAL FOREST

STRUCTURE ■ FUNCTIONS ■ VALUES

U.S. FOREST SERVICE ■ TEXAS FOREST SERVICE SEPTEMBER 2005