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Development, Structure, and Sustainability
of Sacramento's Urban Forest (Part II): Introduction
by Robert W. Miller, Editor
Abstract. This special issue of the Journal of Arboriculture
is the second and final in a series describing the results of an extensive
study of the development, structure, and sustainability of the Sacramento,
California, urban forest. Scientists from the USDA Forest Service, University
of California, and the private sector collaborated on a series of research
projects to provide an in-depth analysis of the Sacramento area from an
ecosystem perspective utilizing the physical, biological, and social science
disciplines. As is the policy of the Journal, each paper was sent to two
members of the Editorial Committee; publication was based on their review
and recommendations. The March 1998 issue of the Journal contains the first
five papers describing this project.
Structure and Sustainability of
Sacramento's Urban Forest
by E. Gregory McPherson
Abstract. The urban forest of Sacramento County, California,
contains approximately 6 million trees. Tree density and basal area decrease
along an urban-rural gradient from city (73 trees/ha, 13.4 m2/ha), to suburban
(64 trees/ha, 4.5 m2/ha), to rural (10 trees/ha, 0.9 m2/ha) sectors. Within
the city and suburban sectors, where 90% of all residents live, approximately
75% of total tree numbers, basal area, and leaf area occurs on residential
land. Sacramento's urban forest is relatively sustainable. Seventy percent
of the trees are in excellent or good condition, the population is well
distributed by age and species, and the most abundant species are reasonably
well suited to local conditions. Factors likely to trigger change in Sacramento's
urban forest during the next 50 years are described (e.g., water conservation,
development patterns, landscape maintenance issues) and species with potential
to thrive in these conditions are listed for future planting and evaluation.
A comparison of canopy cover, density, and basal area of trees in the city
sectors of Sacramento and Chicago, Illinois, reveal surprising similarities.
However, in Sacramento these values decrease along the urban-rural gradient,
while in Chicago they increase. As human influences wane along the gradient,
such factors as climate, soils, competition, and natural regeneration become
more important forces in causing urban forest structure to approach presettlement
conditions.
Comparision of Four Foliar
and Woody Biomass Estimation Methods Applied to Open-Grown Deciduous Trees
by Paula J. Peper and E. Gregory McPherson
Abstract. Concern about global climate change and the effects
that increasing atmospheric carbon dioxide could have on the earth has
risen in recent years. Methods for accurately and efficiently quantifying
carbon storage and annual carbon fluxes are needed to determine what role
urban forests may have in reducing levels of atmospheric CO2. This will
require the development of techniques for estimating foliar and woody biomass
of individual trees. In this study, 2 sampling methods and 2 regression
formulas for estimating foliar and above-ground woody biomass were tested
against the actual above-ground biomass of 8 open-grown deciduous trees
(2 species). There was no significant difference between one of the subsampling
methods and actual foliar, woody, and total above-ground biomass. There
were indications that the method's precision in estimating foliar biomass
could be improved by modifying the sampling method.The second sampling
method predicted foliar biomass of heavily pruned trees within 8% of actual
measurements. For unpruned or lightly pruned trees, one of the regression
equations showed no significant difference between estimates of foliar
biomass and actual biomass.
Urban Forest Impacts on Regional
Cooling and Heating Energy Use: Sacramento County Case Study
by James R. Simpson
Abstract. Urban forests impact energy use for cooling and heating
as a result of their moderating influence on climate. To evaluate the regional
magnitude of these impacts, a large-scale analysis framework was developed
and applied to Sacramento County, California, as a case study. Heating,
cooling, and peak electrical energy use changes resulting from modification
of solar radiation, air temperature, and wind speed by the existing urban
forest were estimated for representative residential and commercial buildings.
This is combined with building age and size, canopy and tree cover, and
tree density (trees/ha) for 71 county subdivisions. Annual cooling savings
are approximately 157 GWh (US$18.5 million) per year -- 12% of total air
conditioning in the county. Net effects on heating are small, with 145
TJ (US$1.3 million) saved annually. Peak energy-use reductions result in
avoided costs of US$6 million. The resulting large-scale analysis incorporates
a manageable level of detail not previously available. Sensitivity of results
to selected input data is demonstrated.
Atmospheric Carbon Dioxide Reduction
by Sacramento's Urban Forest
by E. Gregory McPherson
Abstract. Sacramento County's 6 million trees store 8 million
tons of CO2 (31 t/ha), and annually sequester 238,000 t (0.92 t/ha). Air-conditioning
(157 GWh) and space-heating (145 TJ) savings from the urban forest further
reduce emissions by 75,600 t of CO2 annually (0.29 t/ha). These avoided
emissions are only 32% of the amount sequestered, due to a clean, hydroelectric
energy supply. Annual CO2 release associated with tree maintenance is estimated
at 9,400 t (0.04 t/ha), or 3% of the amount sequestered and avoided. In
net, the urban forest removes approximately 304,000 t (1.2 t/ha) each year,
with an implied value of US$3.3 million ($0.55/tree). Carbon dioxide reduction
by Sacramento's urban forest offsets the total amount of CO2 emitted as
a byproduct of human consumption by 1.8%. Most benefits accrue on residential
lands in the city and suburban sectors, where rates of storage and sequestration
are about one-half those reported for U.S. forests. Guidelines for managing
urban forests to reduce atmospheric CO2 are presented.
Air Pollutant Uptake by Sacramento's
Urban Forest
by Klaus I. Scott, E. Gregory McPherson, and James R. Simpson
Abstract. A dry deposition model was employed to estimate air
pollutant uptake by Sacramento's urban forest. Assuming 1990 air pollutant
concentrations, model simulations estimated that approximately 1,457 metric
tons of air pollutant are absorbed annually, at an implied value of US$28.7
million. The growing season daily uptake for ozone was approximately 2.4
metric tons per day, while particulate matter (10 microns diameter, PM10)
uptake was slightly greater, at 2.7 metric tons per day. Daily uptake of
NO2 and particulate matter represented 1% to 2% of anthropogenic emissions
for the county. Estimated growing-season annual air pollutant uptake rates
averaged 10.9 kg/(ha land area per yr) for the entire study area, 13.9
kg/(ha land area per yr) for urban areas and 4.2 kg/(ha land area per yr)
for rural areas. Pollutant uptake rates decreased with decreasing tree
canopy cover, along an urban-to-rural gradient.
Rainfall Interception by Sacramento's
Urban Forest
by Qingfu Xiao, E. Gregory McPherson, James R. Simpson, and Susan L.
Ustin
Abstract. A one-dimensional mass and energy balance model was
developed to simulate rainfall interception in Sacramento County, California.
The model describes tree interception processes: gross precipitation, leaf
drip, stem flow, and evaporation. Kriging was used to extend existing meteorological
point data over the region. Regional land use/land cover and tree canopy
cover were parameterized with data obtained by remote sensing and ground
sampling. Annual interception was 1.1% for the entire county and 11.1%
of precipitation falling on the urban forest canopy. Summer interception
at the urban forest canopy level was 36% for an urban forest stand dominated
by large, broadleaf evergreens and conifers (leaf area index = 6.1) and
18% for a stand dominated by medium-sized conifers and broadleaf deciduous
trees (leaf area index = 3.7). For 5 precipitation events with return frequencies
ranging from 2 to 200 years, interception was greatest for small storms
and least for large storms. Because small storms are responsible for most
pollutant washout, urban forests are likely to produce greater benefits
through water quality protection than through flood control.