By:
Kristen Wickert, Water Resources Staff Forester, Texas A&M Forest Service -
Houston
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| Figure 1: The Preserve on North Loop, Houston |
Many
posts throughout this blog emphasize the importance of the forest-water
relationship in the rural forests of Texas. However, that does not mean that
the forest-water relationship does not exist in the urban environment. Managing
urban forests can be just as complex as managing rural forests. Specialized
factors such as: increased population, impervious surfaces, and more elaborate pollution contributors, make managing urban forests a difficult task.
This
is why the Texas A&M Forest Service is studying the effects of vegetation
cover in urban environments on water quality and quantity. Our urban foresters
in the Houston office performed a survey of an office complex that is known in
the area for incorporating vegetation and larger developed trees in their
building plan. This complex is called The Preserve. Built in 1971, The Preserve
is located directly in the highly developed concrete jungle of Houston, on the
610 North Loop. Buildings were constructed around already established trees and
replanting occurred in the open spaces between. Most of the trees are now well
established and are larger than six inches in diameter.
Using a public domain software created by the
Northern Research Station of the USDA, called i-tree Eco, the Texas A&M
Forest Service was able to input sampled data from The Preserve property and
generate economic values of the many benefits of the urban forest. A summary
report of the benefits provided by the vegetation include: pollution removal,
carbon storage, oxygen production, runoff reduction, building energy savings,
avoided carbon emissions, and structural values.
Since this is a water resources blog, we will
focus on the reduced runoff from having an urban forest incorporated in a building
plan. There are three main factors that contribute to runoff reduction from
vegetation: Canopy interception of rainfall, water infiltration promotion to
the soil by the root system, and duff accumulation. The
portion of the precipitation that reaches the ground and does not infiltrate
into the soil becomes surface runoff, which costs tax money to clean in waste
water treatment plants.
The i-tree Eco
software calculates annual reduced surface runoff based on rainfall
interception by vegetation, specifically focusing on the difference between
annual runoff with or without vegetation. Although tree leaves, branches, and
bark may intercept precipitation and thus mitigate surface runoff, only the
precipitation intercepted by leaves is accounted for in the i-tree Eco analysis.
The software bases the value of reduced runoff on the U.S. Forest Service's
Community Tree Guide Series.
An excerpt from the report describes the
amount of runoff reduced due to the projected 1,350 trees in the 12 acres of
The Preserve at an estimated 47,200 cubic feet a year with an associated value of $3.14
thousand.” In actuality, this number is higher due to the fact that only leaf
interception is considered, root infiltration is not considered, and the duff
accumulation is absent from the report.
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| Figure 2: Protective duff covering the soil |
The Preserve is a special office complex, because
it is managed mainly without the use of lawn mowers once or twice a week; unlike
most office complexes in urban environments. This enables the office complex
grounds to actually mimic a functioning forest floor by accumulating duff. Duff
is leaf litter and course woody debris that accumulates on the ground to form a
protective shield against rain events and temperature changes. The duff layer
of pine needles, leaves, and small course woody debris slows the infiltration
rate of water into the soil, while still catching and holding the water, increasing
absorption, and keeping moisture in the soil longer. This reduces the amount of
water that passes over the saturated bare soils. Therefore, avoiding runoff into the streets
that could cause erosion and flooding.
The
lessons we learn from our Forest-Water Best Management Practices have more
applications than meets the eye. These valuable lessons are transferable to the
urban environment in scales from small to large. The Texas A&M Forest
Service is working hard to better the lives of rural and urban Texans by
sharing this knowledge.
If
you are interested in learning about more ways in which you can do your part in
urban areas, please feel free to email me at kwickert@tfs.tamu.edu or to call me at 832-530-6468
