Network Connectivity in Resilient Systems

Network Connectivity

Refers to the types and degrees of system interactions that promote robustness and discourage brittleness.

Cultural landscapes and other living systems exist within a complex network of natural and cultural processes, where change in one or more processes has the ability to disrupt others and affect the vitality of the whole. Since network connectivity can be a source of both strength and fragility, cultural landscapes are best managed with a thorough knowledge of relationships, to balance and perpetuate the system interactions that sustain the landscape. Resilient cultural landscapes have greater connectivity to the natural and cultural processes that originally created them and have perpetuated their character through time.

Reestablishing lost tree cover requires consideration of impacts to habitat and understory vegetation, in order to both ensure proper ecosystem function and support network connectivity. At President and Mrs. Hoover’s Rapidan Camp, a cultural landscape within Shenandoah National Park in Madison County, Virginia, the composition of the eastern hemlock (Tsuga canadensis) forest that surrounded the camp during the historic period has been significantly altered by a destructive pest, hemlock woolly adelgid (Adelges tsugae). Shade was one of the most distinctive qualities of the camp during its historic period, creating both a cool atmosphere and a sense of seclusion through physical enclosure.

Replanting of hemlocks would only be feasible with ongoing pesticide applications. Instead, the NPS is replacing missing eastern hemlock trees with tulip poplars (Liriodendron tulipifera) to reestablish tree cover, suppress undesirable pioneering understory species, and encourage regeneration of historically appropriate native forest floor shrub and herbaceous layer.⁹

The forest ecological network of Rapidan Camp is being reconnected by allowing the replacement of diseased eastern hemlock with pioneering tulip poplar.

NPS

Tulip poplars are naturally dominant within exposed areas of the Shenandoah Mountains. The NPS’s decision to selectively thin pioneer species and establish an even-aged stand of tulip poplars was challenging, however, as tulip poplar is significantly different in character from that of the eastern hemlock (a broadleaf deciduous tree versus an evergreen conifer). However, the risk of invasive and undesirable understory species pioneering in exposed areas made the quick reestablishment of an even-aged, long-lived, closed-canopy forest paramount to the character of the cultural landscape. This difficult decision was made following a technical assistance process of inventory, documentation, and analysis provided by the staff and students of the University of Tennessee’s Department of Forestry, Wildlife and Fisheries.¹⁰

At the same time, the NPS has retained a small plot of eastern hemlocks in order to preserve a reservoir of genetic material. This plot is treated regularly with insecticide to suppress woolly adelgid, with the hope that after all untreated hemlocks in the area have succumbed, pest populations will decline to levels that permit reforestation with hemlocks. The contemporary tulip poplars will provide a nurse canopy for the shade-loving eastern hemlock to regenerate. In addition, the United States Forest Service has released natural predatory beetles (Laricobius nigrinus) into the region to control populations of wooly adelgid.

While Shenandoah has already lost approximately 90 percent of its eastern hemlocks since wooly adelgid was first seen in the park in 1988, this new biological control also offers some hope that the tulip poplars may be ecologically succeeded by an eastern hemlock forest at Rapidan Camp once again.

In Washington, DC, the National Mall’s turf panels between Third and Fourteenth Streets are perhaps the most important landscape for civic engagement and First Amendment gatherings in the country. However, the turf panels were not designed for the heavy use to which they are subjected (which include approximately one hundred fifteen special events annually, in addition to regular foot traffic). In 2005, Congress mandated that the NPS prepare a comprehensive plan for the rehabilitation of the National Mall; the resulting plan was completed and approved in 2010.¹¹ The goal for the Mall’s turf panel rehabilitation was to design and install a sustainable system that will function under heavy use. The second phase of the three-phase turf rehabilitation project is currently underway.¹²

The National Mall's turf panels between Third and Fourteenth Streets are perhaps the most important landscape for civic engagement and First Amendment gatherings in the country.

NPS

With a holistic understanding of the landscape’s system requirements, including regular and special uses (operational), materials, and maintenance needs, the NPS worked with consultants to develop an integrated design that could withstand the anticipated levels of use. Soils were engineered to retain structural integrity during heavy use and to drain properly in an area that was historically low-lying and subject to inundation by the Potomac River. Rainwater runoff is harvested through a custom-designed curb catchment system and stored in underground cisterns for use in irrigation. Turf species were selected for drought tolerance and suitability to the engineered soil type.

During special events, modular decking is installed over the rehabilitated turf panels to reduce soil compaction and allow for air circulation. Since phase one, the NPS has hired a turf manager and developed an operations and maintenance manual to guide proper care for the rehabilitated turf. In addition, officials are reviewing the permitting structure for special use to consider sustainable cost recovery from events.

Considered together, the natural and cultural optimal levels of water, soil quality, and access to air allow for ideal growing conditions, while soil structure, managed patterns of use, and modular decking allow for the best surface for heavy active use. Lessons learned during the phase one project have informed refinements to the design of the phase two rehabilitation, including soil profile and composition.

In the information technology field, network connectivity refers to the patterns of connections that facilitate interaction, including linear, hub, tree, web, and star patterns. Each of these typologies has advantages and disadvantages that cause network robustness (an ability to operate in spite of damage) or brittleness (a tendency to break readily, rendering a system nonfunctional).

Understanding the organization and structure of the natural and cultural processes that allow a landscape to function at its optimal level is critical to designing a robust system that can bounce back from adverse impacts. The examples of network connectivity above illustrate the value of research, testing, and experimentation in determining optimal system interactions, whether they involve the relationship of tree canopy cover and understory species distribution or the relationship between turf species, soil structure, soil moisture, and pedestrian carrying capacity. Rehabilitation treatment designs are more resilient when they support the network of natural and cultural interactions that govern landscape function.

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9. John W. Hammond, Cultural Landscape Report for Rapidan Camp (Boston, Mass.: National Park Service, Olmsted Center for Landscape Preservation, 2014).

10. Jennifer Franklin, Rapidan Camp Cultural Landscape Technical Assistance (November 30, 2012).
11. See Secretarial Order 3326 (January 24, 2013).
12. Phase one rehabilitated the 11.3 acres of turf between Third and Seventh Streets and was completed in 2012, just before the second inauguration of President Barack Obama. Phase two includes the rehabilitation of turf panels between Seventh and Twelfth Streets; it began in October 2014.