Located along Buffalo Bayou, the Houston Arboretum & Nature Center is a 155-acre regional resource that serves as a critical refuge for native plants and animals and a hub for environmental education in the city. The Arboretum is a discrete part of the 1,500-acre Memorial Park designed in 1925 by the landscape architecture and planning firm of Hare & Hare. Developed in the 1950s, the site had evolved to present plants and systems related to more idealize ecological habitats. However, recent hurricanes and drought have taken their toll on its landscape as evidenced by 48-percent tree canopy mortality and the significant presence of invasive species. The primary challenge of the Master Plan is to integrate the site’s ecological and cultural history with modern demands into a responsive, flexible plan of renewal.
“The mission of the Arboretum and Nature Center is to provide education about the natural environment to people of all ages and to protect and enhance the Arboretum as a haven and as a sanctuary for native plants and animals.”
Key to crafting a Master Plan that is specific and relevant is starting with an understanding of values and vision. The stakeholder engagement process was extensive, utilizing multiple meetings with a diverse range of public and private sector groups and individuals including teachers, users, Arboretum staff and board members, City of Houston staff, Memorial Park Conservancy board members, donors and others. The process successfully provided an interactive forum for dialogue amongst stakeholders and valuable learning experiences for all participants. At the workshops, stakeholders identified two primary goals – achieving landscape diversity and balancing programmed and “wild” spaces. With a populace so deeply affected by the canopy loss, the landscape architects had to carefully navigate between a desire to restore the previous character of the place and a results-oriented plan founded on the reality of ecological systems in flux. The design team rallied the community around a new trajectory for this landscape—one of a resilient system of varied ecologies changing over time.
The preferred “Corridors” alternative addresses stakeholders’ goals for restoring the natural habitats of the Arboretum in a manner that is authentic to its cultural and ecological context while incorporating interpretive programming activities across the site. The resultant plan lays the foundation for a world-class landscape capable of inspiring visitors and citizen activists who are committed to engagement with the Arboretum and spreading its significance and interpreted messages throughout the region.
Using a forensic approach, the team of landscape architects analyzed the specific site conditions that led to drastic, but not total, canopy mortality. Extensive site analyses of soil-types, drainage patterns, tree canopy, plant diversity and other factors were conducted to understand baseline conditions. The interdisciplinary team of scientific experts and designers conducted research using GIS and field analysis. This analysis revealed that the areas with highest tree mortality rates (those with 70 percent tree mortality or greater) occurred in areas more characteristic of prairie landscapes such as silty clay loam soils and “pimple-dimple” geologic formations where micro-ecologies exist. Canopy trees growing in the “dimple” areas, where water collects, developed shallow andless extensive root systems and experienced the greatest canopy loss during the drought. Paradoxically, canopy trees growing on the raised “pimples” developed deeper and more extensive root systems and survived the drought.
This ecological pattern—revealed only by the recent change in climate—suggested a new landscape mosaic of more varied ecologies. A process of environmental overlays helped determine where optimum conditions existed on-site for varied landscape typologies. For example, prairie currently comprises only 2-percent of the existing site, but 49-percent of the site has characteristics conducive to prairie. This research helped decision makers embrace the radical changes proposed in the plan (23 percent of the site or 12 times the amount of prairie as currently exists) projected to result in a landscape more resilient to future stresses. The new composition of landscape character zones also results in a 420 percent increase in on-site ecotones, a landscape type rich in biodiversity. Prairie, savannah, bog, hardwood forests, and riparian woods are to be developed not as a static spatial condition, but as a potentially shifting and responsive group of interrelated systems. Each is set in motion through specific removals and new plantings arrayed according to particular soil and hydrologic conditions.
The Master Plan improves the infrastructure related to how people use the site in order to diversify their experience, leading to better enjoyment and understanding of the site’s ecology. The Arboretum currently hosts 180,000 visitors a year and is a valued educational and open space asset for the Houston region. School groups visit the site regularly for nature-based education, and there are frequent on-site adult classes. The Master Plan explored expanding the program services offered on-site, but was also very conscious of its carrying capacity to achieve a balance of unprescribed natural areas and programmed areas. Benchmarks with metrics of nature versus programmed space were drawn from relevant botanical gardens and arboreta. Decision makers endorsed the Master Plan’s recommendations that result in similar characteristics as these precedents.
More than 85-percent of the site will be preserved as natural area, and the focus of programmed area will be organized along a diverse network of trails. Currently, there are six trail types based on existing landscape character zones totaling 20,150 linear feet. The plan incorporates 11 educational trails based on new or enhanced landscape character zones and experiences. The linear footage of all trails increased to 39,061 feet (an increase of 94-percent). These changes allow more opportunity for visitors to experience a wider cross-section of the Houston eco-region while visiting the Arboretum.
With expanded infrastructure, however, comes the potential to increase the impervious coverage levels of the site and negatively impact hydrology and water quality, especially as a watershed flowing into Buffalo Bayou. To mitigate the potential effects of increased infrastructure on local ecologies, the Master Plan proposes the demolition of existing asphalt parking areas and the replacement of these permanent parking areas with more permeable surfaces. The plan also proposes stabilized native turf grasses for overflow parking areas and integral rainwater collection to mitigate the effects of impervious cover. The plan integrates new buildings, trails and other built features fitted to the topography in order to minimize erosion that may negatively affect water quality. Wet ponds increase the habitat diversity of the Arboretum through vegetation that filters and cleans water on-site.
As a response to ecological crisis, the Master Plan focuses on developing ecosystem resilience and holds the potential to be a model for southeast Texas and other areas undergoing prolonged drought. As important as the original design of these systems is, the management regime planned to regularly assess ecological performance and brace for potential uncertainty. Utilizing a complex management matrix and an dedicated group of volunteers, the Master Plan defines strategic interventions to initiate new dynamic ecologies. The detritus of tree loss will be chipped and repurposed to help liner stock plantings retain moisture and provide vital organic matter back to a soil degraded by drought. Thickets of invasive species will be removed, fortifying canopy strongholds and other natives. Volunteers will plant drifts of liner stock in waves. Swaths of prairie and savannah will be seeded and regenerated by annual mowing or controlled burnings. The strategy manages toward a resilient set of dynamic and evolving systems, fulfilling the Arboretum’s mission to interpret the land as a collection of natural habitats and interrelated systems rather than as a collection of trees.