When we design a building or a garden, are we doing it for the approval of our clients or our peers or to gain recognition? Who among the enlightened green architects is primarily interested in designing FOR nature? We must establish and work to achieve the overriding principle of sustainable design. As professional designers, we can seek recognition and approval from those who hire us, but until we can protect the non-human world by learning its strategies for survival and its vulnerabilities, we are not practicing sustainable design nor are we acknowledging the significance of design that protects and cherishes natural systems.
When I became interested in native plants, I was drawn to them by their sheer beauty and uniqueness. I was not familiar with them. They seemed exotic and other worldly. I had been designing conventional gardens for fifteen years when I took a hike in the White Mountains of the Eastern Sierra with Dr. Glenn Keator, a renowned field botanist, author, and teacher.
When I first saw the bristle cone pines, I wasn't thinking about the fragile ecosystem that had sustained these trees and enabled them to survive for five thousand years. I was interested in photographing their startling silhouettes against the chalk white limestone slopes. I don't regret one joyful moment that my eyes have experienced in a lifetime of travel and observation, but when it comes to answering the call of appropriate design for the 21st century, we cannot afford to allow our eyes to seduce us. A detailed and thorough understanding of the ecosystem that we are disturbing at a building or landscape site must be in place to guide our minds and eyes to make appropriate choices.
When we build, we disturb. Urbanization and agriculture disturb soils. When we remove native plants from a site, we are altering millions of years of soil microbial activity. Ultimately this disturbance of beneficial soil microbes will affect the quality of the water we drink and the air we breathe. The more we know about how to mitigate soil microbial disruption, the less impact human activity will have on the Earth and its ability to recover from disturbance. Typically, construction practices remove a minimum of two to three feet of native top soil. After construction is completed, perhaps only 2-3 inches are replaced with introduced soil and turf, an exotic mono culture. In addition, the subsoil has been compacted. This compaction limits the air and water pockets, allowing few micro and macro-nutrients to thrive, which produces shallow root growth. Poorly developed root systems produce low evapotransportation and low surface water penetration. So, when rain falls on a disturbed site, 15-30% of the water will evaporate, only 15% of the water will penetrate the soil and up to 70% of the water will run off, carrying with it pesticides, silt, and animal waste.
Conversely, when rainfall hits an undisturbed native soil site, typically 50% of the water will evaporate into the atmosphere and 35% will penetrate the site soils. The water will pass through a top layer of organic matter, then reach the top soil, proceed down through the subsoil until it reaches the ground water. As compared to 70% runoff in disturbed soils, in native soils only 15% of rainfall will run off.
Human development will continue and native soils will continue to be disturbed. How quickly can we mitigate this disturbance and what steps can we take to protect construction sites and reduce the amount of disturbance?
Cut and fill practices may disturb soils to such an extent that within certain fragile ecosystems, they may not recover for hundreds of years.
Heavy earth-moving equipment compacts soils and destroys soil life. This will negatively affect the health and longevity of any remaining vegetation. Minimal use of heavy equipment allows disturbed soils to recover much quicker and protects soil life. When excavation is required, take care to preserve existing top soils, to set them aside in the order in which they were removed. Value and protect site soils during the construction process and return them to the land as close to their original place as possible when construction allows. Cover site soils with organic mulch during the construction process to a depth of at least 6 inches to prevent the intrusion of invasive species. Keep the soils cool and encourage microbial activity.
And most important, landscape with native plants, particularly plants selected from the plant communities of the region. If possible, before construction begins, collect seeds from native plants that are growing on the site. Take cuttings and set up a propagating facility near the site so that the original plants can be returned to the soils after construction. If there are no native plants still existing on the property, learn what species might have grown in these soils, and arrange to have diverse, local species contract grown at a local or regional native plant nursery. All of these steps will speed the process of the restoration of the site soils.
When soils are protected and allowed to return to their native state, we are designing for the protection of the natural world. Healthy native soil is a sponge. It absorbs rain and slows down run-off. It stores and releases water and nutrients as plants require them. It filters, traps and ultimately breaks down urban pollutants such as oil, metal and pesticides. It also filters and purifies the air and water that percolate through it. It perpetuates life on the Earth by supplying valuable nutrients and antioxidants to plants.
If native soils are protected and perpetuated, disease-causing organisms will be kept in check. Native soils convert soluble organic materials into complex stored organic forms. Native soils are the foot soldiers of a healthy eco-system. On the other hand, disturbed soils invite invasive plant species to thrive. Invasive plants affect water quality, species diversity and populations, reduce favorability for species reproduction, and reduce available food sources. Invasive plants accelerate soil erosion and stream sedimentation, absorb precious water sources and affect water quality.
When an exotic plant invades a soil community, it can alter the links between the plants and organisms that are above the ground and the plant parts and organisms that are below the ground. Some exotic plants create more leaf litter and debris. This builds fuel which could increase fire intensity and frequency. In low rainfall areas in particular, this could adversely affect soil communities' structure and function, resulting in a slow recovery from fire and a long delay before a healthy ecosystem returns. In some cases, it may take hundreds of years.
Plants exude secondary compounds from their roots. If the compounds released by an exotic are novel to a soil community, they may alter the composition and function of the soil community. For example, diffuse knapweed, an Eurasian invasive, releases the chemical 8-hydroxyquinoline from its roots. This has been found to act as an antimicrobial agent and has caused shifts in the composition of the beneficial soil microbes in the soils where it has invaded.
A billion soil microbes are found in one teaspoon of soil. Perhaps of those billion soil microbes, there are 4000 different species of bacteria, fungi, nematodes and protozoa. The bacteria bind the finer soil particles together. These become micro-aggregates bound together by fungal vegetative growth. Garlic mustard, another invasive plant, produces glucosinolates. These compounds have been found to cause significant reductions in the abundance and function of arbuscular mycorrhizal fungi. The abundant presence of these symbiotic fungi leads to substantial increases in the nutrient uptake of host plants. Because mycorrhizal fungi can have affects on both individual plants and plant communities, when an invasive plant is able to alter their dynamic, this may affect the long-term relationships of many plant species in a forest.
Exotic plants can directly alter the physical properties of the soil and the attributes of an ecosystem. Certain invasive plant species literally transform ecological communities. A nitrogen-fixing plant that invades a nitrogen-limited plant community will alter nitrogen cycling and the overall composition of the plant community.
Not all exotic plants are invasive, but even non-invasive introduced plants did not evolve with the beneficial soil microbes that are found in the existing site soils. Native plants evolved with these microbes. To restore healthy soils quickly, select and plant natives according to the local and regional plant communities of the site.
When I began designing native gardens twenty years ago, there were few native plant species available. Today my garden designs include hundreds of species of native plants grown by regional native plant nurseries. Any design statement is possible. A knowledgeable native garden designer is not limited by color, size, texture, fragrance, or garden worthiness.
The native gardening movement is only limited by the lack of knowledge of design professionals who learned their craft designing with exotic plants and don't feel comfortable with what they believe native plants can or can't express in the built landscape.
If we hope to create truly sustainable communities, understanding and protecting local ecosystems for future generations can only be accomplished when we restore our native soils by selecting plants that have evolved in those soils for millions of years.
