The Field

Ecological Succession: A Driving Force

Ecological succession (ES) remains one of the most significant determinants of Earth’s biotic life and diversity. Defined as the process of change in the species structure of an ecological community over time, ES drives the environmental shifts of nature and conceives the biological architectures of past, present, and future landscapes.

ES can be broken into three recognized phases: primary succession, secondary succession, and climax community. Primary succession is the series of community changes that occur within an entirely new habitat that has been devoid of life—for example, after a major disturbance such as flood, fire, or volcanic release.

Secondary succession is the process by which an established community is replaced by the next set of biodiversity. Most biological communities remain in a continual state of secondary succession as communities experience minor disturbances, either natural or man-made, that inhibit or reset the successional process.

A climax community represents a stable end product of the successional sequence. Many recognize the Oak-Poplar Forest as a climax community but still acknowledge that any environment can be suddenly disorganized by random variables such as introduced, non-native species. It is said that ES will always remain as Earth is in an ever-changing state.

Today, many forget to recognize the successional phases that are undoubtedly turning all around us. Aesthetic, monetary, and time resources can, at times, skew an image, only accounting for the “now” variables. While this planning stage is necessary, a landscape may be on borrowed time without subsequent conception. Where will the landscape be in one year, one decade, one generation from now? How will it be enjoyed? Will it serve a greater purpose than its original scope? What changes have and will be exerted on this space? Questions such as these can help build upon the natural rhythms of succession while also bridging histories.

Mycorrhizae: The Copilot

The establishment of mycorrhizal fungi throughout trophic levels has been regarded as a primary factor in successional sequence. Literature that addresses plant colonization in successional environments indicate a continuum of mycorrhizal dependency along successional gradients. Facultative mycorrhizal plants are often first to establish on the roots of pioneer plants, replaced by obligately arbuscular mycorrhizal, ectomycorrhizal, and ericoid mycorrhizal host plant species over time. Each of these specialized terms refer to the successional waves of the species.

We know today that an estimated 90 percent of plant families form mycorrhizal associations over their life cycles. These relationships influence plant community structure, increase plant nutrient and water relation, improve plant field survival, improve soil structure and plant tolerance to adverse stresses, and increase plant growth, yield, and reproductive success. With a single mycorrhizal hyphae, the water/nutrient absorbing surface area of a host plant’s roots can increase exponentially while also linking several plant species together. These variables continue to prove that mycorrhizal associations below ground are helping to forge the lasting effects of the biotic biome.

On the Road: Memorial Park, Houston, TX

Working with the staff at Memorial Park Conservancy in Houston, Texas, we had a great opportunity to review the dynamics of ecological succession and the presence of a community of mycorrhizal species. This fungal community is awaiting further processing before relocation to the newly constructed Eastern Glades of Memorial Park.

Due to severe drought conditions, Memorial Park lost thousands of trees during 2010 – 2011, which drastically reset the successional stage of the park’s plant communities. Trees were removed and ground up, resulting in 25,000 yards of mulch byproduct. The material was placed in a two-acre, remote wooded area to allow it to decompose into usable compost material.

During an April 2017 visit, Conservancy staff took myself, Dr. Efren Cazares (Mycologist), and Ted Hartsig (Soil Scientist) to this incredible decomposing wood chip and mulch collection. What we discovered over the course of our stay initiated a change in conversation as we looked at the design of new soils and overarching acclimation of the landscape.

We observed several dead loblolly pine (Pinus taeda) trees within the landscape. Upon further examination, we uncovered pine saplings sprouting from the root zones of their ancestors six years after the initial conception of the space. Dog fennel (Eupatorium capillifolium) and coneflowers (Rudbeckia spp.) were noted growing within the space as well.

Our first task would be to understand how the ecology of the deceased pine species had left behind the necessary elements for future flora establishment. Dr. Cazares collected root samples from the pine saplings, dog fennel, and coneflowers for mycorrhizal colonization analysis.

We discovered that the pine saplings returned averages of 50 percent ectomycorrhizal colonization, with the dog fennel and coneflowers averaging 40 percent vesicular-arbuscular mycorrhiza (VAM). Typical urban landscapes measure anywhere from 15 – 20 percent colonizations.

More research indicated that coneflowers are very often the first species to germinate in a new meadow planting while ecologically producing VAM fungi at high rates. Because of their succession, VAM spores are then left behind to colonize grasses and forbs for the coming growing seasons.

In collaboration with the Memorial Park Conservancy, we developed a plan that will allow us to harvest pine saplings from the site. At the same time we will screen the decaying mulch and separate any quality compost material.

The compost material will then be organized into managed rows where we will install the pine saplings, dog fennel, and coneflowers back into the material along with some clover for nitrogen turnover.

The compost material will mature into a living, green island filled with both ecto- and VAM mycorrhizal spores that are indigenous biology to the park. While the material converts to its anticipated state, it can be used for ecological study and testing until it is placed back into the project as the primary material. No outside sourcing for compost material will be needed due to this method.

Looking Forward

Designing programs such as this one requires the combination of knowledge, experience, and inspiration. It is increasingly important to understand the interconnectedness of the natural living world. In planning, design, and construction phases, nature must be treated as an ever-growing force. The presence of today’s design and care for the landscape can forge an even greater result tomorrow when our thinking is expanded beyond our skill set.

To be continued…

by James Sottilo, Lead Consultant and Founder, Ecological Landscape Management