Pondberry Responds to Light Availability and Soil Flooding

Pondberry can persist under stressful environmental conditions of low light availability and seasonal soil flooding for at least three years. USFS photo.

Pondberry is endangered, but it can persist through environmental stress. It prefers partial sun but can linger in deep shade for years. It can survive long periods of soil flooding, and even flowers while it’s flooded.

“All indications are that this species would benefit from active management,” says Emile Gardiner, a USDA Forest Service research forester. “Passive management of bottomland hardwood forests leads to heavy shading in the understory, which appears to reduce pondberry vigor.”

How might the stress of deep shade interact with the stress of soil flooding? Answering such a question requires soil flooding to be precisely controlled. The Flooding Research Facility in the Lower Mississippi Alluvial Valley allows scientists to do just that. Its 12 one-acre cells can be independently drained and flooded. The facility, which was built on a U.S. Fish and Wildlife Service Refuge, has been operated by SRS since 1993.

The Fish and Wildlife Service listed pondberry (Lindera melissifolia) as an endangered species in 1986. With listing came the need for information about the species’ biology and ecology so that conservation plans could be developed. The current study, published in the journal Forest Science, is part of a larger project designed to address these needs.

SRS scientists at the Center for Bottomland Hardwoods Research began the project in the early 2000s. In 2005, they planted 3,456 six-month-old pondberry plants at the Flooding Research Facility. The plants had been propagated in a lab and represented 20 local genotypes. Flooding regimes varied from either 0, 45, or 90 days of soil flooding.

In each impoundment, the team built shade houses to create three light conditions: deep shade, moderate shade, and light shade. Light shade was 70 percent of ambient sunlight. Moderate shade was 37 percent, and deep shade was 5 percent. In today’s bottomland hardwood forests, pondberry often survives in deep shade, although previous Forest Service research has shown that it is most vigorous given more sunlight.

In fall of 2007, six shrubs from each shade house – 216 in all – were sampled for the current study. The rest of the shrubs continued to grow.

Shoots were clipped at the root-collar, and leaves were separated from stems. Roots and rhizomes were dug up, along with plenty of soil to capture all the fine roots. “It was a tremendous amount of effort to dig up these plants,” says Gardiner.

Digging all 216 plants resulted in about 26 cubic meters of soil, which was stored in a refrigerated 18-wheeler trailer. Pondberry grows in clay soil, so the soil could not be washed off the roots – technicians had to extract roots by hand. It took about three years to extract all the roots. Then the roots, stems, and leaves were weighed and analyzed.

“The way plants grow tells us a lot about how they respond to different environmental stressors,” says Gardiner. “A high proportion of roots tends to indicate they’re moisture stressed and trying to compensate by gathering more moisture.” Light-deprived plants tend to grow proportionately more leaf area in an attempt to capture more energy from the sun.

Seventy percent ambient sunlight was not ideal for pondberry. Although the plant could tolerate it, the root biomass fraction showed that this amount of sunlight caused water stress.

The Flooding Research Facility where the study took place. The structures in the slightly flooded cells are shade houses. Photo by Brian Lockhart.

When the shrubs grew with 37 percent sunlight, the root fraction was less indicative of water stress.

Pondberry grew most in moderate shade and without soil flooding. “In our study, intermediate light, particularly in the absence of soil flooding, appears to have provided the least stressful growing environment for pondberry,” says Gardiner.

Annual soil flooding for 45 days had the greatest impact on shrubs raised in moderate shade. These shrubs had 32 percent less mass than shrubs that did not contend with soil flooding.

However, 45 days of soil flooding did not affect biomass accumulation for pondberry growing in high light. Perhaps acclimation to high light masked their response to the relatively short-term soil flooding – after all, the shrubs grew for two years and were only flooded for 45 days each year.

Without sunlight, biomass accumulation was very slow. Pondberry that wasn’t flooded and received 5 percent light only accumulated 14 grams of biomass – less than half an ounce – over the study period. Their stem biomass fractions indicated they were light-deprived.

“Pondberry is remarkably plastic in its response to light availability and soil flooding,” says Gardiner. “Our study suggests wide flexibility in the development and application of forest stand treatment options that facilitate colony vigor and growth.”

The Army Corps of Engineers provided funding for the project.

Read the full text of the study.

For more information, email Emile Gardiner at emile.gardiner@usda.gov.

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