Unplanned study

Five years and a few days ago, on April 20, 2010, BP’s Deepwater Horizon oil rig exploded in the Gulf of Mexico. Eleven workers were killed, several more were injured, and oil continued to spill from the burst well for the next three months.

NPR reported on the persistent effects of the oil spill earlier this week – despite immediate and on-going cleanup efforts, mangrove islands have disintegrated into tangles of dead roots and crab “traps have been coming up empty.” (Though as the author of that second NPR story points out, because scientists weren’t necessarily studying the area where the disaster occurred in detail before it happened, it’s difficult to trace the exact causes and effects in this situation.)

Among the tangled cascade of human and ecological responses to the oil spill, a team of researchers based in Louisiana selected one thread in particular on which to focus their studies. The scientists, who recently reported the results of their research in the journal Ecohydrology, took advantage of an “incidental test” of what happens to trees growing in coastal cypress swamps during a months-long inundation of fresh water.

In order to keep offshore oil from encroaching on coastal wetlands, water diversions in Louisiana (most of which were designed to control and direct the movement of water and sediment across the landscape) were allowed to release more water than usual following the oil spill, resulting in a freshwater pulse to the coastal swamps located downstream. The researchers focused on one particular diversion, the Davis Pond Diversion near Jean Lafitte National Historical Park and Preserve in coastal Louisiana, which was “operated at six times the normal discharge levels for almost four months.”

In the two years following the freshwater pulse, the dominant tree species in the coastal swamp downstream of the Davis Pond Diversion, baldcypress, produced about two-and-a-half to three times more leaf tissue than the average amount grown during each of the previous three years; reproductive tissues were also higher during the second year after the pulse compared to prior years. Root production decreased following the flood of fresh water, but rebounded the following year; the scientists speculate that the plants may have been preferentially allocating their resources toward leaf growth during the first year.

The Deepwater Horizon oil spill was a tragedy and a major environmental disaster. Amid the myriad negative consequences of the spill, the “unplanned study” these researchers were able to implement as a result of the subsequent response may at least have the small benefit of contributing to our understanding of coastal swamp ecology.

“Concerns are growing because of losses of coastal wetlands along the Gulf Coast,” the authors note. “Special attention should be given to our findings that freshwater release might reinvigorate the production of freshwater coastal vegetation.”

A baldcypress tree in October.

(Image by cm195902 via Flickr)

Intricate combination

An ecosystem is the intricate combination of a community of organisms and their environment. My dictionary (the American Heritage Dictionary, Fourth Edition) defines a ‘system’ as “a group of interactive elements forming a complex whole,” and for an ecosystem, sometimes it’s not immediately obvious how the discrete elements interact.

Scientists have long studied the importance of trees and other vegetation growing on riverbanks, known collectively as riparian vegetation. (A quick Google Scholar search for that term returns more than 57,000 articles and books, the oldest of which were published in the late 1700s.)

“Such vegetation [has] a crucial role in stabilizing streambanks, moderating microclimates and shading streams, delivering litter and large wood to the river, providing habitat and food-web support for a wide range of terrestrial and aquatic animals, and supplying other ecosystem services,” write two scientists from Oregon State University who recently published a paper in the journal Ecohydrology.

Riparian vegetation is a fundamental part of many ecosystems; these researchers were interested in investigating the link between riparian cottonwood trees and the reintroduction of wolves to Yellowstone National Park. Wolves, historically part of the Yellowstone ecosystem, were eliminated from the park in the mid-1920s and reintroduced in the mid-1990s. During their absence, elk were released from the pressure of wolf predation and proliferated, and the consequences ricocheted throughout the ecosystem – the sprouts and seedlings of the plants that herbivores like elk eat, including cottonwoods, were unable to grow into mature trees.

Meandering stream and wide floodplain in Yellowstone National Park with almost no riparian trees. 

(Image by Emily Benson)

The scientists found that riparian cottonwood trees were rebounding in the part of a river valley where elk density had declined following wolf reintroduction, but not in a similar, nearby area with a large bison population; whereas cottonwoods more than five feet tall were absent from both locations prior to the reintroduction of wolves, by 2009 the researchers found about 380 cottonwoods over five feet tall in the section of the river valley with fewer elk, and only seven in the area teeming with bison. At the site where bison were plentiful, they write, “[t]he initial pattern of browsing suppression of young cottonwoods . . . by elk, which began several decades earlier when wolves were absent, is being continued by bison even though wolves are again present.”

Wolves, elk, bison, cottonwood trees, and streambanks are all part of the same ecosystem, their fortunes intertwined in an elaborate web; circumstances that affect one reverberate throughout that web to influence them all.

Bison in Yellowstone National Park.

(Image by Emily Benson)