Salvaging Savanna

Ecologists at The University of Texas at Austin and its Lady Bird Johnson Wildflower Center have observed, studied and used prescribed fires on local savanna. In the process, the research scientists documented a New England-sized region in the heart of Texas that is losing the biodiversity battle. Now the team is examining what needs to be done to help reverse the process.

Savannas, expanses of grassland dotted with some trees and shrubs, constitute about one-fifth of the world’s land surface.

The Central Texas savannas provide an example of a landscape type that has been dramatically altered. Its plight came about from decades of livestock overgrazing, a lack of wildfires to keep saplings in check, and overgrowth of a native tree – Ashe juniper (Juniperus ashei), commonly called cedar – that normally clings to life in areas less prone to fire, like rocky soils.

“We have a very different situation than existed in this area 50, 100 or 500 years ago. A novel ecosystem is being created,” says Dr. Norma Fowler, of the Section of Integrative Biology. She is one of the few experts on the state’s savannas.

Fowler and Dr. Mark Simmons, a Wildflower Center expert who has studied grasslands on three continents, looked at whether the typical ecological model for studying savannas still applies in Texas. In a paper published this spring in Applied Vegetation Sciences, the two reported that savanna in the Edwards Plateau is losing its ability to remain a savanna.

Overgrazing by livestock, lack of fire and the introduction of non-native grasses have whittled away at the presence of native grasses in regions of the plateau, such as the Hill Country.

“The first rule of landscape restoration is ‘keep all the pieces so you can put them back,’” Simmons says, “but in the Hill Country, some of the pieces are gone.”

Without factors such as fire that normally maintain the area’s savannas, the grassy areas are turning into thickets of Ashe juniper. Why does this matter? Besides reducing how much juniper pollen is around to cause allergic reactions, many land managers in the Hill Country want to preserve the region’s iconic vistas for future generations.

From a practical standpoint, savanna grasses have extensive, fibrous roots that make them much better sinks than woodlands when it comes to capturing carbon dioxide, a major greenhouse gas, below ground. Also, juniper thickets attract deer but not quail and other animals that favor open spaces.

Savanna also provide a more diverse landscape than areas dominated by a single non-native grass such as King Ranch bluestem (Bothriochloa ischaemum), which thrives in the changed soils of former savannas after juniper is removed. Biodiverse lands purify storm water better than those with a single-tree species.

During certain times of the year, Simmons and colleagues at the Wildflower Center have used prescribed fire to show that it helps control juniper growth and fosters re-growth of native grasses. However, few municipalities in Central Texas allow prescribed fire in this drought-plagued region.

“We’re going to have to figure out a safe way to burn that gets rid of Ashe juniper,” Fowler says, “or a way to make their mechanical removal more affordable.”

Even without bans, once juniper takes hold, studies by Fowler’s lab and at the Wildflower Center have shown the adult trees can often survive low-intensity ground fires because of water stored in their needles and stems.

Juniper saplings likely will grow nearby due to the way birds spread their seeds. White-tailed deer (Odocoileus virginianus) that prefer cedar thickets will destroy most broad-leaf saplings but avoid Ashe junipers. “I make my students take a taste of it, and they just pucker up – it’s awful,” Fowler notes.

There is some hope for restoring Central Texas savanna to a more natural state. The City of Austin does prescribed burns on its water quality protection lands, and private ranches clear juniper with heavy machinery and set fires when burn bans are not in place. When juniper is thinned out, restoration experts at the Wildflower Center demonstrated that they can foster re-growth of savanna plants such as little bluestem, which hangs on under a canopy of juniper for a long time.

“We are understanding more and more how to manage Central Texas landscapes,” Simmons says. “Native plants can be quite resilient if we take the right steps to help them. The question is whether the cost of turning this ecosystem around will become too high for people to be willing to make that happen.”

This story appeared originally in the Wildflower Center's magazine, Wildflower. Written by Barbra Rodriguez.