Written by Alamo Area Master Naturalist Stan Drezek from materials supplied by Wendy Leonard and Fred Wills.
A savanna is “a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so the canopy does not close [Wikipedia].” Approximately 20% of the earth’s land area is savanna. Savannas tend to be found between tropical rainforest and desert biomes. The open canopy allows light, which supports an understory of grass and herbaceous plants. Our Texas savanna tends to get most of its rain during late spring. Here in Bexar County, in particular, we have a Live Oak-Ashe Juniper savanna whose perennial grasses include Little Bluestem, Curly Mesquite, and Buffalo Grass. Cacti are also common.
PHIL HARDBERER PARK WAS A SAVANNA
For thousands of years prior to the 1850s and the onset of ranching in our area, the majority of the South Texas plains was savanna.
This is beautifully documented in a quantitative study performed by Fred Wills on Texas General Land Office surveys from 1838 to 1857, showing our area was “dominated by open plant communities (grassland and savanna).” Wills goes on to say “…hot summer burns swept the area with a return interval of 7-12 years. These fires must have rather effectively suppressed shrubby as well as arborescent vegetation.” Before the introduction of dairy farming in 1910, Phil Hardberger Park had only 15-30% tree canopy. As recently as 1938 an aerial map shows considerable grassland. However, now the park is roughly 70% tree canopy. What a difference! Looking at our restored savanna, what percentage of tree canopy would you estimate?
SAVANNA DESTROYERS: OVER-GRAZING, FIRE SUPPRESSION, & NON-NATIVES
In another study in Bexar County, Wills showed that juniper-woodland (including forest) has increased at the expense of savanna.
Two actions over the past century increased the tree canopy: over-grazing and fire suppression. These actions by 1950 had reduced the prairies in the US to about 1% of their original extent. Fenced cattle kept grasses short, reducing root structure which limits grasses ability to compete with trees. The active suppression of fires allowed woody plants to proliferate. In addition, droughts proved even more damaging to short-rooted plants, which could not hold soil during periods of heavy rainfall. Erosion dramatically reduced the depth of nutrient-rich topsoil. The lack of abundant grass meant less fuel for fires and decreased control of trees.
Scholes and Archer (1997) stated that “in the absence of disturbances such as repeated fires, clearing by humans, or feeding by large herbivores, the tree cover increases at the expense of grass production until it is limited by tree-tree competition.”
How did these actions change the landscape of natural areas like Phil Hardberger Park and Government Canyon SNA? How do areas of savanna restoration differ from other areas?
Another factor affecting the original savanna was the introduction of non-native grass species such as King Ranch Bluestem, Bermudagrass, and Johnsongrass. These invasive grasses have now become dominant in the non-restored grassland areas. In areas where fire suppression occurred, large stands of Texas Persimmon, Ashe Juniper, and Whitebrush now dominate the landscape. The result is decreased rain absorption, along with reduced native grass and herbaceous plant production. This is a dramatic change from the time when Native Americans set fires to keep woody plants in check and to allow tall grasses like Indiangrass, Big Bluestem, and Switchgrass to flourish, sustaining bison, antelope, and other species.
RESTORING SAVANNA
The concept being evaluated in the trial restoration areas is that a reestablished savanna, using a large diversity of native grasses and other herbaceous plants species will “support herbivores such as grasshoppers, field mice and cottontails. These in turn will support predators such as reptiles, hawks, and foxes.” That is according to Eric Lautzenheiser, former head of the City of San Antonio’s Natural Area office. Leonard and Auken in a recent paper evaluated the effectiveness of restoration in terms of species diversification. A three-acre plot at Phil Hardberger Park (West) was studied three years after the removal of Ashe Juniper and Texas Persimmon by mechanical means. This plot was compared to an adjacent non-cleared woodland. The findings were clear. There were less woody species and nearly three times the number of herbaceous understory species in the plot cleared of shrubs and trees. Avian surveys conducted by Bexar Audubon have noted an increase in wintering sparrows in the savanna since it was first created. However, there have been no formal surveys to determine the impacts of this savanna restoration project on other wildlife in the park. The paper did conclude “Grasslands will not remain as grasslands unless certain conditions are met. The process of change in community composition and structure is community succession and occurs in all plant communities.”
Another example of savanna restoration in a local area is one at Government Canyon SNA. In 2012 450 acres comprised the burn area. Then Superintendent Chris Holms stated “… by reintroducing periodic fires, which kill the junipers and knock back the brush in favor of the fast-growing grasses… [we can create] the benefit of providing more diverse habitat for wildlife.”
In another paper Catherine Mabry and co-authors found that restored savannas support the greatest diversity of bird species when the restoration site is in open country or at an edge, where forest canopy meets open range. Clearly this is exactly the situation for the restored savanna at Phil Hardberger Park (West) as well as along the periphery of the 450 acre savanna at Government Canyon SNA.
In the fall of 2012 and summer of 2013 park staff and community volunteers planted about an additional ten acres of savanna. The savanna at Phil Hardberger Park is being maintained primarily by mowing to 18” in February or March once about every five years or sooner if monitoring suggests a need and by volunteers removing woody shrubs and pulling invasive grasses. Fire is currently not an option. Park naturalists will evaluate the grasses and wildflowers as well as wildlife resulting from the restoration of savanna. If the savanna does well it could be expanded. However; additional acreage beyond the current 13 acres, while desirable, must await further study. How will the park naturalists measure the success of restoration efforts in Phil Hardberger Park?
PHIL HARDBERGER PARK AND THE COMMUNITY
Savanna restoration at Phil Hardberger Park was not simply a park staff project. Instead, thousands of plants were placed in the ground through the efforts of over 400 local volunteers in September 2009. What does this say about the value of Phil Hardberger Park to the San Antonio community?
For additional material for children see Green Grows the Grass by Wendy Drezek, AAMN.
References
Leonard, W. J. & Van Auken, O. W. 2019. Restoration of former grassland in south-central Texas. Texas Journal of Science. 71: Article 1.
Mabry, C.M., Brudvig, L. A., & Atwell, R. C. 2010. The confluence of landscape context and site-level management in determining Midwestern savanna and woodland breeding bird communities. Forest Ecology and Management. 260: 42-51.
Scholes, R.J. & Archer, S. R. 1997. Tree-grass interactions in savannas. Annual Review of Ecology, Evolution, and Systematics. 28: 517-544.
Wills, F. H. 2006. Historic vegetation of Camp Bullis and Camp Stanley, southeastern Edwards Plateau Texas. Texas Journal of Science. 58(3): 219-230.
Wills, F. H. 2006. Plant communities of the South Texas Plains: Early post-settlement conditions. Journal of the Southern Texas Archaeological Association. 32(2): 43-52.