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Plant community structure and dynamics

Juniperus ashei and savanna dynamics

The composition and dynamics of local plant communities are strongly affected by increasing densities of this juniper species. Several aspects of this process and its effects are being currently being studied by members of Fowler's group, including competition, fire, and the dynamics of the encroachment process itself.

Understanding the juniper juggernaut: patterns and mechanisms of shrub encroachment in central Texas" (Fowler and Batchelor 2005)  powerpoint from talk given at the 2005 ESA meeting

Dynamics of woody plant encroachment

Ana González studied the dynamics of woody plant encroachment in central Texas savannas by analyzing sequential aerial photographs with biogically-based algebraic models of the encroachment process.  González & Fowler 2007; González & Fowler 2009

Fowler and Simmons (2008) argue that, although fire presumably maintained the savannas of the eastern Edwards Plateau in the past, under present conditionsthey undergo a one-way conversion to woodlands that meets the textbook definition of succession (Applied Vegetation Science 12:23-31).

We continue to work on modeling the dynamics of encroachment. Currently we are developing a cellular automata model, based on a simple model of patch initiation and growth and parameterized with data from aerial photographs.

digitized aerials

The effects of fire and deer browsing on central Texas woodlands

In addition to its role in central Texas savannas, fire may also have played an important ecological role in the oak-juniper woodlands of the eastern Edwards Plateau. Christina Andruk is working with the staff of the Balcones Canyonlands National Wildlife Refuge, who are using prescribed fire in different woodland communities on an experimental basis. One part of the experimental area is also fenced to exclude deer. She is quantifying plant responses to these treatments.
Andruk & Fowler 2010

REU student Patricia Brignoni (right) participated in the project in the summer of 2011.  REU student Frieda Kay participated in the project in the summer of 2012.

Because prescribed fire is not generally used in Edwards Plateau woodlands, Kevin Doyle took advantage of prescribed savanna fires that burned into neighboring woodlands to study the effects of deer browsing and fire in a range of sites and species.

Doyle, K. 2012. The joint effects of fire and deer herbivory on hardwood regeneration on the eastern Edwards Plateau. M.S. thesis, University of Texas at Austin.

patricia 2011

quercus buckleyi resprout

The effects of pre-wildfire vegetation and prescribed fires, wildfire intensity, and post-wildfire management in the Lost Pines of Texas

This project was initiated in response to the wildfires of September 2011 in the Lost Pines east of Austin. Bastrop State Park has excellent pre-wildfire (longterm FIREMON plots and prescribed burn history) and wildfire  (intensity, initial effects) data. Working with Texas Parks and Wildlife staff, Emily Booth is taking advantage of these to determine the effects of prescribed fires and pre-fire vegetation on post-wildfire recovery. The Park staff are conducting several post-wildfire interventions to reduce erosion and supply seed, whose effects are also being studied.

bastrop SP after fire

oak re-sprout

Facilitation versus competition: Juniperus ashei

Peggy Batchelor found that adult junipers facilitate the survival of seedling junipers, but slow their growth; Quercus fusiformis (live oak) does not facilitate juniper seedlings. Facilitation is probably due to amelioration of water stress. Therefore the pronounced clustering of J. ashei commonly observed in central Texas savannas is apparently mostly due to to bird dispersal and to many seeds remaining under maternal plants.

facilitation field study poster (Batchelor & Fowler 2004)

facilitation garden study poster (Batchelor & Fowler 2005)

The effect of habitat fragmentation on native herbaceous species and on the invasive Eurasian grass Bothriochloa ischaemum

As Juniperus ashei and other woody plants increase in size and abundance in a site, they reduce the habitat available for herbaceous species that cannot live under woody plant canopies.  They also increase the fragmentation of the habitat for these herbaceous species.  Karen Alofs studied the effects of habitat fragmentation on these species. She found that fragmentation reduces native species richness but may also block the spread of B. ischaemum

Alofs, K. M., and N. L. Fowler. 2010. Habitat fragmentation caused by woody plant encroachment inhibits the spread of an invasive grass. Journal of Applied Ecology 47(2):338-347. 

Alofs & Fowler 2007; Alofs & Fowler 2008; 
Alofs & Fowler 2009; Alofs & Fowler 2010

Fire and Juniperus ashei

Jill Noel studied the effects of fire on J. ashei. Fire survival rates increase with plant size, but even the smallest plants (<0.5m tall) are likely to survive a winter fire. It may be, however, that summer fires, if they were allowed, would kill more junipers and therefore maintain savannas.

Noel, J. M., and Fowler, N.L. 2007. Effects of fire and neighboring trees on Ashe juniper. Rangeland Ecology and Management 60:596-603.

The effects of fire on herbaceous savanna species

Christina Andruk is studying the effects of fire on herbaceous plant species of savannas, including its effects on the invasive grass Bothriochloa ischaemum.


Andruk & Fowler 2009Andruk & Fowler 2011

burned savanna

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