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 2009Fowler and Simmons 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 (Fowler and Simmons 2008, 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.
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.
Because prescribed fire is not generally used in Edwards Plateau woodlands, Kevin Doyle is taking 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.
The effects of pre-wildfire vegetation and prescribed fires, wildfire intensity, and post-wildfire management in the Lost Pines of Texas
This is a new project, 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 will take advantage of these to determine the effects of prescribed fires and pre-fire vegetation on post-wildfire recovery. The Park staff are planning several post-wildfire interventions to reduce erosion and supply seed, whose effects will also be studied.
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.
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 & 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.