Woody plant encroachment, primarily by Juniperus ashei, converts savannas to woodlands in central Texas. Prescribed fires and mechanical woody plant removal are used to maintain these savannas. The objectives of this study were to examine the effects of prescribed fire and mechanical removal of woody plants on (1) total woody cover, (2) native herbaceous species, and (3) the invasive Eurasian grass Bothriochloa ischaemum. These were measured in sites under four different management regimes: frequent-fire plus mechanical woody plant removal, frequent-fire only, infrequent-fire only, and control (no fire or removal). We recorded the presence of all vascular plant species in 50 1-meter-radius plots in each management regime at Balcones Canyonlands National Wildlife Refuge. Aerial photographs from the Refuge taken in 1995, 2004, and 2008 were converted into binary images (woody/herbaceous cover) from which percent woody cover was calculated and compared between burned and unburned areas.
Results/Conclusions
Analyses of the aerial photographs indicated that burned areas changed little in woody cover, while unburned areas increased in woody cover 6.62% per year on average. Thus, current Refuge management successfully controls encroachment, but does not restore savannas. As the plot scale, prescribed fires reduced the average number of woody species per plot and the number of plots containing Juniperus ashei. The frequent-fire plus mechanical removal management regime caused the greatest reductions. Most of the differences in herbaceous species among management regimes could be accounted for by the proportion of plots that were ‘open’, defined as plots without either J. ashei or oak (Quercus buckleyi). These open plots were more likely to contain the invasive non-native grass Bothriochloa ischaemum, but also had more native herbaceous species. However, B. ischaemum was significantly negatively associated with several native herbaceous species, including two dominant grasses in open plots. Woody plant removal to maintain or increase native herbaceous species diversity is made less effective because it also favors B. ischaemum. In addition to increasing the proportion of open plots, fires were also associated with higher native herbaceous species richness in plots with J. ashei and/or Q. buckleyi.
