Print PageWoodlands are transient communities that provide a significant reservoir of stored nutrients in many areas around the world. Alterations in the amount of woody biomass have an impact on aboveground carbon stores, but changes in community composition in these areas can also have significant impacts on soil nutrient status.
In a longitudinal study using historical aerial photographs ranging from 1939 to 2004, changes in the proportion of broad vegetation types were analyzed for sites located in the Balcones Canyonlands National Wildlife Refuge near Austin, TX. The analysis examined changes in vegetational composition for areas experiencing fire before, during, and after a severe multi-year drought, and for areas not impacted by fire.
Aerial photographs with a 1-m spatial resolution were classified according to vegetation type (evergreen, deciduous, and herbaceous). Sites were chosen using location data collected on fallen trees utilized for dendrochronological analysis of fire history. For each site, a 20-m by 20-m sample of pixels was extracted from each classified image. Change vectors for each vegetation type were analyzed using spherical statistics. Additionally, landscape trajectories were analyzed based on velocity and acceleration to compare burned areas with unburned areas.
Results/Conclusions
Higher variance implies more randomness in the dispersion of the change vectors and less directionality. Results indicate that dispersion for pre-drought burn sites decreased in the 1950s, but dispersion has steadily increased since then. Dispersion for areas burned during the drought also dropped during the 1950s, but the increase since then has been slower. Dispersion for post-drought burn sites has steadily decreased throughout the study period, only increasing between 1995 and 2004. Unburned areas show a more random pattern of change vector dispersion. Dispersion for unburned sites is generally higher than for burned sites.
Sites not impacted by fire show much less variability in both velocity and acceleration for evergreen and deciduous species. For evergreens in burned areas, velocity has been generally decreasing since 1939, while deciduous species exhibit increased velocity in that time frame with the exception of a decrease in the 1950s.
The results of both analyses indicate that fire, and fire coupled with drought, promote some directionality in vegetation composition during the recovery period. The timing of the fire (before, during, or after) also has an impact on the time frame of the directionality.
