Friday, August 6, 2010

PS 103-108: Drought and fire suppression encourage Juniperus virginiana encroachment in xeric oak forests

Stephen W. Hallgren and Ryan D. DeSantis. Oklahoma State University


The vegetation of the midcontinent transition zone between the eastern forest and western prairie is a mosaic of xeric post oak (Quercus stellata) and blackjack oak (Q. marilandica) woodlands and savannas, and prairie openings.  The vegetation type, referred to as the Cross Timbers, ranges from Kansas through Oklahoma to central Texas covering over 4.8 million ha.  It is an anthropogenic landscape burned by human fire for thousands of years.  Fire regime changed dramatically after Euro-American settlement with fire frequency increasing in some areas and decreasing in others.  We established a study to determine effects of reduced fire on forest stand structure and composition.  Thirty stands across the region first measured in the late 1950s were remeasured after 50 years in the late 2000s.  Trees >7.62 cm diameter at breast height (DBH) were measured by 40 prism plots spaced 22 m apart throughout each stand.  Saplings 2.54 to 7.62 cm DBH and trees were counted by species in 1.83-m wide strip plots between each prism plot.  Increment cores were collected from a random sample of trees and saplings in 11 stands throughout northwestern, north-central, and south-central Oklahoma. Cores were mounted, sanded and crossdated using standard dendrochronological methods to determine year of recruitment


Stand density and composition changed dramatically over 50 years.  Basal area and density of trees doubled while sapling density remained unchanged.  Species richness increased and tree and sapling species composition shifted toward mesophytic species and away from xerophytic species like oaks. Recruitment of Q. stellata and Q. marilandica appeared to be nearly continuous from 1900 to 1980 and Juniperus virginiana did not begin recruiting into the stands until after 1950 when its presence increased dramatically.  Recruitment appeared to have significant pulses after severe droughts such as the 1930s Dust Bowl drought and the locally severe Texas mega drought of the 1950s.  Our results suggested reduced fire led to denser forests dominated by more mesophytic species, and severe droughts may have increased the rate of species change especially for J. virginiana.  These changes are likely to reinforce similar future changes as mesophytic species produce less flammable litter and more canopy closure substantially restricts production of herbaceous fine fuels.  The encroachment of J. virginiana has the potential to dramatically change the nature of these forests.