Abstract:
During the last century, fire suppression, grazing, and climate change have caused sagebrush grasslands to be altered in both function and form; juniper and sagebrush dominate the landscape at the expense of herbaceous plants. Management efforts to reduce juniper and sagebrush overstory in order to enhance herbaceous components of the ecosystem are currently underway. However, such changes may alter the invasion potential for exotic invasive plant species. We examined what effects prescribed fire and woody removal exerted on the spatial mosaic of nutrients essential for plant development, inorganic nitrogen, and if these
practices increased the risk of invasion by a noxious annual grass, Taeniatherum caput-medusae (L.) Nevski (medusahead). Four disjunct sites were selected in central Oregon, treated with prescribed fire and harvesting to remove woody plants, and monitored from 1997 to 1999. We collected monthly soil samples and assessed nitrogen and ammonium concentrations to examine the spatial and temporal dynamics of under canopy
versus interspace resource islands. Simultaneously, we planted medusahead seeds
and monitored their populations monthly. We developed a matrix model of medusahead's population dynamics to compare among the treatments. Average ammonium and nitrate concentrations in canopies were higher than
interspaces for nearly all months of the year. However, this difference was not always significant and was variable through time. We found that these resource islands were enhanced by fire, but not by woody plant removal. After prescribed fire, both ammonium and nitrate remained significantly higher under former canopies than interspaces throughout the study. T. caput-medusae was a serious threat for invasion in undisturbed, burned and juniper-removal areas in both 1998 and 1999. Results of the demographic matrix model indicate T. caput-medusae's population growth rate is higher in
burned areas than control and juniper-removal areas. T. caput-medusae's seedbank
remained small but viable for at least two years under field conditions. T. caput-medusae populations in canopy areas were more fecund than those in interspace areas.
Harvesting juniper trees did not enhance concentrations of inorganic nitrogen in microsites whereas prescribed burning caused concentrations to increase 10-fold during the first four months after burning. Harvesting is not likely to increase the risk of T. caput-medusae invasion above levels found in untreated areas, whereas prescribed fire increased invasion risk. In areas where T. caput-medusae is already present or nearby, harvesting may be a better alternative for woody plant removal than prescribed burning.
