PARENT SESSION
Poster Session # 11: Disturbance Ecology.

Tuesday, August 5 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

Effects of fire on soil nutrients following cheatgrass invasion of Sierra Nevada yellow pine forests.

Wojcik, Nathan ¹, Sanford, Robert^*,1, McGinnis, Thomas², Keeley, Jon², Brooks, Matt³, Belnap, Jayne⁴, ¹ University of Denver, Denver, CO² U.S. Geological Survey, Biological Resources Discipline, Western Ecological Research Center, Sequoia-Kings Canyon National Park, Three Rivers, CA³ U.S. Geological Survey, Biological Resources Discipline, Western Ecological Research Center, Las Vegas Field Station, Las Vegas, NV⁴ U.S. Geological Survey, Biological Resources Discipline, Forest and Rangeland Ecosystem Science Center, Moab, UT

ABSTRACT- Prescribed fire has greatly increased the invasion of cheatgrass (Bromus tectorum L.) in old growth yellow pine forest of Kings Canyon National Park, California. The potential changes in fuels and soil chemistry caused by cheatgrass invasion have important implications for management. By manipulating several variables in 5x5 m forest test plots (n=420), we determine the aspects of fire that have the most effect on cheatgrass, native plants and soil chemistry. This paper focuses on the changes in soil phosphorus (P) and nitrogen (N). Late fall burns in 2001, with light fuel loads and high relative humidity, caused non-significant changes in most forms of soil P and soil N. Soil temperatures 5 cm belowground in the 2001 burn reached a mean of only 18°C. For the summer and fall 2002 burns we added fine fuels in the form of a 5 cm-deep layer of pine needles. Fuel additions caused a significantly greater heat flux belowground, and this increase in soil temperature caused significant changes in both NH₄-N and NO₃-N soil levels (p < 0.05). NH₄-N in fuel treated plots increased to 1.3 g N/g soil post burn, in comparison to control plots with 0.2 g N/g soil (2 cm depth). NO₃-N decreased significantly after burning for both fuel treated and for control plots (0.47 g N/g soil and 0.35 g N/g soil respectively), with the largest decrease in fuel treated plots. We predict that changes in plant available soil P and N levels increase following fires that heat soils to moderate temperatures (100 – 300°C), ultimately increasing the productivity of invasive cheatgrass. We compare our predictions with additional samples from the 2003 growing-season.

Key words: Bromus tectorum, nitrogen, fire, phosphorus