N Deposition, Invasive Species and Impacts on Biodiversity in Southern California Edith B. Allen Department of Botany and Plant Sciences University of California, Riverside

Four major vegetation types are subject to N deposition in southern California Coastal sage scrub and desert scrub are subject to vegetation type conversion by exotic annual grasses. Mixed coniferous forest and chaparral are relatively stable.

Levels of increasing N deposition required to increase invasive species and fire frequency in different vegetation types

What are critical loads for N deposition that promote vegetation type conversion in coastal sage scrub (28-35 cm precipitation, 30 kg ha -1 yr -1 N deposition) vs. Mojave desert scrub ( 8 kg ha -1 yr -1 )?

Soil N gradient from north to south in the Riverside-Perris Plain

Box Springs Mt. with high N deposition dominated by exotic annual grasses Lopez Canyon with low N deposition dominated by native forbs and shrubs

% cover and biomass of grasses following the 1993 fire and N fertilization (60 kg/ha/yr). The threshold for fire is t/ha of fine grass fuel.

% cover of exotic and native forbs following the 1993 fire and N fertilization.

Air pollution at Key’s View, JOTR Dec. 04 Schismus barbata and Brassica tournefortii invasion at Snow Creek following fire

Exotic grass biomass following two years of N fertilization at four sites at Joshua Tree National Park, 2004

Elevated exotic grass biomass above the threshold value of T/ha is most likely responsible for the high incidence of fire in the areas of CSS affected by N deposition. Even in the absence of fire, after a decade of N deposition at 60 kg/ha/yr exotic grasses increased and native species declined. In arid regions N will accumulate in soil, so critical loads are determined by a combination of time X level of N inputs.

Number of plant species in each lifeform in 3 ha plots at seven sites along a N deposition gradient with varying concentrations of soil N (mg/kg)