Critical loads of anthropogenic nitrogen deposition that promote vegetation-type conversion to exotic grassland in coastal sage scrub and desert Edith B. Allen, Leela E. Rao, Robert J. Steers, and Abby G. Sirulnik, Department of Botany and Plant Sciences and Center for Conservation Biology

Tonnesen et al UCR Riverside-Perris Plain, Coastal Sage Scrub Mojave/Colorado Desert

Objectives 1. Test responses of native and invasive plants to N fertilization in coastal sage scrub and desert. 2. Compare levels of N that promote increases in invasive grasses and vegetation type conversion.

Coastal sage scrub vegetation receives up to 30 kg N ha -1 yr -1 mainly as dry deposition; frequent fire, loss of native diversity, exotic annual grass invasion

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

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

Coastal sage scrub (CSS) vegetation was fertilized yearly with 60 kg/ha/yr of N following the 1993 wildfire near Lake Skinner, an area of low N deposition Recovery three years after fire

% cover of native forbs (69 spp.) and exotic forbs (Erodium spp.) following the 1993 fire and N fertilization.

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 (red line).

Soil N concentrations N-N

Air pollution outside Joshua Tree National Park (12/2004) 12 kg N ha -1 yr -1 Schismus barbata invasion outside JTNP (5/2005)

N deposition (kg ha -1 yr -1 ) measured at experimental N fertilization sites in Joshua Tree NP Covington Flat Wide Canyon Pine City Pinto Basin

N fertilizer study sites Covington Flats Pinus-Juniperus Wide Canyon Larrea tridentata Pinto Basin Pine City Higher N depositionLower N deposition

Extractable soil N concentration following N fertilization in July of 2004 (average precipitation) and 2005 (wet year)

Exotic grass biomass after two years of N fertilization, 2004 Covington Flats A A N fertilization (kg/ha) Dry Weight (g/m 2 ) A 0530 Pine City A B N fertilization (kg/ha) A 0530 Wide Canyon A B N fertilization (kg/ha) Dry Weight (g/m 2 ) AB Pinto Basin A B N fertilization (kg/ha) AB 0530

Nitrogen-fertilized plot in Pinto Basin

% cover of exotic and native species in 2005 with N fertilization

Conclusions 1.Exotic annual grasses responded to N fertilizer in both vegetation types within 2 years. 2.In the desert, native forb cover declined significantly in an average year with 30 kg/ha N, and in a wet year with only 5 kg/ha added N. 3.CSS is more resilient to N deposition; native forb cover did not decline significantly until the 11th year of fertilization with 60 kg/ha N. 4.Elevated exotic grass biomass above the threshold value of T/ha is most likely responsible for the high incidence of fire in areas affected by N deposition, and fire causes rapid vegetation type conversion.

Christopher True, Sara Jo Dickens, Heather Schneider, Tracy Tennant, Sheila Kee, Mike Allen, Richard Minnich (UCR) Andrzej Bytnerowicz, Mark Fenn (US Forest Service) Jane Rodgers and many others (NPS) Research funded by National Park Service, National Science Foundation Acknowledgements