PARENT SESSION
Oral Session #66: Invasions: Plants. Mechanisms, patterns, controls. Presiding: R. Mack.
Thursday, August 9, 2001. 1:00 PM to 4:45 PM. Hall of Ideas E.
Multiple limiting soil resources in a California grassland.
Harpole, Stan1, 1
ABSTRACT- Exotic and native grasses in a CA grassland respond differently to multiple limiting resources supporting the hypothesis that coexistence or competitive exclusion between these species can be partly explained in terms of tradeoffs between them in their requirements for limiting resources. In a nutrient addition experiment at Sedgwick Reserve, Santa Ynez CA, I found significant plant productivity and compositional responses to N and at least one of the following nutrients: P, Ca, K, or Mg. In a follow-up experiment at the same site, I added N, P, Ca+K+Mg (Cations), and H2O in a full factorial design in an ungrazed grassland composed of both native and exotic grasses and forbs. Overall productivity increased with addition of N and H2O (60%, 36% biomass increase respectively), and total richness decreased with P. Specifically, percent cover of the exotic annual grass Bromus diandrus increased significantly with added N and with added P, while cover of its congener Bromus hordeaceous decreased with added P. Cover of the native bunch grass Nassella pulchra decreased with added N. This suggests that competition in this grassland is influenced by tradeoffs between species in their requirements for multiple resources, and that competitive outcomes between native and exotic species can be predicted using resource competition theory and resource ratio information. Also, anthropogenic changes in N and H2O supply rates will likely further alter the composition of these grasslands.
KEY WORDS: Resources, Grassland, Invasive, Nutrient addition