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Phenology and seedling competition affect regeneration of perennial grasses in California exotic annual grasslands.
Abraham, Joel*,1, Corbin, Jeffery1, D'Antonio, Carla1, 2, 1 University of California, Berkeley, Berkeley, CA2 USDA-ARS, Reno, Nevada
ABSTRACT- The conversion of California grasslands from perennial-dominated to exotic annual-dominated ecosystems has been well documented. More recently, exotic perennial grasses have invaded both grassland types. One of the theorized reasons for the success of exotic species at the expense of natives is competitive asymmetry caused by earlier germination and more rapid seedling growth of exotic species. The rapid emergence and high densities of annual grasses are thought to reduce resources available to native perennial seedlings, making native restoration difficult. We hypothesized that decreasing the density of annual grasses and delaying their seedling emergence would increase native perennial performance, while nutrient availability would interact with these conditions. Exotic perennial grasses by contrast, have rapid seedling growth much like exotic annual grasses so we hypothesized that they would be less affected by annuals than native perennials grasses would be. To test these hypotheses, we set up a greenhouse experiment in which an exotic annual grass Bromus diandrus, was grown in competition with native perennial grasses (Nassella pulchra or Festuca rubra) or an exotic perennial grass (Holcus lanatus). We maintained Bromus at three competitive densities, manipulated N levels, and manipulated emergence time by delaying planting of Bromus. As predicted, increasing the density of annual competitors decreased perennial aboveground productivity and this effect was not as strong for Holcus. Also, aboveground productivity of Bromus was lower in competition with Holcus than with natives. Delayed annual emergence increased aboveground productivity in Nassella and Holcus, but had no effect on Festuca. N additions in non-competition pots increased aboveground productivity in Holcus and Festuca, while Nassella had no response. However, in competition, Festuca no longer responded to N addition. These results support other studies showing that seedling competition between annuals and perennials may be a critical stage in the regeneration of native grasses in California and suggest that delaying or reducing competition from annuals can increase perennial seedling growth.
Key words: seedling, invasion, grassland, competition