Native grassland restoration and nitrogen deposition

How do competition and propagule pressure influence the continued success of annual exotic grasses in California? Do feedbacks through soil resources or mycorrhizae enable persistence? Does nitrogen deposition intensify these interactions? Can we assist the recovery of native perennial grasses?

Concepts of multiple state equilibria have great applicability to invasion and restoration ecology. While the application of this theory to restoration is appealing, empirical investigations lag far behind the theoretical framework. There are few tests of whether exotic species can push a system across a threshold, resulting in an alternative stable state that is resilient to restoration. The goal of this research is to directly test how multiple stable equilibria in communities relate to invasion at both local and regional scales.

This project addresses the following questions:

  1. How does environmental heterogeneity influence local interactions and regional coexistence of native and exotic species?
  2. Are there environmental conditions where local multiple stable equilibria (MSE) between native and exotics exist?
  3. Do positive feedbacks mediated by species effects on litter and soil microbial processes contribute to invasion dynamics?
  4. How do differences in colonization rates between natives and exotics influence local interaction dynamics?

We focus our tests on California grasslands systems and one of the most widespread and persistent biological invasion in the US. Preliminary results suggest that patterns of species abundance are consistent with multiple stable states, that spatial refuges exist for both native and exotic species over an environmental gradient of nitrogen supply, that interactions between native and exotic species follow MSE dynamics, and that these interactions are driven by positive plant-soil feedbacks.

We conduct our experimental work at the South Coast Research and Extension Center and Irvine Ranch Land Reserve. We explore and explicitly test multiple state dynamics, a new way of thinking about invasion and dominance in ecological systems. Tests include reciprocal invasions into experimental monocultures with different nitrogen supply rates, plant-soil feedback greenhouse experiments, and a larger scale experiment to manipulate colonization dynamics. These experiments are enabling us to parameterize a spatial model of invasion that incorporates positive feedbacks and dispersal, advancing the link between theory and empirical evidence.

People involved: Claudia Stein, Loralee Larios, Dr. Stan Harpole, Iowa State University