Positive feedbacks amplify rates of woody encroachment in mesic tallgrass prairie

Zak Ratajczak,Jesse B. Nippert,Jeffrey C. Hartman,Troy W. Ocheltree

doi:10.1890/es11-00212.1

Abstract

Over the last century, many grasslands worldwide have transitioned from a graminoid to a tree/shrub-dominated state in a short period of time, a phenomenon referred to as woody encroachment. Positive feedbacks and bi-stability are thought to be important drivers of woody encroachment, but there is little empirical evidence to suggest that positive feedbacks accelerate the woody encroachment of mesic grasslands. In mesic tallgrass prairie, shrub establishment does not directly facilitate seedling establishment. Yet, shrub establishment may facilitate the clonal spread of existing shrubs into nearby patches, because clonal reproduction might circumvent barriers that typically limit woody seedlings. Our results show that when Cornus drummondii (the predominate encroacher of mesic tallgrass prairie) extends rhizomatous stems into open grasslands, these stems use the same deep soil water sources as mature stems—thereby avoiding competition with grasses and gaining access to a reliable water source. In addition...

Highlights

In the last century, woody encroachment has been reported in grasslands worldwide and across a range of evolutionary histories and climatic conditions (Archer 1995, Van Auken 2000, Roques et al 2001, Knapp et al 2008), resulting in changes in ecosystem structure and function (Scholes and Archer 1997, Van Auken2000, Knapp et al 2008)
This study addresses the potential for positive feedback mechanisms in mesic tallgrass prairie, a system in which positive feedbacks have not been previously identified
In addition to changes in competition for water, we found evidence that individual C. drummondii may facilitate their own expansion through localized changes in fire frequency/ intensity

Summary

Introduction

Woody encroachment has been reported in grasslands worldwide and across a range of evolutionary histories and climatic conditions (Archer 1995, Van Auken 2000, Roques et al 2001, Knapp et al 2008), resulting in changes in ecosystem structure and function (Scholes and Archer 1997, Van Auken2000, Knapp et al 2008). In semi-arid and arctic grasslands, woody encroachment alters temperature and/or the availability of water and nutrients, which facilitates the establishment of other shrubs and creates a positive feedback loop (Schlesinger et al 1990, Scheffer et al 2001, Ravi et al 2007, Wookey et al 2009, D’Odorico et al 2010; see Archer et al 1988 for mechanisms related to seed dispersal) These and other mechanisms have been found to have a minimal impact on encroachment of tallgrass prairie (Briggs et al 2005, McKinley and Blair 2008). The nature of feedbacks in tallgrass prairie may involve factors that structure mesic grasslands: pronounced belowground competition in the upper soil layers (Nippert and Knapp 2007a, b), frequent and intense fires (Knapp et al 1998, van Wilgen et al 2003, Briggs et al 2005, Bond 2008) and predominance of clonal reproduction (Benson and Hartnett 2006, Dalgleish and Hartnett 2006)

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Results

Discussion

Conclusion