Rapid Re-encroachment by Juniperus virginiana After a Single Restoration Treatment
Introduction
Woody encroachment is the process in which grasslands transition to an alternative, woody-dominated state (Ratajczak et al. 2014; Wilcox et al. 2018). Woody transitions result in fundamental changes in the structure, function, and composition of grassland systems including reductions in herbaceous biomass and diversity, as well as altered nutrient cycles, hydrology, and carbon storage (Scholes and Archer 1997; Jackson et al. 2002; Huxman et al. 2005; Archer and Predick 2014). In response to these threats, managers have turned to woody plant removal, generally known as “brush management” in North and South America, as a restoration tool used to induce a shift back to a grass-dominated state. Indeed, various forms of brush management have proven to be effective in restoring herbaceous biomass and diversity (Archer et al. 2011). However, the lifespan of brush management treatments depends on rates of re-encroachment and recovery (Archer and Predick 2014). One of the biggest risks to sustaining costly brush management programs is if projects are short-lived. Yet rates of re-encroachment are currently unknown for some of the most common encroaching species (Archer et al. 2011), despite substantial investments in brush management.
Here, we implemented the first study to track Juniperus virginiana L. re-encroachment following restoration with high-severity fire. J. virginiana is a notorious encroaching species in the North American Great Plains and is driving a large-scale woodland transition associated with a suite of social-ecological consequences (Briggs et al. 2002; Twidwell et al. 2013b). As the impacts of J. virginiana encroachment become more apparent, some managers have turned to high-severity fire as a cost-effective restoration method at large scales. In this study, we use a regional restoration initiative to quantify rates of J. virginiana re-encroachment 16 yr following woodland collapse with high-severity fire. This study answers several questions concerning the re-encroachment process: 1) Does J. virginiana quickly recover following restoration or is a slow re-establishment process observed? 2) How long are seedlings present in the herbaceous layer before beginning a process of rapid growth in cover? and 3) when does J. virginiana reach heights associated with seed production?
Section snippets
Study system
This study was conducted in the Loess Canyons Experimental Landscape located in central Nebraska (Fig. 1). The Loess Canyons Experimental Landscape spans a 72 843-ha area and consists of private properties that are connected to form a large landowner coalition. Mixed-grass prairie is the dominant vegetation community with an average herbaceous plant height of 0.3 m (Fogarty, unpublished data). Dominant grass species include big bluestem (Andropogon gerardii Vitman), little bluestem (
Results and Discussion
J. virginiana re-encroachment began almost immediately after initial restoration with fire. Seedlings established 1−2 yr after restoration and then increased in density following a logarithmic trend (y = exp[0.4259 + 2.9675 ⋅ ln{x+1}] – 1; R2 = 0.66) (Fig. 2A). Density recovered to levels within the range of unburned woodlands (633−1 267 trees ha−1) within 5−11 yr after restoration (based on bootstrapped 95% confidence intervals), although some individual sites recovered earlier (see Fig. 2A).
Implications
Brush management is a widespread restoration practice in global rangelands, and our study contributes to a growing body of literature showing that restored lands are highly vulnerable to re-encroachment (reviewed by Archer et al. [2011]). A key implication is that sustaining restored grasslands will require follow-up management, consistent with historical fire return intervals, to promote feedbacks that limit woody plants and promote grasslands. Clearly, re-encroachment will influence what it
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We are grateful to the landowners who allowed us to access their properties for this research. The Nebraska Cooperative Fish and Wildlife Research Unit is jointly supported by a cooperative agreement between the US Geological Survey, the Nebraska Game and Parks Commission, the University of Nebraska−Lincoln, the US Fish and Wildlife Service, and the Wildlife Management Institute.
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Funding for this research was provided by Nebraska Game & Parks Commission [grant W‐125‐R‐1], the University of Nebraska's Institute of Agriculture and Natural Resources, the USDA National Institute of Food and Agriculture [grant 2017-67032-26018], and the Arthur W. Sampson Fellowship Fund (University of Nebraska−Lincoln).