Influence of resource availability on Juniperus virginiana expansion in a forest–prairie ecotone

A C Ganguli,P M Mayer,D M Engle,L F Salo

doi:10.1002/ecs2.1433

Abstract

AbstractWoody plant expansion into grasslands and savannas is a global concern. Rapid expansion of Juniperus virginiana, a tree native to North America, has profound ecological consequences. We used transplanted J. virginiana seedlings to investigate the role of resource availability on J. virginiana expansion following the removal of fire, the factor historically limiting range expansion of this fire‐intolerant species. We evaluated J. virginiana seedling survival and seedling growth, two important phases in woody plant expansion, relative to two belowground resource factors, plant‐available soil water (soil clay content, an index of plant‐available soil water) and plant‐available nitrogen (PAN), and an aboveground factor, photosynthetic active radiation (PAR). In three plant communities associated with an oak forest–tallgrass prairie ecotone, we transplanted 2‐yr‐old J. virginiana seedlings in a systematic grid design and measured J. virginiana seedling survival and growth 8, 20, and 30 months following the transplant. We also measured soil clay content, PAN, and PAR in 1‐m2 quadrats centered on each transplanted seedling. We employed path analysis at two spatial scales (144 and 2916 m2) to compare the role of resource factors in seedling growth and survival. Juniperus virginiana seedling survival was about 10% greater in tallgrass prairie and upland oak forest than in the old field, and seedling growth in tallgrass prairie exceeded the two other communities by about a factor of five. Tallgrass prairie in our study area is clearly more vulnerable to expansion of J. virginiana than the other two plant communities. Survival and growth were controlled largely by available light (PAR) and secondarily by plant‐available soil water, indexed by soil clay content. In all three vegetation types and at both fine and coarse scales, soil clay content also exerted an indirect effect, possibly by mitigating the influence of light. Results of this study suggest that soil distribution maps and associated ecological site designations might be useful for directing J. virginiana management in the oak forest–tallgrass prairie ecotone by identifying the potential hotspots of invasion.

Highlights

Woody plant expansion is a global phenomenon in grassland and savanna ecosystems (Archer 1994, Binggeli 1996)
Seedling survival was lowest in old-field grassland and 10% greater in tallgrass prairie and upland oak forest (Table 2)
Our controlled experiment assessed the survival and growth of J. virginiana seedlings transplanted into three contrasting, intermixed vegetation types

Summary

Introduction

Woody plant expansion is a global phenomenon in grassland and savanna ecosystems (Archer 1994, Binggeli 1996). Does infrequent fire accelerate woody plant expansion, restoring historic fire regimes in this region may not reverse woody plant expansion (Briggs et al 2005, Bond 2008). One such example of a rapidly expanding native woody species is Juniperus virginiana L. in North America (Schmidt and Leatherberry 1995, Hoch et al 2002, DeSantis et al 2010). Expansion of J. virginiana is of ecological concern because it changes the plant and animal community composition (Gehring and Bragg 1992, Coppedge et al 2001, Horncastle et al 2005, Alford et al 2012, Powers and Larsen 2014), reduces grassland productivity (Engle et al 1987, Smith and Stubbendieck 1990), alters hydrology (Zou et al 2013, Starks et al 2014), and alters biogeochemistry (Norris et al 2001, 2007, McKinley and Blair 2008)

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