Functional traits of expanding, thicket‐forming shrubs: contrasting strategies between exotic and native species

Sheri A Shiflett,Julie C Zinnert,Donald R Young

doi:10.1002/ecs2.1918

Abstract

AbstractWoody expansion has been documented for decades in many different systems globally, often yielding vast changes in ecosystem functioning. While causes and consequences of woody expansion have been well documented, few studies have addressed plant functional traits that promote dramatic and rapid expansion in range. Our objectives were to investigate plant functional traits that contribute to the colonization, rapid expansion, and thicket formation of an invasive, N‐fixing shrub, Elaeagnus umbellata Thunb. (Elaeagnaceae), and a native, N‐fixing shrub Morella cerifera (L.) Small (Myricaceae) and compare to native, sympatric, non‐expanding shrub species. Quantified functional traits included morphological (e.g., specific leaf area, leaf area) and physiological characteristics (e.g., electron transport rate, hydraulic conductivity) and were linked to two primary resources: light and water, which directly influence plant growth. Elaeagnus umbellata and M. cerifera rely on different strategies to maximize carbon gain, yet resulting physiological efficiency is similar. Elaeagnus umbellata invests a substantial amount of energy into growth during a short amount of time (i.e., deciduous growing season), using an acquisitive trait strategy to outcompete co‐occurring woody species, while M. cerifera is productive year‐round and uses a combination of conservative and acquisitive traits to outcompete co‐occurring woody species. The majority of quantified functional traits of E. umbellata and several of M. cerifera are indicative of efficient light capture, utilization, and internal water movement. These factors contribute to rapid range expansion and thicket formation by promoting enhanced productivity while simultaneously inhibiting colonization and expansion of co‐occurring species. Suites of functional traits are important for expansive success and thicket formation, yet differences in functional traits represent alternative strategies for colonization, rapid expansion, and thicketization.

Highlights

Range expansion of woody species, especially shrubs, has been documented in a variety of ecosystems worldwide (Wessman et al 2004, Briggs et al 2005, Sturm et al 2005, Knapp et al 2008, Ward 2010, Eldridge et al 2011, Heskel et al 2013, Rundel et al 2014, Zinnert et al 2016, Stevens et al 2017)
Few studies have focused on mechanisms, especially plant functional traits related to morphology and physiology, leading to woody expansion and, in some cases, monospecific thicket formation
Our objectives were to investigate functional traits pertaining to light capture, processing, and water use that contribute to successful colonization, expansion, and dense thicket formation of invasive (E. umbellata) and native (M. cerifera) N-fixing shrubs and compare traits to native, sympatric, non-expanding shrubs

Summary

Introduction

Range expansion of woody species, especially shrubs, has been documented in a variety of ecosystems worldwide (Wessman et al 2004, Briggs et al 2005, Sturm et al 2005, Knapp et al 2008, Ward 2010, Eldridge et al 2011, Heskel et al 2013, Rundel et al 2014, Zinnert et al 2016, Stevens et al 2017). Shrubs have a unique growth form, efficient for exploiting horizontal space and minimizing self-shading via formation of a dense vertical array of leaves through the canopy (Knapp et al 2008) In mesic habitats, both leaf area index and annual net primary productivity for shrub-dominated patches can exceed that of most temperate forests (Knapp and Smith 2001, Huxman et al 2004, Knapp et al 2008). Both leaf area index and annual net primary productivity for shrub-dominated patches can exceed that of most temperate forests (Knapp and Smith 2001, Huxman et al 2004, Knapp et al 2008) This shift in growth form is accompanied by alterations in plant functional traits related to resource use (i.e., light, water), which combinedly modify the local environment and alter community structure and function (Throop and Archer 2008, Zinnert et al. September 2017 ❖ Volume 8(9) ❖ Article e01918 SHIFLETT ET AL. Few studies have focused on mechanisms, especially plant functional traits related to morphology and physiology, leading to woody expansion and, in some cases, monospecific thicket formation (i.e., thicketization)

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Conclusion