Plant functional traits suggest novel ecological strategy for an invasive shrub in an understorey woody plant community

Abstract

Summary1. Understanding ecological strategies of invasive species relative to the entire native community is important in understanding and managing both the mechanisms and the potential impacts of invasion, but few studies have taken this approach. 2. We utilize advances in plant ecology to compare functional traits of an invasive shrub species, autumn-olive Elaeagnus umbellata, to those of the understorey native woody plant community in a southeast Michigan forest. We estimate species trait distributions for six leaf functional traits (specific leaf area, leaf laminar area, leaf dry matter content, leaf nitrogen content, leaf carbon: nitrogen ratio and delta 15 N) using a kernel estimator. We then use pairwise trait distance and overlaps to describe the ecological strategy of the invasive shrub relative to that of the native understorey woody plant community. 3. The overlap of the invasive shrub's trait distribution with those of the native species was found to be smaller than the overlap of a native shrub's distribution with other native species, consistent with the empty niche hypothesis of invasion. However, the invasive species' mean leaf functional trait values typically fall outside of the range of most native species' mean trait values, suggesting the invasive shrub extends trait space for understorey individuals of the native woody plant community. 4. The invasive shrub's leaf trait values are characteristic of a sun-demanding species, contradicting its presence in the shaded forest understorey community and its persistence in an empty niche. We propose that this novel use of leaf functional traits by the invasive shrub could indicate that the invasion is facilitated by additional mechanisms such as release from natural enemies, association with nitrogen-fixing bacteria, use of an empty temporal niche (namely the light available to the understorey early in the growing season), bird dispersal or some synthesis of these factors. 5. Synthesis and applications. This study develops plant functional trait theory by studying an invasive species within the context of the entire native woody plant community. The approach can improve our understanding of the potential mechanisms of an invasion event and suggest ecological consequences and related management strategies for the maintenance of native forest communities. Specifically, this study, along with known data on the invasive species, suggests aggressive management in sunny gap and edge habitat provides the best strategy for long-term control.

Plant functional traits of a shrub invader relative to sympatric native shrubs

“…Functional traits reflect species interactions with the environment and have received recent attention for explaining mechanisms of community assemblages and patterns of habitat filtering (de Bello et al 2010, Brym et al 2011, de Bello et al 2012. Functional traits of E. umbellata allow for high resource use as demonstrated by light capture, hydraulic capacity coupled with high water use efficiency, and high nitrogen content that combined allow for its success in a wide variety of habitats.…”

“…A high photosynthetic capacity depends on high stomatal conductance, resulting in increased transpiration rates, which must be supported by a high water supply capacity to the leaves (Santiago et al 2004, Hernández et al 2010. However, E. umbellata does not conform to these expectations as it is characterized as drought resistant and occurs across a range of moisture and light environments (Yates et al 2004, Brym et al 2011. Zhang et al (2012) found similar paradoxical results in arid environment shrubs, which have functional traits indicative of fast growing, highly productive leaves as an adaptation to stressful hot and dry habitats.…”

“…Many studies have reported on the invasiveness of Elaeagnus species discussing the threat to ecological functioning of systems once well established (Katz andShafroth 2003, Yates et al 2004). However, research investigating functional traits and ecophysiology of Elaeagnus species, especially in areas where it is considered invasive, remains limited (Gong et al 2006, Zhao et al 2007, Brym et al 2011. Studies that consider functional trait combinations of expanding species in the context of traits found in cooccurring non-expanding species may reveal selective pressures that are occurring worldwide to cause significant shifts in growth forms (Brym et al 2011).…”

“…Investigation into the functional traits of woody and invasive species has received little attention and may help to mitigate losses of biodiversity and ecosystem function (Brym et al 2011). Our objectives were to investigate functional traits and plant resource strategies that contribute to the rapid expansion of E. umbellata by comparing these traits to two sympatric native shrubs.…”

“…This woody species forms extensive, dense thickets reaching a maximum height of 5-6 m, which limits light under the canopy (Brantley and Young 2010). Elaeagnus umbellata thrives in both disturbed areas and undisturbed habitats including the understory of pre-existing forests (Yates et al 2004, Brym et al 2011. It forms an association with the actinomycete, Frankia, a nitrogen-fixing root nodule forming endosymbiont, which may provide an advantage over co-occurring species with regards to nutrient acquisition, energy resources and stress tolerance (Yates et al 2004).…”