Comparative impacts of aboveground and belowground enemies on an invasive thistle

Abstract

Most research examining how herbivores and pathogens affect performance of invasive plants focuses on aboveground interactions. Although important, the role of belowground communities remains poorly understood, and the relative impact of aboveground and belowground interactions is still debated. As well, most studies of belowground interactions have been carried out in controlled environments, so little is known about the role of these interactions under natural conditions or how these relationships may change across a plant's range. Using the invasive plant Cirsium arvense, we performed a reciprocal transplant experiment to test the relative impacts of above‐ and belowground interactions at three sites across a 509‐km latitudinal gradient in its invaded range in Ontario, Canada. At each site, C. arvense seedlings were protected with above‐ and/or belowground exclosures in a factorial design. Plant performance (biomass, height, stem thickness, number of leaves, length of longest leaf, maximum rhizome length) was greatest when both above‐ and belowground exclosures were applied and lowest when no exclosures were applied. When only one type of exclosure was applied, biomass generally improved more with belowground exclosures than with aboveground exclosures. Despite site‐to‐site differences in foliar damage, root damage, and mesofaunal populations, belowground interactions generally had a greater negative impact on performance than aboveground herbivory alone. These results stress the importance of including both aboveground enemy interactions and plant–soil interactions in studies of plant community dynamics and invader performance.

Soil biota composition and the performance of a noxious weed across its invaded range

“…The three southernmost sites experienced inhibition of plant growth in live soils sourced from both inside and outside invaded patches, although belowground biomass in the Newmarket treatment, a mid-latitude site, exhibited neutral effects. However, other work at this site (Nunes and Kotanen 2018) found negative effects more consistent with other southern Ontario populations. The five northernmost sites experienced improved plant growth in live soils with the exception of plants grown in soils sourced from within invaded patches in Algonquin.…”

Genetically Based Trait Differentiation but Lack of Trade-offs between Stress Tolerance and Performance in Introduced Canada Thistle

“…However, such a hypothesis does not hold for other regions, such as New Zealand, where no native thistles were initially present, and even a transient enemy release may suffice for a successful establishment, until (unfruitful) biological controls are deliberately released ( Nunes and Kotanen, 2018 ). A recent study by Nunes and Kotanen (2017) further indicates that below-ground herbivory has a stronger (negative) impact on plant performance than above-ground herbivory and should therefore be examined more deeply in the future.…”

Using biological invasions to improve plant defense theory

“…Second, a more complete understanding of plant defense requires integrating belowground defenses into tests of plant defense theory. Above-and below-ground defenses may respond differently to shifts in herbivore pressure after introduction to a new range (Huang et al., Yet we still know very little about the prevalence and impact of root feeders, especially across populations of invasive plants (Blossey & Hunt-Joshi, 2003;Nunes & Kotanen, 2018). Verbascum thapsus represents an opportunity to address this knowledge gap, as there is a dramatic release from belowground herbivory in the mountain west region of the USA compared to part of the native range in France (SB Endriss, unpubl.…”