Inhibitory effects of Eucalyptus globulus on understorey plant growth and species richness are greater in non‐native regions

Abstract

Aim: We studied the novel weapons hypothesis in the context of the broadly distributed tree species Eucalyptus globulus. We evaluated the hypothesis that this Australian species would produce stronger inhibitory effects on species from its non-native range than on species from its native range. Location:We worked in four countries where this species is exotic (U.S.A., Chile, India, Portugal) and one country where it is native (Australia). Major taxa studied: Plants. Methods:We compared species composition, richness and height of plant communities in 20 paired plots underneath E. globulus individuals and open areas in two sites within its native range and each non-native region. We also compared effects of litter leachates of E. globulus on root growth of seedlings in species from Australia, Chile, the U.S.A. and India.Results: In all sites and countries, the plant community under E. globulus canopies had lower species richness than did the plant community in open areas. However, the reduction was much greater in the non-native ranges: species richness declined by an average of 51% in the eight nonnative sites versus 8% in the two native Australian sites. The root growth of 15 out of 21 species from the non-native range were highly suppressed by E. globulus litter leachates, whereas the effect of litter leachate varied from facilitation to suppression for six species native to Australia.The mean reduction in root growth for Australian plants was significantly lower than for plants from the U.S.A., Chile and India.Main conclusions: Our results show biogeographical differences in the impact of an exotic species on understorey plant communities. Consistent with the novel weapons hypothesis, our findings suggest that different adaptations of species from the native and non-native ranges to biochemical compounds produced by an exotic species may play a role in these biogeographical differences.

Not a melting pot: Plant species aggregate in their non‐native range

“…Across sites, invaded plots had c. 15% lower native species richness than their adjacent non-invaded plots; a difference smaller than the c. 60% lower native species richness observed when comparing the plots occupied by the 46 focal (Figure 2b). Together, these results suggest a combination of alien species preferentially establishing in species-poor patches, which might pose lower biotic resistance (Levine et al, 2004), but also a negative impact of alien species on native species richness (Becerra et al, 2018;Shah et al, 2014). A negative impact on native species, especially if higher than that on other alien species, could lead to indirect facilitation (Kuebbing & Nuñez, 2016), which might explain the co-occurrence among alien species (Figure 2a), and suggest that an invasional meltdown might be occurring (Simberloff & Von Holle, 1999).…”

“…Across sites, invaded plots had c. 15% lower native species richness than their adjacent non-invaded plots; a difference smaller than the c. 60% lower native species richness observed when comparing the plots occupied by the 46 focal F I G U R E 2 Characteristics of the communities (1 m 2 plots) in which species are found in their native (home) and non-native (away) range. Together, these results suggest a combination of alien species preferentially establishing in species-poor patches, which might pose lower biotic resistance (Levine et al, 2004), but also a negative impact of alien species on native species richness (Becerra et al, 2018;Shah et al, 2014). Note that alien species richness (a) in the home range refers to the number of species that are alien in the site(s) where the focal species is considered native.…”

“…Alternatively, if alien species aggregate with each other instead of merging, they could lead to the replacement of native communities, reduced diversity, and more greatly altered ecosystem functions (Vilà et al, 2011). Evidence suggests that many species have more negative effects on species richness in their non-native ranges compared with their native ranges (Becerra et al, 2018;Shah et al, 2014). Furthermore, by aggregating in the non-native range, their combined effects could lead to even lower native species richness and even greater changes in ecosystem processes in those areas (Kuebbing, Nuñez, & Simberloff, 2013;Simberloff & Von Holle, 1999).…”

Exotic eucalypts: From demonized trees to allies of tropical forest restoration?

“…Only nine out of >700 Eucalyptus and Corymbia species (hereafter referred to as “eucalypts”) comprise >90% of the global planted area (Stanturf, Vance, Fox, & Kirst, ). The prominent environmental concerns associated with the large plantation area and ecological characteristics of exotic eucalypts have motivated several studies to assess their biodiversity value, allelopathic effects, water consumption, and potential for invading unplanted areas (Becerra et al., ; Bremer & Farley, ; Stanturf et al, ). The effects of eucalypts vary, however, with regional climate, previous land use, and plantation management practices (Brockerhoff et al, ).…”

Allelopathic effects of native and invasive Brassica nigra do not support the novel‐weapons hypothesis

“…Plants may release allelopathic compounds through leaf leachates, root exudates, volatiles, and decaying litter (Callaway and Ridenour, 2004; Callaway et al, 2005; Inderjit et al, 2011). Indeed, there is mounting empirical evidence in support of the novel‐weapons hypothesis (e.g., Callaway and Aschehoug, 2000; Ridenour and Callaway 2001; Prati and Bossdorf, 2004; Abhilasha et al, 2008; Gómez‐Aparicio and Canham, 2008; Thorpe et al, 2009; Inderjit et al, 2011a; Becerra et al, 2018; Irimia et al, 2019), suggesting that allelopathic effects could indeed play a crucial role in facilitating the ability of invasive plants to displace their neighbors in the new range. Moreover, if allelopathic effects confer an advantage against naïve competitors as suggested in the novel‐weapons hypothesis, then genotypes that produce high concentrations of allelochemicals might be selected for in the introduced range (Callaway and Ridenour, 2004).…”