Effect of a native tree on seedling establishment of two exotic invasive species in a semiarid ecosystem

Abstract

Theory predicts that in more stressful environments, positive plant-plant interactions should be more important than negative ones. For instance, in arid and semiarid regions, amelioration of soil drought produced by the shade of established plants could facilitate establishment of other species, in spite of light reduction. However, this theory has not been tested widely in the context of plant invasion. In this paper we evaluated the hypothesis that in a semiarid ecosystem of central Chile, the native tree, Lithrea caustica, should facilitate through positive shading effects, the seedling establishment of two widely planted and invasive forestry species, Pinus radiata and Eucalyptus globulus. We assessed the seedling establishment examining two processes: seedling recruitment (including germination) and subsequent seedling survival. We sowed seeds (to assess recruitment) and planted 8 months old seedlings (to assess seedling survival) of each exotic species under Lithrea patches, open sites and under an artificial shade mimicking Lithrea shading. The study was repeated in a north-facing and a southfacing slope in the study area located in a xeric zone within the distribution range of plantations of these species in central Chile. Our results show that in a north-facing slope Lithrea had positive effects on recruitment of both species, which was produced by shading. These effects were counteracted by negative effects on seedling survival but through a different mechanism, which suggests that Lithrea would have no significant effect on the whole seedling establishment process of Pinus radiata nor Eucalyptus globulus in this habitat. In turn, in a south-facing slope Lithrea had no significant effect on recruitment but had a negative effect on seedling survival, which was not produced by shading. This suggests that in this habitat Lithrea has a negative effect on the seedling establishment of these exotic species. Our results suggest that the effect of the native Lithrea caustica on the seedling establishment of these exotic species is dependent upon the life-cycle phase (recruitment or seedling survival) and habitat even within the same semiarid ecosystem. In contrast to the expected positive effects Lithrea is unlikely to facilitate seedling establishment of these exotic species in this area, and in fact in some habitats this effect could be negative. However, our results also suggest that a common mechanism proposed to resist invasion in forest ecosystems such as shading, probably is not sufficient to inhibit invasion in a semiarid region.

The impact of Broussonetia papyrifera (L.) vent. on community characteristics in the forest and forest–savannah transition ecosystems of Ghana

“…Considering the differences in the environmental conditions between the forest and transition sites, it can be argued that B. papyrifera might have produced different effects on the communities, in accordance with the stress-gradient hypothesis (Bertness & Callaway, 1994;Callaway & Walker, 1997;Maestre et al, 2009). It is possible that competition was the main interaction between B. papyrifera and the resident species in the forest site leading to positive impact following its removal (Lenz & Facelli, 2003;Becerra & Bustamante, 2011). On the other hand, at the transition site, B. papyrifera might have produced some positive effects (facilitation) on the resident species as well, producing no significant effects on them following its removal.…”

“…In general, conditions in the transition zone are harsher than in the forest zone (Taylor, 1960;Hall & Swaine, 1981), which given the stress-gradient hypothesis (SGH), should result in appreciable differences in the way the two ecosystems respond to invasion by B. papyrifera. The SGH argues that net competitive effects are more important, or at least more intense, in relatively benign, low-stress environments, whereas facilitative effects are more important in relatively harsh, high-stress environments (Bertness & Callaway, 1994;Lenz & Facelli, 2003;Maestre et al, 2009;Becerra & Bustamante, 2011). Thus, for this study, we expected that competitive interactions between B. papyrifera and indigenous/resident species would dominate in the forest area, whereas facilitative interactions dominate in the transition area.…”

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

“…In contrast, the climate in the native range has precipitation spread more evenly through the year, although it is concentrated in winter. If climate in the native region is more favourable for E. globulus than climate in non‐native regions (Becerra & Bustamante, ), then according to the stress gradient hypothesis more negative effects from E. globulus on species richness would have been more likely in the native range (Cavieres et al, ). However, we found the opposite pattern, stronger negative effects in the exotic ranges; thus, climate does not seem likely to drive the biogeographical differences we found.…”

The community ecology of invasive species: where are we and what's next?

“…Facilitation can be important among invasive species (i.e. invasional meltdown; Flory and Bauer ), but can also occur between invasive and native species, where the invader may act as either the facilitated (Becerra and Bustamante , Wundrow et al , Madrigal‐González et al ) or even the facilitating species (Sun et al , Traveset and Richardson ). Moreover the relative importance of competition and facilitation for invasion is likely to vary along gradients as well.…”