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Positive interactions can drive the assembly of desert plant communities, but we know little about the species-specificity of positive associations between native shrubs and invasive annual species along aridity gradients. These measures are essential for explaining, predicting, and managing community-level responses to plant invasions and environmental change. Here, we measured the intensity of spatial associations among native shrubs and the annual plant community—including multiple invasive species and their native neighbors—along an aridity gradient across the Mojave and San Joaquin Deserts, United States. Along the gradient, we sampled the abundance and species richness of invasive and native annual species using 180 pairs of shrub and open microsites. Across the gradient, the invasive annuals Bromus madritensis ssp. rubens ( B. rubens ), B. tectorum , B. diandrus, Hordeum murinum , and Brassica tournefortii were consistently more abundant under shrubs than away from shrubs, suggesting positive effects of shrubs on these species. In contrast, abundance of the invasive annual Schismus spp. was greater away from shrubs than under shrubs, suggesting negative effects of shrubs on this species. Similarly, native annual abundance (pooled) and native species richness were greater away from shrubs than under shrubs. Shrub-annual associations were not influenced by shrub size or aridity. Interestingly, we found correlative evidence that B. rubens reduced native abundance (pooled), native species richness, and exotic abundance (pooled) under, but not away from shrubs. We conclude that native shrubs have considerable potential to directly (by increasing invader abundance) and indirectly (by increasing negative impacts of invaders on neighbors) facilitate plant invasions along broad environmental gradients, but these effects may depend more upon invader identity than environmental severity.
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Exotic Prosopis juliflora suppresses understory diversity and promotes agricultural weeds more than a native congener
Exotic invasive plant species alter ecosystems and locally extirpate native plant species, and by doing so alter community structure. Changes in community structure may be particularly important if invaders promote species with certain traits. For example, the positive effects of most invaders on soil fertility may promote species with weedy traits, whether native or not. We examined the effects of two co-occurring Prosopis congeners, the native P. cineraria and the exotic invader P. juliflora, on species identified as “agricultural weeds” and species that were not agricultural weeds in the United Arab Emirates. When compared to plots in the open, P. cineraria canopies were associated with lower richness and density of non-weeds while having no impact on agricultural weed species. In contrast, there was lower richness and densities of non-weeds under canopies of P. juliflora, but higher densities of agricultural weeds than in the open surrounding the canopies. These patterns associated with Prosopis congeners and understory plant community composition might be due to the much higher litter deposition, if litter is inhibitory, and shallow root biomass under P. juliflora, or the different soil properties that corresponded with the two Prosopis canopies. In general, soils contained more nitrogen under P. juliflora than P. cineraria, and both understories were more fertile than soil in the open. Our results suggest that evolutionary history may play a role in how exotic invasive species may select for some traits over others in plant communities, with an exotic invader potentially creating reservoirs of agricultural weeds.
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- Award ID(s):
- 1757351
- PAR ID:
- 10158886
- Date Published:
- Journal Name:
- Plant ecology
- ISSN:
- 1385-0237
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/DOI 10.1007/s11258-020-01040-1
