No abstract

Composition and structure of plant communities can be strongly influenced by plant interactions. Interactions among plants commonly comprise positive and negative effects operating simultaneously and bidirectionally. Thus, a thorough understanding of plant interactions requires experimental separation and quantitative assessment of the bidirectional positive and negative effects that add up to the net effects of plant interactions. Using the close spatial association of annual plants with a desert shrub (Ambrosia dumosa) in a sandy area of the Mojave Desert of California as a test system, we separated and quantified negative and positive effects of annuals on shrubs and of shrubs on annuals. We achieved the separation of negative and positive effects with an experimental design that included reciprocal removals of neighbors and simulations of physical effects of neighbors using artificial structures. All experimental manipulations were conducted on space originally occupied by Ambrosia shrubs to focus on immediate effects of neighbors on water availability rather than on long-term microenvironmental effects (e.g., nutrient accumulation). We quantified positive effects by calculating the difference between performance parameters of neighbors growing with artificial structures (thatch to mimic the physical effects of the presence of annuals and artificial canopies to mimic the physical effects of shrubs) and those of neighbors growing alone (removals). We estimated negative effects by calculating the difference between plant performance on control plots (shrubs and annuals growing together) and performances of plants growing with the artificial structures. Annuals had simultaneously strong negative and weak positive effects on shrub water status, growth, and reproductive output. Annuals also had an impact on the sex expression of shrubs by inducing shifts toward a higher male proportion in inflorescences of monoecious Ambrosia. In contrast, we found strong positive and weak or no negative effects of the shrubs on survival, biomass production, and seed production of the entire annual community and of selected annual species (the abundant native Chaenactis fremontii and the two dominant introduced annual species Bromus madritensis ssp. rubens and Schismus barbatus). Overall, in net effect, the interaction between shrubs and annuals can be described as facilitation or positive net effects of shrubs on annuals, and interference or negative net effects of annuals on shrubs. However, during the growing season, the ratios between positive and negative effects shifted. Annual plants benefited from the presence of shrubs to the greatest extent early in the growing season, and initial negative effects of annuals on shrubs declined as annuals senesced later in the season. Results of this study support the view that an experimental resolution of bidirectional positive and negative effects is necessary to achieve an accurate, mechanistic understanding of species interactions.

No abstract

Little is known about the potential for coexistence between native and non-native plants after large-scale biological invasions. Using the example of native perennial bunchgrasses and non-native annual grasses in California grasslands, we sought to determine the effects of interference from non-native grasses on the different life stages of the native perennial bunchgrass Nassella pulchra. Further, we asked whether N. pulchra interferes with non-native annual grasses, and whether competition for water is an important component of these interspecific interactions in this water-limited system. In a series of field and greenhouse experiments employing neighbor removals and additions of water, we found that seedling recruitment of N. pulchra was strongly seed-limited. In both field and greenhouse, natural recruitment of N. pulchra seedlings from grassland soil was extremely low. In field plots where we added seeds, addition of water to field plots increased density of N. pulchra seedlings by 88% and increased total aboveground N. pulchra seedling biomass by almost 90%, suggesting that water was the primary limiting resource. In the greenhouse, simulated drought early in the growing season had a greater negative effect on the biomass of annual seedlings than on the seedlings of N. pulchra. In the field, presence of annuals reduced growth and seed production of all sizes of N. pulchra, and these effects did not decrease as N. pulchra individuals increased in size. These negative effects appeared to be due to competition for water, because N. pulchra plants showed less negative pre-dawn leaf water potentials when annual neighbors were removed. Also, simply adding water caused the same increases in aboveground biomass and seed production of N. pulchra plants as removing all annual neighbors. We found no evidence that established N. pulchra plants were able to suppress non-native annual grasses. Removing large N. pulchra individuals did not affect peak biomass per unit area of annuals. We conclude that effects of interference from non native annuals are important through all life stages of the native perennial N. pulchra. Our results suggest that persistence of native bunchgrasses may be enhanced by greater mortality of annual than perennial seedlings during drought, and possibly by reduced competition for water in wet years because of increased resource availability.

For evaluating climate change impacts on biodiversity, extensive experiments are urgently needed to complement popular non-mechanistic models which map future ecosystem properties onto their current climatic niche. Here, we experimentally test the main prediction of these models by means of a novel multi-site approach. We implement rainfall manipulations—irrigation and drought—to dryland plant communities situated along a steep climatic gradient in a global biodiversity hotspot containing many wild progenitors of crops. Despite the large extent of our study, spanning nine plant generations and many species, very few differences between treatments were observed in the vegetation response variables: biomass, species composition, species richness and density. The lack of a clear drought effect challenges studies classifying dryland ecosystems as most vulnerable to global change. We attribute this resistance to the tremendous temporal and spatial heterogeneity under which the plants have evolved, concluding that this should be accounted for when predicting future biodiversity change.

Summary1 Relatively few studies have looked for patterns of invasion by non-native species within communities. We tested the hypotheses that: (i) some types of microhabitats within a community are more invasible than others; (ii) microhabitat types that differ in invasion also differ in resource availability; and (iii) invasibility is mediated by effects of these resources on competition between native and non-native species. 2 To test the first two hypotheses, we measured plant cover and soils in a coastal grassland in northern California. Consistent with these hypotheses, cover of non-native plants was consistently high where nitrogen-fixing shrubs had recently grown, in the bottoms and sides of gullies and on deep soils, and these microhabitats tended to have relatively high nitrogen or water availability. 3 Cover and number of native species tended to be lower where cover of non-native species was higher, indicating that non-native species as a group negatively affected native species. However, the number of non-native species also tended to be lower where the total cover of non-natives was higher. This suggests that a few non-native species excluded natives and other non-natives alike. 4 To test the third hypothesis, we grew a common non-native, the annual grass Lolium multiflorum , and a common native, the perennial grass Hordeum brachyantherum , at different levels of water and nitrogen. The relative competitive ability of the native was higher at lower nitrogen availability but not at lower water availability. When 10-weekold native plants were grown with non-native seedlings and nitrogen was relatively low, the native out-competed the non-native. However, the non-native out-competed the native at all resource levels when species were both grown as seedlings. Competition between native and non-native grasses in this system may thus help prevent invasion by non-natives in microhabitats where nitrogen availability is low, but invasion may be relatively irreversible.

The ability to selectively avoid competition with members of the same clone should be highly advantageous but has not been demonstrated in plants. We found that physical connection between plants in a clone of the wild strawberry Fragaria chiloensis induced them to segregate their roots, significantly increasing clonal performance. Such increase in performance was not found when plants were grown in containers that artificially divided their rooting zones. There was no effect of connection in a different clone of F. chiloensis with a lower degree of carbon transport between connected plants, suggesting that the mechanism for root segregation depended upon transport of a signal through the strawberry runners. We suggest that clonal integration allows some clones to coordinate below-ground resource foraging with other clone members, thus exhibiting a type of root cooperation.

No abstract