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Net primary productivity (NPP) is a key ecosystem function of plant communities. Climate change is expected to affect NPP both directly and indirectly through associated edaphic and plant community factors. Changes in soil nutrients, plant species richness and/or functional group dominance may amplify or counteract direct climatic effects on NPP, and responses may differ above-versus belowground, making it challenging to predict the net effects on NPP. In this study, we manipulated temperature and precipitation at four sites spanning a latitudinal Mediterranean-climate gradient in the Pacific Northwest, USA, and measured aboveground, belowground, and total NPP responses in experimentally assembled prairie plant communities. Using structural equation models, we disentangled the direct effects of climate from its indirect effects through soil nutrient availability and plant community responses. We found that warming, primarily by reducing soil moisture (that is, drying), had a net negative effect on all aspects of NPP, but these negative effects were partially ameliorated by increasing nitrogen and phosphorus availability, as well as changes to the plant community. Specifically, warming, drying, and greater nutrient availability caused species richness to decline, leading to greater dominance by a restricted set of functional groups, which positively affected aboveground NPP. Furthermore, a shift from perennial grass to annual grass dominance increased the ratio of aboveground to belowground NPP. Our results demonstrate that the indirect effects of climate change can help partially buffer the negative direct effects on NPP in Mediterranean-climate prairies. However, increasing soil moisture limitation may still overwhelm the positive effects of such intermediary pathways. [ABSTRACT FROM AUTHOR]

Endophytic fungi in grasses are often considered to be mutualistic because they can increase host resistance to herbivory and drought. However, not all endophytes are beneficial to their hosts, but may instead be specialist enemies. Brachypodium sylvaticum is an invasive grass in the USA. In its European native range, it is nearly always infected by the host-specific endophyte Epichloë sylvatica. While this fungus decreases herbivory, it also decreases the growth rate and size of infected plants, making them less competitive. After showing that B. sylvaticum has lost its endophyte in the invaded range, we use greenhouse assays to deconfound the effects of endophyte infection and range origin to test assumption of the evolution of increased competitive abilities (EICA) hypothesis. Brachypodium in its invaded range appears to have lost tolerance mechanisms present in the native range, allowing Epichloë to greatly increase seedling mortality and reduce growth rates. Additionally, there is some evidence for increased competitive abilities in the form of increased seedling growth rates in the invasive range. Together, these results provide strong support of the EICA hypothesis. [ABSTRACT FROM AUTHOR]

The popularly cited enemy release hypothesis, which states that non-native species are released from population control by their enemies, has not been adequately tested in plants. Many empirical studies have compared damage to native versus non-native invaders only in the invaded range, which can lead to erroneous conclusions regarding enemy release. Biogeographical studies that have compared natural enemies in native and introduced ranges have typically focused on a small area of the plants' distributions in each range, only one plant species, and/or only one guild of natural enemies. To test enemy release, we first surveyed both pathogens and herbivores in multiple populations in both the native and naturalized ranges of three commonly co-occurring perennial bunchgrasses introduced to the United States from Europe. We then compared our field results to the number of fungal pathogens that have been documented on each species from published host-pathogen data compilations. Consistent with enemy release, our field survey showed less herbivory and denser populations in the naturalized range, but there was no evidence of release from pathogens. In contrast, the published host-pathogen data compilations produced evidence of enemy release from pathogens. The difference in results produced by the two approaches highlights the need for multiple approaches to testing mechanisms of invasions by introduced species, which can enable well supported theory to inform sound management practices. [ABSTRACT FROM AUTHOR]

Wetland restoration is a pressing conservation priority, but there are few replicated field studies that provide a scientific foundation for these activities. We conducted a 3-year, replicated field experiment to examine the effectiveness of initial site preparation techniques (combinations of solarization, herbicide, tilling, and thermal weed control) in restoring native plant biodiversity to an agricultural field in a former wetland prairie in Oregon, USA. Post-treatment, plots were sown with a typical restoration mix of native graminoids and forbs. Treatments were compared to three high-quality managed reference wetlands and the adjacent agricultural field. Site preparation treatments varied in their effectiveness in suppressing extant vegetation and eliminating the residual seed bank. After 1 year, the solarization and fall herbicide application treatments were the most effective at reducing exotic cover. However, after 3 years, plant community composition converged in all treatments due to a loss of annual species and increasing dominance of native perennial bunchgrasses. Plant community composition became more similar to the reference wetlands each year, but diversity and richness diverged, apparently due to a trade-off between the cover of the dominant native bunchgrasses and diversity. Successional theory offers insights into how priority effects and competitive inhibition may influence community trajectories, and offers a useful model for restoring plant communities with high native diversity and dominance. Finding ways to mitigate the tradeoff between native plant cover and diversity by actively managing successional trajectories is an important challenge in wetland restoration that deserves further investigation. [ABSTRACT FROM AUTHOR]

Brachypodium sylvaticum, or false brome, a recent invader of North America, has spread rapidly following establishment in Oregon in 1939 and is now colonizing at least five US states. To understand how ecological processes and environmental context interact to facilitate invasion, we combined a GIS analysis of spatial patterns of invasion with a greenhouse competition experiment under two light and two nutrient levels, and a field competition-removal experiment. Brachypodium sylvaticum was concentrated along human-use corridors and in forests, where our experiments indicate it is a better competitor than co-occurring grasses. Forested areas near trails require the greatest focus for detection and control because the species’ dual propensities to establish and thrive in these locations may make them key source areas for further invasion. Our methodology demonstrates how understanding both the patterns and processes of invasion enables the prescription of context-specific remedial actions. [ABSTRACT FROM AUTHOR]

In prairie ecosystems, abiotic constraints on competition can structure plant communities; however, the extent to which competition between native and exotic plant species is constrained by environmental factors is still debated. The objective of our study was to use paired field and greenhouse experiments to evaluate the competitive dynamics between two native ( Danthonia californica and Deschampsia cespitosa) and two exotic ( Schedonorus arundinaceus and Lolium multiflorum) grass species under varying nutrient and moisture conditions in an upland prairie in the Willamette Valley, Oregon. We hypothesized the two invasive, exotic grasses would be more competitive under high-nutrient, moderate-moisture conditions, resulting in the displacement of native grasses from these environments. In the field, the experimental reduction of competition resulted in shorter, wider plants, but only the annual grass, Lolium multiflorum, produced more aboveground biomass when competition was reduced. In the greenhouse, the two exotic grasses produced more total biomass than the two native grasses. Competitive hierarchies were influenced by nutrient and/or moisture treatments for the two exotic grasses, but not for the two native grasses. L. multiflorum dominated competitive interactions with all other grasses across treatments. In general, S. arundinaceus dominated when in competition with native grasses, and D. cespitosa produced the most biomass in monoculture or under interspecific competition with the other native grass, D. californica. D. californica, D. cespitosa, and S. arundinaceus all produced more biomass in high-moisture, high-nutrient environments, and D. cespitosa, L. multiflorum, and S. arundinaceus allocated more biomass belowground in the low nutrient treatment. Taken together, these experiments suggest the competitive superiority of the exotic grasses, especially L. multiflorum, but, contrary to our hypothesis, the native grasses were not preferentially excluded from nutrient-rich, moderately wet environments. [ABSTRACT FROM AUTHOR]

Pfunder M. and Roy B.A. 2006. Fungal pseudoflowers can influence the fecundity of insect-pollinated flowers on Euphorbia cyparissias. Bot. Helv. 116: 149 – 158. Euphorbia cyparissias is often infected by a rust fungus from the species complex Uromyces pisi. Infected plants do not form flowers but pseudoflowers, rosettes of yellow leaves upon which the fungus presents gametes in a sweet-smelling sugary nectar. Insects feed on the nectar and transfer fungal gametes between mating types. Here we show that pseudoflowers and the flowers of non-infected hosts overlap in “flowering” for more than one month, even though pseudoflowers start “flowering” one month earlier than true flowers. As the fungus and its host also share insect visitors, we hypothesized that they might interact either by facilitating each others’ insect visits or by competing for “pollinators”. We addressed this question by weekly grid-mapping an Euphorbia population near Zermatt in the Swiss Alps and relating the average density and frequency around hosts and pseudoflowers during their “flowering” period to their fitness (success in seed set and spore production). The seed set of uninfected Euphorbia plants was significantly higher when they were surrounded by fewer pseudoflowers. The fungus, on the other hand, was not obviously influenced by the presence of host flowers. Instead, the reproductive success of single pseudoflowers decreased with a higher density of pseudoflower-neighbors. Our results suggest that the fungus might be a pollinator-competitor for Euphorbia flowers. Die Zypressen-Wolfsmilch Euphorbia cyparissias ist häufig mit einem Rostpilz aus dem Formenkreis Uromyces pisi befallen. Die Blütenbildung wird bei infizierten Pflanzen verhindert. Stattdessen bilden sie durch den Einfluss des Pathogens so genannte Pseudoblüten aus, Rosetten von Sprossblättern, auf deren Oberfläche der Pilz seine Gameten in einer süss duftenden Zuckerlösung präsentiert. Insekten ernähren sich von diesem Nektar und transportieren Gameten unterschiedlichen Geschlechts zwischen verschiedenen befallenen Pflanzen. Unsere Feldstudien in den Schweizer Alpen zeigten, dass die “Blühzeit” von Pseudoblüten und nicht infizierten Wirtspflanzen während mehr als einem Monat überlappt, obwohl die Pseudoblüten einen Monat früher mit der Nektarproduktion anfangen. Weil der Pilz und die Wirtspflanzen auch ihre “Bestäuber” teilen, stellten wir die Hypothese auf, dass sie durch gemeinsames “Blühen” positiv interagieren und die Gesamtzahl der Insektenbesuche gegenseitig fördern könnten. Wir nahmen uns dieser Frage an, indem wir an einer Euphorbia-Population in der Nähe von Zermatt wöchentliche Grid-Kartierungen vornahmen und die mittlere Dichte und Häufigkeit der umgebenden Wirtsblüten und Pseudoblüten während ihrer “Blühzeit” mit dem Fortpflanzungserfolg, gemessen in Sporen- und Samenproduktion, in Zusammenhang setzten. Die Samenproduktion von nicht-infizierten Euphorbia-Pflanzen war signifikant höher, wenn sie von weniger Pseudoblüten umgeben waren, während der Pilz nicht offensichtlich von der Gegenwart der Wirtsblüten beeinflusst wurde. Die Daten deuten im Gegensatz eher darauf hin, dass der Reproduktionserfolg einzelner Pseudoblüten mit zunehmender Dichte an umgebenden Pseudoblüten abnahm. Unsere Ergebnisse lassen vermuten, dass der Pilz ein ernst zu nehmender Konkurrent für Bestäuber von nicht-infizierten Euphorbia-Blüten sein könnte. [ABSTRACT FROM AUTHOR]

Genetic variation in plant populations for resistance to pathogens and herbivores might be maintained by parasite-mediated negative frequency-dependent selection (FDS). But it is difficult to observe the time-lagged oscillations between host and parasite genotypes that should result from FDS. To evaluate the potential for FDS, we tested for local adaptation of parasites to common clones, the role of host genetic diversity in resistance to parasites, and genetic correlations among fitness, parasitism, and the frequency of host clones. We studied three populations of Arabis holboellii, a short-lived apomictic (asexual by seed) plant attacked by rust fungi and insect herbivores. To estimate clone frequency, we used polymorphic allozyme markers on 200 individuals in each population in 1990 and in 2000. We also recorded levels of parasitism and host fitness (fruit production). Only the rust fungi showed evidence for local host adaptation; they usually increased in incidence as a function of clone frequency, and they tracked temporal change in clone frequency. In further support of FDS, parasitism was lower in populations with higher genetic diversity. However, total parasitism (herbivory and disease combined) decreased as host clone frequency and fitness increased. Thus, although the highly virulent rust pathogen showed potential for driving the cycles that result from FDS, this apparently does not occur in the populations studied because the host clones were also attacked by herbivores. [ABSTRACT FROM AUTHOR]

Defense costs provide a major explanation for why plants in nature have not evolved to be better defended against pathogens and herbivores; however, evidence for defense costs is often lacking. Plants defend by deploying resistance traits that reduce damage, and tolerance traits that reduce the fitness effects of damage. We first tested the defense-stress cost (DSC) hypothesis that costs of defenses increase and become important under competitive stress. In a greenhouse experiment, uniparental maternal families of the host plant Arabis perennans were grown in the presence and absence of the bunch grass Bouteloua gracilis and the herbivore Plutella xylostella. Costs of resistance and tolerance manifest as reduced growth in the absence of herbivory were significant when A. perennans grew alone, but not in the competitive environment, in contrast to the DSC hypothesis. We then tested the defense-stress benefit (DSB) hypothesis that plant defenses may benefit plants in competitive situations thereby reducing net costs. For example, chemical resistance agents and tolerance may also have functions in competitive interactions. To test the DSB hypothesis, we compared differentially competitive populations for defense costs, assuming that poorer competitors from less dense habitats were less likely to have evolved defenses that also function in competition. Without competitive benefits of defenses, poorer competitors were expected to have higher net costs of defenses under competition in accordance with DSB. Populations of A. perennans and A. drummondii that differed dramatically in competitiveness were compared for costs, and as the DSB hypothesis predicts, only the poor competitor population showed costs of resistance under competition. However, cost of tolerance under competition did not differ among populations, suggesting that the poor competitors might have evolved a general stress tolerance. Although the DSC hypothesis may explain cases where defense costs increase under stress, the DSB hypothesis may explain some cases where costs decrease under competitive stress. [ABSTRACT FROM AUTHOR]