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Many hypotheses address the associations of plant community composition with natural enemies, including: (i) plant species diversity may reduce enemy attack, (ii) attack may increase as host abundance increases, (iii) enemy spillover may lead to increased attack on one host species due to transmission from another host species, or enemy dilution may lead to reduced attack on a host that would otherwise have more attack, (iv) physical characteristics of the plant community may influence attack, and (v) plant vigor may affect attack. Restoration experiments with replicated plant communities provide an exceptional opportunity to explore these hypotheses. To explore the relative predictive strengths of these related hypotheses and to investigate the potential effect of several restoration site preparation techniques, we surveyed arthropod herbivore and fungal pathogen attack on the six most common native plant species in a restoration experiment. Multi-model inference revealed a weak but consistent negative correlation with pathogen attack and host diversity across the plant community, and no correlation between herbivory and host diversity. Our analyses also revealed host species-specific relationships between attack and abundance of the target host species, other native plant species, introduced plant species, and physical community characteristics. We found no relationship between enemy attack and plant vigor. We found minimal differences in plant community composition among several diverse site preparation techniques, and limited effects of site preparation techniques on attack. The strongest associations of community characteristics with attack varied among plant species with no community-wide patterns, suggesting that no single hypothesis successfully predicts the dominant community-wide trends in enemy attack. [ABSTRACT FROM AUTHOR]

Parasite damage strongly affects dynamics of boreal forests. Damage levels may be affected by climate change, either directly or indirectly through changes in properties of host trees. We examined how herbivore and pathogen damage in Alnus viridis subsp. fruticosa (Rupr.) Nym. depend on leaf morphology and chemistry, tree size, and tree neighborhood. Damage and tree properties were measured in 2003 and 2004 on eight trees at each of 20 sites in Interior Alaska. Damage varied significantly among sites and among trees within sites, but Cartesian distances between sites were not correlated with similarity in damage levels. Compared with middle leaves, terminal leaves experienced less damage from phloem-feeding insects and pathogens, whereas leaf-roller damage was largely confined to terminal leaves. Summer drought in 2004 strongly reduced damage from phloem-feeding insects, while damage from chewing insects increased. Overall, herbivore damage was best explained by leaf morphology and chemistry, and pathogen damage by the proximity of other trees; the two damage types were not correlated with each other. Reproduction was negatively correlated with herbivore damage, but positively with pathogen damage. The contrasting relationships found for individual feeding guilds suggest that they must be studied separately in assessing impacts of climate change on parasite damage. [ABSTRACT FROM AUTHOR]

Pathogens and herbivores can severely reduce host fitness, potentially leading to altered succession rates and changes in plant community composition. Thus, to predict vegetation dynamics under climate change, it is necessary to understand how plant pathogens and herbivores will respond. Pathogens and herbivores are predicted to increase under climate warming because the amount of time available for growth and reproduction will increase. To test this prediction, we used a warming experiment in which heaters were suspended over a natural montane meadow for 12 years. In the summer of 2002, we quantified damage by all the observable (aboveground) pathogens and herbivores on six of the most common plant species (Artemisia tridentata, Helianthella quinquenervis, Erigeron speciosus, Potentilla gracilis, Potentilla hippiana, and Lathyrus leucanthus). We found that plants in the earlier melting plots generally had the most damage and were attacked by a larger number of species, which is consistent with predictions. However, although the overall trend was an increase in damage with warmer temperatures and earlier snowmelt, some pathogens and herbivores performed better in cooler or later melting plots. The idiosyncratic response of each species to environmental conditions suggests that there are likely to be changes in community composition as the planet warms. [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]