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Copyright of Journal der Deutschen Dermatologischen Gesellschaft is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Plant monocultures growing for extended periods face severe losses of productivity. This phenomenon, known as 'yield decline', is often caused by the accumulation of above‐ and below‐ground plant antagonists. The effectiveness of plant defences against antagonists might help explain differences in yield decline among species. Using a trait‐based approach, we studied the role of 20 physical and chemical defence traits of leaves and fine roots on yield decline of 4‐ and 18‐year‐old monocultures of 27 grassland species. We hypothesized that yield decline is lower for species with high defences, that root defences are better predictors of yield decline than leaf defences, and that in roots, physical defences better predict yield decline than chemical defences, while the reverse is true for leaves. We additionally hypothesized that the relationship between defences and yield decline increases with time and that species increasing the expression of defence traits after long‐term monoculture growth would suffer less yield decline. We summarized leaf and fine root defence traits using principal component analyses and analysed the relationship between the most informative components along with their temporal changes and monoculture yield decline. The significant predictors of yield decline were traits related to the so‐called collaboration gradient of the root economics space (specific root length and root diameter) as well as their temporal changes and traits related to the leaf physical vs chemical defence tradeoff (leaf dry matter, silicon and cellulose content, toughness and phytochemical diversity). We were unable to unequivocally identify the mechanisms relating the effect of those traits to yield decline as they could mediate plant responses to several stressors such as antagonist accumulation, nutrient depletion or drought. Further studies are needed to differentiate between these alternative mechanisms and to gain a comprehensive understanding of the drivers of yield decline in relation to root and leaf defence traits. [ABSTRACT FROM AUTHOR]

Silicon-based defenses deter insect herbivores in many cultivated and wild grass species. Furthermore, in some of these species, silicon (Si) uptake and defense can be induced by herbivory. Tropical trees also take up Si and leaf Si concentrations vary greatly across and within species. As herbivory is a major driver of seedling mortality and niche differentiation of tropical tree species, understanding antiherbivore defenses is pivotal. Yet, whether silicon is a constitutive and inducible herbivory defense in tropical forest tree species remains unknown. We grew seedlings of eight tropical tree species in a full factorial experiment, including two levels of plant-available soil Si concentrations (-Si/+Si) and a simulated herbivory treatment (-H/+H). The simulated herbivory treatment was a combination of clipping and application of methyl jasmonate. We then carried out multiplechoice feeding trials, separately for each tree species, in which leaves of each treatment combination were offered to a generalist caterpillar (Spodoptera frugiperda). Leaf damage was assessed. Three species showed a significant decrease in leaf damage under high compared to low Si conditions (by up to 72%), consistent with our expectation of Si-based defenses acting in tropical tree species. In one species, leaf damage was increased by increasing soil Si and in four species, no effect of soil Si on leaf damage was observed. Opposite to our expectation of Si uptake and defense being inducible by herbivory damage, simulated herbivory increased leaf damage in two species. Furthermore, simulated herbivory reduced Si concentrations in one species. Our results showed that tropical tree seedlings can be better defended when growing in Si-rich compared to Si-poor soils, and that the effects of Si on plant defense vary strongly across species. Furthermore, Si-based defenses may not be inducible in tropical tree species. Overall, constitutive Si-based defense should be considered part of the vast array of anti-herbivore defenses of tropical tree species. Our finding that Si-based defenses are highly species-specific combined with the fact that herbivory is a major driver of mortality in tropical tree seedling, suggests that variation in soil Si concentrations may have pervasive consequences for regeneration and performance across tropical tree species. [ABSTRACT FROM AUTHOR]