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In areas disturbed by pollution, populations of herbivorous insects may reach high densities. This study was conducted to test one of the hypotheses attempting to explain this phenomenon – that pollution creates an enemy-free space for herbivores. We monitored the population densities of Eriocrania leaf-mining moths on mountain birch, Betula pubescens subsp. czerepanovii, in the vicinity of the nickel–copper smelter in Monchegorsk (Kola Peninsula, NW Russia) over twelve years (1994–2005) and assessed larval mortality from parasitoids, ants and birds. The mean density (mines/birch leaf area) of Eriocrania populations in severely disturbed habitats (industrial barrens) was about 2.7 times higher, and peak densities 2–4 times higher, than in pristine forests. Temporal population variability (measured as the coefficient of variation of log-transformed densities) increased with an increase in pollution load. The proportion of infested trees was not affected by pollution, but mine distribution among trees was more clumped in the polluted sites. Eriocrania populations in disturbed sites fluctuated independently of each other, whereas populations in forest sites fluctuated in synchrony. Larval mortality caused by parasitoids was lower in disturbed sites only during those years when populations of Eriocrania reached high densities; mortality from ants and birds did not differ between disturbed and undisturbed habitats in either high or low density years. In undisturbed forests the rate of population change correlated negatively with previous-year parasitism, suggesting that parasitoids are the key demographic factor in Eriocrania population dynamics. In the habitats heavily disturbed by pollution no such correlation was found, which means that negative feedback with parasitoids is disrupted: parasitoids fail to follow host population growth, thus creating an enemy-free space for Eriocrania leafminers. [ABSTRACT FROM AUTHOR]

The effects of elevated carbon dioxide on plant–herbivore interactions have been summarized in a number of narrative reviews and metaanalyses, while accompanying elevation of temperature has not received sufficient attention. The goal of our study is to search, by means of metaanalysis, for a general pattern in responses of herbivores, and plant characteristics important for herbivores, to simultaneous experimental increase of carbon dioxide and temperature (ECET) in comparison with both ambient conditions and responses to elevated CO2 (EC) and temperature (ET) applied separately. Our database includes 42 papers describing studies of 31 plant species and seven herbivore species. Nitrogen concentration and C/N ratio in plants decreased under both EC and ECET treatments, whereas ET had no significant effect. Concentrations of nonstructural carbohydrates and phenolics increased in EC, decreased in ET and did not change in ECET treatments, whereas terpenes did not respond to EC but increased in both ET and ECET; leaf toughness increased in both EC and ECET. Responses of defensive secondary compounds to treatments differed between woody and green tissues as well as between gymnosperm and angiosperm plants. Insect herbivore performance was adversely affected by EC, favoured by ET, and not modified by ECET. Our analysis allowed to distinguish three types of relationships between CO2 and temperature elevation: (1) responses to EC do not depend on temperature (nitrogen, C/N, leaf toughness, phenolics in angiosperm leaves), (2) responses to EC are mitigated by ET (sugars and starch, terpenes in needles of gymnosperms, insect performance) and (3) effects emerge only under ECET (nitrogen in gymnosperms, and phenolics and terpenes in woody tissues). This result indicates that conclusions of CO2 elevation studies cannot be directly extrapolated to a more realistic climate change scenario. The predicted negative effects of CO2 elevation on herbivores are likely to be mitigated by temperature increase. [ABSTRACT FROM AUTHOR]

1 We hypothesized that the role of positive plant-plant interactions will increase with the environmental stress imposed by long-term pollution. We studied the effects of top- canopy plants on two deciduous ( Vaccinium myrtillus, V uliginosum) and two evergreen ( V vitis-idaea, Empetrum nigrum ssp. hermaphroditum) dwarf shrubs by comparing their growth and reproduction under trees and in gaps in industrial barrens and in unpolluted forests in the Kola Peninsula (NW Russia). 2 Interactions between habitat type and tree proximity on berry yield and reproductive effort were significant for four species combined. In forest habitats both berry yield and reproductive effort were higher in dwarf shrub patches growing in gaps than under tree canopies, but in barren habitats Vaccinium species reproduced better under tree canopies. 3 The interaction of habitat type and tree proximity was also significant for vegetative biomass, mostly due to evergreen species benefiting from neighbouring trees in barren habitats. 4 Overall, all four dwarf shrub species reproduced better in barren habitats than in forests, mostly due to increased reproduction observed under tree canopies in barren sites. 5 Values of the relative neighbour effect indicated that the impact of top-canopy plants on dwarf shrubs shifted from negative in forest habitats to generally positive in barren habitats. [ABSTRACT FROM AUTHOR]

Summary1 Stress may affect compensatory plant growth and plant responses to herbivory may therefore be modified by pollution. We compared the effects of leaf clipping on growth, reproduction and developmental stability of two willow species (Salix borealis and S. caprea) growing at different distances from two copper-nickel smelters (Monchegorsk, NW Russia and Harjavalta, SE Finland). 2 In unpolluted sites, the defoliation of both willow species adversely affected sexual reproduction and increased the formation of epicormic shoots, but did not change either shoot growth or leaf size. Timing and extent of clipping had little effect on the responses. 3 Compensatory responses to leaf clipping in polluted habitats were reduced, especially in S. borealis, as reflected by decrease in shoot growth and fruit production and an increase in leaf fluctuating asymmetry, a non-specific indicator of stress. 4 Activation of dormant buds was a common compensatory response to herbivory in unpolluted sites. The lack of an increase in epicormic shoots in polluted sites may be due to already weakened apical dominance. 5 Compensatory abilities may be affected both by direct damage caused by pollutants and pollution-induced increases in environmental stress. Although S. borealis shows no visible sign of stress in polluted sites, regrowth from dormant buds and enhanced shoot growth were seen under these conditions. Resources were not therefore available for further compensation of damage caused by defoliation. Our results fit the hypothesis that fast growing plants have low compensative abilities. 6 Salix caprea shows more pollution-induced stress than S. borealis, expressed as increased developmental instability near the weaker and shorter shoots near the stronger pollution source. This may explain the differences in their compensatory responses in polluted sites. [ABSTRACT FROM AUTHOR]

1. Fluctuating asymmetry (FA) of birch leaves collected along four pollution gradients in NW Europe was assessed by measuring the difference in width between right and left halves of the leaf. 2. FA of leaves was not related to tree diameter, and values for long and short shoots of the same tree were correlated. 3. FA of silver birch Betula pendula and two subspecies of B. pubescens, white birch B. purbescens ssp. Pubescens and mountain birch B. pubescens ssp. Tortuosa, decreased hyperbolically with distance away from four sources of aerial emission which differed in amounts and composition of pollutants. 4. The extent of the zone of increased FA was higher around polluters which produced more emissions. 5. FA of leaves sampled around two copper-nickel smelters was positively related to foliar nickel concentrations, and regressions from these two site had similar slopes and intercepts. 6. The data suggest that FA of birch leaves may represent a convenient indicator for rapid assessment of environmental quality. [ABSTRACT FROM AUTHOR]