The Effect of Snow on Plant Chemistry and Invertebrate Herbivory: Experimental Manipulations Along a Natural Snow Gradient

Changing snow conditions have strong effects on northern ecosystems, but these effects are rarely incorporated into ecosystem models and our perception of how the ecosystems will respond to a warmer climate. We investigated the relationships between snow cover, plant phenology, level of invertebrate herbivory and leaf chemical traits in Betula nana in four different habitats located along a natural snow cover gradient. To separate the effect of snow per se from other differences, we manipulated the snow cover with snow fences in three habitats. The experimentally prolonged snow cover delayed plant phenology, but not as much as expected based on the pattern along the natural gradient. The positive effect of the snow treatment on plant nitrogen concentration was also weaker than expected, because plant nitrogen concentration closely followed plant phenology. The level of herbivory by leaf-chewing invertebrates increased in response to an increased snow cover, at least at the end of the growing season. The concentration of phenolic substances varied among habitats, treatments and sampling occasions, indicating that B. nana shrubs were able to retain a mosaic of secondary chemical quality despite altered snow conditions. This study shows that the effect of the snow cover period on leaf nitrogen concentration and level of herbivory can be predicted based on differences between habitats, whereas the effect of a changed plant phenology on plant nitrogen concentration is better explained by temporal trends within habitats. These results have important implications for how northern ecosystems should respond to future climate changes.     

Plant–herbivore interactions in Alaskan arctic tundra change with soil nutrient availability

Herbivores in nutrient‐limited systems such as arctic tundra have been suggested to play a minor role in controlling plant growth simply because they are relatively few in number. However, theory predicts that as net primary productivity (NPP) increases because of greater inputs of nutrients or energy, herbivores may have greater effects on plant growth. This prediction has not been tested in the context of climate warming in arctic tundra, which may increase soil nutrient availability and thus NPP. We examined a long‐term experiment that excluded small and large mammalian herbivores and increased soil nutrients in two arctic Alaskan tundra communities: dry heath (DH) and moist acidic tussock (MAT). In the ninth year of manipulations, we measured weekly growth of three plant species of three growth forms: tussock‐forming graminoid, rhizomatous graminoid, and dwarf deciduous shrub, in each community. All species grew better when fertilized. In DH, this increase in growth was exaggerated when plants were protected from herbivores, confirming that herbivory had a negative effect on plant growth under increased nutrient conditions, but was unimportant under ambient soil conditions. However, in MAT, the importance of herbivory differed among species with fertilization. The tussock‐forming sedge at MAT, Eriophorum vaginatum, grew better and flowered more when fenced under both ambient and amended nutrients compared to plants exposed to herbivores. This species decreases in abundance in long‐term fertilized plots when mammals are present, and our results suggest that herbivory may be accounting for at least some of that loss, in addition to shifts in competitive relationships. Although we only focused on individual plants here rather than the entire community, our results suggest that under the increased soil nutrient conditions expected with continued climate warming in the Arctic, herbivores may become more important in affecting several abundant tundra plant populations, and should not be ignored.     

Warming the tundra: reciprocal responses of invertebrate herbivores and plants

Rapid warming in northern ecosystems is simultaneously influencing plants, herbivores and the interactions among them. Recent studies suggest that herbivory could buffer plant responses to environmental change, but this has only been shown for vertebrate herbivores so far. The role of invertebrate herbivory in tundra ecosystems is often overlooked, but can be relevant in determining the structure and dynamics of tundra plant communities and may also affect how plants respond to warming. Invertebrate herbivores are also likely to respond more rapidly to warming than vertebrates because their behaviour and life cycles strongly depend on temperature. We investigated the effects of current season warming on Arctic moth caterpillars, their herbivory rates, and the subsequent responses of two common tundra plants, Salix arctica and Dryas octopetala. We manipulated both herbivore presence and temperature in a full‐factorial field experiment at two elevations, using enclosures and passive warming chambers. Changes in temperature achieved through elevation and/or experimental warming directly affected caterpillars, herbivory and the responses of plants. Caterpillars performed worse (higher respiration rates and lower growth rates) in warmer, lower elevation plots and shifted their diets towards more nutritious foods, such that the relative intensity of herbivory changed for the two studied plants. Within‐season responses of both forage plant species were weak, but invertebrate herbivores affected the responses of plants to elevation or experimental warming. Our results suggest that increased temperatures can reduce the performance of cold‐adapted invertebrate herbivores, with potential consequences to the longer term responses of tundra plants to warming due to changes in herbivory rates and selective foraging.     

Additive and non‐additive effects of birch genotypic diversity on arthropod herbivory in a long‐term field experiment

Herbivores are important drivers of plant population dynamics and community composition in natural and managed systems. Intraspecific genetic diversity of long‐lived plants like trees might shape patterns of herbivory by different guilds of herbivores that trees experience through time. However, previous studies on plant genetic diversity effects on herbivores have been largely short‐term. We investigated how tree genotypic variation and diversity influence herbivory of silver birch Betula pendula in a long‐term field experiment. Using clones of eight genotypes, we constructed experimental plots consisting of one, two, four or eight genotypes, and measured damage by five guilds of arthropod herbivores twice a year over three different years (four, six and nine years after the experiment was established). Genotypes varied significantly for most types of herbivore damage, but genotype resistance rankings often shifted over time, and none of the clones was more resistant than all others to all types of herbivores. At the plot level, birch genotypic diversity had significant positive additive effect on leaf rollers and negative non‐additive effects on chewing herbivores and gall makers. In contrast, leaf‐mining and leaf‐tying damage was not influenced by birch genotypic diversity. Within diverse plots, the direction of genotypic diversity effects varied depending on birch genotype, some having lower and some having higher herbivory in mixed stands. This research highlights the importance of long‐term studies including different feeding guilds of herbivores to understand the effects of plant genetic diversity on arthropod communities. Different responses of various feeding guilds to genotypic diversity and shifts in resistance of individual genotypes over time indicate that genotypic mixtures are unlikely to result in overall reduction in herbivory over time.     

Effects of plant quality and ant defence on herbivory rates in a neotropical ant‐plant

1. Understanding the degree to which populations and communities are limited by both bottom‐up and top‐down effects is still a major challenge for ecologists, and manipulation of plant quality, for example, can alter herbivory rates in plants. In addition, biotic defence by ants can directly influence the populations of herbivores, as demonstrated by increased rates of herbivory or increased herbivore density after ant exclusion. The aim of this study was to evaluate bottom‐up and top‐down effects on herbivory rates in a mutualistic ant‐plant. 2. In this study, the role of Azteca alfari ants as biotic defence in individuals of Cecropia pachystachya was investigated experimentally with a simultaneous manipulation of both bottom‐up (fertilisation) and top‐down (ant exclusion) factors. Four treatments were used in a fully factorial design, with 15 replicates for each treatment: (i) control plants, without manipulation; (ii) fertilised plants, ants not manipulated; (iii) unfertilised plants and excluded ants and (iv) fertilised plants and ants excluded. 3. Fertilisation increased the availability of foliar nitrogen in C. pachystachya, and herbivory rates by chewing insects were significantly higher in fertilised plants with ants excluded. 4. Herbivory, however, was more influenced by bottom‐up effects – such as the quality of the host plant – than by top‐down effects caused by ants as biotic defences, reinforcing the crucial role of leaf nutritional quality for herbivory levels experienced by plants. Conditionality in ant defence under increased nutritional quality of leaves through fertilisation might explain increased levels of herbivory in plants with higher leaf nitrogen.     

Response of galling invertebrates on Salix lanata to reindeer herbivory

Browsing and defoliation often increase the densities of insect herbivores on woody plants. Densities of herbivorous invertebrates were estimated in a long-term grazing manipulation experiment. More then 30-yr-old fences allow us to compare densities of invertebrate herbivores on Salix lanata in areas heavily grazed and areas lightly grazed by reindeer. The number of gall-forming insects ( Pontania glabrifons) and gall-forming mites were higher on the heavily grazed shrubs than on lightly grazed shrubs. In contrast to most short-term studies, the heavily grazed S. lanata had shorter current annual shoots. No difference in leaf size, leaf nitrogen content, or C:N ratio between grazing intensities were detected. However, the enhanced natural δ15N value indicates that heavily grazed shrubs get a higher proportion of their N directly from reindeer faeces. Leaf weight per unit area and relative fluctuating asymmetry of leaf shape increased in heavily grazed S. lanata . Enhanced relative fluctuating asymmetry might indicate higher susceptibility to herbivores. Long-term grazing seems to increase the density of invertebrate herbivory in the same way as short-term grazing, even if the plant responses differ substantially.     

Photosynthetic and phenological responses of dwarf shrubs to the depth and properties of snow

Snow is known to have a major impact on vegetation in arctic ecosystems, but little is known about how snow affects plants in boreal forests, where the snowpack is uneven due to canopy impact. The responses of two dwarf shrubs, the evergreen Vaccinium vitis‐idaea and the deciduous V. myrtillus, to snow conditions were studied in a snow manipulation experiment in southern Finland. The thermal insulation of the snowpack was expected to decrease with partial removal or compression of the snow, while addition of snow was expected to have the opposite effect. The penetration of light was manipulated by partial removal of snow or by formation of an artificial ice layer in the snowpack. CO₂ exchange measurements that were carried out at the time of maximum snow depth in late March indicated significant photosynthetic activity in the leaves of V. vitis‐idaea under snow. Net gain of CO₂ was observed in the daytime on all the manipulation plots, excluding the snow addition plots, where light intensity was very low. The subnivean photosynthesis compensated for a substantial proportion (up to 80%) of the respiratory CO₂ losses. Chlorophyll fluorescence measurements indicated reduced potential capacity of photosystem II in the leaves of V. vitis‐idaea on those plots where snow cover was thin. Neither V. vitis‐idaea nor V. myrtillus suffered from frost damage (assessed as electrolyte leakage) when thermal insulation was reduced by means of snow manipulations. No phenological responses were observed in V. vitis‐idaea, but in V. myrtillus bud burst, leaf unfolding and flowering were advanced by 1–3 days on the addition plots. The results of the present study show that dwarf shrubs respond to not only the thickness of snow but also the physical properties of snow, both of which are expected to change due to climatic warming.     

Insect herbivory and plant adaptation in an early successional community*

To address the role of insect herbivores in adaptation of plant populations and the persistence of selection through succession, we manipulated herbivory in a long‐term field experiment. We suppressed insects in half of 16 plots over nine years and examined the genotypic structure and chemical defense of common dandelion (Taraxacum officinale), a naturally colonizing perennial apomictic plant. Insect suppression doubled dandelion abundance in the first few years, but had negligible effects thereafter. Using microsatellite DNA markers, we genotyped >2500 plants and demonstrate that insect suppression altered the genotypic composition of plots in both sampling years. Phenotypic and genotypic estimates of defensive terpenes and phenolics from the field plots allowed us to infer phenotypic plasticity and the response of dandelion populations to insect‐mediated natural selection. The effects of insect suppression on plant chemistry were, indeed, driven both by plasticity and plant genotypic identity. In particular, di‐phenolic inositol esters were more abundant in plots exposed to herbivory (due to the genotypic composition of the plots) and were also induced in response to herbivory. This field experiment thus demonstrates evolutionary sorting of plant genotypes in response to insect herbivores that was in same direction as the plastic defensive response within genotypes.     

Like herbivores,parasitic plants are limited by host nitrogen content

Herbivores generally benefit from increased plant nitrogen content, because the nitrogen content of animals is much higher than that of plants. Consequently, high plant nitrogen alleviates the profound stoichiometric imbalance that herbivores face in their diets. Parasitic plants provide the opportunity to test this generalization for consumers across kingdoms. We fertilized two microhabitats in a California salt marsh that were dominated by Salicornia virginica or a mixture of S. virginica and Jaumea carnosa. The nitrogen content of both host plants and of the holoparasite Cuscuta salina (dodder) increased in fertilized plots in both microhabitats. Cuscuta preferred to attack Jaumea, although Jaumea had lower nitrogen content than Salicornia. When host nitrogen content was altered by fertilizing plots, however, the percent cover of the parasite doubled. Although parasitic plants and their hosts have similar tissue nitrogen contents, suggesting no stoichiometric imbalance between host and consumer, parasitic plants do not feed on host tissue, but on host xylem and phloem, which are very low in nitrogen. Consequently, parasitic plants face the same dietary stoichiometric constraints as do herbivores, and both herbivores and holoparasitic plants may respond positively to increases in host nitrogen status.     

Palatability, decomposition and insect herbivory: patterns in a successional old-field plant community

We tested the hypothesis that selective feeding by insect herbivores in an old‐field plant community induces a shift of community structure towards less palatable plant species with lower leaf and litter tissue quality and may therefore affect nutrient cycling. Leaf palatability of 20 herbaceous plant species which are common during the early successional stages of an old‐field plant community was assayed using the generalist herbivores Deroceras reticulatum (Mollusca: Agriolomacidae) and Acheta domesticus (Ensifera: Gryllidae). Palatability was positively correlated with nitrogen content, specific leaf area and water content of leaves and negatively correlated with leaf carbon content and leaf C/N‐ratio. Specific decomposition rates were assessed in a litter bag experiment. Decomposition was positively correlated with nitrogen content of litter, specific leaf area and water content of living leaves and negatively correlated with leaf C/N‐ratio. When using phylogentically independent contrasts the correlations between palatability and decomposition versus leaf and litter traits remained significant (except for specific leaf area) and may therefore reflect functional relationships. As palatability and decomposition show similar correlations to leaf and litter traits, the correlation between leaf palatability and litter decomposition rate was also significant, and this held even in a phylogenetically controlled analysis. This correlation highlights the possible effects of invertebrate herbivory on resource dynamics. In a two‐year experiment we reduced the density of above‐ground and below‐ground insect herbivores in an early successional old‐field community in a two‐factorial design by insecticide application. The palatability ranking of plants showed no relationship with the specific change of cover abundance of plants due to the reduction of above‐ or below‐ground herbivory. Thus, changes in the dominance structure as well as potentially associated changes in the resource dynamics are not the result of differences in palatability between plant species. This highlights fundamental differences between the effects of insect herbivory on ecosystems and published results from vertebrate‐grazing systems.     

Effects of above- and belowground herbivory on growth,pollination, and reproduction in cucumber

Plants experience unique challenges due to simultaneous life in two spheres, above- and belowground. Interactions with other organisms on one side of the soil surface may have impacts that extend across this boundary. Although our understanding of plant–herbivore interactions is derived largely from studies of leaf herbivory, belowground root herbivores may affect plant fitness directly or by altering interactions with other organisms, such as pollinators. In this study, we investigated the effects of leaf herbivory, root herbivory, and pollination on plant growth, subsequent leaf herbivory, flower production, pollinator attraction, and reproduction in cucumber (Cucumis sativus). We manipulated leaf and root herbivory with striped cucumber beetle (Acalymma vittatum) adults and larvae, respectively, and manipulated pollination with supplemental pollen. Both enhanced leaf and root herbivory reduced plant growth, and leaf herbivory reduced subsequent leaf damage. Plants with enhanced root herbivory produced 35% fewer female flowers, while leaf herbivory had no effect on flower production. While leaf herbivory reduced the time that honey bees spent probing flowers by 29%, probing times on root-damaged plants were over twice as long as those on control plants. Root herbivory increased pollen limitation for seed production in spite of increased honey bee preference for plants with root damage. Leaf damage and hand-pollination treatments had no effect on fruit production, but plants with enhanced root damage produced 38% fewer fruits that were 25% lighter than those on control plants. Despite the positive effect of belowground damage on honey bee visitation, root herbivory had a stronger negative effect on plant reproduction than leaf herbivory. These results demonstrate that the often-overlooked effects of belowground herbivores may have profound effects on plant performance.     

Seedling Mortality and Herbivory Damage in Subtropical and Temperate Populations: Testing the Hypothesis of Higher Herbivore Pressure Toward the Tropics

Herbivory rates are generally thought to be higher in tropical than in temperate forests. Nevertheless, tests of this biogeographic prediction by comparing a single plant species across a tropical-temperate range are scarce. Here, we compare herbivore damage between subtropical and temperate populations of the evergreen tree Aextoxicon punctatum (Olivillo), distributed between 30° and 43° S along the Pacific margin of Chile. To assess the impact of herbivory on Olivillo seedlings, we set up 29 experimental plots, 1.5 × 3 m: 16 in forests of Fray Jorge National Park (subtropical latitude), and 13 in Guabún, Chiloé Island (temperate latitude). Half of each plot was fenced around with chicken wire, to exclude small mammals, and the other half was left unfenced. In each half of the plots we planted 16 seedlings of Olivillo in December 2003, with a total of 928 plants. Seedling survival, leaf production and herbivory by invertebrates were monitored over the next 16 mo. Small mammal herbivores killed ca 30 percent of seedlings in both sites. Nevertheless, invertebrate herbivory was greater in the temperate forest, thus contradicting the expected trend of increasing herbivore impact toward the tropics. Seedling growth was greater in subtropical forest suggesting better conditions for tree growth or that higher invertebrate herbivory depressed seedling growth in the temperate forest. Invertebrate herbivory increased toward temperate latitudes while small mammal herbivory was similar in both sites. We suggest that comparison of single species can be useful to test generalizations about latitudinal patterns and allow disentangling factors controlling herbivory patterns across communities.     

Variability of invertebrate herbivory on the submerged macrophyte Potamogeton perfoliatus

1. Invertebrate herbivory was studied in twenty-eight populations of the submerged macrophyte Potamogeton perfoliatus in Danish streams and lakes in mid-June. All populations but one experienced invertebrate herbivory and loss ranged from 0 to 11.9% of leaf area among populations. Loss generally increased with leaf age towards the base of the plants, and young apical leaves were rarely damaged. 2. Herbivory loss was significantly higher in streams (mean 5.0%) than in lakes (mean 2.2%), but varied greatly among populations within the same stream or lake and was not correlated to physico-chemical site characteristics, size or density of plant population, or leaf N and P content. High levels of invertebrate herbivory were therefore not associated with certain types of streams or lakes. 3. High herbivore biomass relative to abundance of plants was conducive to high loss. In streams, the biomass of the trichopteran Anabolia nervosa accounted for 50% of the variability in loss. No single species appeared to be equally important in lakes, although loss was correlated to the biomass of the chrysomelid beetle Macroplea appendiculata. Obligate herbivores, such as lepidopteran larvae, apparently exerted little damage on P. perfoliatus, and leaf mining and channelization from specialist feeders were negligible. It is concluded that shredders acting as facultative herbivores were the most important invertebrate herbivores on P. perfoliatus in Danish freshwaters.     

Effects of plant diversity on invertebrate herbivory in experimental grassland

The rate at which a plant species is attacked by invertebrate herbivores has been hypothesized to depend on plant species richness, yet empirical evidence is scarce. Current theory predicts higher herbivore damage in monocultures than in species-rich mixtures. We quantified herbivore damage by insects and molluscs to plants in experimental plots established in 2002 from a species pool of 60 species of Central European Arrhenatherum grasslands. Plots differed in plant species richness (1, 2, 4, 8, 16, 60 species), number of functional groups (1, 2, 3, 4), functional group and species composition. We estimated herbivore damage by insects and molluscs at the level of transplanted plant individuals (“phytometer” species Plantago lanceolata, Trifolium pratense, Rumex acetosa) and of the entire plant community during 2003 and 2004. In contrast to previous studies, our design allows specific predictions about the relative contributions of functional diversity, plant functional identity, and species richness in relation to herbivory. Additionally, the phytometer approach is new to biodiversity-herbivory studies, allowing estimates of species-specific herbivory rates within the larger biodiversity-ecosystem functioning context. Herbivory in phytometers and experimental communities tended to increase with plant species richness and the number of plant functional groups, but the effects were rarely significant. Herbivory in phytometers was in some cases positively correlated with community biomass or leaf area index. The most important factor influencing invertebrate herbivory was the presence of particular plant functional groups. Legume (grass) presence strongly increased (decreased) herbivory at the community level. The opposite pattern was found for herbivory in T. pratense phytometers. We conclude that (1) plant species richness is much less important than previously thought and (2) plant functional identity is a much better predictor of invertebrate herbivory in temperate grassland ecosystems.     

Genotypic variation in growth and resistance to insect herbivory in silver birch (<Emphasis Type="Italic">Betula pendula</Emphasis>) seedlings

If herbivory is unevenly distributed among different types of plants, or if individual plants differ in their response to herbivory, herbivores may affect seedling growth and survival, and consequently plant population structure. In this study we examined variation in resistance to insect herbivory and in growth responses to feeding among 20 silver birch (Betula pendula Roth) genotypes representing variation within a natural population. Birch seedlings were grown in dense stands in random arrangement so that seedlings of different genotypes competed with each other. On insect exposure plots natural colonization of insects was allowed, and insect removal plots were sprayed with insecticide. Resistance to insect herbivory was measured as the inverse of leaf damage, and growth responses of seedlings to feeding were determined as the change in seedling height relative to the amount of damage. Resistance varied significantly among genotypes, but growth responses to feeding did not. In fertilized seedlings, resistance correlated negatively with height, indicating a trade-off between resistance and growth. The absence of such a correlation in non-fertilized seedlings is a sign of environmental effects on the cost of resistance. Growth responses to feeding did not correlate with either resistance or growth. Nevertheless, different effects of the actual damage on height increase changed the positions of the genotypes in the size hierarchy of the experimental populations. Thus, even moderate levels of insect herbivory can change the outcome of competitive interactions between birch genotypes.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.     

Herbivory and Stoichiometric Feedbacks to Primary Production

Established theory addresses the idea that herbivory can have positive feedbacks on nutrient flow to plants. Positive feedbacks likely emerge from a greater availability of organic carbon that primes the soil by supporting nutrient turnover through consumer and especially microbially-mediated metabolism in the detrital pool. We developed an entirely novel stoichiometric model that demonstrates the mechanism of a positive feedback. In particular, we show that sloppy or partial feeding by herbivores increases detrital carbon and nitrogen allowing for greater nitrogen mineralization and nutritive feedback to plants. The model consists of differential equations coupling flows among pools of: plants, herbivores, detrital carbon and nitrogen, and inorganic nitrogen. We test the effects of different levels of herbivore grazing completion and of the stoichiometric quality (carbon to nitrogen ratio, C:N) of the host plant. Our model analyses show that partial feeding and plant C:N interact because when herbivores are sloppy and plant biomass is diverted to the detrital pool, more mineral nitrogen is available to plants because of the stoichiometric difference between the organisms in the detrital pool and the herbivore. This model helps to identify how herbivory may feedback positively on primary production, and it mechanistically connects direct and indirect feedbacks from soil to plant production.     

Impacts of soil temperature,phenology and plant community composition on invertebrate herbivory in a natural warming experiment

Species and community-level responses to warming are well documented, with plants and invertebrates known to alter their range, phenology or composition as temperature increases. The effects of warming on biotic interactions are less clearly understood, but can have consequences that cascade through ecological networks. Here, we used a natural soil temperature gradient of 5–35°C in the Hengill geothermal valley, Iceland, to investigate the effects of temperature on plant community composition and plant–invertebrate interactions. We quantified the level of invertebrate herbivory on the plant community across the temperature gradient and the interactive effects of temperature, plant phenology (i.e. development stage) and vegetation community composition on the probability of herbivory for three ubiquitous plant species, Cardamine pratensis, Cerastium fontanum and Viola palustris. We found that the percentage cover of graminoids and forbs increased, while the amount of litter decreased, with increasing soil temperature. Invertebrate herbivory also increased with soil temperature at the plant community level, but this was underpinned by different effects of temperature on herbivory for individual plant species, mediated by the seasonal development of plants and the composition of the surrounding vegetation. This illustrates the importance of considering the development stage of organisms in climate change research given the variable effects of temperature on susceptibility to herbivory at different ontogenetic stages.     

Interactions between repeated fire, nutrients, and insect herbivores affect the recovery of diversity in the southern Amazon

Surface fires burn extensive areas of tropical forests each year, altering resource availability, biotic interactions, and, ultimately, plant diversity. In transitional forest between the Brazilian cerrado (savanna) and high stature Amazon forest, we took advantage of a long-term fire experiment to establish a factorial study of the interactions between fire, nutrient availability, and herbivory on early plant regeneration. Overall, five annual burns reduced the number and diversity of regenerating stems. Community composition changed substantially after repeated fires, and species common in the cerrado became more abundant. The number of recruits and their diversity were reduced in the burned area, but burned plots closed to herbivores with nitrogen additions had a 14 % increase in recruitment. Diversity of recruits also increased up to 50 % in burned plots when nitrogen was added. Phosphorus additions were related to an increase in species evenness in burned plots open to herbivores. Herbivory reduced seedling survival overall and increased diversity in burned plots when nutrients were added. This last result supports our hypothesis that positive relationships between herbivore presence and diversity would be strongest in treatments that favor herbivory—in this case herbivory was higher in burned plots which were initially lower in diversity. Regenerating seedlings in less diverse plots were likely more apparent to herbivores, enabling increased herbivory and a stronger signal of negative density dependence. In contrast, herbivores generally decreased diversity in more species rich unburned plots. Although this study documents complex interactions between repeated burns, nutrients, and herbivory, it is clear that fire initiates a shift in the factors that are most important in determining the diversity and number of recruits. This change may have long-lasting effects as the forest progresses through succession.     

Intraspecific competition and subterranean herbivory: individual and interactive effects on bush lupine

High mortality of plants growing in dense monospecific stands (i.e. self-thinning) usually results from intense intraspecific competition. However, inconspicuous below-ground insect herbivory might be a potent but overlooked source of mortality within dense stands of plants, particularly if crowding limits a plant's ability to compensate for herbivore damage. Here I ask how high conspecific density influences a plant's ability to cope with heavy below-ground insect herbivory.
I manipulated conspecific density and exposure to an abundant root-borer, the ghost moth ( Hepialus californicus ), and examined the impacts on the fecundity, growth, and survival of bush lupine ( Lupinus arboreus ), a fast-growing shrub that grows in dense monospecific stands in coastal grasslands. Both herbivory and intraspecific competition affected seed production, size, and mortality of bush lupine over the two years of the experiment. Plants consistently produced fewer seeds when growing at high versus low density and ghost moth herbivory also significantly reduced seed production. The negative effects of herbivory on plant fecundity were similar, regardless of plant density. In contrast, plant survival was affected by both competition, herbivory, and the interaction of these factors. In high density plots, plant survival was uniformly low (averaging 0.45–0.50); plants exposed to herbivores died from heavy herbivory, and plants protected from herbivores died due to intense intraspecific competition that compensated for losses due to herbivory. In low density plots, ghost moth herbivory similarly reduced lupine survival, from an average survival probability of 0.94 in plots protected from these herbivores to 0.55 in plots exposed to herbivory. Thus, results show that regardless of plant density, below-ground herbivory can be a potent source of mortality.     

Outbreaks by canopy-feeding geometrid moth cause state-dependent shifts in understorey plant communities

The increased spread of insect outbreaks is among the most severe impacts of climate warming predicted for northern boreal forest ecosystems. Compound disturbances by insect herbivores can cause sharp transitions between vegetation states with implications for ecosystem productivity and climate feedbacks. By analysing vegetation plots prior to and immediately after a severe and widespread outbreak by geometrid moths in the birch forest-tundra ecotone, we document a shift in forest understorey community composition in response to the moth outbreak. Prior to the moth outbreak, the plots divided into two oligotrophic and one eutrophic plant community. The moth outbreak caused a vegetation state shift in the two oligotrophic communities, but only minor changes in the eutrophic community. In the spatially most widespread communities, oligotrophic dwarf shrub birch forest, dominance by the allelopathic dwarf shrub Empetrum nigrum ssp. hermaphroditum, was effectively broken and replaced by a community dominated by the graminoid Avenella flexuosa, in a manner qualitatively similar to the effect of wild fires in E. nigrum communities in coniferous boreal forest further south. As dominance by E. nigrum is associated with retrogressive succession the observed vegetation state shift has widespread implications for ecosystem productivity on a regional scale. Our findings reveal that the impact of moth outbreaks on the northern boreal birch forest system is highly initial-state dependent, and that the widespread oligotrophic communities have a low resistance to such disturbances. This provides a case for the notion that climate impacts on arctic and northern boreal vegetation may take place most abruptly when conveyed by changed dynamics of irruptive herbivores.