Individual‐level differences in generalist caterpillar responses to a plant–plant cue

1. Plant–plant communication has been found to affect interactions between herbivores and plants in several model systems. In these systems, herbivore‐induced volatile chemical cues are emitted and perceived by other plants (receivers), which subsequently change their defensive phenotypes. Most studies have focused on how the effects of volatile cues affect plant damage, whereas herbivore performance has rarely been examined. 2. In this study, it is shown that plant–plant communication between willows reduced the growth rate, feeding rate, and conversion efficiency of some individuals but not others of a generalist caterpillar, Orgyia vetusta. 3. Using a paired, no‐choice trial design, there was substantial variation between caterpillar individuals in their response to willows that had been induced with a volatile plant–plant cue. This variation was explained by feeding parameters of the individual herbivores. Individuals behaved similarly when fed induced and non‐induced willow leaves. Specifically, growth rates of caterpillars that grew rapidly on non‐induced willow leaves were negatively affected by plant–plant cues, but growth rates of caterpillars that grew slowly on non‐induced willow leaves were not affected by the responses to volatiles from neighbouring willows. 4. Induction by volatile plant–plant cues reduced the growth rates of those individual herbivores that caused the greatest damage to willow, but had little effect on weak growers.     

Biomass Harvest from Natural Willow Rings around Prairie Wetlands

The study followed the harvest of natural willow from three wetlands using a prototype modified agricultural round baler nicknamed a Bio-Baler. The study reports fuel characteristics and combustion testing of biomass harvested from natural willow rings. Composition of native willow species in the harvested willow rings was determined. We specifically measured regrowth of the biomass and number of regenerated stems per stump 1 year after harvest to determine how different willow species responded to mechanical biomass cutting with the Bio-Baler. The results of combustion testing for the natural willow were essentially similar to those with “conventional” wood chips or planted willows. The ash content was approximately 1.65%, slightly lower than for planted willow plantations. The calorific value of the natural willow was 19.6 MJ kg^?1 (dry basis) similar to what is expected for wood and planted willow. Four Salix species (Salix bebbiana Sarg., Salix petiolaris Sm., Salix eriocephala Michx., and Salix discolor Muhl) were identified in the willow rings. Stem biomass increased for all species except S. bebbiana after willow was harvested with the Bio-Baler. Overall, willow regeneration was not affected by mechanical harvesting compared to hand cutting. Regenerated stem density was 93 stems per square meter for mechanically harvested stumps compared to 105 stems per square meter for hand-pruned stumps. Based on the results, biomass harvested from natural willow rings has acceptable fuel characteristics when compared to purpose-grown willows and mechanical harvest with a Bio-Baler does not have a negative effect on willow regeneration.     

Herbarium records in Arctic dwarf shrub dendrochronology: Methodological approach and perspectives

Our work aims to investigate whether herbaria resources can be used for the extension of Arctic dwarf shrub chronologies. The current use of herbaria reaches far beyond their initially aims; among the new applications, phenology observations and conservation biology can be mentioned. However, to this date, no studies on the use of herbarium specimens for dendrochronological research have been published. Examples of perennial plants from herbarium sheets that could potentially be used for such studies are dwarf shrubs, samples of which often consist of whole specimens, including the root system, the root collar and branches. Here, we present a protocol for the selection and processing of historical material. Based on the collections from Kew and Copenhagen, which are among of the largest herbaria with Arctic plants, a database of 25 areas from the Atlantic sector of the Arctic was created. Material from the following most common species was collected: grey willow (Salix glauca L.), polar willow (Salix polaris Wahlenb.), dwarf willow (Salix herbacea L.), net-leaved willow (Salix reticulata L.), arctic willow (Salix arctica Pall.), mountain avens (Dryas octopetala L.), dwarf birch (Betula nana L.). We present the preliminary results of a case study using historical samples of Salix arctica from the Thule (Qaanaaq) area, NW Greenland. Dwarf shrubs can commonly reach the age of 80–100 years or beyond, while herbaria resources may allow the extension of such series over the last centuries. Therewith, these resources may provide an excellent proxy data source on the changing natural environment beyond the northern and upper tree limits, where well-replicated proxy time-series remain sparse.     

Plant defence responses to volatile alert signals are population‐specific

Herbivore‐induced volatiles are widespread in plants. They can serve as alert signals that enable neighbouring leaves and plants to pre‐emptively increase defences and avoid herbivory damage. However, our understanding of the factors mediating volatile organic compound (VOC) signal interpretation by receiver plants and the degree to which multiple herbivores affect VOC signals is still limited. Here we investigated whether plant responses to damage‐induced VOC signals were population specific. As a secondary goal, we tested for interference in signal production or reception when plants were subjected to multiple types of herbivore damage. We factorially crossed the population sources of paired Phaseolus lunatus plants (same versus different population sources) with a mechanical damage treatment to one member of the pair (i.e. the VOC emitter, damaged versus control), and we measured herbivore damage to the other plant (the VOC receiver) in the field. Prior to the experiment, both emitter and receiver plants were naturally colonized by aphids, enabling us to test the hypothesis that damage from sap‐feeding herbivores interferes with VOC communication by including emitter and receiver aphid abundances as covariates in our analyses. One week after mechanical leaf damage, we removed all the emitter plants from the field and conducted fortnightly surveys of leaf herbivory. We found evidence that receiver plants responded using population‐specific ‘dialects’ where only receivers from the same source population as the damaged emitters suffered less leaf damage upon exposure to the volatile signals. We also found that the abundance of aphids on both emitter and receiver plants did not alter this volatile signalling during both production and reception despite well‐documented defence crosstalk within individual plants that are simultaneously attacked by multiple herbivores. Overall, these results show that plant communication is highly sensitive to genetic relatedness between emitter and receiver plants and that communication is resilient to herbivore co‐infestation.     

Composition, Stand Structure, and Biomass Estimates of “Willow Rings” on the Canadian Prairies

Many small wetlands are scattered across the Canadian Prairies. These wetlands are often surrounded by a ring of phreatophytic shrubs, of which willows are the dominant genus. Even though many of these willow rings are currently degraded, due to the lack of disturbance factors such as prairie wild fires and bison herds, they are still of great value as a biomass resource without carbon debt. Our study had the objectives to (1) study the distribution of five native willows (Salix bebbiana, Salix discolor, Salix eriocephala, Salix interior, and Salix petiolaris) in 12 willow wetland communities in relation to a moisture gradient and (2)examine the effects of age of the willow ring on biomass potential, stand structure, and species distribution. Annual biomass production ranged from 1.9 to 16.2 odt ha^?1 year^?1 for the 12 sites that were between 9 and 34 years old. Current standing biomass for the 12 willow rings with a total area of 1.5 ha was estimated at 239.2 odt. With age, stand structure changed from multiple small-diameter stems to fewer large-diameter stems, and at the same time the species distribution shifted from one dominated by S. petiolaris to one being dominated by S. discolor. Additionally, the sympatric willows showed a distribution along a moisture gradient, with S. eriocephala, S. interior, and S. petiolaris in moist locations and S. bebbiana and S. discolor in drier locations.     

Melampsora rust species on biomass willows in central and north-eastern Germany

Melampsora willow rusts are the most important fungal pathogens in short rotation coppices of biomass willows. In the past, breeding programmes for rust resistant biomass willows concentrated on the distinction of races within the forma specialis Melampsora larici-epitea f. sp. larici-epitea typica that colonized Salix viminalis and related clones. In a new breeding program that is based on a wider range of willow species it is necessary to identify further Melampsora species and formae specialis that are pathogens of willow species other than S. viminalis. Therefore, three stock collections with Salix daphnoides, Salix purpurea, and other shrub willow species (including S. viminalis) species were sampled in north-eastern Germany. A fourth stock collection in central Germany contributed rusts of tree willows (Salix fragilis and Salix alba) and the large shrub Salix caprea. Out of 156 rust samples, 149 were successfully sequenced for ITS rDNA. A phylogenetic analysis combining Neighbour-Joining, Maximum-Likelihood and Bayesian analysis revealed six species: Melampsora ribesii-purpureae, Melampsora allii-salicis-albae, Melampsora sp. aff. allii-fragilis, Melampsora larici-pentandrae, Melampsora larici-caprearum, and Melampsora larici-epitea. The first four species were found exclusively on the expected hosts. Melampsora larici-caprearum had a wider host range comprising S. caprea and S. viminalis hybrids. Melampsora larici-epitea can be further differentiated into two formae speciales. The forma specialis larici-epitea typica (59 samples) colonized Salix viminalis clones, Salix purpurea, Salix×dasyclados, and Salix×aquatica. In contrast to this relatively broad host range, f. sp. larici-daphnoides (65 samples) was found exclusively on Salix daphnoides. With the distinction and identification of the rust species/formae speciales it is now possible to test for race-specific resistances in a more targeted manner within the determined pairings of rust and willow species.     

Interactions between willows and insect herbivores under enhanced ultraviolet-B radiation

We studied the effects of elevated ultraviolet-B radiation on interactions between insect herbivores and their host plants by exposing two species of phytochemically different willows, Salix myrsinifolia and S. phylicifolia, to a modulated increase in ultraviolet radiation in an outdoor experiment and monitoring the colonisation of insect herbivores on these willows. We examined the effect of increased ultraviolet-B (UV-B) radiation on (1) the quality of willow leaves, (2) the distribution and abundance of insect herbivores feeding on these willows, (3) the resulting amount of damage, and (4) the performance of insect larvae feeding on the exposed plant tissue. Six clones of each of the two willow species were grown in eight blocks for 12 weeks in the UV-B irradiation field. The clones were exposed to a constant 50% increase in UV-B radiation (simulating 20-25% ozone depletion), to a small increase in UV-A radiation or to ambient solar irradiation. We allowed colonisation on the willows by naturally occurring insects, but also introduced adults of a leaf beetle, Phratora vitellinae, a specialist herbivore on S. myrsinifolia. Increased UV-B radiation did not affect any of the measured indices of plant quality. However, numbers of P. vitellinae on S. myrsinifolia were higher in plants with UV-B treatment compared with UV-A and shade controls. In laboratory tests, growth of the second-instar larva of P. vitellinae was not affected by UV-B treatment of S. myrsinifolia, but was retarded on UV-B treated leaves of S. phylicifolia. In addition, naturally occurring insect herbivores were more abundant on willows exposed to elevated UV-B radiation compared to those grown under control treatments. In spite of the increased abundance of insect herbivores, willows treated with elevated UV-B did not suffer more herbivore damage than willows exposed to ambient solar radiation (shade control). The observed effects of UV-B on herbivore abundance, feeding and growth varied significantly due to spatial variation in environment quality, as indicated by the UV-treatment x block interaction. The results suggest that (1) environmental variation modifies the effects of UV-B radiation on plant-insect interactions and (2) specialist herbivores might be more sensitive to chemical changes in their secondary host plants (S. phylicifolia) than to changes in their primary hosts (S. myrsinifolia).     

Correspondence of ectomycorrhizal diversity and colonisation of willows (Salix spp.) grown in short rotation coppice on arable sites and adjacent natural stands

Willows (Salix spp.) are mycorrhizal tree species sometimes cultivated as short rotation coppice (SRC) on arable sites for energy purposes; they are also among the earliest plants colonising primary successional sites in natural stands. The objective of this study was to analyse the degree of colonisation and diversity of ectomycorrhizal (EM) communities on willows grown as SRC in arable soils and their adjacent natural or naturalized stands. Arable sites usually lack ectomycorrhizal host plants before the establishment of SRC, and adjacent natural or naturalized willow stands were hypothesized to be a leading source of ectomycorrhizal inoculum for the SRC. Three test sites including SRC stands (Salix viminalis, Salix dasyclados, and Salix schwerinii) and adjacent natural or naturalized (Salix caprea, Salix fragilis, and Salix?×?mollissima) stands in central Sweden were investigated on EM colonisation and morphotypes, and the fungal partners of 36 of the total 49 EM fungi morphotypes were identified using molecular tools. The frequency of mycorrhizas in the natural/naturalized stands was higher (two sites) or lower (one site) than in the corresponding cultivated stands. Correspondence analysis revealed that some EM taxa (e.g. Agaricales) were mostly associated with cultivated willows, while others (e.g. Thelephorales) were mostly found in natural/naturalized stands. In conclusion, we found strong effects of sites and willow genotype on EM fungi formation, but poor correspondence between the EM fungi abundance and diversity in SRC and their adjacent natural/naturalized stands. The underlying mechanism might be selective promotion of some EM fungi species by more effective spore dispersal.     

The use of non-dormant rods as planting material: A new approach to establishing willow for environmental applications

In this trial we tested a new planting technique for willows used in environmental applications such as green structures in urban settings. Three commercial willow clones were planted in spring 2008 at different periods as non-dormant woody rods. Comparing the main growth parameters at the end of the season, we found that planting date may affect plant establishment depending on the clone. Based on our results, the use of non-dormant willow material collected and planted in spring is possible in our environment with several different clones. Best results were obtained with Salix viminalis (Sv 5027), which can be planted with a delay of about one month without showing any significant decrease in biomass production or survival rate.     

Lead uptake and translocation by willows in pot and field experiments

Plant growth and lead (Pb) uptake by seven willow varieties were investigated in pot and field experiments to assess the suitability of willows for phytoremediation of Pb at heavily contaminated sites such as skeet ranges. Differences in uptake and translocation of Pb in Salix were observed between pot and field experiments. In the pot experiment, willows grown in Pb-contaminated field soil for 6 months showed tolerance to very high soil Pb concentration (21,360 mg kg(-1)), and with the addition of EDTA were able to take up and translocate more than 1000 mg kg(-1) Pb into above-ground tissues. In the field experiment, all willow varieties showed tolerance to heterogeneously high soil Pb concentrations. Plants were also able to take up and translocate Pb into above-ground tissues. However, after 4.5 months, the lead concentration in the above-ground tissues of willows grown in soil amended with EDTA was less than 200 mg kg(-1). The results from the pot experiment suggest that Salix varieties have the potential to take up and translocate significant amounts of Pb into above-ground tissues using EDTA. However, to verify the phytoextraction abilities of Salix in the field, additional research is needed.     

Plant phenology-mediated indirect effects: The gall midge opens the phenological window wider for a leaf beetle

We examined whether larvae of the gall midge Rabdophaga rigidae (Diptera: Cecidomyiidae) can modify the seasonal dynamics of the density of a leaf beetle, Plagiodera versicolora (Coleoptera: Chrysomelidae), by modifying the leaf flushing phenology of its host willow species, Salix serissaefolia and Salix eriocarpa (Salicaceae). To test this, we conducted field observations and a laboratory experiment. The field observations demonstrated that the leaf flushing phenology of the willows and the seasonal dynamics of the beetle density differed between shoots with stem galls and shoots without them. On galled shoots of both willow species, secondary shoot growth and secondary leaf production were promoted; consequently, leaf production showed a bimodal pattern and leaf production periods were 1 to 2 months longer than on non‐galled shoots. The adult beetle density on galled shoots was thus enhanced late in the season, and was found to change seasonally, synchronizing with the production of new leaves on the host willow species. From the results of our laboratory experiment, we attributed this synchrony between adult beetle density and willow leaf flush to beetles’ preference to eat new leaves rather than old. Indeed, beetles consumed five times more of the young leaves when they were fed both young and old leaves. These results indicate that stem galls indirectly enhance the adult beetle density by enhancing food quality and quantity late in the beetle‐feeding season. We therefore conclude that midge galls widen the phenological window for leaf beetles by extending the willows’ leaf flush periods.     

Seasonality of herbivory and communication between individuals of sagebrush

Seasonal changes in herbivore numbers and in plant defenses are well known to influence plant–herbivore interactions. Some plant defenses are induced in response to herbivore attack or cues correlated with risk of attack although seasonal variation in these defenses is relatively poorly known. We previously reported that sagebrush becomes more resistant to its herbivores when neighboring plants have been experimentally clipped with scissors. In this study we asked whether herbivory to leaves of sagebrush varied seasonally and whether there was seasonal variation in natural levels of damage when neighbors were clipped. We found that sagebrush accumulated most chewing damage early in the season, soon after the spring flush of new leaves. This damage was caused by generalist grasshoppers, deer, specialist caterpillars, beetles, gall makers, and other less common herbivores. Sagebrush showed no evidence of preferentially abscising leaves that had been experimentally clipped. Experimental clipping by Trirhabda pilosa beetle larvae caused neighbors to accumulate less herbivore damage later that season, similar to results in which clipping was done with scissors. Induced resistance caused by experimentally clipping a neighbor was affected by season; plants with neighbors clipped in May accumulated less damage throughout the season relative to plants with unclipped neighbors or neighbors clipped later in the summer. We found a correlation between seasonal herbivore pressure, damage accumulated by plants, and induced responses to experimentally clipping neighbors. The causal mechanisms responsible for this correlation are unknown although a strong seasonal effect was clear.     

Clonal growth of sagebrush (Artemisia tridentata) (Asteraceae) and its relationship to volatile communication

To increase systemic resistance to herbivory, some clonal plants transmit internal cues among clonal ramets to prevent widespread damage to genets. Sagebrush (Artemisia tridentata) (Asteraceae) is known to use volatile cues to induce resistance within and between plants (so‐called volatile communication). In the present study, we observed the extent and frequency of clonal growth in a natural sagebrush population in western North America to understand the influence of clonal growth on volatile communication. We used genetic analysis involving microsatellite markers and excavation of the root systems. In addition, we characterized the volatile profiles from the headspace of sagebrush ramets. Excavation of the root system of sagebrush plants revealed that sagebrush propagates clonally below ground and that daughter ramets grow near the mother stem. Volatiles were variable among genetically different ramets, although clonal ramets (genetically identical ramets) released similar volatiles, suggesting a genetic basis for volatile similarity. Sagebrush has been shown to be most responsive to volatiles released from artificially produced clones and suffers less herbivore damage as a result. Therefore, these results, taken into consideration together, imply that volatile communication may occur among genetically identical ramets under natural conditions, and that volatile similarity between the releaser and receiver may be recognized by the receiver and increase resistance against herbivory.     

Leaf Volatiles Induced by Mechanical Damage from Diverse Taxonomic Tree Species

Plant volatiles induced by wounding play key roles in plant-insect and plant-plant interactions. To deeply understand the mechanism of their induction by wounding and their functions in interplant communications, four diverse tree species: ashleaf maples ( Acer negundo L.), hankow willow ( Salix matsudana Koidz.), Chinese white poplar ( Populus tomentosa Carr.) and poplar opera 8277　( P. simonii×P. pyramibalis cv.), were used as materials. The blends of volatiles collected after damage were detected with GC-MS. Most of the induced compounds reach high concentrations in 5 h. They are acyclic monoterpenes, fatty acid derivatives, and aromatic compounds. To authors' knowledge, dimethyl adipate, diisobutyl succinate and benthothiazole have never been reported in previous herbivore insect-plant systems. After being damaged 2 h, green leaf volatiles were released in large amount. The repellents were detected in higher concentration after 24 h. The time of releasing is different within different species, but many kinds of volatiles widely existed in different trees. There were some difference among species. Health ashleaf maple released more terpenoids, but poplars and willow produced more aromatic compounds.     

Selection ofAzolla forms resistant to the water lily aphid,Rhopalosiphum nymphaeae Life history of Rhopalosiphum nymphaeae

Genotypic and environmentally determined differences inSalix viminalis L. suitability forDasineura marginemtorquens (Bremi) (Diptera: Cecidomyiidae) growth and survival were investigated in laboratory and field experiments. Earlier studies have documented high neonate larval mortality on certainS. viminalis genotypes. Here we present results from laboratory experiments which show no difference in larval establishment behaviour on resistant and susceptible willow genotypes. These data indicate that larvae are not able to detect the plant characteristic responsible for resistance. In a field study insect performance was measured on willow shoots of dissmilar size. Adult body size was positively correlated to willow shoot length. In turn, body size was positively correlated to potential fecundity. These results are discussed in relation to observed high densities ofD. marginemtorquens in vigorously growing willows cultivated for biomass production.     

Genetic architecture of susceptibility to herbivores in hybrid willows

We performed a common garden experiment using parental, F1, F2, and backcross willow hybrids to test the hypothesis that hybrid willows experience breakdown of resistance to herbivores. After exposing plants to herbivores in the field, we measured the densities/damage caused by 13 insect herbivores and one herbivorous mite. Using joint-scaling tests, we determined the contribution of additive, dominance, and epistasis to variation in susceptibility to herbivores (measured either as density or damage level) among the six genetic classes. We found the genetic architecture of susceptibility/resistance in the parental species to be complex, involving additive, dominance, and epistasis for each herbivore species. Although genic interactions altered plant susceptibility for each of the 14 herbivores, three distinct patterns of response of herbivores to hybrids were expressed. One pattern, observed in four herbivore species, supported the hypothesis of breakdown of resistance genes in recombinant hybrids. A second pattern, shown by six other herbivore species, supported the hypothesis of hybrid breakdown of host recognition genes. In other words, epistatic interactions for host recognition traits (probably oviposition/feeding stimulants or attractants) appeared to be important in determining herbivore abundance for those six species. The final patterns supported a structure of dominance, either for host recognition traits (in the case of three herbivore species) or for host resistance traits (for one herbivore species). The combination of differing responses of herbivore species, including members of the same genus and tribe, and the ubiquitous importance of epistasis suggests that many genes affect herbivore resistance in this hybrid willow system.     

The specificity of herbivore-induced plant volatiles in attracting herbivore enemies

Plants respond to herbivore attack by emitting complex mixtures of volatile compounds that attract herbivore enemies, both predators and parasitoids. Here, we explore whether these mixtures provide significant value as information cues in herbivore enemy attraction. Our survey indicates that blends of volatiles released from damaged plants are frequently specific depending on the type of herbivore and its age, abundance and feeding guild. The sensory perception of plant volatiles by herbivore enemies is also specific, according to the latest evidence from studies of insect olfaction. Thus, enemies do exploit the detailed information provided by plant volatile mixtures in searching for their prey or hosts, but this varies with the diet breadth of the enemy.     

Establishment and Early Growth of Willow at Different Levels of Weed Competition and Nitrogen Fertilization

To evaluate the effects of weed competition and nitrogen fertilization on the early growth performance of willow, cuttings of the clone Tora (Salix schwerinii x S. viminalis) were planted in buckets together with model weeds (spring barley or white mustard) sown 15, 26, and 30 days after willow planting. The buckets were fertilized with 30 or 90 kg N ha^?1. Willow with weeds sown after 15 days produced less biomass and smaller leaf area and had a lower maximum shoot height compared to willow planted without weeds and willow with weeds sown after 26 or 30 days. Fertilization with 90 kg N ha^?1 gave higher willow biomass production in willow with weeds sown after 26 or 30 days. Type of model weed had no effect on willow performance. Weed biomass and maximum shoot height were higher in weeds planted without willows compared to the willow-weed mixtures. A high nitrogen level gave more weed biomass when planted without willows and in the willow-weed mixture with weeds sown after 15 days. We conclude that for the given high density of willow, competition from weeds emerging soon after willow planting had strong effect on early production. Furthermore, if there is a risk of weed infestation, fertilization should be delayed.     

Rapid gall midge adaptation to a resistant willow genotype

Abstract 1 We conducted two experiments to investigate why a basket willow Salix viminalis L. genotype, known to be highly resistant to the leaf-roller gall midge Dasineura marginemtorquens (Bremi), should support very high gall densities in a field plantation at Tälle, south Sweden.
2 The first experiment was a field test of the hypothesis of fine-scale host adaptation in the gall midge/willow system. Support for the hypothesis would be established if midges originating from resistant willows and those originating from nearby susceptible willows differed in their abilities to initiate galls and complete development on resistant plants.
3 The objective of the second experiment was to explore whether there was a genetic basis to the trait for virulence in the midge population and to investigate any potential trade-offs this trait may entail.
4 Our results indicate that there was a fine-scaled microgeographic genetic structure to the midge population at Tälle. Midges originating from resistant plants had a heritable trait that enabled them to establish galls on resistant plants.
5 Midges able to initiate galls on the resistant genotype had longer developmental time on the susceptible genotype. This suggests that there is a physiological cost associated with being adapted to the resistant willow genotype.
6 We suggest that driving forces behind the observed host adaptation are selection imposed on the midge population by very strong willow resistance and restricted gene flow in the midge populations due to the special life history features of D. marginemtorquens .     

Herbivores Influence the Growth,Reproduction, and Morphology of a Widespread Arctic Willow

Shrubs have expanded in Arctic ecosystems over the past century, resulting in significant changes to albedo, ecosystem function, and plant community composition. Willow and rock ptarmigan (Lagopus lagopus, L. muta) and moose (Alces alces) extensively browse Arctic shrubs, and may influence their architecture, growth, and reproduction. Furthermore, these herbivores may alter forage plants in such a way as to increase the quantity and accessibility of their own food source. We estimated the effect of winter browsing by ptarmigan and moose on an abundant, early-successional willow (Salix alaxensis) in northern Alaska by comparing browsed to unbrowsed branches. Ptarmigan browsed 82–89% of willows and removed 30–39% of buds, depending on study area and year. Moose browsed 17–44% of willows and browsed 39–55% of shoots. Browsing inhibited apical dominance and activated axillary and adventitious buds to produce new vegetative shoots. Ptarmigan- and moose-browsed willow branches produced twice the volume of shoot growth but significantly fewer catkins the following summer compared with unbrowsed willow branches. Shoots on browsed willows were larger and produced 40–60% more buds compared to unbrowsed shoots. This process of shoot production at basal parts of the branch is the mechanism by which willows develop a highly complex “broomed” architecture after several years of browsing. Broomed willows were shorter and more likely to be re-browsed by ptarmigan, but not moose. Ptarmigan likely benefit from the greater quantity and accessibility of buds on previously browsed willows and may increase the carrying capacity of their own habitat. Despite the observed tolerance of willows to browsing, their vertical growth and reproduction were strongly inhibited by moose and ptarmigan. Browsing by these herbivores therefore needs to be considered in future models of shrub expansion in the Arctic.