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The North American beaver (Castor canadensis Kuhl) and cottonwoods (Populus spp.) are foundation species, the interactions of which define a much larger community and affect a threatened riparian habitat type. Few studies have tested the effect of these interactions on plant chemistry and a diverse arthropod community. We experimentally examined the impact of beaver foraging on riparian communities by first investigating beaver food preferences for one cottonwood species, Fremont cottonwood (P. fremontii S. Watson), compared to other locally available woody species. We next examined the impact of beaver foraging on twig chemistry and arthropod communities in paired samples of felled and unfelled cottonwood species in northern Arizona (P. fremontii) and southwestern Colorado (narrowleaf cottonwood, P. angustifolia James, and Eastern cottonwood, P. deltoides W. Bartram ex Marshall). Four major patterns emerged: (1) In a cafeteria experiment, beavers chose P. fremontii six times more often than other woody native and exotic species. (2) With two cottonwood species, we found that the nitrogen and salicortin concentrations were up to 45% greater and lignin concentration 14% lower in the juvenile resprout growth of felled trees than the juvenile growth on unfelled trees (six of seven analyses were significant for P. fremontii and four of six were significant for P. angustifolia). (3) With two cottonwood species, arthropod community composition on juvenile branches differed significantly between felled and unfelled trees, with up to 38% greater species richness, 114% greater relative abundance and 1282% greater species diversity on felled trees (six of seven analyses with P. fremontii and four of six analyses with P. angustifolia were significant). The above findings indicate that the highest arthropod diversity is achieved in the heterogenous stands of mixed felled and unfelled trees than in stands of cottonwoods, where beavers are not present. These results also indicate that beaver herbivory changes the chemical composition in 10 out of 13 chemical traits in the juvenile growth of two of the three cottonwood species to potentially allow better defense against future beaver herbivory. (4) With P. deltoides, only one of five analyses in chemistry was significant, and none of the four arthropod community analyses were significant, suggesting that this species and its arthropod community responds differently to beaver. Potential reasons for these differences are unknown. Overall, our findings suggest that in addition to their impact on riparian vegetation, other mammals, birds, and aquatic organisms, beavers also may define the arthropod communities of two of three foundation tree species in these riparian ecosystems. 

Mycorrhizal restoration benefits are widely acknowledged, yet factors underpinning this success remain unclear. To illuminate when natural regeneration might be sufficient, we investigated the degree mycorrhizal fungi would colonizePopulus fremontii(Fremont cottonwood) 2 years after the restoration of a riparian corridor, in the presence of an adjacent source. We compared colonization levels across plant populations and ecotypes, and from trees in the planted area to those in natural source populations. Four findings contribute to the theory and application of host–symbiont interactions. (1) Median ectomycorrhizal colonization of trees in the planted area was less than one‐tenth of that within natural source populations (p < 0.05), suggesting that even with adjacent intact habitat, sluggish regeneration would make proactive mycorrhizal restoration beneficial. (2) Within the planted area, median ectomycorrhizal and arbuscule colonization of trees sourced from greater distances were less than one‐third of that for trees sourced locally (p < 0.05), suggesting translocation poses barriers to symbioses. (3) Changes in colonization did not align with plant ecotypes, suggesting that geographic scales of selection for plants and fungi differ. (4) Slight increases in median mycorrhizal colonization (from 0% to 5%) were strongly correlated with increased survival for the plant provenance with lowest survival (r² = 46% andr_s = 48%,p < 0.05), suggesting mycorrhizae are particularly beneficial when plants are under stress (including translocation‐induced stress). This study is novel in demonstrating that mycorrhizal regeneration is slow even in the presence of adjacent intact habitat, and that when colonization could seem negligible, it may still have biological significance.