Fig 1.
A. ANOVA results of differences in arthropod species richness and abundance for a community composed of 61 species. Trees at low elevation sites supported significantly greater arthropod species richness than trees at medium and high elevation sites in both the wild and in a 20 yr old common garden (Wild: F(2,27) = 9.61, P = 0.0007) (Garden: F(2,27) = 6.09, P< 0.007). Trees at low elevation/distance sites also supported significantly greater arthropod abundance than trees at medium and high elevation sites in both the wild and common garden (Wild: F(2,27) = 4.34, P = 0.02) (Garden: F(2,27) = 20.21, P< 0.0001). Uppercase letters indicate significant differences among trees in their respective locations (Wild vs. Common Garden) (example: Arthropod Species Richness at the low elevation site in the wild (A) was significantly different from species richness at the medium and high sites in the wild (a)). B. ANOVA results of differences in arthropod species richness and abundance for 13 ubiquitous species. Trees at the lower elevation site and trees in the common garden of low elevation/distance origin supported higher arthropod abundance than trees from higher elevation origins (Wild-F(2,27) = 9.78, P< 0.001, Garden- F(2,27) = 31.40, P< 0.0001). However, when we examined the 13 ubiquitous species community we found only a marginal difference in species richness among trees in the wild (Wild-F(2,27) = 2.97, P< 0.06) while trees from low elevation/distance origin in the common garden supported significantly higher species richness than medium and high trees (Garden- F(2,27) = 4.24, P< 0.02). Uppercase letters indicate significant differences among trees in their respective locations (Wild vs. Common Garden) (example: Arthropod Abundance at the low elevation site in the common garden (B) was significantly different from abundance at the medium and high sites in the common garden (b)).
Table 1.
Results of nested ANOVA analysis.
A grouping variable was used to combine the effects of site elevation and tree genotype origin where genotype was a random effect nested within all trees and site was a fixed effect used as our predictor variable. Trees from low elevation and low origin supported significantly higher arthropod species richness and abundance when compared to medium and high elevation trees.
Fig 2.
Non-Metric Multidimensional Scaling (NMDS) figure comparing arthropod communities on trees along an elevation gradient and in a common garden.
Communities that are more similar appear closer together while communities that are more dissimilar appear farther apart. Centroids represent community genotype means while error bars represent 95% confidence limits. All wild communities were significantly different from their common garden communities (P<0.05). Arthropod communities found on trees from the low elevation site were more similar to the communities found on trees from low elevation origin that were grown in a 20 yr. common garden (Anosim-R = 0.319)(R values range from 0.0–1.0. R values closer to 1.0 represent more dissimilar communities and R values closer to 0.0 represent more similar communities. Additionally, the communities found on trees from the medium elevation site and garden were more dissimilar than between low elevation trees (Anosim-R = 0.681). While communities found on trees of high elevational in the wild and of high origin but grown in a lower, warmer common garden were the most dissimilar (Anosim-R = 0.804).
Fig 3.
Bar graph showing Shannon’s diversity index of arthropod communities.
(Shannon’s Diversity H: Elevation gradient in the wild = 2.04, Garden = 1.50)(t-test: F = 108.53, p<0.0001). High elevation trees in the wild (C) had significantly less diverse arthropod communities than high elevation trees in the common garden (c). Diversity of arthropod communities on low and medium site trees in the wild (A, B) were not significantly different from their corresponding tree genotypes in the garden (A, B).
Fig 4.
A. Plot of linear regression of the effects of leaf area on arthropod species richness within a common garden. Leaf area varies with elevation and is positively correlated with arthropod species richness (P = 0.04, R2 = 0.14). B. Plot of linear regression of the effects of leaf area on arthropod abundance within a common garden. Leaf area, a genetically controlled trait, varies with elevation and is positively correlated with differences in arthropod abundance (P = 0.03, R2 = 0.15).
Fig 5.
Bar graph of differences in leaf area among the different elevation sites where arthropod surveys were conducted in the wild.
Leaf area was significantly greater at the low elevation site where arthropod abundance and species richness were also greater (F = 125.53, P< 0.0001).
Fig 6.
Conceptual diagram of stepwise progression procedure for assisted migration of host plants while maintaining their home site arthropod communities.
If home site arthropod communities are to be kept intact then a stepwise progression is likely needed. To maintain site of origin arthropod communities we suggest assisted migration efforts for narrowleaf cottonwood trees should not exceed distances greater than 50km and a change in elevation greater than 250m in elevation relative to their original location Table 1.