No abstract

We used multiscale plots to sample vascular plant diversity and soil characteristics in and adjacent to 26 long-term grazing exclosure sites in Colorado, Wyoming, Montana, and South Dakota, USA. The exclosures were 7-60 yr old (31.2 Ϯ 2.5 yr, mean Ϯ 1 SE). Plots were also randomly placed in the broader landscape in open rangeland in the same vegetation type at each site to assess spatial variation in grazed landscapes. Consistent sampling in the nine National Parks, Wildlife Refuges, and other management units yielded data from 78 1000-m 2 plots and 780 1-m 2 subplots. We hypothesized that native species richness would be lower in the exclosures than in grazed sites, due to competitive exclusion in the absence of grazing. We also hypothesized that grazed sites would have higher native and exotic species richness compared to ungrazed areas, due to disturbance (i.e., the intermediate-disturbance hypothesis) and the conventional wisdom that grazing may accelerate weed invasion. Both hypotheses were soundly rejected. Although native species richness in 1-m 2 subplots was significantly higher (P Ͻ 0.05) in grazed sites, we found nearly identical native or exotic species richness in 1000-m 2 plots in exclosures (31.5 Ϯ 2.5 native and 3.1 Ϯ 0.5 exotic species), adjacent grazed plots (32.6 Ϯ 2.8 native and 3.2 Ϯ 0.6 exotic species), and randomly selected grazed plots (31.6 Ϯ 2.9 native and 3.2 Ϯ 0.6 exotic species). We found no significant differences in species diversity (Hill's diversity indices, N1 and N2), evenness (Hill's ratio of evenness, E5), cover of various life-forms (grasses, forbs, and shrubs), soil texture, or soil percentage of N and C between grazed and ungrazed sites at the 1000-m 2 plot scale. The species lists of the long-ungrazed and adjacent grazed plots overlapped just 57.9 Ϯ 2.8%. This difference in species composition is commonly attributed solely to the difference in grazing regimes. However, the species lists between pairs of grazed plots (adjacent and distant 1000-m 2 plots) in the same vegetation type overlapped just 48.6 Ϯ 3.6%, and the ungrazed plots and distant grazed plots overlapped 49.4 Ϯ 3.6%. Differences in vegetation and soils between grazed and ungrazed sites were minimal in most cases, but soil characteristics and elevation were strongly correlated with native and exotic plant diversity in the study region. For the 78 1000-m 2 plots, 59.4% of the variance in total species richness was explained by percentage of silt (coefficient ϭ 0.647, t ϭ 5.107, P Ͻ 0.001), elevation (coefficient ϭ 0.012, t ϭ 5.084, P Ͻ 0.001), and total foliar cover (coefficient ϭ 0.110, t ϭ 2.104, P Ͻ 0.039). Only 12.8% of the variance in exotic species cover (log 10 cover) was explained by percentage of clay (coefficient ϭ Ϫ0.011, t ϭ Ϫ2.878, P Ͻ 0.005), native species richness (coefficient ϭ Ϫ0.011, t ϭ Ϫ2.156, P Ͻ 0.034), and log 10 N (coefficient ϭ 2.827, t ϭ 1.860, P Ͻ 0.067). Native species cover and exotic species richness and frequency were also significantly positively correlated with percentage of soi...

We used multiscale plots to sample vascular plant diversity and soil characteristics in and adjacent to 26 long-term grazing exclosure sites in Colorado, Wyoming, Montana, and South Dakota, USA. The exclosures were 7-60 yr old (31.2 Ϯ 2.5 yr, mean Ϯ 1 SE). Plots were also randomly placed in the broader landscape in open rangeland in the same vegetation type at each site to assess spatial variation in grazed landscapes. Consistent sampling in the nine National Parks, Wildlife Refuges, and other management units yielded data from 78 1000-m 2 plots and 780 1-m 2 subplots. We hypothesized that native species richness would be lower in the exclosures than in grazed sites, due to competitive exclusion in the absence of grazing. We also hypothesized that grazed sites would have higher native and exotic species richness compared to ungrazed areas, due to disturbance (i.e., the intermediate-disturbance hypothesis) and the conventional wisdom that grazing may accelerate weed invasion. Both hypotheses were soundly rejected. Although native species richness in 1-m 2 subplots was significantly higher (P Ͻ 0.05) in grazed sites, we found nearly identical native or exotic species richness in 1000-m 2 plots in exclosures (31.5 Ϯ 2.5 native and 3.1 Ϯ 0.5 exotic species), adjacent grazed plots (32.6 Ϯ 2.8 native and 3.2 Ϯ 0.6 exotic species), and randomly selected grazed plots (31.6 Ϯ 2.9 native and 3.2 Ϯ 0.6 exotic species). We found no significant differences in species diversity (Hill's diversity indices, N1 and N2), evenness (Hill's ratio of evenness, E5), cover of various life-forms (grasses, forbs, and shrubs), soil texture, or soil percentage of N and C between grazed and ungrazed sites at the 1000-m 2 plot scale. The species lists of the long-ungrazed and adjacent grazed plots overlapped just 57.9 Ϯ 2.8%. This difference in species composition is commonly attributed solely to the difference in grazing regimes. However, the species lists between pairs of grazed plots (adjacent and distant 1000-m 2 plots) in the same vegetation type overlapped just 48.6 Ϯ 3.6%, and the ungrazed plots and distant grazed plots overlapped 49.4 Ϯ 3.6%. Differences in vegetation and soils between grazed and ungrazed sites were minimal in most cases, but soil characteristics and elevation were strongly correlated with native and exotic plant diversity in the study region. For the 78 1000-m 2 plots, 59.4% of the variance in total species richness was explained by percentage of silt (coefficient ϭ 0.647, t ϭ 5.107, P Ͻ 0.001), elevation (coefficient ϭ 0.012, t ϭ 5.084, P Ͻ 0.001), and total foliar cover (coefficient ϭ 0.110, t ϭ 2.104, P Ͻ 0.039). Only 12.8% of the variance in exotic species cover (log 10 cover) was explained by percentage of clay (coefficient ϭ Ϫ0.011, t ϭ Ϫ2.878, P Ͻ 0.005), native species richness (coefficient ϭ Ϫ0.011, t ϭ Ϫ2.156, P Ͻ 0.034), and log 10 N (coefficient ϭ 2.827, t ϭ 1.860, P Ͻ 0.067). Native species cover and exotic species richness and frequency were also significantly positively correlated with percentage of soi...

No abstract

Frankia strains are nitrogen-fixing soil actinobacteria that can form root symbioses with actinorhizal plants. Phylogenetically, symbiotic frankiae can be divided into three clusters, and this division also corresponds to host specificity groups. The strains of cluster II which form symbioses with actinorhizal Rosales and Cucurbitales, thus displaying a broad host range, show suprisingly low genetic diversity and to date can not be cultured. The genome of the first representative of this cluster, Candidatus Frankia datiscae Dg1 (Dg1), a microsymbiont of Datisca glomerata, was recently sequenced. A phylogenetic analysis of 50 different housekeeping genes of Dg1 and three published Frankia genomes showed that cluster II is basal among the symbiotic Frankia clusters. Detailed analysis showed that nodules of D. glomerata, independent of the origin of the inoculum, contain several closely related cluster II Frankia operational taxonomic units. Actinorhizal plants and legumes both belong to the nitrogen-fixing plant clade, and bacterial signaling in both groups involves the common symbiotic pathway also used by arbuscular mycorrhizal fungi. However, so far, no molecules resembling rhizobial Nod factors could be isolated from Frankia cultures. Alone among Frankia genomes available to date, the genome of Dg1 contains the canonical nod genes nodA, nodB and nodC known from rhizobia, and these genes are arranged in two operons which are expressed in D. glomerata nodules. Furthermore, Frankia Dg1 nodC was able to partially complement a Rhizobium leguminosarum A34 nodC::Tn5 mutant. Phylogenetic analysis showed that Dg1 Nod proteins are positioned at the root of both α- and β-rhizobial NodABC proteins. NodA-like acyl transferases were found across the phylum Actinobacteria, but among Proteobacteria only in nodulators. Taken together, our evidence indicates an Actinobacterial origin of rhizobial Nod factors.