Impact of sudden oak death on tree mortality in the Big Sur ecoregion of California

The Big Sur ecoregion in coastal California is a botanically and ecologically diverse area that has recently experienced substantial mortality of oak (Quercus spp.) and tanoak (Lithocarpus densiflorus) trees due to the emerging forest disease sudden oak death, caused by the invasive pathogen Phytophthora ramorum. In response to the urgent need to examine environmental impacts and create management response strategies, we quantified the impact of P. ramorum invasion on tree mortality across the Big Sur ecoregion using high-resolution aircraft imagery and field data. Using the imagery, we mapped all detectable oak and tanoak trees possibly killed by P. ramorum infection within redwood-tanoak forests and mixed oak woodlands. To validate and improve our remote assessment, we quantified the number, size, and infection status of host trees in 77 field plots (0.25 ha). The field data showed that our remote assessment underestimated mortality due to the occurrence of dead trees in the forest understory. For each forest type, we developed regression models that adjusted our remote assessments of tree mortality in relation to field observations of mortality and local habitat variables. The models significantly improved remote assessment of oak mortality, but relationships were stronger for mixed oak woodlands (r ² = 0.77) than redwood-tanoak forests (r ² = 0.66). Using the field data, we also modeled the amount of dead tree basal area (m²) in relation to the density of mapped dead trees in mixed oak woodlands (r ² = 0.73) and redwood-tanoak forests (r ² = 0.54). Application of the regression models in a GIS estimated 235,678 standing dead trees in 2005 and 12,650 m² of tree basal area removed from the ecoregion, with 63% of mortality occurring in redwood-tanoak forests and 37% in mixed oak woodlands. Integration of the remote assessment with population estimates of host abundance, obtained from an independent network of 175 field plots (0.05 ha each), indicated similar prevalence of mortality in redwood-tanoak forests (20.0%) and mixed oak woodlands (20.5%) at this time. This is the first study to quantify a realistic number of dead trees impacted by P. ramorum over a defined ecological region. Ecosystem impacts of such widespread mortality will likely be significant.

Evaluation of several commercial biocontrol products on European and North American populations of Phytophthora ramorum

Five commercially available biological control products were tested in vitro with seven isolates of Phytophthora ramorum from North American (NA1, NA2), and European (EU1) populations. The in vitro tests included dual culture methods and detached leaf assays on wounded Rhododendron and Camellia leaves. Variability in response to biocontrol agents among isolates of P. ramorum from North American and European populations was examined. In dual culture tests, both Bacillus subtilis products (Companion® and Serenade®) resulted in better inhibition of the NA1 group than NA2 and EU1. Actinovate® (Streptomyces lydicus) was the least effective of the three bacterial biocontrol agents and there was no difference in percent inhibition among P. ramorum lineages. Two products containing Trichoderma spp. were tested: Plant Helper® (T. atroviride) caused 100% inhibition of all lineages of P. ramorum, while SoilGard? (T. virens) was only about 30% effective. There was great variability among P. ramorum isolates in their response to biocontrol agents. All treatments reduced P. ramorum lesion size on both Rhododendron and Camellia. Combined treatments of Actinovate® with one other BCA did not perform as well as either treatment used individually. Best results were obtained with Serenade® on Rhododendron and Camellia foliage, especially against the NA1 group. Lack of a linear relationship between percent inhibition of P. ramorum by BCAs in vitro and foliar treatments on detached Rhododendron and Camellia leaves indicates that in vitro testing is a poor predictor of BCA performance on plant material.     

Ancient isolation and independent evolution of the three clonal lineages of the exotic sudden oak death pathogen Phytophthora ramorum

The genus Phytophthora includes some of the most destructive plant pathogens affecting agricultural and native ecosystems and is responsible for a number of recent emerging and re-emerging infectious diseases of plants. Sudden oak death, caused by the exotic pathogen P. ramorum , has caused extensive mortality of oaks and tanoaks in Northern California, and has brought economic losses to US and European nurseries as well due to its infection of common ornamental plants. In its known range, P. ramorum occurs as three distinct clonal lineages. We inferred the evolutionary history of P. ramorum from nuclear sequence data using coalescent-based approaches. We found that the three lineages have been diverging for at least 11% of their history, an evolutionarily significant amount of time estimated to be on the order of 165 000 to 500 000 years. There was also strong evidence for historical recombination between the lineages, indicating that the ancestors of the P. ramorum lineages were members of a sexually reproducing population. Due to this recombination, the ages of the lineages varied within and between loci, but coalescent analyses suggested that the European lineage may be older than the North American lineages. The divergence of the three clonal lineages of P. ramorum supports a scenario in which the three lineages originated from different geographic locations that were sufficiently isolated from each other to allow independent evolution prior to introduction to North America and Europe. It is thus probable that the emergence of P. ramorum in North America and Europe was the result of three independent migration events.     

Investigating the distribution of prairie dogs in an urban landscape

Habitat fragmentation is a prevalent threat to biological diversity, and urbanization is a primary agent of fragmentation and a leading cause of species endangerment. Landscape biogeographic and local habitat characteristics can be important determinants of the distribution of species in habitat patches in urban landscapes. However, the specifics of which characteristics are most critical to maintaining biological diversity are not fully known for prairie ecosystems, especially in fragmented urban habitat. This study focuses on black-tailed prairie dogs along an urban gradient in Denver, CO. Prairie dogs have declined precipitously throughout the region and are an essential part of the prairie ecosystem, making them excellent study subjects. We identified a series of habitat fragments along a gradient of urbanization in the fully urbanized areas and south suburbs of Denver, CO, both containing and not containing prairie dogs. Local characteristics, including fragment slope and vegetative cover, and landscape characteristics, including fragment size, age and connectivity, were measured on each fragment. We used likelihood-based methods to explore which variables most accurately predicted prairie dog occurrence within our study area. Multiple factors influenced the distribution of prairie dogs in urban settings, with colony connectivity the strongest predictor of occupancy. Large and recently isolated fragments near other prairie dog colonies, flat areas and those with high graminoid cover were most likely to support prairie dog populations. Our study provides the first attempt to model prairie dog occurrence in highly fragmented urban habitat and has important implications for the management and conservation of prairie dogs.     

Effects of environment,habitat configuration and forest continuity on the distribution of forest plant species

In present day European landscapes many forest plant species are restricted to isolated remnants of a formerly more or less continuous forest cover. The two major objectives of this study were (1) to determine the relative importance of habitat quality (mainly in terms of soil parameters), habitat configuration (patch size and isolation) and habitat continuity for the distribution of herbaceous forest plant species in a highly fragmented landscape and (2) to examine if groups of species with different habitat requirements are affected differently. Deciduous forest patches in northwestern Germany were surveyed for the presence of a large set of forest species. For each patch, habitat quality, configuration and continuity were determined. Data were analysed by Redundancy Analysis with variation partitioning for effects on total species composition and multivariate logistic regression for effects on individual species, for two different data sets (base‐rich and base‐poor forest patches). Overall, we found strong effects of habitat quality (particularly of soil pH, water content and topographic heterogeneity in the base‐rich forest patches; and of calcium content and disturbance in the base‐poor patches), but only relatively weak effects of habitat configuration and habitat continuity. However, a number of species were positively affected by patch area and negatively affected by patch isolation. Furthermore, the relative importance of habitat configuration tended to be higher for species predominantly growing in closed forests compared to species occurring both in the forest and in the open landscape.     

Recent range expansion and agricultural landscape heterogeneity have only minimal effect on the spatial genetic structure of the plant pathogenic fungus Mycosphaerella fijiensis

Understanding how geographical and environmental features affect genetic variation atboth the population and individual levels is crucial in biology, especially in the case ofpathogens. However, distinguishing between these factors and the effects of historicalrange expansion on spatial genetic structure remains challenging. In the present study, weinvestigated the case of Mycosphaerella fijiensis—a plant pathogenic fungusthat has recently colonized an agricultural landscape characterized by the presence ofpotential barriers to gene flow, including several commercial plantations in which diseasecontrol practises such as the use of fungicides are applied frequently, and low hostdensity areas. We first genotyped 300 isolates sampled at a global scale on untreatedplants in two dimensions over a 50 × 80-km area. Using two different clusteringalgorithms, no genetic structure was detected in the studied area, suggesting expansion oflarge populations and/or no influence of potential barriers. Second, we investigatedthe potential effect of disease control practises on M. fijiensis diversity bycomparing populations sampled in commercial vs food-crop plantations. At this local scale,we detected significantly higher allelic richness inside commercial plantations comparedwith the surrounding food-crop plantation populations. Analysis of molecular varianceindicated that 99% of the total genetic variance occurred within populations. Wediscuss the suggestion that high population size and/or high migration rate betweenpopulations might be responsible for the absence of any effect of disease controlpractises on genetic diversity and differentiation.