Quercus Agrifolia Research Articles

Balancing detection and eradication for control of epidemics: sudden oak death in mixed-species stands.

Culling of infected individuals is a widely used measure for the control of several plant and animal pathogens but culling first requires detection of often cryptically-infected hosts. In this paper, we address the problem of how to allocate resources between detection and culling when the budget for disease management is limited. The results are generic but we motivate the problem for the control of a botanical epidemic in a natural ecosystem: sudden oak death in mixed evergreen forests in coastal California, in which species composition is generally dominated by a spreader species (bay laurel) and a second host species (coast live oak) that is an epidemiological dead-end in that it does not transmit infection but which is frequently a target for preservation. Using a combination of an epidemiological model for two host species with a common pathogen together with optimal control theory we address the problem of how to balance the allocation of resources for detection and epidemic control in order to preserve both host species in the ecosystem. Contrary to simple expectations our results show that an intermediate level of detection is optimal. Low levels of detection, characteristic of low effort expended on searching and detection of diseased trees, and high detection levels, exemplified by the deployment of large amounts of resources to identify diseased trees, fail to bring the epidemic under control. Importantly, we show that a slight change in the balance between the resources allocated to detection and those allocated to control may lead to drastic inefficiencies in control strategies. The results hold when quarantine is introduced to reduce the ingress of infected material into the region of interest.

Hilda Raz : A Celebration

Hilda Raz:A Celebration Mari L'Esperance (bio) My first book, The Darkened Temple (2008, University of Nebraska Press) owes much to Hilda Raz, who selected it for the 2007 Prairie Schooner Book Prize in Poetry from among hundreds of worthy contestants. When I recently learned during a visit to Lincoln that Hilda plans to retire from academic life at the end of the year, after twenty-two years at the helm of Prairie Schooner as editor in chief, I couldn't at first quite fathom the news. How could this be, I thought in mild alarm? How on earth will Prairie Schooner possibly continue without Hilda when, after all, Prairie Schooner IS Hilda, or so it has come to seem? And who could possibly fill her infinitely able editorial shoes? The idea felt impossible, unreal, and still does, even as I write this. For it is difficult to imagine any but Hilda's keen eye and ear, sharp mind, and generous heart shaping each issue of Prairie Schooner, poem by poem, story by story, "hand over hungry hand" (to quote the late Lucille Clifton). [End Page 13] Until recently I'd mostly thought of my relationship with Hilda as beginning on June 28, 2007, when I received a voicemail message from her informing me that she was "very excited" to speak with me directly. With a mixture of hope, terror, and denial, I dared to presume that she was calling to tell me that my manuscript had managed to place in the 2007 Prairie Schooner Book Prize competition. When I finally returned her call later that morning (as my husband and I drove north along Highway 101 from San Luis Obispo to our home in the Bay Area, past golden hills dotted with coast live oaks), I was enthusiastically informed by Hilda that she had selected my manuscript for the 2007 book prize in poetry and that she hoped I was as happy at the news as she was. I must sheepishly admit now that my response was lackluster at best, the unfortunate and untimely result of dumb shock, disbelief, and plain old nerves. I imagine that Hilda must have wondered to herself why I did not scream my head off with unbounded surprise and elation, as does every good book prize winner worth her salt? Alas, I did no such thing. In any case, Hilda's initial impression of me beyond my manuscript could not have been stellar. But I emerged from shell shock and thus ensued several phone conversations and e-mails in which Hilda gave me sharp and insightful editorial suggestions for revising some of the poems in my manuscript, many of which I incorporated, many of which I didn't. This collaborative phase in the manuscript's life, before production began, felt simultaneously special and intensely focused, in a good way. I was keenly aware of the significance of our work together and impressed by Hilda's empathy and expert editorial skill, how intimately she seemed to know my poems, and her willingness to permit me the ultimate creative say in the name of the poem, something which I appreciate to this day and will hold as an editorial standard in the future. As a first-book poet, I feel fortunate to have had Hilda guide and support me through what can often be a challenging and intimidating process: transforming a manuscript into a publishable book. It is a process that I will never take for granted. What escaped my awareness at the time of Hilda's first phone call to me was that my relationship with her had actually begun several weeks earlier, when my manuscript first crossed her desk (a magical event in itself) and she began the process of reading and responding to it, eventually selecting it for the book prize. During these weeks, I was to learn later from her, Hilda had lived and breathed my book, spent hours reading and rereading the manuscript, [End Page 14] considered the book's breadth and depth and each individual poem carefully and thoughtfully. As a poet sending my manuscript to contests for the first time, it didn't once occur to me that...

Bird rookeries have different effects on different feeding guilds of herbivores and alter the feeding behavior of a common caterpillar

Aggregations of nesting birds are common in many landscapes and have a broad impact on their surrounding habitat through the nutrient input and disturbance of their guano depositions. Ecological theory makes specific predictions about how disturbances or nutrient pulses will affect the interactions between different trophic levels of organisms. This study dissects the effects of a multi-species bird rookery on plant-herbivore interactions on coast live oak trees (Quercus agrifolia). I found that different feeding guilds of herbivores were affected in opposing directions by the rookery. I observed less chewing damage within the rookery than outside, but more damage from piercing/sucking and galling herbivores within the rookery than outside. To understand why chewing guild herbivores are negatively affected by the environmental impacts of the rookery, I explored the behavioral response of a specialist caterpillar (Phryganidia californica) to various rookery conditions. Bird guano had a direct negative impact on P. californica foraging. P. californica movement was impaired by guano addition to twigs, and caterpillars preferred to eat leaves without guano on them. The rookery also had an indirect negative effect on P. californica foraging, as P. californica preferred clean leaves from forest areas outside of the rookery over clean leaves from within the rookery. This study suggests that while it may be possible to make accurate predictions about the effect of large events (such as rookery formation) on different trophic levels, understanding the response of individual species within that trophic level requires an understanding of aspects of their natural history—such as feeding mode and behavior.

Survival rates of immature Ixodes pacificus (Acari: Ixodidae) ticks estimated using field-placed enclosures

Survival rates of immature Ixodes pacificus (Acari: Ixodidae) ticks estimated using field-placed enclosures

Influence of acorn woodpecker social behaviour on transport of coast live oak (Quercus agrifolia) acorns in a southern California oak savanna

Summary 1. Many plant species depend upon animals for seed dispersal, yet animals disperse seeds in pursuit of their own social and behavioural agendas. Animal social behaviour affects where and how they forage, so it must also shape patterns of seed dispersal. 2. At Sedgwick Reserve, California, USA, we established a study population of Quercus agrifolia to determine patterns of acorn foraging by the acorn woodpecker (Melanerpes formicivorous). This cooperative breeder lives in social groups that defend territories surrounding arboreal seed caches (granaries), foraging communally within these territories. 3. We genotyped pericarp tissue of 568 acorns, as well as 285 adult Q. agrifolia trees, including all adults within 150 m of 16 focal granaries. After quantifying genotyping error, we identified a genetically reliable subset of 524 acorns. We assigned a source tree to each acorn and estimated the number of seed sources per granary and seed source sharing among granaries. 4. We found one to eight distinct seed‐source genotypes per granary, and an effective source diversity ranging from 1.0 to 6.6 seed sources. Of all transport events, 96.5% involve source trees within 150 m of the granaries. For one granary, all sampled acorns were transported from five trees located more than 1.3 km away, with all source trees within 90 m of each other. No measure of seed‐source diversity was associated with density of potential seed sources, and the pattern of acorn movement fits three traditional dispersal curves poorly. 5. Woodpecker groups rarely collected acorns from overlapping sets of maternal sources. Some pairs of neighbouring granaries shared maternal sources, and we identify those that were probably maintained by the same woodpecker group. 6. Synthesis. Territoriality of woodpecker groups restricts both the spatial area of foraging and the sharing of seed sources. This foraging behaviour limits distances and directions of acorn transport from oaks located within woodpecker territories. Dispersal agents with this type of social structure will create a high degree of local genetic structure. Extreme behavioural variations may result in anomalous long‐distance dispersal events that increase genetic connectivity, but are likely to do so in an episodic and erratic fashion.

Responses of oaks and tanoaks to the sudden oak death pathogen after 8 y of monitoring in two coastal California forests

Sudden oak death, caused by Phytophthora ramorum, is widely established in mesic forests of coastal central and northern California. In 2000, we placed 18 plots in two Marin County sites to monitor disease progression in coast live oaks ( Quercus agrifolia), California black oaks ( Q. kelloggii), and tanoaks ( Lithocarpus densiflorus), the species that are most consistently killed by the pathogen in these areas. Through early 2008, the numbers of newly infected trees increased for all species. The infection rate for trees that were asymptomatic in 2000 was 5.0% y −1 for coast live oaks, 4.1% y −1 for black oaks and 10.0% y −1 for tanoaks. Mortality rates were 3.1% y −1 for coast live oaks, 2.4% y −1 for black oaks, and 5.4% y −1 for tanoaks. Mortality not attributed to P. ramorum was 0.54% y −1 for coast live oaks, and 0.75% y −1 for tanoaks. Weibull survival models of trees that were asymptomatic in 2000 provided overall median survival times of 13.7 y for coast live oaks, 13.8 y for black oaks, and 8.8 y for tanoaks. Survival of infected (bleeding) trees declined to 9.7 y for coast live oaks, 6.2 y for black oaks, and 5.8 y for tanoaks. Ambrosia beetle attacks on bleeding trees further reduced modeled survival times by 65–80%, reaffirming the earlier finding that beetle attacks on bleeding cankers considerably reduce survival. Across all plots, the modeled time for 90% of trees that were asymptomatic in 2000 to become infected is 36.5 y for coast live oaks and 15.4 y for tanoaks. There was a trend toward higher infection rates as tree diameter increased. Greater than 90% of living coast live oaks that failed during the study had extensive beetle tunneling at the site of the break. Disease intensity in coast live oaks at the plot level was positively associated with bay laurel ( Umbellularia californica) basal area and negatively associated with Pacific madrone ( Arbutus menziesii) basal area. This study demonstrates the use of survival modeling to characterize the effects of epidemic disease on different species and to project the future of forests infected with tree pathogens.

Similar breakdown rates and benthic macroinvertebrate assemblages on native and Eucalyptus globulus leaf litter in Californian streams

Summary1. Eucalyptus globulus, a tree species planted worldwide in many riparian zones, has been reported to affect benthic macroinvertebrates negatively. Although there is no consensus about the effects of Eucalyptus on aquatic macrobenthos, its removal is sometimes proposed as a means of ecological restoration.2. We combined the sampling of macroinvertebrates with measurement of the colonisation of leaf packs in mesh bags, to examine the effects of riparian Eucalyptus and its litter on benthic macroinvertebrates in three small streams in California, U.S.A. Each stream included one reach bordered by Eucalyptus (E‐site) and a second bordered by native vegetation (N‐site).3. The macrobenthos was sampled and two sets of litter bags were deployed at each site: one set with Eucalyptus litter (Euc‐bags) and one with mixed native tree litter (Nat‐bags) containing Quercus, Umbellularia, Acer and Alnus. Bags were exposed for 28, 56 and 90 days and this experiment was repeated in the autumn, winter and spring to account for effects of changing stream flow and insect phenology.4. Litter input (average dry mass: 950 g m−2 year−1 in E‐sites versus 669 g m−2 year−1 in N‐sites) was similar, although in‐stream litter composition differed between E‐ and N‐sites. Litter broke down at similar rates in Euc‐bags and Nat‐bags (0.0193 day−1 versus 0.0134 day−1), perhaps reflecting the refractory nature of some of the leaves of the native trees (Quercus agrifolia).5. Summary metrics for macroinvertebrates (taxon richness, Shannon diversity, pollution tolerance index) did not differ significantly between the E and N sites, or between Euc‐bags and Nat‐bags. No effect of exposure time or site was detected by ordination of the taxa sampled. However, distinct seasonal ordination clusters were observed in winter, spring and autumn, and one of the three streams formed a separate cluster.6. The presence of Eucalyptus was less important in explaining the taxonomic composition of the macrobenthos than either ‘season’ or ‘stream’. Similarly, these same two factors (but not litter species) also helped explain the variation in leaf breakdown. We conclude that patches of riparian Eucalyptus and its litter have little effect on stream macrobenthos in this region.

Beyond the tropics: forest structure in a temperate forest mapped plot

Question: How do the diversity, size structure, and spatial pattern of woody species in a temperate (Mediterranean climate) forest compare to temperate and tropical forests? Location: Mixed evergreen coastal forest in the Santa Cruz Mountains, California, USA. Methods: We mapped, tagged, identified, and measured all woody stems (� 1 cm diameter) in a 6-ha forest plot, following Center for Tropical Forest Science protocols. We compared patterns to those found in 14 tropical and 12 temperate forest plots. Results: The forest is dominated by Douglas-fir (Pseudotsuga menziesii) and three species of Fagaceae (Quercus agrifolia, Q. parvula var. shrevei ,a ndLithocarpus densiflorus), and includes 31 woody species and 8180 individuals. Much of the diversity was in smalldiameter shrubs, treelets, and vines that have not been included in most other temperate forest plots because stems o5-cm diameter had been excluded from study. The density of woody stems (1363 stems ha � 1 )w as lower than that in all but one tropical plot. The density of large trees (diameter � 30 cm) and basal area were higher than in any tropical plot. Stem density and basal area were similar to most other temperate plots, but were less than in low-diversity conifer forests. Rare species were strongly aggregated, with the degree of aggregation decreasing with abundance so that the most common species were significantly more regular than random. Conclusions: The patterns raise questions about differences in structure and dynamics between tropical and temperate forests; these need to be confirmed with additional temperate zone mapped plots that include small-diameter individuals.

A comparative study of oak (Quercus, Fagaceae) seedling physiology during summer drought in southern California

Natural recruitment of oaks appears to be declining throughout the northern hemisphere. Summer drought poses a potentially important barrier to oak recruitment in southern California. To evaluate this barrier, we grew evergreen Quercus agrifolia and deciduous Q. lobata from seeds near parental trees. We measured water relations, chlorophyll fluorescence, and gas exchange during these seedlings' fourth and fifth summers and compared them to neighboring adults. Most seedlings had substantially lower values for predawn xylem pressure potential (Ψ(pd)), minimum photosystem II (PSII) quantum efficiency (Φ(PSIIMIN)), maximum quantum efficiency for PSII under dark-adapted leaf conditions (Fv/Fm), and maximum photosynthetic assimilation (Amax), and higher values for maximum nonphotochemical quenching (NPQmax) than did conspecific adults. The high, unvarying Ψ(pd) values of the adults suggest they use perennially available groundwater. Quercus lobata seedlings commonly had lower values for Ψ(pd) than did Q. agrifolia, and values for Ψ(pd) and Φ(PSIIMIN) were significantly related to size in Q. lobata but not in Q. agrifolia. These data suggest important interspecific differences in root architecture. Lower values for Φ(PSIIMIN), Fv/Fm, and higher NPQmax in Q. agrifolia indicate that Q. agrifolia seedlings were usually under more stress than Q. lobata, which typically had higher Amax rates than did Q. agrifolia seedlings. Diurnal photosynthesis curves were quite flat for Q. agrifolia, but they peaked in the morning for Q. lobata. Established seedlings appeared to be under more stress than adults, but this stress did not appear severe enough to cause death. Access to perennially available groundwater may be crucial for the seedling to sapling transition.

Topographic position modulates the mycorrhizal response of oak trees to interannual rainfall variability

California coast live oak (Quercus agrifolia) forms tripartite symbiotic associations with arbuscular (AMF) and ectomycorrhizal (EMF) fungi. We selected oak individuals differing in topographic position and depth to groundwater (mesic valley vs. xeric hill sites) to investigate changes of tree mycorrhizal status in response to interannual rainfall variability. EMF root colonization, as well as hyphal abundance and viability in upper rhizosphere soil (0-30 cm), were negatively affected by severe multi-year drought, although not to the same extent in each topographic location. Oak trees growing in hill sites showed EMF colonization levels <1% in upper roots during drought. By contrast, oaks in valley sites maintained much higher EMF colonization (>19%) in upper roots during drought. EMF root colonization increased sharply at both topographic positions during the ensuing wet year (78% in valley, 49% in hill), which indicates that the mycorrhizal status of roots in upper rhizosphere soil is highly responsive to interannual rainfall variability. Across sites and years, percentage EMF colonization and soil hyphal density and viability were strongly positively correlated with soil moisture potential, but percentage AMF root colonization was not. Interestingly, changes in percentage EMF root colonization and density of viable hyphae between a wet and a dry year were proportionally much greater in xeric hill sites than in mesic valley sites. The mycorrhizal status of oak trees was particularly responsive to changes in soil moisture at the hill sites, where roots in upper rhizosphere soil shifted from almost exclusively AMF during severe drought to predominantly EMF during the ensuing wet year. By contrast, the mycorrhizal status of oaks in the valley sites was less strongly coupled to current meteorological conditions, as roots in upper soil layers remained predominantly EMF during both a dry and a wet year. Canopy shading and hydraulic lift by oaks in valley sites likely contributed to maintain the integrity and viability of EMF roots and extraradical hyphae in upper rhizosphere soil during extended drought. Our results suggest that oak woodlands in water-limited ecosystems may become increasingly reliant on the AMF symbiosis under future climate change scenarios for the U.S. southwest and other world regions.

Efficacy of phosphonic acid, metalaxyl‐M and copper hydroxide against Phytophthora ramorum in vitro and in planta

The ability of metalaxyl‐M, phosphonic acid in the form of phosphonate, and copper hydroxide to inhibit different stages in the life cycle of Phytophthora ramorum, the causal agent of sudden oak death (SOD), was tested in vitro using 12 isolates from the North American forest lineage. In addition, experiments were conducted in planta to study the ability of phosphonic acid injections and metalaxyl‐M drenches to control pathogen growth on saplings of California coast live oak (Quercus agrifolia), and of copper hydroxide foliar sprays to control infection of California bay laurel (Umbellularia californica) leaves. Phytophthora ramorum was only moderately sensitive to phosphonic acid in vitro, but was highly sensitive to copper hydroxide. In planta experiments indicated the broad efficacy of phosphonic acid injections and of copper hydroxide sprays in preventing growth of P. ramorum in oaks and bay laurels, respectively. Finally, although metalaxyl‐M was effective in vitro, drenches of potted oak trees using this active ingredient were largely ineffective in reducing the growth rate of the pathogen in planta.

Forest stand dynamics and sudden oak death: Mortality in mixed-evergreen forests dominated by coast live oak

Sudden oak death (SOD), caused by the recently discovered non-native invasive pathogen, Phytophthora ramorum, has already killed tens of thousands of native coast live oak and tanoak trees in California. Little is known of potential short and long term impacts of this novel plant–pathogen interaction on forest structure and composition. Coast live oak ( Quercus agrifolia) and bay laurel ( Umbellularia californica) form mixed-evergreen forests along the northern California coast. This study measured tree mortality over a gradient of disease in three time periods. Direct measurements of current mortality were taken during 2004, representing a point-in-time estimate of present and ongoing mortality. Past stand conditions, c. 1994, were estimated using a stand reconstruction technique. Future stand conditions, c. 2014, were calculated by assuming that, given a lack of host resistance, live trees showing signs of the disease in 2004 would die. Results indicate that coast live oaks died at a rate of 4.4–5.5% year −1 between 1994 and 2004 in highly impacted sites, compared with a background rate of 0.49% year −1, a ten-fold increase in mortality. From 2004 to 2014, mortality rates in the same sites were 0.8–2.6% year −1. Over the entire period, in highly impacted sites, a 59–70% loss of coast live oak basal area was predicted, and coast live oak decreased from 60% to 40% of total stand basal area, while bay laurel increased from 22% to 37%. Future stand structures will likely have greater proportions of bay laurel relative to coast live oak.

Phytophthora ramorum: a pathogen with a remarkably wide host range causing sudden oak death on oaks and ramorum blight on woody ornamentals

Phytophthora ramorum is an oomycete plant pathogen classified in the kingdom Stramenopila. P. ramorum is the causal agent of sudden oak death on coast live oak and tanoak as well as ramorum blight on woody ornamental and forest understorey plants. It causes stem cankers on trees, and leaf blight or stem dieback on ornamentals and understorey forest species. This pathogen is managed in the USA and Europe by eradication where feasible, by containment elsewhere and by quarantine in many parts of the world. Genomic resources provide information on genes of interest to disease management and have improved tremendously since sequencing the genome in 2004. This review provides a current overview of the pathogenicity, population genetics, evolution and genomics of P. ramorum. Phytophthora ramorum (Werres, De Cock & Man in't Veld): kingdom Stramenopila; phylum Oomycota; class Peronosporomycetidae; order Pythiales; family Pythiaceae; genus Phytophthora. The host range is very large and the list of known hosts continues to expand at the time of writing. Coast live oak and tanoak are ecologically, economically and culturally important forest hosts in the USA. Rhododendron, Viburnum, Pieris, Syringa and Camellia are key ornamental hosts on which P. ramorum has been found repeatedly, some of which have been involved in moving the pathogen via nursery shipments. Disease symptoms: P. ramorum causes two different diseases with differing symptoms: sudden oak death (bleeding lesions, stem cankers) on oaks and ramorum blight (twig dieback and/or foliar lesions) on tree and woody ornamental hosts. http://nature.berkeley.edu/comtf/, http://rapra.csl.gov.uk/, http://www.aphis.usda.gov/plant_health/plant_pest_info/pram/index.shtml, http://genome.jgi-psf.org/Phyra1_1/Phyra1_1.home.html, http://pamgo.vbi.vt.edu/, http://pmgn.vbi.vt.edu/, http://vmd.vbi.vt.edu./, http://web.science.oregonstate.edu/bpp/labs/grunwald/resources.htm, http://www.defra.gov.uk/planth/pramorum.htm, http://www.invasive.org/browse/subject.cfm?sub=4603, http://www.forestry.gov.uk/forestry/WCAS-4Z5JLL.

Attraction of ambrosia and bark beetles to coast live oaks infected by Phytophthora ramorum

Abstract1 Sudden oak death is caused by the apparently introduced oomycete, Phytophthora ramorum. We investigated the role of bark and ambrosia beetles in disease progression in coast live oaks Quercus agrifolia.2 In two Marin County, California sites, 80 trees were inoculated in July 2002 with P. ramorum and 40 were wounded without inoculation. Half of the trees in each group were sprayed with the insecticide permethrin [cyclopropanecarboxylic acid, 3‐(2,2‐dichloroethenyl)‐2,2‐dimethyl‐(3‐phenoxyphenyl) methyl ester] to prevent ambrosia and bark beetle attacks, and then were sprayed twice per year thereafter. After each treatment, sticky traps were placed on only the permethrin‐treated trees. Beetles were collected periodically in 2003.3 Inoculated trees accounted for 95% of all beetles trapped. The ambrosia beetles Monarthrum scutellare and Xyleborinus saxeseni and the western oak bark beetle Pseudopityophthorus pubipennis were the most abundant of the seven species trapped.4 Permethrin treatment delayed initiation of beetle attacks and significantly reduced the mean number of attacks per tree. Beetles did not attack any wounded or noncankered inoculated trees.5 Trees with larger cankers trapped more beetles early in the disease. Once permethrin lost effectiveness, the number of beetle entrance tunnels was a more reliable predictor of subsequent trap catch than was canker size.6 Beetles were initially attracted to P. ramorum cankers in response to kairomones generated in the host‐pathogen interaction. After beetles attacked the permethrin‐treated trees, aggregation pheromones most probably were the principal factor in beetle colonization behaviour.

Spatial pattern dynamics of oak mortality and associated disease symptoms in a California hardwood forest affected by sudden oak death

Sudden oak death is a disease affecting coastal forests in California and southern Oregon. The spatial pattern of disease dynamics is important for forest and landscape pathology; in this work we investigated the interaction across landscape scales of disease symptomology in coast live oaks, Quercus agrifolia, (trunk bleeding, presence of beetles, and presence of the fungus Hypoxylon thouarsianum) and tree mortality through time. We used two-dimensional spatial analysis tools with data gathered in point-centered-quarter format in 2001 and 2004 to quantify the population density of the disease through time; to examine the spatial pattern of tree mortality across scales through time; and to examine the spatial co-occurrence of disease symptoms with crown mortality through time. Early in the study period dead trees were strongly clustered at smaller scales (~300 m) and after three years this clustering was less pronounced. Bleeding on trees occurred in clusters away from dead trees, particularly in 2004, likely indicating a new cohort of infected trees. The presence of H. thouarsianum was strongly related to overstory mortality through time. Beetle-infested trees co-occurred with mortality in 2001. By 2004, they occurred throughout the forest, and were less strongly correlated with overstory tree mortality, suggesting a future peak of tree mortality.

Geographic distribution, disease symptoms and pathogenicity of Phytophthora nemorosa and Phytophthora pseudosyringae in California, USA

SummaryDuring the course of surveys for Phytophthora ramorum in coastal forests of California and Oregon, P. nemorosa and P. pseudosyringae were frequently isolated from foliage and stems of the same hosts as P. ramorum. Both species ranged from central California to Oregon within 50 km of the Pacific Ocean. Both were also found in the Sierra Nevada Mountains. Phytophthora nemorosa was primarily found infecting trees in coast redwood forests and was most often isolated from bay laurel leaves (Umbellularia californica), bleeding cankers on the main bole of tanoak (Lithocarpus densiflorus), and leaf and small stem tissue of redwood (Sequoia sempervirens). Phytophthora pseudosyringae was primarily isolated from hosts found in coast live oak woodlands. Bay laurel was the most common host while infection of coast live oak (Quercus agrifolia) stems was less frequent. Inoculation studies confirmed the pathogenicity of P. nemorosa and P. pseudosyringae on their most common hosts.

Evidence for the role of synchronicity between host phenology and pathogen activity in the distribution of sudden oak death canker disease.

Variations in synchronicity between colonization rate by the pathogen and host phenology may account for unexplained spatial distribution of canker disease. The hypothesis that synchronous pathogenicity and host development are necessary for incidence of sudden oak death disease was tested by correlating seasonal variations in host cambial phenology and response to inoculation with Phytophthora ramorum. Response to infection was estimated by inoculating branch cuttings from coast live oak (Quercus agrifolia) trees at nine dates through a full annual cycle in 2003-2004. Host phenology was estimated from measurements of bud burst and cambial activity in spring 2006. Lesions were largest in the spring soon after the cambium resumed activity. A moderate genetic component to lesion size was detected. Variation among trees in date of largest lesions correlated with variation in timing of bud burst and cambial phenology. The data support the hypothesis that active host cambial tissue is a necessary requisite for successful infection with the pathogen that causes sudden oak death canker disease. Genetic variation in host phenology will buffer coast live oak against epidemics of this disease.

Multi‐scale patterns of human activity and the incidence of an exotic forest pathogen

Summary Plant pathogens can have major impacts on diverse taxa and ecological systems world‐wide, and some of the most conspicuous of these are invasive non‐native species. Although many factors are known to influence the distribution and abundance of plant pathogens, the extent to which humans play a role is less well‐known but still critical for understanding the dynamics of invasive pathogens in natural systems. One invasive pathogen of great concern world‐wide is Phytophthora ramorum, which causes Sudden Oak Death, an emerging forest disease. Here, we examined the influence of humans and a range of environmental factors on the distribution of P. ramorum at three distinct spatial scales in northern and central California. At the local scale, P. ramorum more commonly occurred in soil on hiking trails used heavily by humans than in soil from adjacent areas off trails. These results support the hypothesis that humans dispersed the pathogen within already infected areas and into areas lacking local sources of inoculum. At the landscape scale, using a network of 202 randomly located plots across a 275 km2 area, we found that forests on public land open to recreation exhibited higher prevalence of disease in a critical infectious host tree (bay laurel, Umbellularia californica), than forests on private lands. Infection levels were also higher in plots surrounded by large amounts of forest with warm climatic conditions and greater potential soil moisture. Although prevalence of diseased canker hosts (Quercus agrifolia, Q. kelloggii and Lithocarpus densiflora) was positively associated with dominance of bay laurel and rainy season temperatures, it was not significantly related to public access, and we hypothesize that this occurred because our study area was in the early stages of infection. At the regional scale, the probability of disease occurrence at 165 sites distributed across the geographic range of P. ramorum in California increased significantly as human population density increased in the surrounding area. Chances of infection also increased significantly with precipitation and presence of bay laurel. Synthesis. Our results suggest that human activity – along with temperature, moisture and host composition – is associated with increased prevalence of an influential exotic forest pathogen. These results indicate that there may be conflicts between humans and disease, and that efforts to address this issue may require aggressive management of human activity.

The relative importance of factors affecting age-specific seedling survival of two co-occurring oak species in southern California

As has been reported in other oak habitats throughout the Northern Hemisphere, natural recruitment rates of young oaks in California are very low for some species and in some regions. The majority of experimental studies that contribute to our understanding of this oak recruitment pattern in California have been relatively short-term, conducted on a small-scale. Thus, while we have valuable information about the array of factors that are able to limit seedling establishment, we know much less about their relative importance, and how they vary across years, sites, or age classes. To investigate the impacts of factors limiting seedling and sapling establishment of valley oak ( Quercus lobata) and coast live oak ( Quercus agrifolia) across a landscape and over time we replicated experiments in four different years in Santa Barbara County, California. Experimental factors manipulated included cattle grazing, and access by deer and small mammals. Plots were distributed across three sites with a cumulative area of 200-ha, and seedling survival was recorded for a minimum of 5 years. We used classification and regression tree analysis (CART) to examine the relative importance of factors influencing survival of planted oaks at different life stages. The relative importance of limiting factors varied among age classes for both species. For initial seedling emergence and survival to 6 months planting year was the most important factor and rodent access was the second most important factor for both Q. lobata and Q. agrifolia. For survival of seedlings through their 1st year rodents, planting year, and site were major limiting factors, though their relative importance varied between the two species. For survival from 1.5 to 5 years, Q. lobata was strongly affected by rodents and site, while Q. agrifolia was mainly affected by site, deer browsing (which reduced survival), and indirect effects of winter–spring cattle grazing (which improved survival). Contrary to our expectations, based on observed patterns of natural recruitment, Q. agrifolia had equivalent or lower survivorship than Q. lobata in all seedling age classes. Overall, in addition to other factors, there were strong year and site effects controlling oak seedling emergence and survival and the relative importance of limiting factors depended on seedling age. Our results suggest that large-scale, long-term experiments encompassing multiple sites will improve our understanding of controls on recruitment in oak woodlands in California and elsewhere.

Spectroscopic determination of health levels of coast live oak (Quercus agrifolia) leaves

Three sets of coast live oak (Quercus agrifolia) leaf samples were collected on three dates: 20 April 2002, 23 July 2002 and 11 September 2001, respectively, for Sudden Oak Death (SOD) monitoring. A total of 330 reflectance spectra (covering 350–2500 nm) were measured in the laboratory with a spectrometer FieldSpec®Pro FR. In this study, the spectroscopic determination of two health levels of the coast live oak leaves was conducted with three sets of spectra. We used two classification algorithms, penalized discriminant analysis (PDA) and cross correlogram spectral matching (CCSM), to discriminate between healthy and infected leaves. PDA is a penalized version of Fisher's linear discriminant analysis (LDA) and can considerably improve upon LDA when it is used for the classification of hyperspectral data. CCSM is practised by calculating the cross correlation at different match positions between a test spectrum and a reference spectrum and is also suitable for processing hyperspectral data. Experimental results indicate that the PDA algorithm has produced approximately 7% higher classification accuracy than that produced by CCSM, although both are very low. When considering the subtle spectral differentiation between the two health levels, the PDA method demonstrates its promise as a classification algorithm. Among the 10 spectral ranges, some higher accuracies are produced by both PDA and CCSM algorithms from those spectral range wavelengths shorter than 1400 nm. Based on our experimental results and previous work, existing remote sensing techniques, including airborne or satellite remote sensing and multispectral or hyperspectral remote sensing, may be insufficient for monitoring and mapping disease-induced moisture stress in trees that have recently been infected. However, this does not preclude the analysis of trees at very advanced stages of disease, and the practicality of finding trees within weeks of dying is considerable.