California Oak Research Articles

California black oak responses to fire severity and native conifer encroachment in the Klamath Mountains

Fire exclusion in forests across the western United States has resulted in widespread vegetation change, often with notable increases in density and cover of fire-sensitive species. In California oak woodlands, encroachment by the more shade-tolerant native Douglas-fir (Pseudotsuga menziesii) is particularly common, and results in the piercing and eventual overtopping of crowns of shade-intolerant trees. We investigated canopy competition, paired tree ages, and post-fire effects in a recently burned, encroached California black oak (Quercus kelloggii) woodland. Pre-fire woodland overstory was heavily dominated by Douglas-fir, which commonly pierced and overtopped California black oak crowns. Trees that pierced overstory oak crowns were younger (mean difference=43.6years, p<0.001) than their paired oaks. Fire effects were variable, leaving some areas unchanged and others with complete above-ground stem mortality. Although above-ground stem-kill was often high for California black oak, the majority (82%) re-sprouted from the root collar. Logistic modeling revealed a significant relationship between California black oak mortality and neighboring Douglas-fir height and plot heat load index. The probability of California black oak mortality increased as neighboring Douglas-fir height increased and decreased with increasing heat load index. Probability of mortality for Douglas-fir decreased with increasing tree size, while the probability of California black oak top-kill was much higher than that for 20–30cmdbh Douglas-fir across a continuous measure of char height. Results indicate that competitive pressure from encroaching trees may compromise California black oak’s ability to survive fire while resilience of encroaching Douglas-fir improves with greater size. Caution should be taken when planning and implementing restoration activities in California black oak woodlands to minimize loss of compromised, remnant oaks while still achieving adequate removal of encroaching conifers.

Tree shelters and weed control enhance growth and survival of natural blue oak seedlings

Blue oak is regenerating poorly in portions of its range. Techniques to artificially regenerate trees by collecting acorns, growing seedlings in a nursery and then planting them are effective but costly. Improving the growth and survival rate of existing volunteer seedlings in woodlands could be more cost efficient and therefore more widely used. We tested tree shelters and weed control treatments over 3 years at six woodland sites to evaluate whether they helped blue oak seedlings grow into saplings. The tree shelters enhanced height growth, and weed control improved survival. Together, these two techniques can improve the chances for managing blue oak sustainably and conserving this native California oak for future generations.

Can the destruction of California’s oak woodlands be prevented? Potential for biological control of the goldspotted oak borer, Agrilus auroguttatus

The goldspotted oak borer (GSOB), Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), is an introduced and aggressive phloem/wood borer infesting native oaks in southern California. Elevated levels of oak mortality have occurred continually for the last nine years on three oak species in San Diego Co., California, USA. Biological control is being assessed as an option for long-term and widespread management of the invasive population of GSOB. Foreign exploration in the native ranges of GSOB and a related sibling species (Agrilus coxalis Waterhouse) was conducted to determine life history information, to assess the natural enemy complex, and to collect specimens for molecular analyses that could help to identify the area of origin of California’s introduced population. Two species of parasitoids, Calosota elongata Gibson (Eupelmidae) and Atanycolus simplex Cresson (Braconidae), were discovered with GSOB populations in Arizona and California. No insect natural enemies were found with populations of A. coxalis in southern Mexico. However, Quercus conzatti Trel. and Quercus peduncularis Nee in Oaxaca and Chiapas, respectively, were recorded as the first known hosts of A. coxalis. A comparative analysis of our understanding of the natural enemy complexes for other pestiferous Agrilus with that of GSOB suggests that more effort should be directed at uncovering potential egg parasitoids and microbial pathogens of GSOB. Analyses of mitochondrial and nuclear ribosomal DNA (rDNA) revealed that the California population of GSOB was more similar to the Arizona population. Specimens of A. coxalis from southern Mexico were confirmed as a separate species. Additional surveys and sampling are needed across the complete native range of the GSOB species complex to develop a comprehensive inventory of parasitoid species that could be considered for use in a classical biological control program in California and to delineate the area of origin of California’s population.

Reactivity of Litter Leachates from California Oak Woodlands in the Formation of Disinfection By‐Products

Litter materials from forested watersheds can be a significant source of dissolved organic matter (DOM) to surface waters that can contribute to the formation of carcinogenic disinfection by-products (DBPs) during drinking-water chlorination. This study characterized the reactivity of DOM from litter leachates of representative vegetation in oak woodlands, a major plant community in the Foothill Region of California. Leachates from fresh and decomposed litter (duff) from two oak species, pine, and annual grasses were collected for an entire rainy season to evaluate their reactivity to form DBPs on chlorination. Relationships among specific ultraviolet absorbance (SΔUVA), fluorescence index (FI), specific differential ultraviolet absorbance (SΔUVA), specific chlorine demand (SCD), and the dissolved organic carbon:dissolved organic nitrogen (DOC:DON) ratio to the specific DBP formation potential (SDBP-FP) were examined. The DOM derived from litter materials had considerable reactivity in forming trihalomethanes (THMs) (1.80-3.49 mmol mol), haloacetic acid (HAAs) (1.62-2.76 mmol mol(-1)), haloacetonitriles (HANs) (0.12-0.37 mmol mol(-1)), and chloral hydrate (CHD) (0.16-0.28 mmol mol). These values are comparable to other identified watershed sources of DBP precursors reported for the California Delta, such as wetlands and organic soils. Vegetation type and litter decomposition stage (fresh litter versus 1-5 yr-old duff) were key factors that determined characteristics of DOM and their reactivity to form DBPs. Pine litter had significantly lower specific THM formation potential compared with oak and grass, and decomposed duff had a greater DON content, which is a precursor of HANs and other nitrogenous DBPs. The SΔUVA and SDBP-FP were temporally variable and dependent on vegetation type, degree of decomposition, and environmental conditions. Among the optical properties of DOM, SΔUVA was the only parameter that was consistently correlated with SDBP-FP.

Collection History and Comparison of the Interactions of the Goldspotted Oak Borer,Agrilus auroguttatusSchaeffer (Coleoptera: Buprestidae), with Host Oaks in Southern California and Southeastern Arizona, U.S.A.

An invasive population of the goldspotted oak borer, Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), is colonizing and killing three species of oaks in San Diego Co., California. However, the interactions of A. auroguttatus with oaks in its native range in southeastern Arizona have not been recorded. We present a complete inventory of the North and Central American collection records of A. auroguttatus and Agrilus coxalis Waterhouse from the literature and from a survey of the holdings of 27 museum and personal collections. We also discuss the relationship between this collection history and the behavior of A. auroguttatus as an intracontinental invasive species. Surveys of native populations of A. auroguttatus in oak forest stands from four mountain ranges in southeastern Arizona revealed injury patterns on Emory oak, Quercus emoryi Torrey, and silverleaf oak, Quercus hypoleucoides A. Camus, similar to those observed on other “red” oaks in California. No damage was observed on “white” oaks in Arizona, and observed only rarely on a white oak, Quercus engelmannii Greene, in California. In Arizona, adult emergence was confirmed from bark removed from Q. emoryi, representing the first developmental record of A. auroguttatus from a native host. Late instars of Agrilus sp. were also recovered from Q. hypoleucoides, but they were not reared to the adult stage for species identification. Nonetheless, our observations of damage and the presence of larvae in the same configuration and location in the outer bark as we would expect for A. auroguttatus suggest that Q. hypoleucoides is also likely a host. Two hymenopteran parasitoids, Calosota elongata Gibson (Eupelmidae) and Atanycolus simplex Cresson (Braconidae), and two likely coleopteran predators (Trogossitidae and Elateridae) emerged from, or were collected in southeastern Arizona from, Q. emoryi bark infested with A. auroguttatus. Based on the museum survey results, the morphological similarity of individuals from the California and Arizona populations, the spatial dynamics of the pattern of infestation in California, the geographic isolation of hosts in California from native populations of the beetle, and the proximity of San Diego Co. to southeastern Arizona, we hypothesize that A. auroguttatus was introduced to California from Arizona or less likely from the Mexican states of Baja California, Chihuahua, or Sonora, and that the introduction most likely occurred on oak firewood. Further, we hypothesize that the oak mortality in southern California is occurring from this intracontinental invasive species because the beetle is filling a vacant niche by colonizing and developing in non-coevolved trees with low host resistance in the absence of a diverse and coevolved insect natural enemy complex.

First Report of Bot Canker Caused by Diplodia corticola on Coast Live Oak (Quercus agrifolia) in California.

Sharp decline and mortality of coast live oak (Quercus agrifolia) has been observed in San Diego County, CA since 2002. Much of this decline has been attributed to a new pest in California, the goldspotted oak borer (GSOB, Agrilus coxalis) (1). Symptoms include crown thinning, bark cracking and/or peeling, patches of stain (1 to 10 cm in diameter), bleeding on the bole, and tree death and are most often observed on trees with a diameter at breast height (DBH) >30 cm. In 2008, a Botryosphaeria sp. was recovered from necrotic tissue of bleeding bole cankers from GSOB-affected trees in Jamul, CA. Zone lines separated dead and live tissue in affected phloem and xylem. Pycnidia were observed on the bark surface of the infected host. Fifty conidia averaging 32 × 18 μm, one-septate with age, and morphologically similar to conidia described by Úrbez-Torres et al. were observed (4). Oak stands with tree mortality were surveyed in GSOB-infested and -uninfested sites over eight locations throughout San Diego and Riverside counties in 2009 and 2010. Symptomatic tissue or conidia from pycnidia of affected trees, plated onto potato dextrose agar amended with 0.01% tetracycline and incubated at 25°C for 1 week, consistently produced cultures with dense, wooly, olive-green mycelium. Mycelia fit the description of Botryosphaeria corticola A.J.L. Phillips, Alves et Luque (anamorph Diplodia corticola) (2). The resulting amplified ITS4/5 region of two sequences matched 100% to published D. corticola sequences (GU799472 and GU799460) (4). These sequences were deposited with NCBI GenBank (HM104176 and HM104177). Koch's postulates were conducted by inoculating 2-mm-diameter holes on five coast live oak trees with D. corticola. Holes were drilled to the cambium at 2 to 4 locations per tree within 1 to 2 m up the bole using a 0.157-cm portable electric drill. Trees ranged from 3.7- to 32.4-cm DBH. Either single agar plugs from two isolates each of a 7-day-old culture (UCR454 and UCR793) or noncolonized agar plugs as uninoculated controls were inserted into the holes and then covered with petroleum jelly and Parafilm. Average temperature was 10°C, relative humidity of 64%, and no precipitation during inoculation. Inoculations were conducted at a location in San Diego County uninfested by GSOB and repeated twice. After 3.5 months, bark was removed from inoculation sites. Average lesion length was not significantly different between inoculations, thus data were combined (one way analysis of variance [ANOVA]; P = 0.05). Lesions averaged 13.9 × 2.3 cm and were significantly different (n = 30; one way ANOVA; P = 0.05) from controls that measured 0.31 × 0.3 cm. Staining was observed around the inoculation points on all trees and three trees exhibited bleeding. Necrotic tissue was observed in the phloem and 3 mm into the xylem tissue, where the lesion had extended up and down the grain. D. corticola was consistently reisolated from necrotic tissue but not from control treatments. B. corticola was originally described as a canker pathogen on Quercus spp. in the western Mediterranean (2), and is known to contribute to the decline of cork oak (Q. suber) in the region (3). To our knowledge, this is the first report of D. corticola causing bot canker on coast live oak in California.

Foraging patterns of acorn woodpeckers (Melanerpes formicivorus) on valley oak (Quercus lobata Née) in two California oak savanna-woodlands

Landscape characteristics and social behavior can affect the foraging patterns of seed-dependent animals. We examine the movement of acorns from valley oak (Quercus lobata) trees to granaries maintained by acorn woodpeckers (Melanerpes formicivorus) in two California oak savanna-woodlands differing in the distribution of Q. lobata within each site. In 2004, we sampled Q. lobata acorns from 16 granaries at Sedgwick Reserve in Santa Barbara County and 18 granaries at Hastings Reserve in Monterey County. Sedgwick has lower site-wide density of Q. lobata than Hastings as well as different frequencies of other Quercus species common to both sites. We found acorn woodpeckers foraged from fewer Q. lobata seed source trees (K g = 4.1 ± 0.5) at Sedgwick than at Hastings (K g = 7.6 ± 0.6) and from fewer effective seed sources (N em* = 2.00 and 5.78, respectively). The differences between sites are due to a greater number of incidental seed sources used per granary at Hastings than at Sedgwick. We also found very low levels of seed source sharing between adjacent granaries, indicating that territoriality is strong at both sites and that each social group forages on its own subset of trees. We discovered an interesting spatial pattern in the location of granaries. At Sedgwick, acorn woodpeckers situated their granaries within areas of higher-than-average tree density, while at Hastings, they placed them within areas of lower-than-average tree density, with the outcome that granaries at the two sites were located in areas of similar valley oak density. Our results illustrate that landscape characteristics might influence the number of trees visited by acorn woodpeckers and the locations of territories, while woodpecker social behavior, such as territoriality, shapes which trees are visited and whether they are shared with other social groups.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-010-1828-5) contains supplementary material, which is available to authorized users.

Trade-offs between vegetative growth and acorn production in Quercus lobata during a mast year: the relevance of crop size and hierarchical level within the canopy.

The concept of trade-offs between reproduction and other fitness traits is a fundamental principle of life history theory. For many plant species, the cost of sexual reproduction affects vegetative growth in years of high seed production through the allocation of resources to reproduction at different hierarchical levels of canopy organization. We have examined these tradeoffs at the shoot and branch level in an endemic California oak, Quercus lobata, during a mast year. To determine whether acorn production caused a reduction in vegetative growth, we studied trees that were high and low acorn producers, respectively. We observed that in both low and high acorn producers, shoots without acorns located adjacent to reproductive shoots showed reduced vegetative growth but that reduced branch-level growth on acorn-bearing branches occurred only in low acorn producers. The availability of local resources, measured as previous year growth, was the main factor determining acorn biomass. These findings show that the costs of reproduction varied among hierarchical levels, suggesting some degree of physiological autonomy of shoots in terms of acorn production. Costs also differed among trees with different acorn crops, suggesting that trees with large acorn crops had more available resources to allocate for growth and acorn production and to compensate for immediate local costs of seed production. These findings provide new insight into the proximate mechanisms for mast-seeding as a reproductive strategy.

Testing the environmental prediction hypothesis for mast-seeding in California oaks

We analyzed 29 years of acorn production by five species of California oaks (genus Quercus ) to test the hypothesis that trees produce large seed crops prior to wet years, conditions facilitating seedling germination and survival. The mean crop of three of the species correlated positively and nontrivially with the following year’s rainfall, but none was statistically significant. Including the acorn crop 1 and 2 years earlier yielded several significant relationships between the acorn crop and future rainfall, but none held up when applied to a second, independent site. Across individuals, acorn production by 7% of trees correlated significantly with subsequent rainfall. Although these trees differed from other trees in several characters, differences were not sufficient to discriminate between trees that correlated significantly with subsequent rainfall from those that did not. We conclude that acorn production by California oaks does not forecast wet years and does not support the environmental prediction hypothesis.

Groundwater uptake by woody vegetation in a semiarid oak savanna

Groundwater can serve as an important resource for woody vegetation in semiarid landscapes, particularly when soil water is functionally depleted and unavailable to plants. This study examines the uptake of groundwater by deciduous blue oak trees (Quercus douglasii) in a California oak savanna. Here we present a suite of direct and indirect methods that demonstrate its occurrence and quantify its rates. The study site is underlain by a thin soil layer and fractured metavolcanic bedrock. Typical depth to groundwater is approximately 8 m. A variety of water storage and flux measurements were collected from 2005 to 2008, including groundwater levels, soil moisture contents, sap flows, and latent heat fluxes. During the dry season, groundwater uptake rates ranged from 4 to 25 mm month−1 and approximately 80% of total ET during June, July, and August came from groundwater. Leaf and soil water potentials supported these results, indicating that groundwater uptake was thermodynamically favorable over soil water uptake for key portions of the growing season. These findings strongly suggest that blue oaks should be considered obligate phreatophytes and that groundwater reserves provide a buffer to rapid changes in their hydroclimate, if these assets are not otherwise depleted by prolonged drought or human consumption. While groundwater uptake may provide for short‐term protection, it should be viewed not as a mechanism for continued plant growth. It allows the woody vegetation to subsist during the summer but not to flourish.

P.2.a.025 Dopamine transporter gene play a risk factor in patient with major depressive disorder?

The western black-legged tick, Ixodes pacificus Cooley and Kohls, commonly bites humans in the far western U.S. In addition to transmitting Lyme borreliosis and anaplasmosis, it is a host of nonpathogenic bacteria as well as some of unknown pathogenicity. In this study, we report the detection, prevalence, and burden of 2 rickettsial phylotypes with unknown pathogenicity in I. pacificus ticks from 6 California counties using real-time quantitative PCR with phylotype-specific primers and probes. Prevalence of rickettsial phylotypes G021 and G022 from 247 I. pacificus ticks was 100% and 2.0%, respectively. The median burden of phylotype G021 was 7.3 per tick cell, whereas the burden of phylotype G022 was 0.8 per tick cell. The burden of phylotype G021 significantly differed between collection sites and between vegetation habitats. Ticks collected from the coastal sage scrub habitat of southern California had a lower burden of phylotype G021 when compared to central California oak woodland, northern California oak woodland, and mixed evergreen and ponderosa pine-oak habitats of northern California. No significant correlation was found between the burden of the phylotype G021 in the presence and absence of the phylotype G022 in I. pacificus, suggesting that the presence of these Rickettsia species do not interfere with each other in I. pacificus.

Determining Soil–Tree–Grass Relationships in a California Oak Savanna Using Eco‐Geophysics

Savanna ecosystems have long fascinated ecologists due to the codominance of trees and grasses. This codominance is often explained by the vertical juxtaposition of rooting depths leading to niche separation for water uptake. The tree–grass spatial patterns observed are often investigated within the competition, disturbance, and resource heterogeneity framework. Ecohydrologic studies are developing models to describe patterns based on this framework and the complex interplay between climate, soil moisture, and resource heterogeneity. A common impediment to unraveling the relative importance of each of these factors is a lack of detailed spatial and temporal data describing resource heterogeneity. In this research, we investigated the use of electromagnetic‐induction (EMI) imaging as a way of determining soil spatial resource heterogeneity. The aim was to determine if EMI signal response, reflecting soil texture and moisture, can provide a rapid method for identifying soil spatial resource heterogeneity, thus providing a novel method to help differentiate between the importance of competition, disturbance, and resource heterogeneity in controlling tree–grass pattern development. We found a significant difference (t = 10.18, P = 1.57 × 10−21) between signal response under grass and trees. Higher bulk soil electrical conductivity (ECa) values occurred under grass (∼32 mS m−1), indicating more clay, while lower ECa values (∼21 mS m−1) occurred under trees where clay content was less. Significant ECa differences were also observed between the oak (Quercus spp.) and buckeye [Aesculus californica (Spach) Nutt.] tree communities, but not between the evergreen and deciduous oak communities. Soil ECa provides a rapid measurement method to identify soil resource heterogeneity in savanna tree–grass systems and could provide a vital tool for woodland restoration planning.

First Report of Diplodia corticola Causing Grapevine (Vitis vinifera) Cankers and Trunk Cankers and Dieback of Canyon Live Oak (Quercus chrysolepis) in California.

The botryosphaeriaceous fungus Diplodia corticola A. J. L. Phillips, Alves & Luque was shown to be the most prevalent canker and dieback pathogen in cork oaks (Quercus suber L.) in the Iberian Peninsula causing a general decline of the trees as a consequence of canker formation in the trunks (1). In addition, D. corticola has been recently reported as a grapevine pathogen causing cankers in the vascular tissue of 1-year-old canes, spurs, and cordons in Texas (3). In 1998, Jacobs and Rehner reported one isolate of D. corticola from oak in California, but no information regarding the oak species from which the isolate was obtained and its virulence were available in the study (2). In 2009, D. corticola was isolated on potato dextrose agar (PDA) amended with 0.01% tetracycline hydrochloride from symptomatic grapevine cordons and on acidified PDA from the trunk of a canyon live oak tree from Sonoma and Plumas counties, respectively. Two grapevine isolates (UCD1260So and UCD1275So) and one oak isolate (CDFA519) were examined and morphologically compared with previously identified D. corticola isolates CBS678.88 and UCD2397TX from cork oak from Spain and grapevine in Texas, respectively. D. corticola colonies from California were characterized by moderately fast-growing, dark olivaceous, and dense aerial mycelium on PDA. Conidia were obtained from pycnidia formed on pine needles placed on 2% water agar. Conidia were hyaline, contents granular, aseptate, thick walled, ellipsoidal, sometimes becoming dark brown and septate with age. Nucleotide sequences of three genes (ITS1-5.8S-ITS2, a partial sequence of the beta-tubulin gene BT2, and part of the translation elongation factor EF1-α) from D. corticola isolates UCD1260So, UCD1275So, and CDFA519 from California were amplified. All DNA sequences from grapevine and oak tree isolates from California showed 99 to 100% homology with D. corticola isolates previously identified and deposited into GenBank. All DNA sequences obtained from Californian isolates were also deposited into GenBank. Pathogenicity tests were conducted by inoculating detached Vitis vinifera cv. Red Globe dormant canes and canyon live oak branches with agar plugs of isolates UCD1260So, UCD1275So, and CDFA519 (10 inoculations per isolate per host) as described by Úrbez-Torres et al. (3). The same number of grapevine canes and oak branches were inoculated with noncolonized agar plugs as controls. Six weeks after inoculation, the extent of vascular discoloration that developed from the point of inoculation was measured. D. corticola isolates UCD1260So, UCD1275So, and CDFA519 caused an average vascular lesion length of 30.4, 29.6, and 24 mm and 15, 13.2, and 8.6 mm in grapevine dormant canes and oak branches, respectively. Furthermore, D. corticola isolates from grapevine were pathogenic in oak branches and vice versa. Reisolation of D. corticola from discolored vascular tissue of inoculated material was 100%. The extent of vascular discoloration from inoculated grapevine canes and oak branches was significantly greater (P < 0.05) compared with the controls (1.8 and 2 mm, respectively). No fungi were reisolated from the slightly discolored tissue of the controls. To our knowledge, this is the first report of D. corticola causing grapevine cankers and oak trunk cankers in California.

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.

Litter Contributions to Dissolved Organic Matter and Disinfection Byproduct Precursors in California Oak Woodland Watersheds

Export of dissolved organic matter (DOM) from California oak woodland ecosystems is of a great concern because DOM is a precursor for carcinogenic disinfection byproducts (DBPs) formed during drinking water treatment. Fresh litter and decomposed duff materials for the four dominant vegetation components of California oak woodlands: blue oak (Quercus douglassi H. & A.), live oak (Quercus wislizenii A. DC.), foothill pine (Pinus sabiniana Dougl.), and annual grasses, were exposed in natural condition for an entire rainy season (December to May) to evaluate their contributions of particulate (POC) and dissolved (DOC) organic carbon, particulate (PON) and dissolved (DON) organic nitrogen, inorganic nitrogen (NH4+ and NO3-), and trihalomethane (THM) and haloacetonitrile (HAN) formation potentials, to surface waters. Litter and duff materials can be significant sources of DOC (litter=29-126 mg DOC g(-1) C; duff=6.5-37 mg DOC g(-1) C) and THMs and HANs (up to 4600 mg-THMs g-C(-1) and 137 microg-HANs g-C(-1)). Blue oak litter had the highest yield of DOC, THM, and HAN precursors. When scaled to the entire watershed, leachate production yielded 445 kg-DOC ha(-1), as compared to DOC export via streams of 5.25 kg-DOC ha(-1). DOC transport to surface waters is facilitated by subsurface lateral flow through A horizons during storm events. The majority of DOM and DBP precursors was leached from plant materials in the initial rainfall events and thus may explain the seasonal stream pattern of a DOC pulse early in the rainy season.

Exurban development influences woodland bird composition

One of the fastest growing types of land-use change is exurban development—low-density housing outside urban service boundaries. However, how individual species are responding to exurban development remains uncertain. We monitored birds for 5 years across three housing density levels in northern California oak woodlands. We compared community and species responses to exurban development (4–16 ha parcels) with suburban and undeveloped natural areas. We found that individual species and groups of species exhibited variable responses to exurban development. Some species and guilds were impacted by exurban development to the same extent as suburban development while others were less sensitive to this type of land use. For example, the proportion of the bird community composed of tree-and-shrub feeders was similar between exurban and natural areas, whereas proportions of temperate migrants showed significant reductions at both suburban and exurban sites. Similarly, Northern Flicker, Hutton's Vireo, and Orange-crowned Warbler were equally rare in exurban and suburban sites, making large, undeveloped parcels essential for their conservation. By explicitly measuring ecological changes associated with parcel size and density this research provides valuable information to land-use planners on the consequences of zoning for biodiversity conservation.

Effects of long‐term consumer manipulations on invasion in oak savanna communities

Consumer-plant interactions can alter the outcome of biological invasions when native and exotic plants differ systematically in their resistance to and/or tolerance of consumer impacts. Given evidence for indirect interactions and shifts in plant communities from the few existing long-term studies, it is clear that long-term studies are a critical component for understanding the role of consumers in plant invasions. Moreover, studies of the role of consumers in mediating invasions have focused on the effects of exotic consumers, while the effects of native consumers on invasion have received little attention. Here we examine the long-term impact of a largely native vertebrate consumer community on native and exotic understory plants and recruitment of native oaks in a California oak savanna. We sampled plant community composition, oak recruitment, and soils inside and outside of 10 exclosures (mean area = 1000 m2) that had been in place for an average of 32 years. Plots with consumers present had 41% more exotic species, 31% higher cover of exotic species, and 33% lower richness of native herbaceous perennials, suggesting that native consumers may play an important role in mediating invasion in this system. The presence of oak canopies had a strong impact on the plant community independent of consumer effects, with greater recruitment of oaks, higher cover of native shrubs, and lower cover of exotic species cover under oak canopies. The concordant variation of native tree canopy and native woody plants suggests that adult oaks provide a refuge for their seedlings and other native woody plants. Thus, the widespread loss of native oaks has likely increased exotic invasion into an important refuge for native species in the California oak savanna ecosystem.

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.

Science-based outreach helps stem sudden oak death

One of the biggest challenges in controlling sudden oak death is that prevention is the best treatment, but most efforts begin only after trees are already infected. “People don't deal with this disease except in crisis mode,” says Janice Alexander, outreach coordinator for the California Oak Mortality Task Force. “We're trying to reach people on the leading edge of the pathogen before it's a crisis.”

Preferences for 15N-ammonium, 15N-nitrate, and 15N-glycine differ among dominant exotic and subordinate native grasses from a California oak woodland

Following an invasion of exotic annual grasses into California oak woodlands, grass species dominance shifted from native perennials to exotic annuals. In combination with other ecosystem and species characteristics, species-specific N preferences may influence species coexistence and dominance. If species N preferences follow dominance patterns in California oak woodlands, then the more dominant exotic grasses should prefer the most abundant inorganic soil N form (NH 4 +), while the subordinate native grasses should prefer the less available inorganic (NO 3 −) or organic (glycine) soil N forms. To investigate this prediction, we applied 15N-labeled NH 4 +, NO 3 −, and glycine to soil and measured % 15N recovery by two dominant annual grasses ( Bromus diandrus and Bromus hordeaceus) and two subordinate perennial grasses ( Elymus glaucus and Nassella pulchra). As expected, shoots of B. diandrus recovered more 15N-NH 4 + (74%) than either 15N-NO 3 − (51%) or 15N-glycine (39%). B. diandrus also captured at least 3.2 times more 15N-NH 4 + than subordinate grasses. Dominant B. hordeaceus, however, demonstrated no N form preferences. As hypothesized, shoots of subordinate E. glaucus and N. pulchra recovered 2.1–2.3 times more 15N-NO 3 − than 15N-NH 4 + and increased %N by 4.8–5.7% in response to the application of 15N-NO 3 −. Both subordinate grasses did not prefer 15N-glycine over 15N-NH 4 +, suggesting that the importance of this N form in structuring species coexistence in California oak woodlands is minimal. These results support our theory that species N preferences follow dominance patterns in California oak woodlands. To further understand the role of these species-specific N preferences in structuring dominance, the importance of N form versus such characteristics as rooting distribution and species phenologies needs to be explored in the presence of interspecific competition.