California Oak Woodland Research Articles

Aspect Differences in Vegetation Type Drive Higher Evapotranspiration on a Pole‐Facing Slope in a California Oak Savanna

AbstractQuantifying evapotranspiration (ET) is critical to accurately predict vegetation health, groundwater recharge, and streamflow generation. Hillslope aspect, the direction a hillslope faces, results in variable incoming solar radiation and subsequent vegetation water use that drive ET. Previous work in watersheds with a single dominant vegetation type (e.g., trees) have shown that equator‐facing slopes (EFS) have higher ET compared to pole‐facing slopes (PFS) due to higher evaporative demand. However, it remains unclear how differences in vegetation type (i.e., grasses and trees) influence ET and water partitioning between hillslopes with opposing aspects. Here, we quantified ET and root‐zone water storage deficits between a PFS and EFS with contrasting vegetation types within central coastal California. Our results suggest that the cooler PFS with oak trees has higher ET than the warmer EFS with grasses, which is counter to previous work in landscapes with a singule dominant vegetation type. Our root‐zone water storage deficit calculations indicate that the PFS has a higher subsurface storage deficit and a larger seasonal dry down than the EFS. This aspect difference in subsurface water storage deficits may influence the subsequent replenishment of dynamic water storage, groundwater recharge and streamflow generation. In addition, larger subsurface water deficits on PFS may reduce their ability to serve as hydrologic refugia for oaks during multi‐year droughts. This research provides a novel integration of field‐based and remotely‐sensed estimates of ET required to properly quantify hillslope‐scale water balances. These findings emphasize the importance of resolving hillslope‐scale vegetation structure within Earth system models, especially in landscapes with diverse vegetation types.

Downed wood removal effects on survival and site fidelity of woodrats in a California oak woodland

AbstractOwing to the increasing intensity and frequency of wildfires in the western United States, the removal of woody debris (downed dead wood and snags) from fire‐prone landscapes is being evaluated for wildfire mitigation. Consequently, the study of the ecological value of coarse woody debris to sustain dependent species has become of foremost importance. From 2004–2009, we used a before‐after control‐impact (BACI) study design to assess the effects of downed wood removal on a population of big‐eared woodrats (Neotoma macrotis) in an oak woodland (Quercus spp.) in coastal‐central California, USA. Using Pollock's robust design mark‐recapture analyses (with 12 primary capture occasions represented by trapping each spring and fall, and secondary occasions composed of 3 trap nights), we estimated survival and emigration rates. Further, applying mixed‐effects models, we evaluated the effects of 7 habitat attributes on woodrat abundance and reproduction. Following the experimental removal of downed wood from the 11 randomly selected treatment plots, woodrat survival was higher (P = 0.013), and emigration was lower (P = 0.007) among 11 control plots on which downed wood was retained. Woodrat abundance within plots was best predicted by stem density, demonstrating positive demographic associations with habitat complexity. Plot‐level reproductive success (i.e., the presence of juveniles within a plot during spring) was more likely with increasing snag density. These findings indicate that snags and downed dead wood are key components of high‐quality habitat for big‐eared woodrats. Management of woody debris to mitigate wildfire risk should consider the ecological value of this habitat attribute for big‐eared woodrats and other dependent species.

Decline in small mammal species richness in coastal-central California, 1997-2013.

The richness and composition of a small mammal community inhabiting semiarid California oak woodland may be changing in response to climate change, but we know little about the causes or consequence of these changes. We applied a capture-mark-recapture model to 17 years (1997-2013) of live trapping data to estimate species-specificabundances. The big-eared woodrat was the most frequently captured species in the area, contributing 58% of total captures. All small mammal populations exhibited seasonal fluctuations, whereas those of the California mouse, brush mouse, and pinyon mouse declined during the study period. We also applied a multispecies dynamic occupancy model to our small mammal detection history data to estimate species richness, occupancy (), detection (p), local extinction (), and colonization () probabilities, and to discern factors affecting these parameters. We found that decreased from 0.369 ± 0.088 in 1997 to 0.248 ± 0.054 in 2013; was lower during the dry season (May-September) than the wet season (October-April) and was positively influenced by total seasonal rainfall (slope parameter, = 0.859 ± 0.371; 95% CI = 0.132-1.587). Mean mammalian species richness decreased from 11.943 ± 0.461 in 1997 to 7.185 ± 0.425 in 2013. With highly variable climatic patterns expected in the future, especially increased frequency and intensity of droughts,itis important to monitor small mammal communities inhabiting threatened California oak woodlands.

A highly contiguous genome assembly for the California quail (Callipepla californica).

The California quail (Callipepla californica) is an iconic native bird of scrub and oak woodlands in California and the Baja Peninsula of Mexico. Here, we report a draft reference assembly for the species generated from PacBio HiFi long read and Omni-C chromatin-proximity sequencing data as part of the California Conservation Genomics Project (CCGP). Sequenced reads were assembled into 321 scaffolds totaling 1.08 Gb in length. Assembly metrics indicate a highly contiguous and complete assembly with a contig N50 of 5.5 Mb, scaffold N50 of 19.4 Mb, and BUSCO completeness score of 96.5%. Transposable elements (TEs) occupy 16.5% of the genome, more than previous Odontophoridae quail assemblies but in line with estimates of TE content for recent long-read assemblies of chicken and Peking duck. Together these metrics indicate that the present assembly is more complete than prior reference assemblies generated for Odontophoridae quail. This reference will serve as an essential resource for studies on local adaptation, phylogeography, and conservation genetics in this species of significant biological and recreational interest.

Conifer encroachment increases foliar moisture content in a northwestern California oak woodland

Background Foliar moisture content influences crown fire ignition and behaviour. Some spatial variation in foliar moisture is attributable to differences in stand conditions but evidence describing this role is lacking or contradicting. Aims To examine the role of stand conditions on tree physiology and foliar moisture content in Oregon white oak (Quercus garryana) and Douglas-fir (Pseudotsuga menziesii). Methods We monitored foliar moisture content in both species, and tree physiology (stomatal conductance and leaf water potential) in oak, across three stand conditions, including intact stands (unencroached), stands invaded by Douglas-fir (encroached), and thinned stands with Douglas-fir removed. Key results Encroached stands had higher foliar moisture content than intact or thinned stands. Higher stand density was associated with higher foliar moisture content in both species and foliage ages. Encroached stands also had higher midday leaf water potential compared with intact or thinned stands. Conclusions These findings provide strong evidence that stand conditions and thinning treatments can influence foliar moisture content in Oregon white oak ecosystems, with likely implications for other ecosystems. Implications Better understanding of the role of stand conditions on foliar moisture content may contribute to improved spatial and temporal prediction of foliar moisture content and modelling of potential crown fire behaviour.

Factors driving California pocket mice (Chaetodipus californicus) population dynamics

AbstractUnderstanding how demographic parameters respond to climatic variables is essential for predicting species’ response to changing environmental conditions. The California pocket mouse (Chaetodipus californicus) is an inhabitant of coastal-central California oak (Quercus spp.) woodland that is undergoing a rapid anthropogenic transformation while also facing effects of global climate change. We analyzed the population dynamics of the California pocket mouse by applying Pradel’s temporal symmetry model to a 10-year (2004 – 2013) capture–mark–recapture data set to estimate survival and recruitment rates and realized population growth rate. The overall monthly apparent survival probability (ϕ) was 0.76 ± 0.01 SE and was slightly higher in the dry season (0.79 ± 0.02 SE) than the wet season (0.74 ± 0.01 SE). Coefficients of variation (CV) of temperature and rainfall (with and without a one-season lag), average seasonal temperature, and regional climatic variation (El Niño index) positively influenced ϕ. Overall monthly recruitment rate (f) was 0.17 ± 0.01 SE but varied seasonally; f was substantially higher during the dry season (0.39 ± 0.04 SE) than the wet season (0.09 ± 0.02 SE). Average seasonal temperature, CV of temperature and rainfall (without a one-season lag), and total seasonal rainfall (with a one-season lag) positively influenced recruitment, whereas regional climatic variation (El Niño index), total seasonal rainfall (without a one-season lag), and CV of rainfall (with a one-season lag) had a negative effect on f. Monthly realized population growth rate (λ) was 1.00 ± 0.02 SE for the entire study period, but it varied temporally. Our study provides the first estimates of demographic parameters for the California pocket mouse and tests for the influence of climatic variables on these parameters. Although the California pocket mouse population remained relatively stable during our study (as indicated by λ = 1.00), changing climate and anthropogenic influences on California oak woodland could adversely influence demographic parameters and population dynamics and might also indicate effects of climate change on its ecologically sensitive habitat.

Hotter Drought Escalates Tree Cover Declines in Blue Oak Woodlands of California

California has, in recent years, become a hotspot of interannual climatic variability, recording devastating climate-related disturbances with severe effects on tree resources. Understanding the patterns of tree cover change associated with these events is vital for developing strategies to sustain critical habitats of endemic and threatened vegetation communities. We assessed patterns of tree cover change, especially the effects of the 2012–2016 drought within the distribution range of blue oak (Quercus douglasii), an endemic tree species to California with a narrow geographic extent. We utilized multiple, annual land-cover and land-surface change products from the U.S. Geological Survey (USGS) Land Change Monitoring, Assessment and Projection (LCMAP) project along with climate and wildfire datasets to monitor changes in tree cover state and condition and examine their relationships with interannual climate variability between 1985 and 2016. Here, we refer to a change in tree cover class without a land-cover change to another class as “conditional change.” The unusual drought of 2012–2016, accompanied by anomalously high temperatures and vapor pressure deficit, was associated with exceptional spikes in the amount of both fire and non-fire induced tree cover loss and tree cover conditional change, especially in 2015 and 2016. Approximately 1,266 km2 of tree cover loss and 617 km2 of tree cover conditional change were recorded during that drought. Tree cover loss through medium to high severity fires was especially large in exceptionally dry and hot years. Our study demonstrates the usefulness of the LCMAP products for monitoring the effects of climatic extremes and disturbance events on both thematic and conditional land-cover change over a multi-decadal period. Our results signify that blue oak woodlands may be vulnerable to extreme climate events and changing wildfire regimes. Here, we present early evidence that frequent droughts associated with climate warming may continue to affect tree cover in this region, while drought interaction with wildfires and the resulting feedbacks may have substantial influence as well. Consequently, efforts to conserve the blue oak woodlands, and potentially other vegetation communities in the Western United States, may benefit from consideration of climate risks as well as the potential for climate-fire and vegetation feedbacks.

TREE MORTALITY IN BLUE OAK WOODLAND DURING EXTREME DROUGHT IN SEQUOIA NATIONAL PARK, CALIFORNIA

Blue oak woodlands in California have been a focus of conservation concern for many years. Numerous studies have found that existing seedling and sapling numbers are inadequate to sustain current populations, and recent work has suggested that blue oak woodlands might be particularly vulnerable to a warming climate. California has recently experienced a drought of historically unprecedented severity, resulting in the mortality of tens of millions of trees, including an apparent spike in mortality in oak communities. Here we present the results of a survey of tree mortality and composition in blue oak woodlands in Sequoia National Park. We found that 18% (95% CI = 14–24,) of all standing trees and 23% (95% CI = 17–30) of standing Quercus douglasii Hook. & Arn. (blue oak) were dead, substantially higher than proportions of dead trees recorded in pre-drought datasets, which showed 4% (95% CI = 2–9) standing dead for all trees and 5% (95% CI = 4–7) dead or 8% (95% CI = 4–16) standing dead for blue oak. Furthermore, much of this mortality appeared to be recent. Based on foliage or fine twig retention, 19% (95% CI = 14–26) of blue oak and 23% (95% CI = 16–31) of Quercus wislizeni A. DC. (interior live oak) appear to have died recently. In contrast, only 5% (95% CI = 3–8) of Aesculus californica (Spach) Nutt. (California buckeye) and 5% (95% CI = 2–11) of Fraxinus dipetala Hook. & Arn. (California ash) appear to have died recently. Even after such high mortality, with blue oak basal area dropping by 26% (from 9.5 m2/ha [95% CI = 7.4–11.6] to 7.0 m2/ha [95% CI = 5.3–8.7]), blue oak remains the dominant species in these ecosystems. However, given the lack of recruitment and the apparent vulnerability to extreme drought, blue oak populations may be at risk for severe decline if such mortality events become more frequent.

Spatio‐temporal variation of biotic and abiotic stress agents determines seedling survival in assisted oak regeneration

Abstract Mediterranean oak woodlands are currently undergoing considerable anthropogenic changes that globally threaten their long‐term persistence. Restoration efforts via assisted regeneration depend on species traits and type of stress. However, how different sources of stress vary across space (microsites) and time (intra‐ and inter‐annual variation) for co‐occurring oak species remains largely unknown. We examined spatio‐temporal variation of biotic (herbivory, seed predation) and abiotic (drought) stress agents to evaluate seedling survival across different regeneration microsites, and considering two climatically contrasting years. We used two co‐occurring species of contrasting ecology and leaf traits in California oak woodlands: the deciduous Quercus lobata (valley oak) and the evergreen Q. agrifolia (coast live oak). Most oak seedlings (98.8%) suffered from some type of stress. For both species, summer drought was the main cause of mortality, followed by rodent damage, ungulate browsing and insect damage. Rodents represented the predominant biotic stress, as all rodent‐affected seedlings eventually died. Interactions among stress factors were nuanced: insect and ungulate‐browsing stresses were greater during dry year, whereas rodent damage was similar in dry and average years. Intra‐annually, rodent damage was the main stress agent in early spring, whereas water stress and ungulate‐browsing were greater from late spring to early fall. Plant species traits were also determinant: overall, the evergreen species had greater survival probability than the deciduous one. Across microsites, water stress showed higher occurrence in open grassland habitats, while rodent damage was prevalent under palatable shrubs and ungulate browsing under conspecific trees. Shrub and tree cover reduced damage attributed to water stress but increased rodent and ungulate damage, respectively. Interestingly, lower water stress but higher rodent damage was found further inside the shrub cover. Synthesis and applications. Intra‐ and inter‐annual variation are critical determinants of stress type and intensity on oak recruitment. Spatial variation (microsite identity) is highly useful to manipulate stress agent impact. We conclude that assisted regeneration should consider not only the predominance and lethality of each stress type and microsite, but also the interactions thereof and thus manipulate protective microsites accordingly.

Climatic drivers of pinyon mouse Peromyscus truei population dynamics in a resource‐restricted environment

AbstractHighly variable patterns in temperature and rainfall events can have pronounced consequences for small mammals in resource‐restricted environments. Climatic factors can therefore play a crucial role in determining the fates of small mammal populations. We applied Pradel's temporal symmetry model to a 21‐year capture–recapture dataset to study population dynamics of the pinyon mouse (Peromyscus truei) in a semi‐arid mixed oak woodland in California, USA. We examined time‐, season‐ and sex‐specific variation in realized population growth rate (λ) and its constituent vital rates, apparent survival and recruitment. We also tested the influence of climatic factors on these rates. Overall monthly apparent survival was 0.81 ± 0.004 (estimate ± SE). Survival was generally higher during wetter months (October–May) but varied over time. Monthly recruitment rate was 0.18 ± 0.01, ranging from 0.07 ± 0.01 to 0.63 ± 0.07. Although population growth rate (λ) was highly variable, overall monthly growth rate was close to 1.0, indicating a stable population during the study period (λ ± SE = 0.99 ± 0.01). Average temperature and its variability negatively affected survival, whereas rainfall positively influenced survival and recruitment rates, and thus the population growth rate. Our results suggest that seasonal rainfall and variation in temperature at the local scale, rather than regional climatic patterns, more strongly affected vital rates in this population. Discerning such linkages between species' population dynamics and environmental variability are critical for understanding local and regional impacts of global climate change, and for gauging viability and resilience of populations in resource‐restricted environments.

Climatic influences on demography of the California mouse (Peromyscus californicus) in semiarid oak woodland

The California mouse (Peromyscus californicus) inhabits semiarid oak (Quercus spp.) woodlands and shrublands of California's southern Coast Ranges and lower slopes of the western Sierra Nevada. From 1993 to 2014, we studied the demography of California mice in semiarid oak woodland in coastal-central California. Using capturemark-recapture (CMR) methods, we estimated apparent survival (ϕ), recruitment (f), and realized population growth (λ), and tested for the effects of climatic covariates on these demographic rates. Monthly ϕ averaged 0.869, but it varied temporally between 0.507 and 0.954. Monthly f averaged 0.146 and ranged from 0.027 to 0.556. Monthly λ averaged 0.996; however, λ also exhibited strong temporal variation, ranging from 0.636 to 1.373. Tests of singular covariate models revealed that ϕ was positively influenced by total rainfall (regression coefficient, β = 0.449), but negatively influenced by average temperature (β = -0.394) and by variability in temperature (β = -0.412) and rainfall (β = -0.147). Recruitment was positively affected by total rainfall (β = 0.203) and rainfall variability (β = 0.120); it was negatively affected by temperature variability (β = -0.162). However, the effect of rainfall and temperature on California mice varied across seasons. During winter, high temperature increased both ϕ (β = 0.690) and f (β = 0.754), but in summer these hot conditions reduced ϕ (β = -1.087) and f (β = -0.517). Rainfall positively affected ϕ in both seasons (β = 0.141 in winter; β = 0.614 in summer). Although rainfall positively affected f in winter (0.319), it had no discernible effect in summer. Warmer, drier, and more variable climatic conditions, together with increasingly pervasive habitat alterations, especially wildfire, may threaten the persistence of some California mouse populations.

Risk of vector tick exposure initially increases, then declines through time in response to wildfire in California

AbstractIdentifying the effects of human‐driven perturbations, such as species introductions or habitat fragmentation, on the ecology and dynamics of infectious disease has become a central focus of disease ecologists. Yet, comparatively little is known about how the ecology of zoonotic disease systems responds to catastrophic disturbance events such as wildfires or hurricanes. In California, wildfire disturbance is centrally important to the ecology of forests and oak woodlands and is projected to increase in severity and extent under future climate change. Here, taking advantage of a recent wildfire as a natural experiment, we investigate the effects of wildfire disturbance on the ecology of tick‐borne disease in California oak woodlands. We find that wildfire leads to elevated abundance of questing adult and nymphal western blacklegged ticks (Ixodes pacificus) in the year following fire, relative to unburned control plots, but that vector tick abundance declines sharply in the following two years. We find that the abundance of non‐competent hosts (western fence lizards) for the Lyme disease bacterium is unaffected by fire, but that the abundances of important reproductive hosts (deer) for ticks and reservoir hosts (dusky‐footed woodrats) for tick‐borne pathogens are significantly negatively affected by fire. We found ticks and hosts infected with Borrelia burgdorferi sensu lato only within the burn extent and only in the year following the wildfire, though rates of infection were exceedingly low representing little risk to humans. In aggregate, due to the differential effects of this catastrophic disturbance event on different key host species and vector tick life stages in the transmission of tick‐borne pathogens, we conclude that wildfire may potentially increase risk of exposure to vector ticks in the first year following wildfire in California, but that risk is dampened substantially in following years due to tick population declines and loss of key reservoir hosts from the system.

Short-term facilitation of microbial litter decomposition by ultraviolet radiation

Solar radiation plays an important role in carbon cycling by increasing the decomposition rates of plant litter and soil organic matter (i.e. photodegradation). Previous work suggests that exposure to radiation can facilitate microbial decomposition of litter by altering litter chemistry and consequently litter degradability (i.e. photopriming). However, it remains unclear to what extent photopriming contributes to litter decomposition processes and on what timescale photopriming operates. We conducted laboratory experiments to compare the effects of UV photopriming at two temporal scales (months versus days). In one experiment, we found that four months of UV exposure induced a significant but small (3–4%) mass loss in two of three litter species commonly found in California oak savanna; however, UV exposure did not alter litter degradability as measured by microbial respiration in an incubation experiment. We also found that UV exposure had limited effects on lignin and other cell wall structures, but one month of microbial decomposition (in absence of UV exposure) significantly reduced lignin β-aryl ether inter-unit linkages and acetylated xylans. These results indicate that abiotic photodegradation alone was ineffective at breaking down lignin. In another experiment, litter of a common grass was exposed to either alternating UV radiation and dark conditions or constant darkness for 128days. We found that the alternating UV exposure increased litter CO2 production in both dark and UV phases over that observed in constant darkness. This led to a 35% greater release of CO2 from the alternating UV exposure treatment between days 65 and 128 of the experiment. These results demonstrate that alternating UV exposure with dark conditions is key to enabling photopriming on a timescale of days. Overall, we identify short-term photopriming as a novel mechanism behind photodegradation. Our results also challenge the conventional hypothesis that abiotic processes are primarily responsible for degrading lignin during photodegradation.

Hydrologic refugia, plants, and climate change.

Climate, physical landscapes, and biota interact to generate heterogeneous hydrologic conditions in space and over time, which are reflected in spatial patterns of species distributions. As these species distributions respond to rapid climate change, microrefugia may support local species persistence in the face of deteriorating climatic suitability. Recent focus on temperature as a determinant of microrefugia insufficiently accounts for the importance of hydrologic processes and changing water availability with changing climate. Where water scarcity is a major limitation now or under future climates, hydrologic microrefugia are likely to prove essential for species persistence, particularly for sessile species and plants. Zones of high relative water availability - mesic microenvironments - are generated by a wide array of hydrologic processes, and may be loosely coupled to climatic processes and therefore buffered from climate change. Here, we review the mechanisms that generate mesic microenvironments and their likely robustness in the face of climate change. We argue that mesic microenvironments will act as species-specific refugia only if the nature and space/time variability in water availability are compatible with the ecological requirements of a target species. We illustrate this argument with case studies drawn from California oak woodland ecosystems. We posit that identification of hydrologic refugia could form a cornerstone of climate-cognizant conservation strategies, but that this would require improved understanding of climate change effects on key hydrologic processes, including frequently cryptic processes such as groundwater flow.

Save water or save wildlife? Water use and conservation in the central Sierran foothill oak woodlands of California, USA

This project was funded as part of the NSF Coupled Human Natural Systems Program, Project Award Number 1115069, Wetlands in a Working Landscape, with Professor Steve Beissinger as Principal Investigator. J.L. Oviedo’s involvement in this study was also funded by the Salvador de Madariaga program (grant number PRX16/00452) of the Spanish Ministry of Education, Culture and Sports.

Effect of Contact Insecticides Against the Invasive Goldspotted Oak Borer (Coleoptera: Buprestidae) in California.

The goldspotted oak borer, Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), was linked in 2008 to ongoing tree mortality in oak woodlands of southern California. Mortality of coast live oak, Quercus agrifolia Née, and California black oak, Q. kelloggii Newb., continues as this exotic phloem borer spreads in southern California. Management options are needed to preserve high-value oaks and maintain management objectives. From 2009 to 2012, we tested four contact insecticide formulations in four experiments against A auroguttatus in California. The impact of contact insecticides was evaluated ∼<1, 8, and 12 mo postapplication against A auroguttatus adults in no-choice leaf-feeding or walking bioassays. At <1 mo postapplication, bifenthrin, carbaryl, lambda-cyhalothrin, and permethrin all reduced adult survival and feeding in leaf-feeding and walking bioassays. At 8 mo postapplication, only bifenthrin reduced adult feeding, but had no effect on survivorship. At 12 mo postapplication, adult A auroguttatus survived fewer days and fed less in leaf-feeding bioassays with bifenthrin, carbaryl, and permerthin. These results support the annual application of contact insecticides prior to A auroguttatus' flight period to reduce adult leaf maturation feeding and activity on the bark surface (e.g., oviposition), but additional studies are needed to show these contact treatments can prevent tree mortality from this invasive species.

Habitat quality and nest-box occupancy by five species of oak woodland birds

Habitat quality can have important consequences for avian communities through impacts on survival and annual reproductive success. However, habitat quality is often hard to measure, leading to the use of occupancy as a proxy. We compared habitat use of 5 avian species that used nest boxes in the oak woodlands of central coastal California, USA, to determine which habitat characteristics best predicted box occupancy. We focused on the relationship between habitat characteristics and occupancy for five species—Ash-throated Flycatcher (Myiarchus cinerascens), House Wren (Troglodytes aedon), Oak Titmouse (Baeolophus inornatus), Violet-green Swallow (Tachycineta thalassina), and Western Bluebird (Sialia mexicana)—for which we had 12 consecutive years of data on nest boxes spread over a 700 ha study area. We also examined whether the physical habitat characteristics and box occupancy rates were good predictors of reproductive success, to infer whether they were useful indicators of habitat quality. The habitat characteristics influencing nest-box occupancy differed among the 5 species. Ash-throated Flycatchers were associated with fragmented habitats with less grassland. House Wrens were associated with riparian vegetation, as were Oak Titmice, which were also associated with chaparral. Violet-green Swallows were associated with chaparral but tended to nest farther from riparian corridors than Oak Titmice. Western Bluebirds nested away from riparian corridors and in areas with more grassland and oak woodland. Finally, occupancy rate was a better predictor than habitat characteristics of reproductive success, which suggests that occupancy can be a valuable proxy for habitat quality for these 5 species.

The Effect of Nitrogen Deposition on Plant Performance and Community Structure: Is It Life Stage Specific?

Nitrogen (N) deposition is a key global change factor that is increasing and affecting the structure and function of many ecosystems. To determine the influence of N deposition on specific systems, however, it is crucial to understand the temporal and spatial patterns of deposition as well as the response to that deposition. Response of the receiving plant communities may depend on the life stage-specific performance of individual species. We focus on the California oak savanna because N deposition to this system is complex—characterized by hotspots on the landscape and seasonal pulses. In a greenhouse experiment, we investigated the relative influence of N deposition on plant performance during early growth, peak biomass, and senescent life stages across different soil types, light, and community compositions. To represent the community we used three grass species—a native, naturalized exotic, and invasive exotic. At early growth and peak biomass stages performance was measured as height, and shoot and root biomass, and at the senescent stage as seed production. Simulated N deposition 1) increased shoot biomass and height of the native and, even more so, the naturalized exotic during early growth, 2) positively affected root biomass in all species during peak biomass, and 3) had no influence on seed production at the senescent stage. Alone, N deposition was not a strong driver of plant performance; however, small differences in performance among species in response to N deposition could affect community composition in future years. In particular, if there is a pulse of N deposition during the early growth stage, the naturalized exotic may have a competitive advantage that could result in its spread. Including spatial and temporal heterogeneity in a complex, manipulative experiment provides a clearer picture of not only where N management efforts should be targeted on the landscape, but also when.

Using realistic nitrogen deposition levels to test the impact of deposition relative to other interacting factors on the germination and establishment of grasses in the California oak savanna

Nitrogen (N) deposition is a spatially and temporally heterogeneous process. Thus, the response to N deposition by plant communities can also be expected to vary in space and time. The total amount of deposition received by a plant community may vary regionally due to proximity to emission sources and locally due to vegetation structure. Temporal variation such as a seasonal peak in N deposition may coincide with specific life stages of the plant and drive community response to N deposition. Using the California oak savanna as a model ecosystem, a greenhouse experiment simulating realistic levels of N deposition received across the region was conducted to test the relative effects of N fertilization, soil type, light, and community mixture on the germination and seedling establishment of grasses. Seedling mixtures were composed of 3 common grass species—Stipa pulchra, Hordeum murinum, and Elymus caput-medusae—grown in monoculture and in 2- and 3-species combinations. Nitrogen fertilization alone had no effect on germination or seedling establishment, with one exception that high N increased seedling establishment of E. caput-medusae when that species was grown in monoculture. This limited effect of N deposition in a complex and realistic experimental setting indicated that deposition levels in oak savannas are not affecting most plants during early growth. Rather, early growth stages are species-specific and respond most strongly to neighbor identity.

Hemiparasite--host plant interactions in a fragmented landscape assessed via imaging spectroscopy and LiDAR.

Species interactions are susceptible to anthropogenic changes in ecosystems, but this has been poorly investigated in a spatially explicit manner in the case of plant parasitism, such as the omnipresent hemiparasitic mistletoe-host plant interactions. Analyzing such interactions at a large spatial scale may advance our understanding of parasitism patterns over complex landscapes. Combining high-resolution airborne imaging spectroscopy and LiDAR, we studied hemiparasite incidence within and among tree host stands to examine the prevalence and spatial distribution of hemiparasite load in ecosystems. Specifically, we aimed to assess: (1) detection accuracy of mistletoes on their oak hosts; (2) hemiparasitism prevalence within host tree canopies depending on tree height, and (3) spatial variation in hemiparasitism across fragmented woodlands, in a low-diversity mediterranean oak woodland in California, USA. We identified mistletoe infestations with 55-96% accuracy, and detected significant differences in remote-sensed spectra between oak trees with and without mistletoe infestation. We also found that host canopy height had little influence on infestation degree, whereas landscape-level variation showed consistent; non-random patterns: isolated host trees had twice the infestation load than did trees located at the core of forest fragments. Overall, we found that canopy exposure (i.e., lower canopy density or proximity to forest edge) is more important than canopy height for mistletoe infestation, and that by changing landscape structure, parasitic prevalence increased with woodland fragmentation. We conclude that reducing fragmentation in oak woodlands will minimize anthropogenic impact on mistletoe infestation at the landscape level. We argue that advanced remote sensing technology can provide baselines to quantitatively analyze and monitor parasite-host trajectories in light of global environmental change, and that this is a promising approach to be further tested in other temperate and tropical forests.