Overstory Oak Research Articles

Spatial and Temporal Variability of Throughfall among Oak and Co-Occurring Non-Oak Tree Species in an Upland Hardwood Forest

Canopy throughfall comprises the largest portion of net precipitation that is delivered to the forest floor. This water flux is highly variable across space and time and is influenced by species composition, canopy foliage, stand structure, and storm meteorological characteristics. In upland forests throughout the central hardwoods region of the Eastern United States, a compositional shift is occurring from oak-hickory to more mesic, shade-tolerant species such as red maple, sweetgum, and winged elm. To better understand the impacts of this shift on throughfall flux and the hydrologic budget, we monitored throughfall for one year in Northern Mississippi under the crowns of midstory and overstory oak (post oak and southern red oak) and non-oak species (hickory, red maple, and winged elm). In general, oak had more throughfall than co-occurring non-oak species in both canopy levels. In the overstory during the leaf-off canopy phase, white oak had relatively higher throughfall partitioning (standardized z-score = 0.54) compared to all other species (z-score = −0.02) (p = 0.004), while in the leaf-on canopy phase, red maple had relatively lower throughfall (z-score = −0.36) partitioning compared to all other species (z-score = 0.11). In the midstory, red maple was the only species to exhibit a difference in throughfall between canopy phases, with much lower throughfall in the leaf-off compared to the leaf-on canopy phase (z-score = −0.30 vs. 0.202, p = 0.039). Additionally, throughfall under oak crowns was less variable than under non-oak crowns. These results provide evidence that the spatial and temporal distribution of throughfall inputs under oak crowns are different than non-oak species, likely due to differences in crown architecture (i.e., depth and density). As oak dominance diminishes in these forests, it is possible that the portion of rainfall diverted to throughfall may decrease as well. The net impacts to watershed hydrology are still unknown, but these results provide one mechanism by which the distribution of water resources may be affected.

Scale and strength oak-mesophyte interactions in a transitional oak-hickory forest.

Forests in eastern North America are undergoing rapid compositional changes as they experience novel climate, disturbance, and pest conditions. One striking pattern is the replacement of canopy oaks (Quercus spp.) by mesic, fire-sensitive, shade-tolerant species like red maple (Acer rubrum). To gain insight into the successional patterns driving stand-level canopy oak replacement we ask two questions: (i) What is the spatial association of oak and mesophyte recruitment compared to oak and mesophyte overstory individuals, and (ii) How do oaks and mesophytes differentially respond to canopy openings. We analyzed census data from a 23 ha forest plot surveyed in 2003, 2008 and 2014. We show that oak recruits are negatively associated with overstory red maples and black cherries (Prunus serotina), while mesophytic recruits were positively associated with overstory oaks. Second, we found that proximity to a dead overstory tree increased growth and survival for black cherries, increased growth for red maples, but had no effect on oaks. Black cherries and red maples are therefore better suited than oaks to take advantage of canopy openings and the moderate light available under adult oaks. These same fine scale competitive processes are contributing to canopy oak replacement across eastern North America.

A Dendroecological Investigation of an Upland Oak-Dominated Forest within the Grand Prairie Region of Illinois

ABSTRACT We studied an upland forest located within the Grand Prairie region of Illinois and utilized tree-ring analysis to document tree cohort development and radial growth in relation to stand disturbance and climate. The overstory within the Humiston Woods Nature Center (Livingston County, Illinois) study area was dominated by oak (Quercus spp.), hickory (Carya spp.), and ash (Fraxinus spp.), while the subordinate crown classes were principally composed of elm (Ulmus spp.) and sugar maple (Acer saccharum). The study area contained tree cohorts originating during two recruitment periods. Overstory oaks and hickories primarily established between 1865 and 1900. A second tree cohort was associated with a timber harvest occurring in the late 1960s. In contrast to the 1865–1900 recruitment period, trees within the 1965–1980 cohort were primarily ash, elm, sugar maple, and black cherry (Prunus serotina). RWI values were most strongly correlated with Palmer Drought Severity Index, precipitation, and temperat...

Impacts of Wildfire Recency and Frequency on an Appalachian Oak Forest

Cabwaylingo State Forest in southern West Virginia has experienced numerous anthropogenic wildfires over the past 36 years. In this case study, we assessed the relationship between fire frequency and recency and stand composition and structure, with emphasis on oak and its competitors. Frequent and recent fire was significantly correlated with reduced red maple overstory stem density and basal area. Overstory oak density did not significantly vary with either fire frequency or recency. Total overstory basal area was greatest in areas of either no fire or nonrecent fire. Oak sapling density was significantly greater with high frequency and recent fire. Red maple sapling densities were greatest when fires were infrequent and recent, and red maple seedlings were greatest in no fire and low-frequency nonrecent fire areas. Our results suggest that recurring fire can enhance the development of large oak advanced reproduction. However, frequent fires without a sufficient fire-free interval could prevent the recruitment of oaks into the overstory.

Changes in stand structure and tree vigor with repeated prescribed fire in an Appalachian hardwood forest

Without large scale disturbances to alter forest structure and open the canopy, historically oak-dominated forests of the central and Appalachian hardwood regions of eastern North America are shifting to dominance by shade-tolerant, ‘mesophytic’ species. In response, prescribed fire is applied with increasing frequency and spatial extent to decrease non-oak species and promote dominance of oak species. However, relatively few studies have examined impacts of repeated fire to forest structure and tree vigor across multiple years and varied terrain. In this study, we examined tree vigor, tree mortality, and stand structure in response to different burn treatments: Frequent (burned 4 times in eight years), Less Frequent (burned 2 times in eight years), and Fire-Excluded. We hypothesized that fire-driven decreases in stem density and basal area would be greatest for small size classes, especially of shade-tolerant species on drier landscape positions, and would increase with burn frequency and fire temperature. We expected trees surviving fire to exhibit increased crown vigor over time since fire. Prescribed fire effects depended on tree size-class and landscape position. About 60% of surviving midstory trees (10–20cm diameter at breast height (DBH)) and 25% of overstory trees (⩾20cm DBH) on sub-xeric and intermediate landscape positions experienced crown dieback. Fire-Excluded sites had fewer trees with crown dieback (11–28% across size classes) compared to burned sites (21–87%). Throughout the duration of the study, midstory and overstory maples had significantly greater likelihood of increased crown dieback compared to oaks. Paradoxically, midstory maples had a higher survival probability than similarly-sized oaks, while overstory maples had lower survival than overstory oaks. The greatest reductions in density and basal area occurred in saplings (trees 2–10cm DBH) and midstory trees on sub-xeric and intermediate (but not sub-mesic) landscape positions. Both Less Frequent and Frequent burning reduced density and basal area of sapling and mid-story shade-tolerant species, but also of mid-story chestnut oaks. Individual tree mortality was positively correlated with char height after the first burn regardless of burn frequency. A large and significant initial sprouting response to fire dissipated over time and with repeated burning. Future assessments of mortality and vigor of residual trees following fire are essential for evaluating the long-term effectiveness of prescribed fire management in shifting species composition away from ‘mesophytic’ species and toward oaks, and could help guide management choices regarding repeated prescribed burning.

Effects of Tornado Damage, Prescribed Fire, and Salvage Logging on Natural Oak (Quercusspp.) Regeneration in a Xeric Southern USA Coastal Plain Oak and Pine Forest

ABSTRACT: Due in large part to fire exclusion, many oak-dominated (Quercus spp.) forests, woodlands, and savannas throughout eastern North America are being replaced by less diverse forest ecosystems. In the interior coastal plain of the southern United States, these forests are dominated in the mid- and understory by mesophytic species such as Acer rubrum L. and Liquidambar styraciflua L., which may eventually displace overstory oaks. Oak regeneration has been shown to respond positively to open canopies and prescribed burning. In 2008, a severe tornado damaged previously established study plots in a xeric oak and pine (Pinus spp.) forest in northern Mississippi. Some damaged and undamaged plots were treated with prescribed fire in 2010. Additional plots were established in damaged areas, and these plots were salvage-logged. Species composition and growth of saplings were measured to assess the impact of tornado damage and the treatments on oak regeneration. Tornado damage increased overall sapling densi...

Effect of gap size on mid-rotation stand structure and species composition in a naturally regenerated mixed broadleaf forest

This paper is an assessment of the effect of gap size on stand structure and species composition 48 years following treatment in a mixed broadleaf upland forest. Established in 1960, the study tests three circular openings, 15.2 m (0.02 ha), 45.7 m (0.16 ha), and 76.2 m (0.46 ha). Forty-eight years following treatment (2008) basal area, top height, and quadratic mean diameter were significantly lower in 15.2 m openings. Maple (Acer spp.) species had the highest mean importance value across treatments (0.40). Trends suggest that species composition of dominant and codominant trees among opening sizes may have been influenced by shade tolerance adaptations of the species groups present. Whereas 15.2 m openings were dominated by shade tolerant maple species, 45.7 and 76.2 m openings produced a mixture of commercial species including shade intolerant species such as yellow-poplar (Liriodendron tulipifera L.), trees of intermediate shade tolerance like oak (Quercus spp.), and shade tolerant maple. Data further suggest the density of overstory oak was highest in the intermediate opening size (45.7 m), while yellow-poplar increased in the larger opening sizes. Evaluation of species shifts between 1981 and 2008 showed that relative basal area of maple increased across all treatments. Relative basal areas from 45.7 to 76.2 m openings suggest declines in yellow-poplar and other non-commercial species were balanced by increases in oak and maple.

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.

Characterizing spatial–temporal tree mortality patterns associated with a new forest disease

A new forest disease called Sudden Oak Death, caused by the pathogen Phytophthora ramorum, occurs in coastal hardwood forests in California and Oregon. In this paper, we analyzed the spatial–temporal patterns of overstory oak tree mortality in China Camp State Park, CA over 4 years using the point patterns mapped from high spatial resolution remotely sensed imagery. Both univariate and multivariate spatial point pattern analyses were performed with special considerations paid to the spatial trends illustrated in the mapped point patterns. In univariate spatial point pattern analyses, we investigated inhomogeneous K-functions and Neyman–Scott point processes to characterize and model the spatial dependence among dead oak trees in each year. The results showed that the point patterns of dead oak trees are significantly clustered at different scales and spatial extents through time; and that both the extent and the scale of the clustering patterns decrease with time. In multivariate spatial point pattern analyses, we developed two simulation methods to test the spatial–temporal dependence among dead oak trees over time and the spatial dependence between dead oak trees and California bay trees, an important host for the pathogen. The results showed that new dead oak trees tend to be located within up to 300 m of past dead oak trees; and that a strong spatial association between oak tree mortality and California bay trees exists 150 m away.

Modeling sprout-origin oak regeneration in the central Appalachians

We describe the development of individual-tree models to estimate, before overstory removal, the contribution of common central Appalachian oak species (northern red oak (Quercus rubra L.), black oak (Quercus velutina Lam.), chestnut oak (Quercus montana Willd.), and white oak (Quercus alba L.)) to stand stocking in the third decade (20–30 years) after harvest. The models incorporate three aspects of sprout-group development: (1) the initial sprouting of overstory oaks (≥5 cm DBH) following cutting, (2) sprout-group survival, and (3) sprout-group size at the end of the regeneration period. Logistic regression was used to model initial sprouting as a function of parent tree species and diameter using measurements on 1173 stumps. Sprout-group survival was extrapolated from 4 year remeasurements on 321 stumps. Sprout-group size, expressed as a percentage of stocking, was estimated from measurements on 429 third-decade sprout groups. Predicted sprout-origin stocking values calculated for mature oak stands compared favorably with stocking values observed in third-decade stands. The models provide forest managers with essential information on stand development following planned regeneration treatments. A similar modeling approach may be useful in other plant communities where sprouting plays an important role in post-disturbance stand development.

Mechanical Land Clearing to Promote Establishment of Coastal Sandplain Grassland and Shrubland Communities

Abstract The decline in grasslands and other species‐rich early successional habitats on the coastal sandplains of the northeastern United States has spurred management to increase the area of these declining plant communities. We mechanically removed overstory oak and applied seed from a nearby sandplain grassland on the island of Martha’s Vineyard, Massachusetts, to evaluate this technique for creating an open oak community able to support sandplain herbaceous species. We compared vegetation structure and composition before and after clearing in an area of total tree removal (clearcutting), an area where 85% of tree basal area was removed (savanna cutting), and in adjacent coastal oak forest. Plant responses to clearcutting and savanna cutting were similar. Sandplain herbs colonized at high frequencies after seeding and increased herbaceous cover from less than 7% before clearing to 22–38% three growing seasons later. Pennsylvania sedge (Carex pensylvanica) increased in cover approximately 6‐fold, accounting for 84–90% of the increased herbaceous cover. Other native ruderals and exotic herbs reached 2 and less than or equal to 1%, cover, respectively, after three years. Species richness across cleared treatments increased from 30 to 79 species. All forest species were retained. Forest shrubs and trees initially declined from their dominant cover but rebounded after three years. Tree clearing plus seeding appeared to be a viable management practice for increasing cover of herbaceous sandplain species while causing minimal increases in exotic herbaceous cover. The long‐term persistence of sandplain herbs may require periodic disturbances that limit woody regrowth.

Stand structure over 9 years in burned and fire-excluded oak stands on the Cumberland Plateau, Kentucky

Upland oak-dominated forests in the central Appalachian region are being managed increasingly with prescribed fire, ostensibly to reduce midstory stem density of fire-sensitive species and to promote oak regeneration, creating the need to document the effectiveness of fire alone for accomplishing these objectives. On three ridges in the Daniel Boone National Forest, Kentucky, changes in stand structure and tree regeneration were examined from 1995 to 2003 in oak-dominated stands where fire-sensitive red maple ( Acer rubrum L.) and eastern white pine ( Pinus strobus L.) were gaining dominance. Fire-excluded and burn treatments were applied to the three ridges, such that each ridge contained three units receiving one of the following treatments: fire-excluded, burned 2×, or burned 3×. Burning reduced midstory (2–10 cm DBH) stem density by 91% and midstory basal area by 86%, whereas on fire-excluded units, midstory stem density decreased 24% and basal area 28%. Burning reduced overstory (≥10 cm DBH) stem density 30% but did not significantly affect overstory basal area. Overstory stem density on fire-excluded units did not change but overstory basal area decreased 10%. Repeated burning reduced midstory red maple stem density 94%, compared to fire-excluded units, which experienced a 39% decrease in midstory red maple stem density and an 85% increase in overstory red maple stem density and basal area. Burning reduced midstory oak ( Quercus spp.) stem density 82%, while on fire-excluded units, a decline in midstory oak stem density was not significant. Overstory oak stem density decreased 25% on both fire-excluded and burned units. Damaged and dead stems on burned units responded by sprouting vigorously, with red maple being the most prolific. The number of basal sprouts/stem increased with each additional fire and the basal sprout density (sprouts/ha) was highest after the third fire. Burning also affected tree regeneration by eliminating eastern white pine stems in the sapling layer (≥50 cm height and <2 cm DBH). Red maple stem density in the sapling-layer was highest on the 3×-burned units primarily due to basal sprouting. Seedling-layer (<50 cm height) oak and flowering dogwood stems were also highest on units burned 3×. Multiple prescribed fires successfully reduced midstory densities of fire-sensitive species such as red maple, but also negatively affected oaks. Additional silvicultural methods to augment prescribed fire, or changing the timing and/or intensity of burning may be necessary for restoring oak-dominated stands.

Impact of multiple fires on stand structure and tree regeneration in central Appalachian oak forests

The National Guard Training Center at Fort Indiantown Gap (NGTC-FIG) near Harrisburg, PA, has experienced frequent fires since the 1950s on the ridges and 1980s in the valleys as a result of military training exercises. This represented a unique opportunity to investigate the role of recent and repeated fire in oak ( Quercus) forests in the eastern USA. We investigated four frequently burned and two unburned sites replicated in ridge and valley ecosystems. Burned sites generally had lower tree density and a higher proportion of overstory oak species (64–92% relative importance value) than unburned stands (47–49% importance). Oak saplings averaged 875 ha −1 in burned forests and 31 ha −1 in unburned forests. Red maple ( Acer rubrum, L.), the most aggressive oak replacement species in the eastern USA, had overstory importance of 7% and 24% in burned and unburned stands, respectively. Oak saplings ranged from 824 to 1545 ha −1 in three of the four burned stands and 0–62 ha −1 in the unburned stands. Oak sapling density was only 62 ha −1 one recently (2002) burned stand where fire had not resulted in reduced tree density; this stand had the highest tree density of all sampled stands. There were no red maple saplings in three of the four burned stands. Oak saplings were most abundant when overstory density was less than 400 trees/ha and understory tree density was less than 200 trees/ha. When overstory or understory tree density exceeded 400 and 200 trees/ha, respectively, oak regeneration was virtually absent. The results of this study suggest that periodic fire often reduces overstory and understory stand density and promotes successful regeneration of relatively shade intolerant oak species in the eastern USA. However, high tree density in forests will retard the development of oak understories and subsequent recruitment, even if periodic burning occurs.

Classification and Successional Changes of Mixed-Oak Forests at the Mohn Mill Area, Pennsylvania

A vegetation study was conducted within central Pennsylvania’s Ridge and Valley Province at the Mohn Mill natural area, an area that harbors the federally endangered northeastern bulrush, Scirpus ancistrochaetus. A non-metric multidimensional scaling ordination identified two gradients of species replacement distinguished by differences in forest canopy species and groundcover. Basal-diameter histograms for red maple (Acer rubrum), eastern hemlock (Tsuga canadensis), and witch hazel (Hamamelis virginiana) showed distributions indicative of populations recruiting new individuals, whereas basaldiameter distributions for white oak (Quercus alba), chestnut oak (Q. montana), and red/black oak (Q. rubra/Q. velutina) indicate declining populations with limited recruitment. Consequently, as current populations of overstory oaks age and ultimately die, they will be replaced by red maple and eastern hemlock. This transition has major implications for herbaceous plants, including the persistence of the northeastern bulrush, given that ground-level light levels will decrease due to the denser canopies of red maple and eastern hemlock recruits.

Spatial and Temporal Disturbance Characteristics of Oak-Dominated Old-Growth Stands in the Central Hardwood Forest Region

Abstract Using a radial growth averaging technique, decadal-scale growth changes of 150- to 350-yr-old overstory oaks were used to identify canopy disturbance events in five old-growth stands, and canopy disturbance intervals were estimated using frequency analysis. Stem maps and tree-establishment and canopy-accession chronologies were then used to reconstruct the size of canopy openings. Between 1700 and 1990, median canopy disturbance interval was approximately 3 yr for each stand; larger disturbances involving two or more trees occurred about every 16 yr per stand. Based on age distribution of the oldest cohorts, three stands became established after a major disturbance, but the majority of multiple-tree disturbances were associated with gaps &amp;lt; 200 m2 in area. Historic disturbance frequency and size distribution of canopy gaps suggest that the oak components of these stands persisted by utilizing a variety of growth strategies appropriate to large and small openings. The absence of significant changes in overstory disturbance frequencies further suggests that increases in the level of understory competition are responsible for the present-day decline in oak dominance.

Oak establishment and canopy accession strategies in five old-growth stands in the central hardwood forest region

Using a radial growth averaging technique, decadal-scale changes in growth rates of overstory oaks were used to identify canopy accession events at five old-growth sites. A review of tree-ring chronologies yielded three growth strategies: (1) one-half of the oaks originated in a large opening and achieved overstory status before canopy closure; (2) 38% originated in a smaller opening and required a second gap event to attain overstory status; and (3) 13% achieved overstory status after an extended period of very low growth in understory shade and one or two subsequent gap releases. For trees that required a major canopy release, understory residence times averaged 89, 54, 50 and 38 years for white oak, northern red oak, black oak, and chestnut oak, respectively. Average diameters (inside bark) at canopy accession were 13.3, 11.7, 10.0, and 6.2 cm for the four oak species, respectively. Although there is some historical precedence for these values, few contemporary second-growth forests contain understory oaks of this age, particularly red oak. These long understory residence times suggest that the level of understory shade, and by inference, the abundance of shade tolerant understory species, was considerably less before 1900.

The Mortality of Midwestern Overstory Oaks as a Bioindicator of Environmental Stress

The Mortality of Midwestern Overstory Oaks as a Bioindicator of Environmental Stress

THE MORTALITY OF MIDWESTERN OVERSTORY OAKS AS A BIOINDICATOR OF ENVIRONMENTAL STRESS

Tree mortality rates are potential indicators of the impact of environmental stresses on forests. But the relationship between environmental stress and tree mortality is poorly understood, particularly in forests not experiencing epidemic mortality. To determine whether there is a sufficiently direct link between stress and tree mortality to allow mortality rate to serve as a stress bioindicator, I addressed the question: Are trees more likely to die during years when environmental stress is more intense? Subject trees were 62 overstory oaks (Quercus spp. L., '20 cm diameter at breast height) that died during 1968-1987. These trees were from seven oak-hickory forests in the midwestern USA that were ex- periencing low rates of tree mortality. A previous study had found these oaks to be growing more slowly than surviving oaks from the same forests, suggesting that these now-dead trees may have been physiologically compromised and particularly vulnerable to stress. Based on two Monte Carlo method-based tests, there was no consistent positive association between the likelihood of tree death in a year and the intensity of stress during that year or the previous year, where stress was represented by the growth rates of surviving neighbor trees (mean chronologies), growing season precipitation, or the mean Palmer drought se- verity index during the growing season. An evaluation of the statistical power of one of the two tests found a low probability of failing to detect a modest positive association between year of tree death and stress intensity. The results indicate the need for additional research on the link between stress and tree mortality if overstory tree mortality rates are to serve effectively as a bioindicator of environmental stresses.

Effects of Seasonal Prescribed Fires on Residual Overstory Trees in Oak-Dominated Shelterwood Stands

Abstract A study was initiated in 1994 to evaluate the degree of bole damage and crown decline residual overstory trees would experience because of prescribed burning of shelterwood stands. Three oak-dominated shelterwood stands, partially harvested 2 to 4 yr earlier, were divided into four treatments (unburned control, spring burn, summer burn, and winter burn). Fifteen permanent sampling points were systematically located in each 5 to 12 ac treatment area, and overstory trees were selected from these points with a 10 BAF prism. Before burning, each tree was evaluated for lower bole and crown condition and reevaluated two growing seasons after the fires. Hickory, oak, and yellow-poplar were largely unaffected by the winter and summer prescribed fires but displayed bole damage and crown decline following spring burning. American beech and red maple declined after all fire treatments. Fire damage to oak, hickory, and yellow-poplar was strongly associated to presence of logging slash near a tree's base. Directional felling or moving slash should minimize injury to these trees. This research will aid resource managers wishing to use prescribed fire in shelterwood stands to favor oak regeneration while minimizing damage to residual overstory oaks. South. J. Appl. For. 23(2):88-93.

Gypsy Moth (Lepidoptera: Lymantriidae) Control with Ground-Based Hydraulic Applications of Gypchek, In Vitro-Produced Virus, and Bacillus thuringiensis

Gypchek, a registered microbial insecticide for aerial and ground-based application against the gypsy moth, Lymantria dispar L., was field-tested in 1996 and 1997 at 2 doses (1011 and 1012 polyhedral inclusion bodies (PIB) per 379 liters (100 gallons) ) and with and without a sunscreen. An in vitro-produced strain of gypsy moth virus was tested in 1997 at the 1O11-PIB dose; however, the effective dose was lower because in vitro-produced PIBs contained 5-fold fewer virions than Gypchek PIBs. Treatments were applied to overstory oak, Quercus spp., trees with a truck-mounted hydraulic sprayer. Larval mortality was significantly greater, and larval density was significantly lower, on trees treated with both Cypchek and in vitro-produced virus than on untreated trees.