Red Oak Group Research Articles

Evaluating the impact of prescribed surface fire on seedlings in the Central Hardwood Region, USA

Abstract Oak (Quercus) is being successionally replaced by maple (Acer) across much of the eastern deciduous forest. Past research on the close relationship between oak and fire has led forest managers to use prescribed surface fires to deter this replacement. However, there has not been a comprehensive evaluation of the effectiveness of prescribed fire by managers, particularly in the Central Hardwood Region. For example, it is not known how many repeat applications of prescribed fire are needed to accumulate enough oak reproduction to successfully re-establish oak-dominated stands. Tree reproduction and midstory composition were systematically surveyed across 63 mature, oak-dominated stands ranging in prescribed fire histories and aspects on the Hoosier and Wayne National Forests in southern Indiana and Ohio, respectively. These stands represent the vast majority of stands burned to promote oak regeneration since 1990 in these two National Forests. Across all sites, seedlings in the white oak group (section Quercus) and red oak group (section Lobatae) comprised 47.8 ± 3.3 per cent and 8.2 ± 1.6 per cent (mean ± SE), respectively, of all seedlings. In comparison, common competing species (maples and American beech) comprised 5.4 ± 1.4 per cent of all seedlings. Mid- and overstory basal area, percent slope, years since last burn, and total number of burns significantly affected oak seedling density. Greater increases in taller, competitive oak seedling density occurred after sites had been burned three times and with high frequency (≤4 years between burns). However, results also suggest that if an established midstory already exists or fire has not been used on the landscape for almost a decade, repeat applications of prescribed fire will not likely increase oak regeneration in the short term without concurrent treatment of midstory stems.

Impacts of oak-focused silvicultural treatments on the regeneration layer nine years posttreatment in a productive mixed-oak southern Appalachian forest

Oaks (Quercus spp.) are foundational species in forests and woodlands in the eastern USA. An oak regeneration bottleneck has occurred throughout its range in recent decades, and refining silvicultural treatments to localized conditions has become a focus in addressing this problem. This study was developed to determine species regeneration dynamics among oak and oak competitors on productive sites in response to silvicultural treatments in oak-dominated southern Appalachian mountain forests. The treatments were: an oak shelterwood treatment (25–30% basal area [BA]) reduction through midstory removal with herbicides); a prescribed fire treatment (two, late-dormant season prescribed fires occurring over a 9-yr period); a shelterwood and burn treatment (one, late-dormant season prescribed fire 3–5 yr following 30–40% BA removal); and an unmanaged control. To determine treatment impacts on the regeneration layer, changes in relative and absolute importance values and stems ha-1 (germinants up to stems ≤ 4.9 cm diameter at breast height [DBH]) were calculated at the species group and individual species level 0 and 9 yr postinitial treatment. The greatest relative increases in importance values were 1,401% and 2,995% for the red oak group and yellow-poplar (Liriodendron tulipifera), respectively, in the shelterwood and burn (SWB). Changes in all species groups were predominantly influenced by the smallest size-class (< 0.6 m tall), with the exception of northern red oak (Q. rubra) and yellow-poplar in the SWB. The SWB significantly reduced importance values of all shade-tolerant species groups and was the only treatment to decrease red maple (Acer rubrum) importance value and density over the study duration. The prescribed fire (RXF) treatment increased the red oak group importance value, while simultaneously decreasing yellow-poplar's importance value and increasing red maple's importance value. Changes in the red oak group in the SWB and the RXF were driven by northern red oak. Treatments did not significantly change the importance value of the white oak group. The SWB was the only treatment to significantly decrease overstory BA. The RXF and SWB treatments improved the competitive status of only some oak species, but modifications to these treatments might result in better control of yellow-poplar and red maple competition, further improving oak's competitive status. Although the SWB resulted in modest recruitment of northern red oak saplings, all treatments appear in need of additional follow-up vegetation control to further improve the competitiveness and recruitment of oak into large size-classes.

Effects of oak wilt (Bretziella fagacearum) on post harvest Quercus regeneration

Oak wilt is a serious disease affecting oaks, especially those in the red oak group. Despite concern around a lack of oak regeneration and increasing mortality from oak wilt, there has been little effort to connect the two in relation to common forest management practices to regenerate oak. Oak management commonly includes clearcut harvesting, particularly within lower quality sites where sprouting is a viable regeneration method, but also where oak wilt is most problematic. We found that clearcut harvesting that encompassed pockets of oak wilt infected trees did not facilitate expansion or long-term perpetuation of oak wilt, but rather could serve as a containment mechanism for oak wilt. We found that stumps around 30 cm produced the greatest number of sprouts and that relatively minor differences in stump size determined if a stump was initially viable or never produced sprouts. Strong effects of stump size, and sprouting ability, remained significant drivers of survival (and mortality) over an extended 10-year period. Forest practices for oak on poor sites, such as those we studied, tend to be managed on short rotations which is likely well suited to capturing the maximum sprouting ability and long-term survival of oaks to regenerate stands.

Growth, survival and sunfleck response of underplanted red oaks (Quercus spp., section Erythrobalanus) along a topographic gradient in southern New England

An understanding of the regeneration niche is critical to our ability to refine silvicultural approaches aimed at regenerating and maintaining diverse, mixed species stands. However, few studies have directly tested differences in habitat associations among closely related trees of the same genus, particularly to assess habitat partitioning at the regeneration stage. To evaluate differences in regeneration niche, we established a common garden experiment with five red oak species (Quercus spp., section Erythrobalanus) under-planted across a topographic gradient (valley, midslope and ridge positions) in mixed hardwood forests of Southern New England. Represented were three red oak species common to intermediate site conditions of southern New England (Q. rubra L, Q. velutina Lam., and Q. coccinea Münchh.), and two species commonly associated with more extreme site conditions - Q. ilicifolia Wangenh. (xeric, skeletal ridge tops) and Q. palustris Münchh. (river floodplains). Seedlings were caged to prevent deer browse, and seedling height and survival were measured annually for nine growing seasons. Response to sunflecks, a primary source of light for advance regeneration-dependent species that establish in the forest understory, was measured using a LiCor 6400 photosynthesis system. Overall, survival was highest for Q. rubra, followed by Q. velutina, Q. coccinea, Q. ilicifolia and finally Q. palustris. The pattern among species held for all topographic positions, with higher survival rates on the ridgetops, and lower survival in valley positions. Height development was highest in the valley, intermediate in the ridge, and lowest in the midslope positions. Survival patterns were positively correlated with light availability, and negatively correlated with site fertility. Based on net photosynthetic rates, Q. rubra, Q. velutina, and Q. coccinea were more responsive to sunflecks than Q. ilicifolia and Q. palustris, and maintained higher photosynthetic rates over the measurement period. Greater survival of all species in upper topographic position reflects in part adaptations of the red oak group in this region to higher light and lower soil fertility conditions. But sunflecks likely play a critical role as well, judging from the positive correlation of survival with understory light (43–80% of which occurs as sunflecks), the strong height development in the light-limited valleys (where sunflecks should be most critical), and the clear alignment of species sunfleck response with species survival patterns. Differences among the oaks in photosynthetic response to sunflecks may thus help explain their habitat associations and contribute to niche partitioning among these related species. These findings have important silvicultural implications, particularly for enrichment planting of different red oaks and the timing of overstory removal treatments to facilitate establishment of regeneration for growth into the canopy.

Mortality, scarring, and growth in an oak woodland following prescribed fire and commercial thinning in the Ozark Highlands

Oak-dominated (Quercus Spp.) woodlands are commonly thinned and burned in the Ozark Highlands to prevent canopy closure and regenerate desired species, despite a lack of information regarding tree mortality, scarring, and growth in residual stands. Our study compared stand- and tree-level responses after two prescribed burns across four treatments: control, burn, thin, and thin+burn. Results showed that two prescribed fires led to 19% greater cumulative mortality than in unburned stands. In the burn treatment, 19.3% of residual live overstory trees were scarred, compared to 32.4% of trees in the thin+burn treatment. Analysis of scar area revealed that thinning before burning significantly increased the surface area (cm2) of fire scars. In general, trees in the red oak group (Erythrobalanus spp.) had the greatest percentage of scarred trees, followed by the white oak group (Leucobalanus spp.), hickories (Carya spp.), and shortleaf pines (Pinus echinata Mill.). Our data indicate that two fires did not significantly decrease the radial growth of white oaks, except in stands which were thinned prior to prescribed burns. Average percent change in ring-widths (mm) suggest a 1.9% growth decrease in control, 1.4% decrease in burn, 84% increase in thin, and a 35% increase in thin+burn. Covariates such as age, slope, surrounding basal area, canopy openness, and fire scars were analyzed, but tree diameter was the only significant predictor of growth response. Overall, results suggest that effects of prescribed burning are more pronounced in thinned stands as a function of increased fuel loads and fire intensity, causing greater mortality, fire scarring, and reductions in potential growth than fire or thinning alone. This study highlights important tradeoffs between prescribed fire and thinning in oak dominated ecosystems, especially where fire related damages are in potential conflict with other stand objectives.

Two decades of compositional and structural change in deciduous old-growth forests of Indiana, USA

Aims Using a network of permanent plots, we determined how multiple old-growth forests changed over an 18–19-year period at a statewide scale. This examination of change allowed us to assess how the compositional and structural stability of each forest varied with site characteristics (topography, physiography and productivity) and stochastic disturbance. Methods In 2011, we resampled 150 plots distributed across five oldgrowth hardwood forests in Indiana, USA that were originally sampled in 1992–1993. Within each plot, we relocated and remeasured the diameter at breast height (dbh) of all trees (≥10.0 cm) present during the 1992–1993 sample, w hich allowed us to track their individual fates through time for growth and mortality calculations. Trees that grew to ≥10.0 cm dbh since plot establishment were designated as ingrowth. The dbh and species of all saplings (stems ≥2.0 cm but <10.0 cm dbh) were also recorded. For each forest, we calculated density (stems ha −1 ), basal area (BA; m 2 ha −1 ) and importance value (relative density + relative BA)/2) of trees by species. For saplings, density per ha was calculated by species for each forest. We also calculated annual mortality rate (AMR) for three diameter classes (10–29.9, 30–59.9 and ≥60 cm) and species richness (S), evenness (E) and Shannon–Weiner diversity (Hʹ) for the tree and sapling layers. Differences between years were compared for each forest using paired t-tests and Wilcoxon signed rank tests. Important Findings Although we observed commonality in changes across some sites, our results suggest that these forests differ in their rates and trajectories of change. Changes in total stand BA and density varied across sites and were influenced by past disturbance and mortality rates. We observed a general increase in the overstory dominance of Acer saccharum coupled with a general decrease in the dominance of Quercus section Lobatae (red oak group) species. Mortality of overstory trees present in 1992–1993 ranged from 27 to 49% over the study period (mean AMR 1.6–3.7%). Most sites experienced greater mortality of early and mid-successional species, but one site experienced heavy mortality of Fagus grandifolia, a shade-tolerant late-successional species. Shade tolerant species, A. saccharum in particular, dominated the sapling layer at most sites. However, recruitment of this species into larger size classes did not occur uniformly across all sites and the species was comparatively uncommon at one site. Overall, our results suggest that oldgrowth remnants, even within a single state, cannot be viewed as equivalent units with regard to research or management. Stochastic disturbance events and surrounding land use may have amplified effects on small scattered remnants. Therefore, continued monitoring of these rare, but biologically important forests is critical to their long-term management and protection.

Modeling and Mapping Oak Advance Reproduction Density Using Soil and Site Variables

Regenerating oaks (Quercus spp.) has remained a widespread and persistent problem throughout their natural range. Research shows that abundant oak advance reproduction is crucial for success. Although it is recognized that oak advance reproduction accumulation is inversely related to site quality, there has been little effort to model oak advance reproduction density as a function of measured levels of light, water, or nutrient supply. The objective of this study was to determine whether oak advance reproduction could be modeled and mapped with site variables. The study was conducted on the Sinkin Experimental Forest in southeastern Missouri in 20 5-ha experimental units. Vegetation and site data were collected in 120 0.5-ha circular plots with nested subplots for the inventory of the midstory (0.01 ha) and reproduction (0.004 ha). Site variables included soil available water capacity, pH, photosynthetically active radiation in the understory, forest stocking, terrain shape, and slope-aspect. Oak advance reproduction abundance was related to soil acidity and available water capacity and to other site information such as slope-aspect. Models for the red oak group species generally exhibited better fit than those for the white oaks. There also was evidence that estimates of soil acidity and available water capacity can be obtained from the SSURGO database and used in these oak advance reproduction models along with other site information to generate maps of estimated oak reproduction densities. These maps could be used for planning silvicultural interventions to increase the abundance and size of oak advance reproduction before forest regeneration.

Spatio-Temporal Trends of Oak Decline and Mortality under Periodic Regional Drought in the Ozark Highlands of Arkansas and Missouri

At the forest landscape/region level, based on annual Forest Inventory and Analysis plot data from 1999 to 2010, oak decline and mortality trends for major oak species (groups) were examined in the Ozark Highlands of Arkansas and Missouri. Oak decline has elevated cumulative mortality of red oak species to between 11 and 15 percent in terms of relative density and basal area of standing dead oak trees, respectively. These values are three to five times higher than for white oak group and non-oak species. Oak decline and associated escalating mortality have occurred primarily in red oak species while the white oak group has maintained a relatively stable mortality rate that is comparable to non-oak species. Cross-correlation analyses indicate that mortality in the red oak group was significantly correlated with the growing season Palmer drought severity index (PDSI) and usually lagged two to three years following single drought events. Moreover, based on the past 17 years PDSI data, it appears that the cumulative impacts of drought may last up to 10 years. The Ozark Highlands experienced a severe drought extending from 1998 to 2000 and another milder drought from 2005 to 2006. These drought events triggered the escalation of mortality starting around year 2000. Spatially, high red oak mortality sites (hot spots with proportional basal area mortality &gt; 0.12) initially occurred in the central Ozarks and spread gradually over most of the Ozark Highlands as regional droughts continued. In contrast, sites with elevated white oak and non-oak mortality occurred sporadically, mainly in the southern portion (Arkansas) of the Ozark Highlands. During the most recent inventory period (2006–2010), over 60%, 7% and 5% of red oak, white oak and non-oak groups, respectively, had relative mortality rates of &gt; 12%.

Calcium oxalate crystal types in three oak species (Quercus L.) in Turkey

Crystal types from 3 oak species representing 3 different sections of the genus Quercus were identified. Crystals were studied with scanning electron microscopy after localisation by light microscopy. The crystals were composed of calcium oxalate silicates as whewellite (calcium oxalate monohydrate) composites. In Quercus macranthera Fisch. et Mey. subsp. syspirensis (C.Koch) Menitsky (İspir oak) from the white oaks (section Quercus: Leucobalanus), whewellite and trihydrated weddellites coexisted. Axial parenchyma cells included 30 crystal chains with walls as chambers in uniseriate ray cells; some cells had many crystals without any chambers and were small. Long thin crystals were found adherent to the membrane in the tracheary cell lumens of latewood. Quercus cerris L. var. cerris (hairy oak/Turkey oak) from the red oak group (section Cerris Loudon) is extremely rich in rhomboidal crystals. In species Quercus aucheri Jaub. et Spach (grey pırnal) from the evergreen oak group (section Ilex Loudon) rhomboidal crystals were found in axial parenchyma and multiseriate ray cells. Lignified wall layers were not observed around the crystals with chambers.

Bulk Density (Scaling Factor) of 14 Hardwood Species in Arkansas

With the monetary exchange for hardwood trees and logs changing from log-scale board feet to tons, it is important that bulk density values (also called scaling factors) be developed to aid in determining the value of standing trees and logs. This study included 14 species of hardwoods and a total of 325 trees. Ten species were oak, which were divided into a red oak group (seven species) and a white oak group (three species). The red oak group had an average bulk density of 80.7 lb/ft3, and the white oak group had an average bulk density of 79.2 lb/ft3. A significant difference was found between the groups, as were significant differences between species within each group. Southern red oak (Quercus falcate) had the highest value (83.0 lb/ft3), and white oak (Quercus alba) had the lowest (77.0 lb/ft3). Three hickory types had an average bulk density of 79.4 lb/ft3. Sweetgum (Liquidambar styraciflua) had a bulk density of 78.3 lb/ft3.

Temporal Patterns of Oak Mortality in a Southern Appalachian Forest (1991–2006)

The sustainability of eastern oak-dominated forests is threatened by high oak mortality rates and widespread oak regeneration failure, and presents a challenge to natural area managers. We tracked the rate and cause of mortality of 287 mature oak trees of five species for 15 years to determine the temporal patterns and sources of mortality. We observed a 15.3% total mortality rate during the study period. Mortality was due to oak decline (7.3% of trees) and high-intensity wind events (6.6% of trees). Decline-related mortality was gradual, averaging 0.5% annually. Windthrow was episodic, occurring during hurricane-related weather events in 1995 and 2004. Within species, total mortality was disproportionately high for scarlet oak (Quercus coccinea Muenchh) (41.2%) compared to other species in the red oak group (13.8% for northern red oak (Q. rubra L.); 12.5% for black oak (Q. velutina Lam.)) or the white oak group (10.4% for white oak (Q. alba L.); 5.7% for chestnut oak (Q. prinus L.)). Decline-related mortality was highest for scarlet oak (15.7%) followed by black oak (8.3%), white oak (7.5%), northern red oak (6.9%), and chestnut oak (2.3%). Within the red oak group, the average age of decline-affected and surviving trees did not differ, but average dbh of decline-affected trees was smaller. Decline-affected trees in the white oak group were on average older, but average dbh did not differ from surviving trees. Wind-related mortality also was higher for scarlet oak (21.6%) than for northern red oak (5.2%), black oak (4.2%), white oak (3.0%) or chestnut oak (2.3%). Windthrown red oaks were smaller than survivors, but windthrown trees in the white oak group did not differ in size from survivors. Average age did not differ between windthrown and surviving trees for either group. Oak mortality rates observed in this study, coupled with oak regeneration failure, could result in a substantial reduction in the proportion of mature canopy oaks and change the relative abundance of oak species in southern Appalachian forests over the long-term.

Effects of alternative silviculture on stump sprouting in the southern Appalachians

Stump sprouts are an important form of regeneration for a number of species in the southern Appalachians, especially the oaks ( Quercus spp.). Alternative regeneration systems to clearcutting such as shelterwood and leave-tree systems are being implemented in many hardwood stands in the Appalachians. However, the effects of these alternative silvicultural systems on stump sprouts are not known. Therefore, we evaluated the impact of three silvicultural systems: a clearcut, leave-tree, and shelterwood on stump sprouting. These treatments were implemented in seven stands in Virginia and West Virginia in the Appalachian Plateau (AP) and Ridge and Valley (RV) physiographic provinces. The stands were even-aged oak dominated Appalachian hardwood stands with ages ranging from 62 to 100 years. Species were placed into six groups: (1) red oak ( Quercus spp.), (2) chestnut oak ( Q. prinus L.), (3) white oak ( Q. alba L.) and hickory ( Carya spp.), (4) red maple ( Acer rubrum L.), (5) mixed mesic, and (6) midstory groups. Partial harvesting also reduced the number of sprouts per stump for the red oak group and red maple. Sprouting probabilities were generally less in the Appalachian Plateau than the Ridge and Valley, particularly for the oaks ( Quercus spp.). Partial harvesting systems decreased sprouting in both physiographic provinces. However, the sprouting in specific species groups varied between the two physiographic provinces. In the Ridge and Valley, the highest sprouting rates were in the clearcut for the red oak (60%), chestnut oak (77%), white oak–hickory (26%), and midstory (33%) species groups. Red maple sprouting was highest in the leave-tree (67%) in the Ridge and Valley. The mixed mesic and midstory groups were only reduced in the Ridge and Valley. Sprouting was negatively correlated with residual basal area for the red oak group, chestnut oak, and red maple. For the all oak species except white oak, sprouting was reduced by about 2% for every 1m 2/ha increase in residual basal area.

Influence of physiographic and climatic factors on spatial patterns of acorn production in Maryland and Virginia, USA

AbstractAim The aims of this study were to identify the effects physiographic differences have on the spatial synchrony of acorn production within red (Quercus rubra) and white (Quercus alba) oak subgenus groups, to identify climatic factors associated with acorn production patterns, and to assess if and how these relationships vary across a distinct physiographic boundary.Location Maryland and Virginia in the eastern United States.Methods Using data from 36 survey sites in Virginia (1973–2000) and Maryland (1977–2000), we described the spatial synchrony of annual acorn production based on Mantel tests and modified correlograms at three spatial scales: (1) the entire study area, (2) within two ecological regions (mountain and Piedmont), and (3) between these ecoregions. Using climate data obtained from the National Climate Data Centre, we described the relationship between climate and acorn production within each ecological region using linear regression.Results Spatial synchrony in the white oak group acorn production was strong among sites closest together, and declined with distance. The average April temperature during the year of acorn drop was positively related to acorn crop size within the mountain and Piedmont ecoregions. Spatial synchrony in the red oak group was strong among sites closest together and declined with distance in the mountain region. Synchrony was weaker in the Piedmont than in the mountain ecoregion and declined slightly with increasing distance. Between the mountain and Piedmont ecoregions, synchrony was not detected among the closest sites, but was detected with increasing strength as the distance between site pairs increased. In the Piedmont ecoregion, the number of April freeze events and total July precipitation, both 2 years prior to acorn drop, were negatively related to red oak acorn production. In the mountains, average July temperature 2 years prior to acorn drop was positively related to acorn production.Main conclusions Physiographic differences affected the spatial acorn production synchrony in the red oak but not in the white oak subgenus group. Climatic factors related to annual acorn production variability differed between subgenus groups and physiographic regions. The physiographic differences between the mountain and Piedmont ecoregions may define spatial patterns of acorn production synchrony in some oak species and mediate climatic influences on acorn production.

Occurrence of Treehoppers (Hemiptera: Membracidae: Smiliinae) on Oaks in Delaware Water Gap National Recreation Area, 2004–2006

A total of 870 treehoppers and 24 species from the tribe Smiliini (Hemiptera: Membracidae: Smiliinae) were collected from various oaks in the Delaware Water Gap National Recreation Area in 2006 using yellow sticky cards. Combining all years and collecting methods, 27 species were found in the park. A majority of the specimens collected in 2006 were males of Cyrtolobus vau and Ophiderma pubescens, as in previous years. Most of the treehoppers were caught in mid to late June, comparable to 2004 and 2005. It appears that many species are segregated either temporally or by oak group; some treehopper species show preference for either the red or white oak group rather than for one species of oak. Color photographs for 27 treehopper species (many including both sexes) are included.

Oak mortality associated with crown dieback and oak borer attack in the Ozark Highlands

Oak decline and related mortality have periodically plagued upland oak–hickory forests, particularly oak species in the red oak group, across the Ozark Highlands of Missouri, Arkansas and Oklahoma since the late 1970s. Advanced tree age and periodic drought, as well as Armillaria root fungi and oak borer attack are believed to contribute to oak decline and mortality. Declining trees first show foliage wilt and browning, followed by progressive branch dieback in the middle and/or upper crown. Many trees eventually die if severe crown dieback continues. In 2002, more than 4000 living oak trees ≥11 cm dbh in the relatively undisturbed mature oak forests of the Missouri Ozark Forest Ecosystem Project (MOFEP) were randomly selected and inventoried for tree species, dbh, crown class, crown width, crown dieback condition (healthy: <5% crown dieback, slight: >5–33%, moderate: 33–66%, and severe: >66%) and number of emergence holes created by oak borers on the lower 2.4 m of the tree bole. The same trees were remeasured in 2006 to determine their status (live or dead). In 2002, about 10% of the red oak trees showed moderate or severe crown dieback; this was twice the percentage observed for white oak species. Over 70% of trees in the red oak group had evidence of oak borer damage compared to 35% of trees in the white oak group. There was significant positive correlation between crown dieback and the number of borer emergence holes ( p < 0.01). Logistic regression showed oak mortality was mainly related to crown width and dieback, and failed to detect any significant link with the number of oak borer emergence holes. Declining red oak group trees had higher mortality (3 or 4 times) than white oaks. The odds ratios of mortality of slightly, moderately, and severely declining trees versus healthy trees were, respectively, 2.0, 6.5, and 29.7 for black oak; 1.8, 3.8, and 8.3 for scarlet oak; and 2.6, 6.5 and 7.1 for white oaks.

Individual Tree and Stand Level Influences on the Growth, Vigor, and Decline of Red Oaks in the Ozarks

Abstract Repeated oak decline and mortality events have occurred in the Ozark region for decades and probably longer. We sampled an age sequence of 1,259 black and scarlet oaks (Quercus velutina Lam. and Quercus coccinea Muench.) to better describe the process of oak decline and mortality in the red oak group (subgenera Erythrobalanus). Trend in basal area increment (BAI) over the most recent 40 years was used to establish three vigor classes for trees with decreasing, stable, or increasing growth (Declining, Stable, or Healthy). We compared crown condition measures with absolute BAI and boundary line BAI, a measure of radial growth adjusted for tree size. A pulse of mortality was found to occur just subsequent to the most recent drought, although decline often started decades previously. Time series of individual tree BAI suggests that half of all oak decline events were incited by one or two drought-related step-changes in growth and variance. Predisposing factors to decline generally showed significant but weak relationships with crown conditions. Surviving oaks growing in high-mortality stands had poorer crown conditions and grew more slowly than trees in low-mortality stands. When recently dead trees were accounted for, the same high-mortality stands had significantly greater predecline basal area and stocking than low-mortality stands. Thus, a less competitive growth environment may afford some buffer to drought stress before oak decline but does not appear to help afflicted stands improve their growth and vigor.

The role of environmental factors in oak decline and mortality in the Ozark Highlands

Oak decline is a chronic problem in Missouri Ozark forests. Red oak group species are most susceptible and decline is reportedly more severe on droughty, nutrient-poor sites. However, it was not clear whether greater decline severity was caused by poor site conditions or is simply due to the greater abundance of red oak group species found on poorer sites. We conducted this study to determine whether oak decline severity in oak-dominated Missouri Ozark forests is related to factors strongly influencing site quality including soil, landform position, and slope-aspect. We monitored the survival of 6606 dominant or co-dominant oak trees for 10 years and surveyed crown dieback on more than 1995 oak trees during a single year to determine if mortality and the presence of decline symptoms were related to site factors. Analysis confirmed that red oak group species had more crown dieback and greater mortality than did white oak group species. We also found greater red oak mortality on upper slope positions and where soils were gravelly and low in base cations. However, the abundance of red oaks was also greater in these same locations. Further analysis showed that if the initial abundance of red oaks was included as a covariate in the model, the site factors no longer were significant effects related to oak mortality. Moreover, we found that frequency of oaks exhibiting crown dieback was the same or sometimes greater on high quality sites. These findings show that red oak mortality is more prevalent on droughty and nutrient-deficient sites because red oak group species are more abundant there. Rather than simply predisposing oaks to decline, droughty and nutrient-deficient site conditions most likely favored the establishment and growth of red oaks following the extensive logging during the early 1900s. The extensive oak decline occurring on droughty and nutrient-deficient soils today appears to due to the high abundance of mature red oak group species on these sites.

Oak decline in the Boston Mountains, Arkansas, USA: Spatial and temporal patterns under two fire regimes

A spatially explicit forest succession and disturbance model is used to delineate the extent and dispersion of oak decline under two fire regimes over a 150-year period. The objectives of this study are to delineate potential current and future oak decline areas using species composition and age structure data in combination with ecological land types, and to investigate how relatively frequent simulated fires and fire suppression affect the dynamics of oak decline. We parameterized LANDIS, a spatially explicit forest succession and disturbance model, for areas in the Boston Mountains of Arkansas, USA. Land type distribution and initial species/age class were parameterized into LANDIS using existing forest data. Tree species were parameterized as five functional groups including white oak ( Quercus alba L., Quercus stellata Wangenh., Quercus muehlenbergii Engelm.), red oak ( Qurecus rubra L., Quercus marilandica Muenchh., Quercus falcata Michx., Quercus coccinea Muenchh.), black oak ( Quercus velutina Lam.), shortleaf pine ( Pinus echinata Mill), and maple ( Acer rubrum L., Acer saccharum Marsh.) groups. Two fire regimes were also parameterized: current fire regime with a fire return interval of 300 years and a historic fire regime with an overall average fire return interval of 50 years. The 150-year simulation suggests that white oak and shortleaf pine abundance would increase under the historic fire regime and that the red oak group abundance increases under the current fire regime. The black oak group also shows a strong increasing trend under the current fire regime, and only the maple group remains relatively unchanged under both scenarios. At present, 45% of the sites in the study area are classified as potential oak decline sites (sites where red and black oak are >70 years old). After 150 simulation years, 30% of the sites are classified as potential oak decline sites under the current fire regime whereas 20% of the sites are potential oak decline sites under the historic fire regime. This analysis delineates potential oak decline sites and establishes risk ratings for these areas. This is a further step toward precision management and planning.

Using ecological land types to examine landscape-scale oak regeneration dynamics

The long-standing interest in regenerating oaks stimulated the development of a number of research studies during the past several decades. Most studies have focused on addressing oak regeneration problems and many of these suggested that oak regeneration failures occur where site conditions favor the establishment and growth of competing species that capture the growing space following crown release. Because ecosystem classification regimes incorporate information about site conditions including many of the biophysical factors that largely govern site quality, some forest scientists have proposed that they be used for predicting oak regeneration dynamics. We used a large data set from a replicated landscape-scale experiment comprising nine, 400-ha forest management compartments to systematically examine oak regeneration dynamics following the application of a range of regeneration methods (clearcutting, group and single-tree selection) across differing ecological land types. We found that species of the red oak group appeared to be regenerating successfully only in stands harvested with clearcutting and that regeneration success was greatest in clearcut stands located on ecological land types of lower site quality where the density of competitors was relatively low. In contrast, species of the white oak group were regenerated with clearcutting and with the combination of group and single-tree selection but their regeneration response did not appear to be related to ecological land type. We conclude that an ecological classification framework is valuable for partitioning the landscape into finer-scale land units each having a similar propensity to (1) accumulate red oak reproduction in advance of harvesting and (2) respond to various regeneration methods each in a uniform manner. This framework was less important for examining species in the white oak group, which had similar regeneration dynamics across land types.

Oak mortality risk factors and mortality estimation

Managers are often concerned about oak mortality in maturing mixed-oak forests, but they often lack explicit information about mortality risk for oaks that differ in species, size, crown class, competitive status, and growth rate. In eastern North America, tree species in the red oak group ( Quercus Section Lobatae) are typically shorter lived than those in the white oak group (Section Quercus), and red oak group mortality can be greatly exacerbated during episodes of oak decline. We analyzed factors associated with red oak group mortality for two large data sets from the Ozark Highlands and identified groups of trees that differed significantly in mortality risk. One data set was from a large, upland Ozark forest with more than four decades of uneven-aged management, and the other was representative of relatively undisturbed upland oak forests in the same ecoregion. Neither location experienced significant episodes of oak decline during the period of observation, and our results represent endemic mortality for mature (sawtimber size) forests in the region. We found that mortality rates for trees in the red oak group were four to six times greater than mortality rates for associated trees in the white oak group. We also found that red oak group mortality was significantly related to tree crown class, tree diameter, and basal area of larger competitors. When recent tree dbh growth was known, it significantly increased the ability to predict future mortality for some classes of trees. Annual mortality rates for different classes of trees in the red oak group ranged from 0.4 to 10.7%. Classes of trees in the red oak group that were most likely to die (in rank) were as follows: trees with dbh growth <0.2 cm per year; suppressed trees; trees with intermediate crown class and basal area of larger competitors >14 m 2/ha; dominant and codominant trees >25 cm dbh with basal area of larger competitors <14 m 2/ha; trees with intermediate crown class and basal area of larger competitors <14 m 2/ha; dominant and codominant trees >25 cm dbh with basal area of larger competitors <15 m 2/ha; dominant and codominant trees <25 cm dbh with basal area of larger competitors >15 m 2/ha; dominant and codominant trees <25 cm dbh with basal area of larger competitors <15 m 2/ha; and trees with annual dbh growth greater than 0.6 cm (assuming recent dbh growth is known). This information can be used when preparing silvicultural prescriptions and/or forest management plans to identify and treat classes of trees that are at high risk of mortality.