Changes in Forest Structure Associated with Oak Decline in Severely Impacted Areas of Northern Arkansas

that 286,000 ha in Arkansas were affected by the oak decline. In 2002 and 2003, Guldin et al. (2006) established 181 field plots in Arkansas, Missouri, and Oklahoma and estimated that 33% of red oak density (trees/ha) and 30% of red oak basal area in the region were dead or dying. Kabrick et al. (2004) studied nine forested sites in southern Missouri ranging in size from 312 to 514 ha. They reported that 20.8% of red oaks and 5.5% of white oaks died between 1992 and 2002, although not all mortality was associated with decline. The magnitude and spatial distribution of mortality indicate this decline is a landscape phenomenon. Based on previous oak decline events in the Ozark Mountains (Law and Gott 1987, Starkey and Oak 1989, Jenkins and Pallardy 1995), it is likely that oaks will remain an important forest component at the regional scale. Over time, however, it is unclear how various levels of oak decline will affect forest structure, species composition, and regeneration dynamics at the local scale, particularly in areas exhibiting high levels of mortality. Our objectives in this study were to examine four northern red oak (Quercus rubra L.)–white oak (Quercus alba L.) stands in northern Arkansas that were severely impacted by oak decline to (1) describe the decline-associated changes in forest structure and species composition and (2) examine the regeneration status of red and white oak within these heavily disturbed stands. Methods Study Areas The four stands selected for study—Rotary Ann (59 ha), Chinquapin Knob (39 ha), Pilot Knob (46 ha), and Sand Gap (40 ha)—are located in the Boston Mountains of Arkansas, a heavily forested region in the southwestern portion of the Ozark Plateau. The Boston Mountains are characterized by a sharply dissected landscape and rugged terrain, which forms a band 48–64 km wide and 320 km long from north central Arkansas westward into eastern Oklahoma. Stands are in Pope and Johnson Counties at latitude 3535 to 3543 N and longitude 92 55 to 9315 W. All stands are located on the Bayou Ranger District of the Ozark National Forest. Stands were chosen because they were (1) mature oak stands, (2) very badly damaged by oak decline (according to local Forest Service personnel), and (3) located on southto west-facing upper slopes and ridgetops. Similarity in aspect and topographic position was an important criterion because the stands also are replicates in a long-term prescribed burning project. However, no burning took place before this study. The distance between stands ranged from 6 to 30 km. Elevations varied from 488 to 610 m with slopes of 20–40%. Soils were well drained, gravelly, or stony fine sandy loams in the Nella, Enders, Mountainburg, and Linker series (Garner et al. 1977, Vodrazka et al. 1981). Sampling Design From May to August 2003, 30 field plots were established at Rotary Ann, 18 at Chinquapin Knob, 18 at Pilot Knob, and 21 at Sand Gap. Depending on stand shape, parallel transects were spaced at 80to 380-m intervals, and plots were located along these transects every 55–240 m. In each plot, using a 2.3 m/ha basal area prism, we tallied living trees more than 14 cm dbh and dead trees more than 14 cm dbh that were judged to have died in the past 3 years. Each tree was recorded by species, dbh, and crown condition. For crown condition, tree crowns were assessed visually from the ground and placed into one of the following categories: less than 25% dieback, 25–50% dieback, more than 50% dieback but still alive, and dead. We define “healthy” trees as trees with 50% or less dieback and “dying” trees as living trees with more than 50% dieback. Understory trees and seedlings were also tallied in each stand. Three 0.002-ha understory plots and three 0.0004-ha seedling plots were nested within each prism plot at plot center and 8 m east and west of plot center. Understory trees 1.5–14.0 cm dbh were tallied by species and dbh. Seedlings less than 1.5 cm dbh and more than 60 cm tall were tallied by species and height class (61–90 cm, 91–120 cm, 121–150 cm, and more than 150 cm). Data on red oak borer abundance were gathered from living and dead/dying red oak that were more than 14 cm dbh in four, eight, five, and four randomly chosen prism plots at Rotary Ann, Chinquapin Knob, Pilot Knob, and Sand Gap, respectively. Sampling intensity varied across stands because of availability of field personnel. Red oaks in these plots were examined carefully for red oak borer emergence holes created in 2001 and/or 2003. A total of 27 healthy and 41 dead/dying red oaks were examined. For each tree, the number of borer holes was counted on the lower 2 m of the bole. Emergence hole data were tallied in four classes: 0 holes/tree, 1–5 holes/tree, 6–20 holes/tree, or more than 20 holes/tree (Fierke et al. 2005b). Data Analysis Plot data in each stand were summarized to describe the health of trees more than 14 cm dbh and the density of understory trees and seedlings. We also wanted to determine whether small-diameter trees were more or less likely to be impacted by decline than largediameter trees. For red oaks and white oaks within each stand, the proportion of dead/dying trees 15–25 cm dbh was compared with the proportion of dead/dying trees more than 25 cm dbh. Comparisons were made using two-sample tests of proportion. In addition, a chi-square test was used to examine differences between the number of red oak borer emergence holes in healthy and dead/dying red oak. For all tests, significance was accepted at the P 0.05 level. Results Dead/dying trees more than 14 cm dbh in the four stands ranged from 26 to 38% of total density and from 23 to 42% of total basal area (Table 1). Red oaks (predominantly northern red oak with scattered black oak [Quercus velutina Lam.]) were particularly impacted. There were 79 dead/dying red oak trees/ha at Rotary Ann, 86 trees/ha at Chinquapin Knob, 54 trees/ha at Pilot Knob, and 103 trees/ha at Sand Gap. This represents 51–75% of red oak density in each stand and 40–70% of red oak basal area. Red oak damage exceeded white oak damage in every stand. Impacts on white oak were most pronounced at Rotary Ann and Sand Gap, where 15–27 white oak trees/ha, 25–26% of density, and 13–14% of basal area were affected, respectively. About 10% of hickory (Carya spp.) stems were dead/dying in two stands, but no hickory damage was observed in the other two stands (Table 1). Decline resulted in a shift in species importance. Before decline, red oak density and basal area were greater than any other species in all stands (Table 1). However, we measured more healthy white oak trees than red oak in three stands, and more healthy white oak basal area than red oak in two stands. At Rotary Ann, which was the stand having the greatest proportion of dead/dying red oak, healthy red 18 SOUTH. J. APPL. FOR. 31(1) 2007 oak density decreased from first to fourth in magnitude. Nevertheless, red oak remained an important species in all stands, comprising 15–41% of total density and 27–43% of total basal area (Table 1). In general, the relationship between tree size and health differed between red and white oak. For red oak, there was a high frequency of dead/dying stems across a wide range of diameters (Figure 1). In two stands (Chinquapin Knob and Sand Gap), there was a significantly higher proportion (P 0.03) of dead/dying small dbh (15–25 cm) red oak than large dbh (more than 25 cm) red oak. However, 41 and 47% of large dbh red oak in these stands were dead or dying. Thus, both small and large red oaks were severely impacted in all stands. In contrast, dead/dying white oak trees more than 25 cm dbh were uncommon (Figure 1). At Rotary Ann and Sand Gap, 37–41% of small dbh white oak were dead/dying, but only 4–7% of large dbh trees. Averaging all stands, the proportion of damaged small white oak was three times greater than the proportion of damaged large white oak, although this difference was not significant (P 0.26). There was a significant difference (P 0.001) in the number of red oak borer emergence holes on the boles of healthy and dead/dying red oak (Figure 2). Healthy trees had fewer holes than dead/dying trees. In fact, 78% of the sampled healthy red oak had five or less borer holes, and no healthy tree had more than 20 holes. For dead/dying red oak, 76% of the sampled trees had six or more exit holes, and only three trees had no holes at all. There was a well-developed stratum of understory trees in the study areas (Table 2). The number of trees 1.5–14 cm dbh ranged from 810 to 1,316 stems/ha. There were 37 times more nonoaks than oaks in the understory at Rotary Ann, 6 times more at Chinquapin Knob, and 52 times more at Sand Gap. Important competitors included blackgum (Nyssa sylvatica Marsh.), red maple (Acer rubrum L.), hickory, black cherry (Prunus serotina Ehrh), and flowering dogwood (Cornus florida L.). Only at Pilot Knob was oak density (319 stems/ha) nearly equal to nonoak density (446 stems/ha). Including all stands, density of understory red oak averaged 38 stems/ha, and understory white oak averaged 87 stems/ha. White oak was more numerous than red oak in three stands. There also was a high number of seedlings at each site, with densities of 5,039 –10,018 stems/ha (Table 3). As in the overstory, common competitors to oaks included blackgum, red maple, hickory, black cherry, and flowering dogwood, along with sassafras (Sassafras albidum [Nutt.] Nees). These and other nonoak species made up 64 –94% of the total seedlings and 73–98% of seedlings taller than 150 cm. Among taller seedlings (more than 150 cm) at the four stands, red and white oak density averaged 116 and 63 stems/ha, respectively. Density of tall red oak seedlings equaled or exceeded that of tall white oak in three stands. Table 1. Density and basal area of healthy and dead/dying trees more than 14 cm dbh at four upland oak stands in Arkansas. Site/species Density (trees/ha) Basal area (m/ha) Healthy Dead/dying Dead/dying (%) Healthy Dead/dying Dead/dying (%) Rotary Ann Red oak 26 79 75 3 7 70 White oak 45 15 25 4 1 13 Hickory 12 0 0 1 0 0 Blackgum 52 0 0 2 0 0 Black cherry 6 0 0 1 0 0 Red maple 28 4 13 1 1 8 Others 2 1 33 1 1 50 Totals 171 99 37 11 8 42 Chinquapin Knob Red oak 66 86 57 5 4 44 White oak 44 1 3 3 1 4 Hickory 29 3 10 2 1 8 Blackgum 7 0 0 1 0 0 Black cherry 0 0 0 0 0 0 Red maple 16 0 0 1 0 0 Others 0 0 0 0 0 0 Totals 162 90 36 11 5 31 Pilot Knob Red oak 52 54 51 3 2 40 White oak 88 6 6 5 1 8 Hickory 59 6 9 2 1 12 Blackgum 1 0 0 1 0 0 Black cherry 3 0 0 1 0 0 Red maple 0 0 0 0 0 0 Others 3 7 70 1 1 31 Totals 205 73 26 10 3 23 Sand Gap Red oak 67 103 61 6 6 50 White oak 75 27 26 5 1 14 Hickory 8 0 0 1 0 0 Blackgum 23 0 0 1 0 0 Black cherry 9 0 0 1 0 0 Red maple 31 0 0 1 0 0 Others 3 0 0 1 0 0 Totals 216 130 38 14 6 30 a Mostly northern red oak with scattered black oak b Includes shortleaf pine (Pinus echinata Mill.), white ash (Fraxinus americana L.), black locust (Robinia pseudoacacia L.), and sweetgum (Liquidambar styraciflua L.). SOUTH. J. APPL. FOR. 31(1) 2007 19 Discussion In the severely impacted stands we examined, red oak was more affected than white oak. Other studies of this decline and previous declines also reported a greater susceptibility by red oak to decline (Starkey and Oak 1989, Stringer et al. 1989, Kabrick et al. 2004). The reduction in red oak has shifted these formerly red oak-dominated stands toward a more mixed assemblage of white oak, hickory, red oak, blackgum, and red maple. Dead/dying red oak occurred over a wide range of tree dbh, but dead/dying white oak was most conspicuous among trees 15–25 cm dbh. A similar pattern in the current decline was observed in Arkansas, Missouri, and Oklahoma (Heitzman 2003, Heitzman and Guldin 2004, Guldin et al. 2006). Although no age data were collected, it is likely that many small red and white oak trees in the four stands were about the same age as the larger dbh oaks. Many mature oak forests in northern Arkansas originated after timber harvests and/or wildfires in the early 1900s (Sutton 2001). In the resulting even-aged stands, both red and white oak can persist for extended periods as small trees in lower canopy positions (Soucy et al. 2004, 2005). In this study, such suppressed and presumably older stems were particularly vulnerable to oak decline. Stressed, low vigor oaks are especially susceptible to attack by a variety of organisms (Dunn et al. 1986, Bruhn et al. 2000). That larger red oak also were severely impacted may be due, in part, to the physiological maturity of this cohort. Northern red oak is shorter-lived than white oak (Burns and Honkala 1990). Figure 1. Diameter distributions of healthy and dead/dying red and white oak at four upland oak stands in Arkansas. Figure 2. Number of red oak borer emergence holes in 27 healthy and 41 dead/dying red oaks at four upland oak stands in Arkansas. 20 SOUTH. J. APPL. FOR. 31(1) 2007 Dead/dying red oaks were associated with high populations of the red oak borer. Individual trees with low borer populations generally were healthy, but trees supporting higher numbers of borers usually were dead or dying. The unprecedented insect densities in the phloem and sapwood probably weakened already stressed red oak and contributed to tree death. Because borer holes were examined only on red oak, the influence of the red oak borer on the health of white oak is unknown. White oak has been reported as a host for the borer, albeit an uncommon one (Galford 1983, Fierke et al. 2005a). Furthermore, we did not examine borer-infested red oak for the presence of other possible contributing factors such as Armillaria root rot or the two-lined chestnut borer. It is possible that one or both of these interact with the red oak borer to cause tree death. The canopy gaps created by the death of overstory trees will increase the availability of resources for the abundant understory trees and seedlings at the four stands. It remains unclear whether oaks will compete successfully for these resources and eventually replace dead oaks in upper canopy positions. On the one hand, smaller oaks were greatly outnumbered by faster-growing species such as blackgum, red maple, and black cherry. Field observations indicated that overstory mortality, although widespread, generally was patchy in distribution and rarely included groups of more than several large, dead trees. Because oaks are intermediate in shade tolerance, the size of the openings may be too small for oak recruitment. For successfully regenerating oaks using the group selection method, an average opening diameter of at least twice the height of the surrounding overstory trees is favored by most authorities (Trimble 1973, Miller et al. 1995). Thus, a circular opening among 23-m tall trees should be at least 0.17 ha in size. It appeared that most canopy gaps at the four stands were smaller than this, suggesting that additional disturbances may be needed for successful oak recruitment into larger size classes. On the other hand, oak saplings and seedlings may be well positioned to regenerate these damaged stands. With the exception of Sand Gap, there were over 250 oaks/ha taller than 150 cm (including understory trees) at the study areas. Sander (1972) suggested that oak advance regeneration at least 150 cm tall is likely to compete successfully after a harvest cutting. Furthermore, the four stands are located on southto west-facing upper slopes and ridgetops. Such relatively xeric sites favor oak regeneration over nonoak species (Sander et al. 1984). These findings of abundant oak advance regeneration in decline-impacted areas contrast with other regional studies (Heitzman 2003, Heitzman and Guldin 2004). Those studies, which were not limited to xeric sites, indicated that oak decline was accelerating a change in species importance to nonoak species. The extent to which site factors such as topography and aspect influence oak decline severity is poorly understood. A number of investigators in the southern United States have reported that ridgetops and/or dry aspects had the greatest amount of oak decline (Starkey and Oak 1989, Stringer et al. 1989, Oak et al. 1996). However, more recent work from Arkansas and Missouri suggests that factors other than (or Table 2. Density of understory trees 1.5–14 cm dbh at four upland oak stands in Arkansas. Species Density (trees/ha) Rotary Ann Dbh class (cm) Chinquapin Knob Dbh class (cm) Pilot Knob Dbh class (cm) Sand Gap Dbh class (cm) 2.5 5.0 7.5 10.0 12.5 2.5 5.0 7.5 10.0 12.5 2.5 5.0 7.5 10.0 12.5 2.5 5.0 7.5 10.0 12.5 Red oak 33 0 0 0 0 0 0 0 9 9 9 27 27 9 27 0 0 0 0 0 White oak 0 5 0 0 0 27 18 37 9 9 37 91 37 37 18 8 0 0 16 0 Hickory 126 22 17 5 5 55 27 9 18 9 64 73 55 18 18 71 16 16 8 0 Blackgum 115 99 82 27 22 27 37 9 9 0 46 0 9 0 0 157 149 63 16 8 Black cherry 66 60 17 5 0 18 0 18 0 0 27 37 0 0 0 110 23 16 0 0 Red maple 154 66 33 39 0 101 27 18 18 9 18 0 0 0 0 133 63 39 23 0 Dogwood 22 27 22 11 0 46 46 9 9 9 9 0 0 9 0 39 47 8 0 0 Others 165 49 11 11 0 137 27 0 0 0 18 9 0 27 18 102 102 47 0 0 Totals 681 328 182 98 27 411 182 100 72 45 228 237 128 91 81 620 400 189 63 8 a Mostly northern red oak with scattered black oak. b Includes white ash, black locust, shortleaf pine, elm (Ulmus sp.), Carolina buckthorn (Frangula caroliniana Walter A. Gray), sassafras, pawpaw (Asimina triloba L. Dunal), downy serviceberry (Amelanchier arborea Michx.f. Fern.), and red buckeye (Aesculus pavia L.) Table 3. Density of seedlings more than 60 cm tall and less than 1.5 cm dbh at four upland oak stands in Arkansas. Species Density (trees/ha) Rotary Ann Height class (cm) Chinquapin Knob Height class (cm) Pilot Knob Height class (cm) Sand Gap Height class (cm) 61–90 91–120 121–150 150 61–90 91–120 121–150 150 61–90 91–120 121–150 150 61–90 91–120 121–150 150 Red oak 412 138 109 193 91 138 138 46 321 138 91 183 314 79 40 40 White oak 138 82 0 27 640 274 91 183 778 230 91 0 78 0 40 40 Hickory 412 356 247 329 595 321 46 46 230 230 46 0 314 432 195 472 Blackgum 356 329 138 274 595 183 412 91 274 183 138 91 393 746 274 746 Black cherry 301 385 220 440 46 46 46 91 91 46 46 91 158 40 158 314 Red maple 576 850 603 961 1647 961 321 412 138 91 46 46 785 1,215 393 1,020 Dogwood 301 220 54 82 230 0 91 0 138 0 0 0 235 119 0 79 Sassafras 0 0 0 0 320 183 138 274 183 138 46 91 0 79 0 40 Others 274 326 163 356 0 46 0 0 457 138 46 183 79 274 237 590 Totals 2,770 2,686 1,534 2,662 4,164 2,152 1,283 1,143 2,610 1,194 550 685 2,356 2,984 1,337 3,341 a Mostly northern red oak with scattered black oak. b Includes white ash, black locust, elm, Carolina buckthorn, pawpaw, downy serviceberry, red buckeye, plum (Prunus sp.), and witch-hazel (Hamamelis virginiana L.). SOUTH. J. APPL. FOR. 31(1) 2007 21 perhaps in addition to) topography and aspect, such as species composition, tree age, and crown position, are important determinants of oak decline that confound the influences of site (Kabrick et al. 2004, Poole et al. 2006). Although decline is most severe on xeric sites such as those we studied, not all xeric sites in the Boston Mountains display the high levels of damage we have reported. Conclusion In the heavily impacted stands we examined, red oak was no longer the dominant species it was before the decline event. However, red oak was not eliminated from upper canopy positions and remained a common overstory tree at all four sites. Given the complex species composition and densities of understory trees and seedlings and the relatively short period of time since the stands were disturbed, it is difficult to predict whether decline-associated mortality will stimulate oak regeneration or accelerate a transition to nonoak forest types. 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