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Dendroecology, Forest Composition, and Land-Use History of a Suburban Cross Timbers Forest in Central Oklahoma
Chad B. King and Justin Cheek

Urban Naturalist, No. 6 (2015): 1–20

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Urban Naturalist 1 C.B. King and J. Cheek 22001155 URBAN NATURALIST No. 6:N1–o2. 06 Dendroecology, Forest Composition, and Land-Use History of a Suburban Cross Timbers Forest in Central Oklahoma Chad B. King1,* and Justin Cheek1 Abstract - Previous research has indicated that the Cross Timbers are in a state of transition from Quercus-dominance to becoming co-dominant with non-Quercus species due to fire suppression and climate change. However, there were no reports that documented suburban Cross Timbers patches were undergoing such shifts in composition. Herein we show that a suburban Cross Timbers forest is likely exhibiting a similar transition due to historic land-use practices. We investigated the dendroecology and land-use history of a Cross Timbers patch surrounded by a commercial and residential matrix at E.C. Hafer Park, Edmond, OK. This land had a history of agriculture dating to the early 20th century that transitioned to an unmanaged recreational park during the early 1980s. The most-important canopy species continue to be Quercus stellata (Post Oak; relative importance value = 32.81) and Q. marilandica (Blackjack Oak; relative importance value = 27.52), but these are absent as saplings. Native and introduced shrub species had the highest densities (1254 stems/ha) in the understory, creating a second canopy that is contributing to reduced Quercus recruitment. Several non-Quercus species recruited beginning in the 1950s and have probably increased in importance, particularly Juniperus virginiana (Eastern Redcedar; relative importance value = 13.6). The results suggest Q. marilandica and Q. stellata dominance in this surburban area is transitioning to a forest with increasing importance of non-Quercus species. Introduction Studies of stand dynamics are often conducted in old-growth forests that provide a deeper understanding of development and succession in eastern deciduous forests (Abrams and Copenheaver 1999, King and Muzika 2014, Ruffner and Abrams 1998) and the western terminus of the eastern deciduous forest (Abrams 1986, Hallgren et al. 2012). Often tied to forest succession is the pattern of land-use history and how land-use has contributed to the contemporary structure (Hall et al. 2002). However, studies that specifically address dendroecology and forest structure are rare in urban and suburban settings of the United States (Copenheaver et al. 2013, Loeb et al. 2015). The structure of forests in suburban parks reflect ecological and cultural values of the community (Lawrence 1993). Succession of these areas occur with prevailing disturbance regimes (wind, ice, insect outbreaks, fire, herbivory) but also with a suite of anthropogenic effects (pollution, exotic species, park maintenance). Therefore, the existing forest may retain historic legacies but also include species that had not been previously documented and which may lead to a transition in species composition (Dolan 2015, Fahey et al. 2012). Additionally, changes in tree density 1Department of Biology, University of Central Oklahoma, Edmond, OK 73034. *Corresponding author - cking24@uco.edu. Manuscript Editor: Katalin Szlavecz Urban Naturalist C.B. King and J. Cheek 2015 No. 6 2 and species composition are attributable to how the urban forest is used by humans (Loeb et al. 2015) and the effect of urbanization on forests and savannas (McBride and Jacobs 1986). While studies of the dynamics and dendroecology of urban forests and natural areas are becoming more common, there is a lack of data in these research areas pertaining to urban and suburban forests in the Cross Timbers of Oklahoma and Texas. Studies of the historic Cross Timbers of the south-central United States have emphasized the contemporary structure (Bragg et al. 2012, Rosiere et al. 2013, Stahle et al. 2007), long-term changes in structure (DeSantis et al. 2010a, 2011), historic disturbance dynamics (Allen and Palmer 2011, DeSantis et al. 2010b, Stambaugh et al. 2009), and contemporary disturbance effects (Shirakura et al. 2006). The Cross Timbers, located from southeastern Kansas to west-central Texas (Hoagland et al. 1999, Rice and Penfound 1959), is a mosaic of upland oak forest, savanna, and prairie dominated by Quercus stellata (Post Oak) and Q. marilandica (Blackjack Oak). One of the current concerns in the Cross Timbers ecoregion is that fire suppression during the 20th century has led to changes in species composition (DeSantis et al. 2011). With the recognition of the importance of fire on the Cross Timbers landscape, there has been an increasing effort to reintroduce fire (Burton et al. 2010, 2011; Hallgren et al. 2012). Although prescribed fire can have benefits by reducing fuel and non-Quercus species densities while promoting Quercus regeneration (Hallgren et al. 2012) in the Cross Timbers ecoregion, there are likely limitations to reintroducing fire in Cross Timbers patches that are in a recreational park setting surrounded by a residential and commercial matrix. The limitations of using prescribed fire in suburban Cross Timbers forests provides an opportunity to study development in the absence of fire. We investigated a patch of Cross Timbers forest within a matrix of suburban development in central Oklahoma to understand the contemporary structure and historic dynamics of this forest during the 20th century. We used land-use–history records and dendroecology to understand the development of the contemporary forest in a suburban park. Land-use history provides indirect evidence of pre-European and Euro-American settlement activity that is exhibited in vegetation change (Hart et al. 2008, King and Muzika 2014, Ruffner and Abrams 1998). Federal land patents (transfer of land titles from the US federal government to individuals), General Land Office (GLO) surveys, United States Census records, and historical maps can provide valuable information about land ownership and land use that can be related to the quantitative analysis of forest structure and dynamics. Dendroecology can be used to identify naturally occurring events (wind, ice, fire, insect outbreaks) and anthropogenic events (logging, grazing, fire) that occurred decades to centuri es earlier. The objectives of our study were to (1) quantify the contemporary structure and composition of the Cross Timbers at E.C. Hafer Park and (2) compare the historical development and dynamics of this forest using land-use records and dendroecology in order to understand the current structure. We hypothesized that given the isolated nature of this area, the forest is exhibiting a transition from Quercus dominance due to land-use changes during the 20th century. The contemporary structure and Urban Naturalist 3 C.B. King and J. Cheek 2015 No. 6 composition of the forest can be influenced by historical land-use practices (Dolan 2015, Fahey et al. 2012) prior to the designation of the park at the study site. It is unknown if urban Cross Timber forests are exhibiting similar responses to fire suppression and changes in land-use practices documented in other Cross Timbers studies (DeSantis et al. 2010a, Rice and Penfound 1959). Little to no management occurs in the park’s forest. Trees along asphalt trails are trimmed, but the forest interior is absent of any management. This provides an opportunity to explore the current structure and dendroecology and infer changes based on the land-use history of the study site. Site Description E.C. Hafer Park (35°38'34"N, 97°27'18"W) is located within the city limits of Edmond, OK (2014 population = 81,405 people). The City of Edmond is located in the northern portion of the greater Oklahoma City area. The community park is 48.9 ha (121 acres) in size and was commissioned in 1979. According to the USGS topographic map of 1964, this area was a sewage treatment location adjacent to Spring Creek for the city of Edmond. Further exploration of the site revealed that the City of Edmond purchased the land in 1952 and decommissioned the disposal site in 1972. Treated wastewater was released into nearby Spring Creek, and treated biosolids were shipped to local farms (Chris Neifing, Water Resources Superintendent, City of Edmond, OK, pers. comm.) The location for the current study is a ~7.1-ha (17.5-acre) forest within the park. The park is surrounded by commercial and residential development. E.C. Hafer Park is located on the western terminus of the Cross Timbers region, specifically the Post Oak–Blackjack Oak vegetation type that his torically included a mosaic of forest, woodland, and grassland (Hoagland 2008). Climate in this region is considered continental with a mean annual temperature of 15.63 °C (60.1 °F) and mean annual precipitation of 91.4 cm (36 inches). Monthly mean precipitation is bimodal with the greatest amounts of precipitation during May–June and again during September–October (Oklahoma Climatological Survey 2014). Soils at the study site are generally classified as Stephenville-Darnell-Niotaze, which include shallow, sandy and loamy, well-drained soils that are moderately acidic (Carter and Gregory 2008, Dominick 2003). Elevation of the study site exhibits a gradual slope from the north park boundary (347 m) to the south park boundary along Spring Creek (329 m). Methods Land-use history Land ownership and land-use was traced to the Oklahoma Land Run of the “Unassigned Lands” of 1889. The Oklahoma Land Run was a series of events in the late 19th century that opened land in present-day Oklahoma to Euro-American settlement (Goins and Goble 2006). The Bureau of Land Management provided original Federal Land Patents. Federal Land Patents indicate the transfer of federal land to private Urban Naturalist C.B. King and J. Cheek 2015 No. 6 4 ownership and gives an indication of early land owners of the study site. Federal Land Patents were recorded in 1893 based on the location of the study site (SW ¼, Section 31, Township 14N, Range 2W; (BLM 2014). For purposes of understanding pre-Euro-American tree composition in the township, we accessed the General Land Office (GLO) Survey of 1871. Notes provided information about tree species at section corners and quarter-sections that provided a general description of tree species found in the vicinity of the study site (BLM 2014). Percentage of species identified in the township was compiled for comparing to the contemporary forest structure at the study site. To infer land-use patterns at the study site, we accessed online plat maps and land deed records through the Oklahoma County Assessor’s Office (2014). We traced land ownership to a 1905 plat map of the township, which indicated a transfer of ownership at some point between 1893 and 1905. Other land deeds provided information on land transfer from private ownership to City of Edmond ownership during the 1970s. Personal communications with the City of Edmond Urban Forester (R. Ochsner) and Water Resources Superintendent (C. Neifing) provided a timeline of land-use patterns by the City of Edmond from the 1950s to present day. We utilized United States Census Records (Ancestry 2014) to record the employment type of the landowner during the early 20th century. Employment of the land owner via the census records provided information on possible land-use practices at the study site. Forest structure To analyze forest structure and composition, we collected data on eighteen 0.04- ha (radius = 11.28 m) fixed-area plots within the 7.1-ha study site. We established two 450-m transects, 100 m apart, from north to south, and systematically located plot centers at 50-m intervals along the transects. To characterize the overstory, we measured the girth of all trees with diameter at breast height (DBH, 1.37 m above ground surface) >10 cm and identified them to species. We calculated relative density (trees/ ha), relative dominance (m2/ha), and relative frequency for each species (Cottam and Curtis 1956) and relative importance values ([relative density + relative dominance + relative frequency] / 3) for all overstory species (Ruffner and Abrams 1998). We collected increment cores from 5 to 6 trees >10 cm DBH at 30 cm above ground level for age determination and radial-growth analysis in each plot. To characterize understory structure and species composition, we established nested 0.01-ha and 0.002-ha plots to categorize saplings (>1.37 m height, <10 cm DBH) and seedlings (<1.37 m height), respectively. We marked out 2 nested 0.01- ha (radius = 5.64 m) plots at 5.64 m north and south of the center of each 0.04-ha plot, and sampled for seedlings within 0.002-ha (radius = 2.52 m) plots at the center of each of the 0.01-ha plots. Prior to initiation of this study, reconnaissance demonstrated several woody shrub species in the understory, and these were included within the respective sapling or seedling category. Radial growth analysis We collected increment cores (n = 115) at 30 cm above ground level from the 5 to 6 largest diameter trees in each plot. Multiple species were represented in the increment- core data, including Post Oak (n = 37), Blackjack Oak (n = 35), Juniperus Urban Naturalist 5 C.B. King and J. Cheek 2015 No. 6 virginiana (Eastern Redcedar; n = 24), Celtis laevigata (Sugarberry; n = 5), Ulmus rubra (Slippery Elm; n = 9), Sideroxylon lanuginosum (Chittamwood; n = 2), Juglans nigra (Black Walnut; n = 2), and Catalpa sp. (n = 1). In the event of heart rot or a hollow tree, an additional increment core was collected at 60 cm above ground level. We prepared increment cores for analysis in the laboratory following the methods of Stokes and Smiley (1996). Increment cores were air-dried, glued to wooden mounts, and sanded with progressively finer sandpaper (80-grit to 1200- grit) to expose the cellular structure of the increment core when viewed under a binocular microscope. We used multiple crossdating procedures in order to accurately assign age of each tree. We created skeleton plots (Stokes and Smiley 1996) for crossdating of Eastern Redcedar. This species is well known for having false-ring formation (Edmondson 2010, Kuo and McGinnis 1973) and pinching rings that can make crossdating difficult. We measured tree-ring width (mm) to the nearest 0.001 mm for all samples using a Velmex TA Unislide System (Velmex, Inc., Bloomfield, NY), binocular microscope, and J2X measurement software (Voortech Consulting, Holderness, NH). We compared measurement series visually using the list method (Yamaguchi 1991), and statistically using the program COFECHA (Grissino-Mayer 2001, Holmes 1983). In the event of a missing pith, we used the methods of Duncan (1989) to estimate the number of missing tree-rings to pith. To assess the patterns of radial growth at the study site, we standardized ring-width indices using the program ARSTAN (Cook and Holmes 1986). We interactively detrended using a negative exponential curve or linear regression line to each series to remove age-related growth trends. If the negative exponential curve does not fit the data, the program then applies a linear function to each ring-width series. We created a master ring-width index (mean = 1) for the 3 species with highest relative importance values and greatest sample depth in order to assess patterns of above and below-average growth during the 20th century. We assessed changes in radial-growth patterns using growth-release analysis to infer canopy-disturbance dynamics. Growth releases (a proxy for canopy disturbance) are increases in radial growth of surviving trees that exceed a predetermined threshold via a percent growth change equation (Hart and Grissino-Mayer 2008, Nowacki and Abrams 1997, Rubino and McCarthy 2004). This approach can assess the frequency and magnitude of growth-release events that are related to current forest structure. Using raw-ring widths for each tree-ring series measured via the Velmex system, we analyzed the percent change in radial growth by comparing running raw ring-width medians of the previous and subsequent 10 years (Hart and Grissino-Mayer 2008). We categorized growth-release events in overstory trees (>10 cm DBH) as >25% increases in growth in which the increase in median growth was sustained for at least 10 years (Nowacki and Abrams 1997). To categorize understory (<10 cm DBH) growth releases, we used the methods of Lorimer and Frelich (1989) in which major releases (>100% increase in radial growth) last 15 years and moderate releases (>50% increases in radial growth) last 10–15 years. Urban Naturalist C.B. King and J. Cheek 2015 No. 6 6 Results Pre-European settlement and land-use history Post Oak and Blackjack Oak were most frequently cited in the 1871 GLO survey of Lincoln Township (T14N, R2W), accounting for 76% of all trees recorded at quarter sections and section corners (Table 1). These 2 species were most often cited in upland areas of the township, while Q. macrocarpa (Bur Oak), Black Walnut, and Salix nigra (Black Willow) were identified along riparian areas. There was no reference to Eastern Redcedar in the GLO notes for Lincoln Township; however, this omission does not indicate the absence from the township. The GLO notes make reference to the physiognomy of Lincoln Township including “prairie”, “scattered timber”, and “heavy timber” throughout the township. Post Oak, Blackjack Oak, and Ulmus spp. were noted along the section lines that included our study site (Section 31, T14N, R2W). European settlement of the township was tied to the original Land Run of 1889 in the “Unassigned Lands” of Oklahoma. However, Federal Land Entries did not begin at our study site until 1893 (Table 2) under the name Joseph Quein. By 1905, ownership of the study site passed to Henry Habben and remained in the Habben family for approximately 35 years. Habben described his employment as “farming” and “general farming” in the US Census (Table 2). The 1905 plat map indicates 2 structures existing on the property. (Table 2). The land was purchased by the City of Edmond in 1952 for the purpose of a wastewater treatment facility that was decommissioned in 1972 (C. Neifing, pers. comm.). The sewage lagoon (now a pond) and water tower are still present at the park. The City of Edmond continued to purchase small tracts of land in and around the study site during the early 1970s. E.C. Hafer Park was commissioned in 1979, and several trails were built in the park in the early 1980s. Forest structure and composition Total basal area of trees >10 cm DBH was 36.72 m2/ha, and tree density was 419 trees/ha (Table 3). The most important species were Post Oak and Blackjack Table 1. Species composition for 146 trees identified in the 1871 General Land Office survey of Lincoln Township (T14N R2W), Oklahoma County, OK. Probable species indicates likely species identified based on contemporary distributions in central Oklaho ma. Name in GLO notes Probable species % composition Post oak Quercus stellata Wangenh. (Post Oak) 39.7 Blackjack oak Quercus marilandica Münchh. (Blackjack Oak) 36.3 Black oak Quercus velutina Lam. (Black Oak)/ 15.1 Quercus shumardii Buckland (Shumard Oak) White oak Quercus muehlenbergii Engelm. (Chinkapin Oak) 3.4 Bur oak Quercus macrocarpa Michx. (Bur Oak) 2.1 Elm Ulmus americana L. (American Elm)/ 2.1 Ulmus rubra Muhl. (Slippery Elm) Walnut Juglans nigra L. (Black Walnut) 0.7 Willow Salix nigra Marshall (Black Willow) 0.7 Urban Naturalist 7 C.B. King and J. Cheek 2015 No. 6 Oak (Table 3). These 2 species along with Eastern Redcedar comprised 74% of the total relative importance. Ten other species were noted in the overstory of the park but only accounted for a minor proportion of relative importance (Table 3). The 2 largest individuals based on DBH were a Black Walnut (36.4 cm) and a Post Oak (35.2 cm). Post Oak, Blackjack Oak, and Eastern Redcedar had the highest tree densities and >83% of trees at the study site. Post Oak had the highest basal area (15.15 m2/ha) including >41% of the total relative dominance. Eastern Redcedar, Black Walnut, and Slippery Elm followed Post Oak and Blackjack Oak in importance, together comprising 19.7% of the total relative dominance (Table 3). The smallest diameter class (10.0–12.9 cm) of the five most important overstory species (Post Oak, Blackjack Oak, Eastern Redcedar, Slippery Elm, Black Walnut) accounted for >86 trees/ha (Fig. 1). These same 5 tree species comprised >89% of the total tree density (trees/ha) in the 4 largest diameter classes (Fig. 1). In the understory, the sapling and seedling layer displayed stem densities of 3926 stems/ha and 10,191 stem/ha, respectively (Table 4). Twelve species were identified in the understory and 13 species in the seedling layer. Ligustrum sinense (Chinese Privet) had the highest density in the understory, comprising 27% of the total stem density. There were no Post Oak and Blackjack Oak saplings identified in the sapling layer. Post Oak had the highest density in the seedling layer with 1568 stems/ ha and accounted for 15% of the total seedling density (Table 4). Shrubs in the understory included Symphoricarpos orbiculatus (Coralberry), Cornus drummondii (Roughleaf Dogwood), and Zanthoxylum americanum (Common Prickly-ash), and the introduced Chinese Privet. Shrubs comprised >29% of the total seedling density and 47% of the understory density (Table 4). Table 2. Land-use history of E.C. Hafer Park area (SW ¼, Section 31, Township 14 North, Range 2 West). Date Historical events Source 1871 Original land survey BLM (2014) 1889 Land run of the “unassigned lands” Goins and Goble (2006) 1893 First Federal land entries under the BLM (2014) Homestead Act of 1862 1905 Private ownershipA 1905 Lincoln Township, Oklahoma Plat Map (www.oklahomacounty.org/assessor) 1910 “Farming”A 1910 US Census (Ancestry 2014) 1940 Private OwnershipA “General farming” 1940 US Census (Ancestry 2014) 1952–1972 City of Edmond sewage treatment 1964 USGS Topographic Map (University facility of Central Oklahoma Library Archives); Water Resources Superintendent, City of Edmond 1971–1975 Land deeds to City of Edmond Oklahoma County Assessor (2014) 1979 E.C. Hafer Park commissioned City of Edmond website (www.edmondok. com) ARecords indicate that Henry Habben and descendants owned this property. Urban Naturalist C.B. King and J. Cheek 2015 No. 6 8 Table 3. Density (trees/ha), relative density, basal area (m2/ha), relative dominance, relative frequency and relative importance values of trees >10 cm at E.C. Hafer Park, Edmond, OK. Relative importance = (relative density + relative dominance + relative frequency) / 3 (Ruffner and Abrams 1998). I.V. = importance values. Density Relative Basal area Relative Frequency Relative Relative Species (trees/ha) density (%) (m2/ha) dominance (%) (# of plots) frequency (%) I.V. importance (%) Quercus stellata (Post Oak) 153 36.50 15.15 41.25 18 20.69 98.44 32.81 Quercus marilandica (Blackjack Oak) 139 33.20 11.37 30.98 16 18.39 82.57 27.52 Juniperus virginiana L. (Eastern Redcedar) 56 13.40 3.31 9.01 16 18.39 40.80 13.60 Ulmus rubra (Slippery Elm) 19 4.53 1.38 3.76 8 9.20 17.40 5.83 Catalpa spp. 10 2.39 0.40 1.10 6 6.90 10.40 3.46 Juglans nigra (Black Walnut) 10 2.39 2.54 6.93 5 5.75 15.10 5.02 Ulmus americana (American Elm) 8 1.91 0.53 1.44 5 5.75 9.10 3.03 Sideroxylon lanuginosum Michx. (Chittamwood) 6 1.43 0.92 2.51 3 3.45 7.39 2.46 Celtis occidentalis L. (Common Hackberry) 6 1.43 0.18 0.49 3 3.45 5.37 1.79 Morus rubra L. (Red Mulberry) 4 0.95 0.27 0.74 2 2.30 3.99 1.33 Celtis laevigata Willd. (Sugarberry) 4 0.95 0.13 0.35 2 2.30 3.60 1.20 Quercus muehlenbergii (Chinkapin Oak) 3 0.72 0.41 1.13 2 2.30 4.15 1.38 Populus deltoides W.Bartram ex Humphry Marshall 1 0.24 0.12 0.33 1 1.15 1.72 0.57 (Eastern Cottonwood) Totals 419 100.00 36.72 100.00 87 100.00 300.00 100.00 Urban Naturalist 9 C.B. King and J. Cheek 2015 No. 6 Dendroecology A total of 115 increment cores were collected from 9 species. We were unable to crossdate 6 samples (3 Blackjack Oak, 3 Eastern Redcedar) due to rot or missing rings. The oldest trees by species that were >10 cm DBH included Post Oak (112 years: 1901–2013), Blackjack Oak (105 years: 1908–2013), Chittamwood (84 years: 1929–2013), Black Walnut (63 years: 1950–2013), Eastern Redcedar (56 years; 1957–2013), Slippery Elm (35 years: 1978–2013), Sugarberry (34 years: Figure 1. Diameter distributions for all trees and the 4 mostimportant species >10 cm DBH at E.C. Hafer Park, Edmond, OK. The “all other species” category includes Catalpa spp., Sugarberry, Black Walnut, Slippery Elm, Common Hackberry, American Elm, Chinkapin Oak, Chittamwood, and Eastern Cottonwood. Note different y-axis scale for “all trees” graph. Urban Naturalist C.B. King and J. Cheek 2015 No. 6 10 1979–2013), Celtis occidentalis (Common Hackberry; 29 years: 1984–2013), and Catalpa spp. (22 years: 1991–2013). Recruitment of Post Oak and Blackjack Oak was continuous between 1910 and 1970 (Fig. 2). Of particular note was the recruitment of 35% (n = 38) of trees during the 1950s (Fig. 2). Eighty-one percent (n = 35) of current trees >10 cm DBH that recruited after 1960 were non-oak species (Eastern Redcedar, Catalpa spp., Chittamwood, Black Walnut, Sugarberry, Common Hackberry, Slippery Elm; Fig. 3). Radial-growth analysis of the 3 most important species (Post Oak, Blackjack Oak, Eastern Redcedar) exhibited periods of variable growth during the 20th century (Fig. 4). Periods of increasing radial growth during the mid-1910s, early 1920s, and late 1930s (Fig. 4) was consistent with recruitment of Post Oak, Blackjack Oak, and Chittamwood prior to the 1950s (Figs. 2, 3). An increase in radial growth during the late 1950s occurred with a recruitment pulse of oak species and one Eastern Redcedar. Reduced radial growth and recruitment occurred during the late 1960s and early 1970s. Following a reduced period of growth during the late 1970s and early 1980s (Fig. 4), increases in radial growth were concurrent with recruitment of non-oak species into the early 1990s (Figs. 2, 3). A total of 49 growth-release events were identified between 1920 and 1990. Release events occurred in Post Oak (n = 28 releases), Blackjack Oak (n = 17 releases), Black Walnut (n = 2 releases), and Eastern Redcedar (n = 2 releases). Approximately 53% of release events were moderate (>50% increase in radial growth for 10–15 years; Fig. 5). Growth releases were historically rare between Table 4. Stem densities (stems/ha) of seedlings, saplings, shrubs, and trees at E.C. Hafer Park, Edmond, OK. Seedling Sapling Tree Species name density density density Quercus stellata (Post Oak) 1568 0 153 Sideroxylon lanuginosum (Chittamwood) 1411 118 6 Symphoricarpos orbiculatus Moench (Coralberry) 1372 0 0 Ulmus spp. (elms) 1254 470 27 Cercis canadensis L. (Eastern Redbud) 862 235 0 Ligustrum sinense Lour. (Chinese Privet) * 706 1058 0 Celtis spp. 784 517 10 Cornus drummondii C.A. Mey (Roughleaf Dogwood) 862 627 0 Quercus marilandica (Blackjack Oak) 667 0 139 Juniperus virginiana (Eastern Redcedar) 510 431 56 Robinia pseudoacacia L. (Black Locust) 78 0 0 Quercus muehlenbergii (Chinkapin Oak) 78 118 3 Zanthoxylum americanum Mill. (Common Prickly-ash) 39 157 0 Morus rubra (Red Mulberry) 0 78 4 Juglans nigra (Black Walnut) 0 78 10 Ailanthus altissima (Mill.) Swingle (Tree of Heaven)* 0 39 0 Catalpa spp. 0 0 10 Populus deltoids (Eastern Cottonwood) 0 0 1 Total 10191 3926 419 *Indicates introduced species. Urban Naturalist 11 C.B. King and J. Cheek 2015 No. 6 1900 and 1980, with 3 being the greatest number of trees exhibiting a release during a decade. During the early 1980s, 35% of trees (n = 34) exhibited a growth release (Fig. 5). Discussion Forest structure The 2 most important species identified were Post Oak and Blackjack Oak (Table 3), typical of upland Cross Timbers forests (Dyksterhuis 1948, Hoagland et al. 1999). Our study site also exhibited 13 species in the overstory, a degree of diversity similar to that found in other studies of the Cross Timbers (Bragg et al. 2012, Karki 2007, Roserie et al. 2013). Of interest in the Cross Timbers at our study site is the high density of Eastern Redcedar (Table 3). Studies have highlighted the increases in Eastern Redcedar density and basal area in Cross Timbers forests during the 20th century (DeSantis et al. 2010a, Hallgren et al. 2012) attributable to changes in fire regimes (DeSantis et al. 2011) that likely promoted Eastern Redcedar populations. Additionally, we note a suite of tree species (e.g., Ulmus spp., Celtis spp.) that may indicate a change in species composition that reflects fire suppression and land-use change (Nowacki and Abrams 2008). Our results demonstrate an increase in non-oak recruitment during City of Edmond ownership (Fig. 3). DeSantis et al. Figure 2. Decade of recruitment at 30 cm above ground for trees in E.C. Hafer Park, Edmond, OK. The category “non-oak species” includes other species that were cored in plots: Eastern Redcedar, Catalpa spp., Sugarberry, Black Walnut, Slippery Elm, Common Hackberry, American Elm, and Chittamwood. Urban Naturalist C.B. King and J. Cheek 2015 No. 6 12 (2010) demonstrated an increase in density and basal area of non-oak species across several sites in the Oklahoma Cross Timbers attributable to anthropogenic changes on the landscape. The forest at our particular study site experiences general park maintenance that only consists of tree trimming along asphalt trails. The lack of repeated disturbances (e.g., fire, thinning, agriculture) at this site has likely contributed to the increase in non-oak species since the 1960s (Fig. 3). Figure 3. (A) Age-diameter relationships for 109 cored trees at E.C. Hafer Park, Edmond, OK. QUMA = Quercus marilandica (Blackjack Oak); QUST = Quercus stellata (Post Oak). Non-oak trees include Eastern Redcedar, Catalpa spp., Sugarberry, Black Walnut, Slippery Elm, Common Hackberry, American Elm, and Chittamwood. (B) Timeline of land-use patterns during the 20th century. Urban Naturalist 13 C.B. King and J. Cheek 2015 No. 6 The successional transition that is occurring at the study site has been documented in other urban forests in the eastern deciduous forest. Breen (2015) demonstrated that succession in suburban deciduous communities in Maryland was dependent on the time since agricultural abandonment. Shade-intolerant species dominated in recently abandoned lands while shade-tolerant species dominated areas that had a longer time since abandonment. Loeb et al. (2015) suggested that the structure of 3 ridge forests in Nashville, TN, were a product of trampling by hiking and hunting Figure 4. Mean ring-width indices that show radial growth patterns for the 3 most important (based on relative importance values, Table 3) species at E.C. Hafer Park, Edmond, OK. Horizontal line represents mean = 1 for each ring-width index that allows for analysis of above-average (above horizontal line) and below-average growth (below horizontal line) for each species. Sample depth (shaded area) shows the number of cross-dated samples in the ring-width index for each species. Urban Naturalist C.B. King and J. Cheek 2015 No. 6 14 practices that changed the effects of herbivory. Our results suggest that agricultural practices in the early 20th century favored the shade-intolerant oak species. Stem densities for oak increased following agricultural abandonment in 1952 and nonoak stem densities began to increase during the 1970s (Figs. 2, 3). Seedling and sapling densities were within a similar range as that found by Clark and Hallgren (2003). Based on 3 sites, they indicated an average of 10,297 seedlings and 1904 sapling stems. Of interest was the lack of Post Oak and Blackjack Oak saplings in our study (Table 4). DeSantis et al. (2010a) indicated a 43% and 29% decrease in Blackjack Oak and Post Oak sapling densities, respectively, between the 1950s and 2000s. McGrath (2012) also reported declines in oak sapling densities along with sapling mortality rates >70% between 1998 and 2008 at a Cross Timbers site in northeast Oklahoma. These declines in sapling densities are attributed to reduced light availability because both Post Oak and Blackjack Oak are moderately shade intolerant. At our study site, oak saplings are competing for light availability due to 2 developing canopy layers: an overstory layer and a shrub layer. Shrubs accounted for 47% of the understory density (Table 4) and likely create a second canopy layer that is affecting oak recruitment. Of particular concern in this suburban forest is the presence of Chinese Privet, a non-native shrub species that is native to southeast Asia (Merriam and Feil 2003). Figure 5. Frequency of major (>100% increase in growth for minimum 15 years), moderate (>50% increase in growth for 10–15 years), and overstory (>25% increase in growth for minimum 10 years) release events by decade at E.C. Hafer Park, Edmond, OK. Number above each bar indicates number of trees that exhibited a release. Release events occurred in Post Oak (n = 28 releases), Blackjack Oak (n = 17 releases), Black Walnut (n = 2 releases: 1960s, 1980s), and Eastern Redcedar (n = 2 releases: 1970s, 1980s). Urban Naturalist 15 C.B. King and J. Cheek 2015 No. 6 This shrub is known for its shade tolerance in closed-canopy forests (Morris et al. 2002), high stem densities in the forest understory (Hart and Holmes 2013), and prolific seed production (Westoby et al. 1983). This non-native species had the highest stem density of all species in the understory (Table 4). The development of a second canopy in the understory due to the high stem density of Chinese Privet along with a closed overstory canopy is likely contributing to reduced success of oak from seedling to sapling. Another factor may be contributing to reduced success of oak from seedling emergence to sapling. Lorimer (1993), in his review of oak-regeneration problems, indicates that deer herbivory can be a contributing factor that affects advanced regeneration of oak in eastern forests. He specifically notes that on the Allegheny Plateau of Pennsylvania there are up to 78 deer per km2 (30 deer per mi2 ). There are approximately 500,000 Odocoileus virginianus (Zimmermann) (White-tailed Deer) in Oklahoma (18.4 per km2 [7.1 per mi2 ]) based on a 2014 estimate (Masters et al. 2014). While deer can affect oak regeneration, their effect at the study site is likely limited. Forest dynamics and dendroecology The 1871 General Land Office survey for Lincoln Township indicated a strong Post Oak /Blackjack Oak component indicative of Cross Timbers forests. A small component (3.4%) of the survey indicated “white oak”. These trees were probably not Quercus alba L. (White Oak) because the edge of its contemporary distribution is >200 km to the east of our study site. Rather, the surveyor may have been identifying Q. muehlenbergii (Chinkapin Oak) as “white oak” based on bark coloration or using less-precise classification (Table 1). The composition of our study site during the early 20th century was dominated by Post Oak and Blackjack Oak. The oldest living tree found at the study site dated to the late 19th century (Fig. 2). The oak composition of the early 20th century may be explained by the use of fire by the landowner (Table 2) that maintained an oak savanna (Abrams 1992) at the site. Previous fire-history studies in the Oklahoma Cross Timbers found that surface fires were frequent events occurring on average every 1 to 5 years during the late 19th and early 20th century (Allen and Palmer 2011, Clark et al. 2007, DeSantis et al 2010b, Stambaugh et al. 2009). We did not find evidence of historic fires (charcoal, fire scars) at our study site. However, based on fire-scarred remnant wood, we have documented 11 fires that burned between 1905 and 1948 at another site approximately 6 km east of the present study site (C.B. King, 2014 unpubl. data). Another possible explanation for the relatively young age of this forest is land-use change. With European settlement came land clearing for agricultural purposes. Our historical documents indicated farming was present during the early 20th century (Table 2, Fig. 3). We noted increased recruitment following the purchase of the property in 1952 that is likely reflective of changes in land-use practices (Fig. 3). Similarly, Glitzenstein et al. (1990) in New York and Copenheaver and Abrams (2002) in Michigan noted an increase in tree density following agriculture abandonment indicative of secondary succession. Urban Naturalist C.B. King and J. Cheek 2015 No. 6 16 DeSantis et al. (2011) also found an increase in recruitment of Post Oak, Blackjack Oak, and Eastern Redcedar during the late 1950s (Figs. 2, 3). They noted increased recruitment of these 3 species subsequent to a severe drought in the 1950s in the south-central United States. They argued that drought-induced mortality of overstory trees, especially Blackjack Oak (Rice and Penfound 1959), and subsequent increase in soil moisture and precipitation during the late 1950s and early 1960s explained increased recruitment. Other studies have also noted increased recruitment during the late 1950s and early 1960s in the Arkansas (Bragg et al. 2012) and Texas Cross Timbers (Stahle et al. 2007). Another explanation for the increased recruitment during the 1950s may reflect land-use change. Prior to the 1950s, the property was privately owned and farmed (Table 1). During the early 1950s, a portion of this property was converted to a sewage treatment facility for the City of Edmond. This change in ownership and land-use likely promoted increased tree recruitment in the absence of farming practices. Stahle et al. (2007) also suggested land-use changes during the 1950s as an explanation for increases in tree recruitment in a Texas Cross Timbers stand. We noted few moderate growth releases during the early 20th century (Fig. 5). However, we found 70% of detected growth releases and 35% of trees exhibited a release during the early 1980s. One explanation for the releases during this time period was the development of an asphalt and primitive trail system at the park (Oklahoma Historical Society 2014) that resulted in the complete or partial removal of the overstory along the trail system. A second explanation for the releases during the early 1980s is increases in precipitation that occurred following a moderate drought during the late 1970s and early 1980s in central Oklahoma (NOAA 2014). Below-average radial growth in Post Oak, Blackjack Oak, and Eastern Redcedar were identified during the late 1970s and early 1980s that occurred during the drought (Fig. 4). Subsequent to this time period, there were increases in radial growth that also corresponded with increases in precipitation in central Oklahoma. Our study site is a reflection of the current succession of many Cross Timbers forests in the absence of historic fire regimes and changing land-use practices. These shifts in forest species composition can occur abruptly (DeSantis et al. 2011) due to changes in prevailing disturbance regimes. Land-use changes (farming to a recreational park) have led to the changes that will likely have ramifications on continued oak recruitment and dominance. A high stem density of Chinese Privet, a non-native shrub, is likely contributing to the reduced oak understory. While this study occurred at one site in Oklahoma, it represents another example of changes that are occurring in upland oak forests across the eastern United States (Abrams 1992). Changes in land-use patterns at the study site from agriculture in the early to mid-20th century to an unmanaged recreational park in the late 20th century have coincided with and likely contributed to a developing shift from oak dominance to an increasing non-oak component. 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