Cross Timbers forests, typically dominated by Quercus stellata Wangenh. and Q. marilandica Muenchh., are the transition zone between eastern deciduous forest and prairie in the southern Great Plains. Fire exclusion beginning in the mid-1900s has led to increasing stand density and encroachment of fire-intolerant Juniperus virginiana L. and mesic hardwood. We measured current forest structure and tree ages of 25 stands (130 plots) in north-central Oklahoma to characterize the extent and dynamics

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... of encroachment. The respective basal area and stand density of the overstory (diameter at breast height; dbh > 10 cm) were 19.0 m 2 ha −1 and 407 trees ha −1 with Q. stellata comprising 43% of basal area and 42% of stand density. Quercus marilandica represented only 3% of basal area and 4% of overstory density. Juniperus virginiana represented 7% of basal area and 14% of stand density while mesic hardwoods, e.g., Celtis spp., Ulmus spp., Carya spp., 33% of basal area and stand density. The sapling layer was dominated by mesic hardwoods (68%) and J. virginiana (25%) while the seedling layer was dominated by mesic hardwoods (74%). The majority of Quercus recruited into the overstory between 1910-1970, while recruitment of J. virginiana and mesic hardwoods began more recently (post 1950s). Growth rate, based on the relationship between age and dbh, was faster for mesic hardwoods than for J. virginiana and Q. stellata. These results indicate that removal of recurrent surface fire as a disturbance agent has significantly altered forest composition in the Cross Timbers region by allowing encroachment of J. virginiana and fire-intolerant, mesic hardwoods. This increases wildfire risk because J. virginiana is very flammable and will alter how these forests respond to future drought and other disturbance events. Forests 2018, 9, 75 2 of 17 following canopy gaps that developed as a result of widespread drought in the 1950s, the Cross Timbers forests have seen an increase in stand basal area and an increase in fire-intolerant species such as Juniperus virginiana (eastern redcedar), Celtis spp., and Ulmus spp. [9] . This change in forest composition and structure in the Cross Timbers parallels the "mesophication" of much of the rest of the eastern deciduous forest, where fire-tolerant oak species are replaced with less fire-tolerant species due to fire exclusion [10] . In particular, the encroachment of highly flammable, evergreen J. virginiana into Cross Timbers forests is of serious concern. Re-measurement of stands initially sampled from 1953-1957 by Rice and Penfound [11] indicated that J. virginiana increased from 0.05 to 2.71 m 2 ha −1 in terms of basal area and from 0.73 to 23.85 stems ha −1 , while the overall stand basal area and stand density roughly doubled [9, 12] . Juniperus virginiana is typically considered a shade-intolerant species; however, within the forest interior of the Cross Timbers, J. virginiana finds enough light availability in canopy gaps and is physiologically active during the winter/spring oak leaf-off period [13] . Once J. virginiana establishes, it shades out herbaceous vegetation [14] and adds litter to the forest floor that decomposes differently from oak leaves [15] , alters fungal communities and litter chemistry [16] , and burns much less readily than either oak litter or herbaceous vegetation. This reduces the available fine fuel load around J. virginiana trees and saplings and results in a forest that is incapable of maintaining a consistent, low-intensity surface fire. These low-intensity surface fires are necessary to purge J. virginiana seedlings (and other fire intolerant species), which become difficult to kill with prescribed fire once the trees reach a height of approximately 2 m [17, 18] . In contrast to its effects on surface fire, J. virginiana encroachment increases the risk of wildfire. Nowacki and Abrams [10] proposed an alternative stable state for the oak forests of the eastern United States where fire exclusion leads to a more mesic forest that is resistant to all types of fire due to reduced fine fuel loading. However, the encroaching J. virginiana is more xeric, resinous, and burns extremely well under drought/wildfire conditions [19] . Hoff et al. [20] found that J. virginiana encroachment in north-central Oklahoma increased available fuel loading for wildfires by 38% through the inclusion of J. virginiana foliage and small branches. This reduction in ability for fine fuels to support a surface fire combined with the increased risk of crown fire from J. virginiana in the midstory sets the stage for wildfires that will be harder to suppress and more dangerous to human health and property. Increased risk and severity of wildfires also may change the regeneration pattern of Cross Timbers forests by replacing surface fires with stand-replacing fires [21] . Understanding the extent of encroachment of fire-intolerant species into Cross Timbers forests and the timeline of the invasion process is relevant to understanding regional forest dynamics and is important information for land managers. Previous work by Stambaugh et al. [21] and DeSantis et al. [9], based on tree aging, documented an increase in J. virginiana within Cross Timbers forests beginning in the mid 1900s. However, neither study looked at the timeline for invasion of more mesic species such as Celtis occidentalis L., Celtis laevigta Willdenow, or Ulmus americana L. In addition, the majority of studies on Cross Timbers forests have occurred on government controlled and managed lands (e.g., [4, 5, 7, 8, 22] ) that do not reflect the typical disturbance pattern for land owned by small private landowners, which represents the vast majority of land holdings in the region. By knowing the dynamics of J. virginiana and mesic species invasion into Cross Timbers forests, land managers can prioritize and schedule management activities based on current conditions and projected rates of encroachment to reduce J. virginiana, reduce wildfire risk, and/or restore historical ecosystem composition or structure. To address the issue of J. virginiana and mesic species encroachment into Cross Timbers forests, we measured current forest structure and condition during the summer of 2016 on Bureau of Indian Affairs (BIA) properties in north-central Oklahoma. In addition, we determined the age structure of the various species and related that to the dynamics of J. virginiana and mesic hardwood encroachment. Our objectives were to (1) quantify the current forest overstory, sapling, and seedling composition, Forests 2018, 9, 75 3 of 17 (2) determine the extent of encroachment by fire-intolerant species, (3) establish a sub-decadal timeline of encroachment, and (4) compare growth rates of major species groups.