Spatiotemporal Analysis of Fire Regimes in Yellow Pine Stands, Craig Creek Valley, Virginia, USAGeorgina DeWeese Wight1, Henri D. Grissino-Mayer1,

Charles W. Lafon2, and Elaine Kennedy Sutherland3

IntroductionThe deterioration of fire-adapted yellow pine stands, and in particular Table Mountain pine (Pinus pungens Lamb.) stands, in the central Appalachians Mountains has prompted concern by land management agencies. Table Mountain pines require medium to medium-high intensity fires to open their serotinous cones and release seeds, open the canopy to provide sunlight, and reduce the duff layer on the forest floor to expose mineral soils. With the advent of successful fire suppression practices in the 20th century, Table Mountain pine stands are now in decline. The absence of fire has increased litter accumulation and allowed fire-intolerant species to move into and dominate Table Mountain pine stands. The objective of this study is to gain a better understanding of historic fire regimes,

including fire return intervals, in Central Appalachian yellow pine stands.

Study SitesYellow pine stands were selected on south-southwest facing ridges (Fig. 1) of Brush Mountain, north of Blacksburg, Virginia (Fig. 3). The stands were relatively open with an understory sparsely populated by mountain laurel (Kalmia latifolia L.) and serviceberry (Amelanchier spp.). Table mountain pine (Pinus pungens Lamb.), Virginia pine (Pinus virginiana Mill.), scarlet oak (Quercus coccinea Muenchh.), chestnut oak (Quercus prinus L.), and black oak (Quercus velutina Lam.) comprise the canopy, while black gum (Nyssa sylvatica Marsh.), red maple (Acer rubrum L.), white pine (Pinus strobus L.), and white oak (Quercus alba L.) were common associates. North Mountain is located between Roanoke and New Castle, Virginia, and is east-northeast of Brush Mountain. The ridges sampled faced south-southwest. The yellow pine stands here are denser and the terrain more steep than on Brush Mountain. The understory contains a thick cover of mountain laurel, blueberry (Vaccinium spp.), huckleberry (Vaccinium spp.), bear oak (Quercus ilicifolia Wangenh.), and greenbrier (Smilax rotundifolia L.). Understory trees include serviceberry, black gum, red maple, and hickory (Carya spp.). Table Mountain pine, Virginia pine, scarlet oak, black oak, chestnut oak, and black locust (Robinia pseudoacacia L.) comprise the canopy.

MethodsFour south-southwest facing ridges were samples on each mountain. Cross-sections were collected from fire-scarred trees at both sites to reconstruct fire events (Figs. 2, 4). Two macroplots (100 m X 20 m) were established on each mountain to inventory stand composition, determine the stand age structure, and determine forest succession dynamics in the overstory of yellow pine stands. Increment cores were then taken from randomly selected tree within the plots. All cores and sections were mounted, sanded, measured, and crossdated (Fig. 5). These cores and cross-sections were used to create separate Brush Mountain and North Mountain tree-ring chronologies. The chronologies were used to date fire-scarred cross sections. Fire dates were loaded into FHX2 software for analysis. Age structure was assessed using frequency distributions.

Results• The Brush Mountain tree-ring chronology extends from 1732 to 2002, has a interseries correlation of 0.59, and

an average mean sensitivity of 0.30. The North Mountain tree-ring chronology extends from 1743 to 2002, has an interseries correlations of 0.60, and an average mean sensitivity of 0.34.

• From the age structure analysis, we determined that one major cohort established on Brush Mountain in the 1930s (Fig. 6), while the fire history suggested another major cohort established in the mid-1850s (Fig. 8). On North Mountain, we found evidence of only one major cohort that established in the 1930s (Figs. 7, 9).

• To assess fire frequency, we used the Weibull Median Interval (MEI) rather than the Mean Fire Interval because the latter is influenced by skewed distributions. We dated 43 fire-scarred sections from Brush Mountain. When assessing all fire-scarred samples, the MEI was 3.3 yrs, while the lower and upper exceedance intervals (which bracket the range of variation) were 1.0 and 8.0 yrs, respectively. Excluding those fire years when only one tree was scarred, however, the MEI was 7.6 yrs, while the lower and upper exceedance intervals were 2.2 and 17.7 yrs, respectively.

• Twenty-seven fire-scarred samples were dated from North Mountain. For all samples, the MEI was 3.8 yrs, while the lower and upper exceedance intervals were 1.1 and 8.6 yrs, respectively. Assessing only those fire years when at least two trees recorded fires, the MEI was 11.4 yrs, while the lower and upper exceedance intervals were 2.6 and 30.6 yrs, respectively.

1 Department of Geography, University of Tennessee, Knoxville, Tennessee; 2 Department of Geography, Texas A&M University, College Station, Texas; 3 Rocky Mountain Research Station, USDA Forest Service, Missoula, Montana

Figure 1: Brush Mountain. The southwest facing slopes show yellow pine stands.

Figure 2: Henri Grissino-Mayer sampling a fire-scarred Table Mountain pine.

Figure 6: Age class distributions for Brush Mountain.

Figure 7: Age class distributions for North Mountain.

Figure 3: Yellow pine study sites, Jefferson National Forest, Virginia.

Figure 8: Master fire chart for Brush Mountain. Figure 9: Master fire chart for North Mountain.

• The establishment of cohorts occurs after two major fire events on Brush Mountain. A major establishment in the 1850s followed a trio of fires in 1851, 1852, and 1853. The 1853 fire was a late season fire, whereas most other fires occurred in the dormant season. A late season fire would have been more intense and destructive because of higher temperatures and drier fuels. A second establishment on Brush Mountain occurred in the 1930s following the major 1926 and 1934 fires.

• On North Mountain, fires appear to be patchier as indicated by fire years in which only one tree recorded a fire. Cohort establishment does not appear to be related to major fires. This could be because the majority of fires on North Mountain took place in the dormant season.

• Fire frequency increased on both mountains between ca. 1900-1925, likely due to human ignitions. On Brush Mountain, the last major fire occurred in 1934. Since then, fuels have likely increased to unprecedented levels, ensuring that the next fire will be of greater severity. A similar, though less severe situation exists on North Mountain where the last major fire occurred in 1963. The situation on Brush Mountain is further complicated by neighborhood development.

Discussion

Figure 5: Fire-scarred cross-section of Table Mountain pine.

Figure 4: Field Assistant Preston Roberts transporting cross sections off North Mountain.