Influence of Rocky Landscape Features and Fire Regime on Vegetation Dynamics in Appalachian Quercus Forests

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Journal of Vegetation Science 17: 675-684, 2006 © IAVS; Opulus Press Uppsala. - Influence of interactions between landscape features and fire regime on vegetation dynamics - 675

Influence of rocky landscape features and fire regime on vegetation dynamics in Appalachian Quercus forests

Signell, Stephen A.1,2* & Abrams, Marc D.1

1307 Forest Resources Building, School of Forest Resources, Pennsylvania State University, University Park, PA 16802, USA; 2Present address: Adirondack Ecological Center, SUNY Environmental Science and Forestry, 6312 State Route 28N, Newcomb, NY 12852, USA; *Corresponding author; Fax +1 5185822181; E-mail [email protected]

Abstract Introduction Question: How do interactions between rocky landscape features and fire regime influence vegetation dynamics? Low intensity surface fires are thought to have been Location: Continental Eastern USA. common in eastern Quercus forests during the era pre- Methods: We measured vegetation, disturbance and site char- ceding European settlement (Abrams 1992; Lorimer acteristics in 40 pairs of rocky and non-rocky plots: 20 in recently burned stands, and 20 in stands with no evidence of 2001; Brose et al. 2001). Quercus first became abundant recent fire (‘unburned’ stands). Two-way analysis of variance in the Central Appalachians ca. 8000 years ago and has (ANOVA) was used to assess the main and interaction effects maintained dominance well into the 20th century of fire and rock cover on plant community composition. (Delcourt et al. 1984; Webb 1988; Watts 1979; Loeb Results: In burned stands, rock cover had a strong influence 1989). During this period, fire-sensitive genera such as on vegetation. Non-rocky ‘matrix’ forests were dominated by Acer, Betula and Nyssa were typically present at low Quercus, and had abundant ground cover and advance regen- levels. (Delcourt et al. 1984; Webb 1988; Watts 1979; eration of early and mid-successional tree species. Burned Loeb 1989, Abrams & Ruffner 1995; Black & Abrams rocky patches supported greater density of fire-sensitive species 2001; Cogbill et al. 2002; Whitney & DeCant 2003). such as Acer rubrum, Sassafras albidum and Nyssa sylvatica and had little advance regeneration or ground cover. Quercus Frequent fire is thought to have facilitated regeneration had fewer fire scars and catfaces (open, basal wounds) on of relatively light-demanding Quercus by removing fire- rocky patches, suggesting that rocky features mitigate fire sensitive competitors and allowing more light to reach severity. In unburned stands, differences between rocky and the understorey (Lorimer 1993). Many have theorized non-rocky patches were less distinct, with both patch types that the current Quercus regeneration problem and the having sparse ground cover, little tree regeneration, and high abundance of fire sensitive species in Quercus understorey densities of relatively shade tolerant A. rubrum, understoreys are the result of 20th century fire suppres- N. sylvatica and Betula lenta. sion efforts (Abrams 1992; Lorimer 1993). Conclusion: Under a sustained fire regime, heterogeneity in Riparian areas, coves and ridge tops have been sug- rock cover created a mosaic where fire-adapted species such as Quercus dominate the landscape, but where fire-sensitive gested as potential spatial refuges for fire-sensitive species persisted in isolated pockets of lower fire severity. species during presettlement fires in the Eastern US Without fire, species and landscape richness may decline as (Abrams & McCay 1996; Abrams 2003). Another can- early-mid successional species are replaced by more shade didate is small-scale rocky features, which are widely tolerant competitors. distributed across ridge sites in the central Appalachians (Hack & Goodlett 1960; Clark 1968a; Middlekauff 1991). Rocky patches have been identified as spatial Keywords: Disturbance interaction; Habitat island; Outcrop; refuges in other ecosystems, for example in the western Patch; Pennsylvania; Periglacial; Ridge; Spatial refugium. US and Australia (Burkhardt & Tisdale 1976; Clarke 2002; Price et al. 2003). Following fires, species are Nomenclature: Rhoads & Block (2000). able to expand their range outward into the surrounding fire-adapted communities (Burkhardt & Tisdale 1976). Long periods without fire can result in a loss of landscape diversity as fire-adapted plant communities disappear. For example, Niklasson et al. (2002) found that Quercus-Pinus forests in Sweden had been com- pletely replaced by Fagus-Picea forests following the 676 Signell, S.A. & Abrams, M.D. cessation of fires during the late 18th century. Fagus had Study area survived fires in a refuge made up of large boulders in the centre of the study area, expanding outward when This study was conducted in Quercus-dominated fires stopped. Once such fundamental shifts in land- stands in the National Guard Training Center at Fort scape pattern occur, the original patchwork of commu- Indiantown Gap (NGTC-FIG), Lebanon and Dauphin nities may be difficult, if not impossible to restore Counties, south central Pennsylvania, (Fig. 1). NGTC- (Heinselmann 1973; Heyerdahl et al. 2001). In this FIG lies across the southernmost extent of the Ridge and paper, we investigate the role of rocky patches as refugia Valley physiographic province of the Appalachian Moun- for fire-sensitive species in frequently burned central tain chain. The climate is humid continental, with a mean Appalachian Quercus-dominated forests. annual temperature of 10 °C, a winter range between –7 The forests at the National Guard Training Center at °C and 19 °C and a summer range between –2 °C and 30 Fort Indiantown Gap (NGTC-FIG) near Harrisburg, °C. Annual precipitation averages 1070 mm with ca. 60% Pennsylvania, offer a unique opportunity to investigate falling from April to October; Anon. 2002). interactions between fire, landscape features and veg- The study sites were located on the south-facing etation in central Appalachian Quercus forests. An abun- slope of Second Mountain, a long ridge that runs across dance of ignition sources has resulted in multiple forest the northwestern portion of the NGTC-FIG property. fires in parts of NGTC-FIG during the past 70 years. This ridge was formed from resistant shales, sandstones, Most NGTC-FIG forest fires have originated in the and conglomerates (Anon. 1981). The soils were of the ‘impact area’, a restricted access zone where training Laidig and Buchanan series (mesic Typic and Hydric exercises often include use of live ammunition. Histori- Fragidults, respectively), derived in colluvium from cally, fires that escaped the impact area were not imme- sandstone, siltstone or shale (Anon. 2002). These very diately suppressed, but were allowed to burn until reach- deep soils are most often found in gently sloping areas ing a firebreak such as a road or ridge top. Thus, stands on lower slopes or mountain terraces, and are character- with strongly contrasting fire histories often exist in ized as well drained to very well-drained. Laidig and close proximity, e.g., on opposite sides of a road. On Buchanan soils typically have rocks at or near the sur- ridges, stands also contain considerable heterogeneity face (Ciolkosz et al. 1979). Within the study sites rocky in rock cover, with non-rocky areas forming a ‘matrix’ patches were composed of large rock blocks, many of surrounding discrete, isolated rocky patches ranging in which were oriented vertically and anchored firmly in size from 0.03 to 0.5 ha. The presence of rocky and non- the subsoil. Such patterned ground in Pennsylvania is rocky areas within burned and unburned stands consti- typically periglacial in origin, having formed as a result tuted a ‘natural experiment,’ enabling us to test the of repeated freezing and thawing of upper soil horizons hypothesis that rocky patches serve as fire refugia. during Quaternary glaciations (Clark 1968b; Ciolkosz et al. 1986; Clark et al. 1992). Periglacial features occur often uncorrelated with present-day topographic features (Hack & Goodlett 1960).

Fig. 1. Map of Fort Indiantown Gap (NGTC-FIG) showing locations of study sites. Burned stands lie uphill from the impact area (hatched polygon) and have ex- perienced several fires in the last 50 years. Unburned stands, located further from the impact area, have had no fires during this time. - Influence of interactions between landscape features and fire regime on vegetation dynamics - 677

Methods locating the paired matrix plot so close to the rocky patch, we minimized spatial variability in other factors, Sampling scheme and site selection e.g., seed source availability. Distance between paired plot centres varied from 20 to 50 m, depending on the We used Two-way analysis of variance (ANOVA) size of the rocky patch. to study the main and interaction effects of fire and rock To quantify fire history, two or three trees were cover on vegetation. Our balanced design consisted of selected at random in the immediate vicinity of each 40 pairs of rocky and non-rocky plots: 20 in recently plot. Basal cross-sections were brought back to the lab burned stands, and 20 in stands with no evidence of to be dried and sanded with increasingly finer sand recent fire (‘unburned’ stands). We controlled for as- paper. The samples were examined for signature years pect, slope and soil by selecting only stands with low to help identify missing, partial, or false rings (Stokes & slope, south aspect, and belonging to the Laidig or Smiley 1996). Signature years are years where radial Buchanan soil series. Two recently burned (Burned 1 & growth shows an abrupt decrease across all samples, 2) and two recently unburned stands (Unburned 1 & 2) and are often associated with years of extreme drought. met the above criteria (Fig. 1). In June 2004, a complete Two years were used to cross date the samples: 1962, a inventory was made of rocky patches (area > 0.02 ha, drought year, and 1977, a year of heavy damage by the rock cover >75%) within these stands. The locations of gypsy moth (Lymantria dispar). After annual rings were 57 suitable rocky patches were recorded using a hand- counted and cross-dated, the year and seasonality of held Trimble GPS receiver. Only eight suitable sites each fire scar was recorded and used to assemble a fire were found in Burned 2; all of these were selected for record for each tree (Smith & Sutherland 1999; Shumway inclusion. A random number generator was used to et al. 2001). We assigned dormant season fires to the select the remaining 32 plots: 12 in Burned 1, ten in year following the scarring event, since most fires in Unburned 1 and ten in Unburned 2. Pennsylvania occur in the spring (Haines et al. 1978).

Vegetation Statistical analysis

We used a handheld GPS unit to relocate each rocky Fire scar data obtained from the basal cross-sections patch and obtained an estimate of its size by measuring were combined to create figures showing fire occur- the longest and shortest axes of the patch. We then rence in each stand. In order to highlight differences located a point three meters in a random direction from between burned and unburned stands, two separate fire the approximate center of each rocky patch, which served return intervals (FRI) were calculated; one for the pe- as the center for a circular 0.02 ha (8 m radius) overstorey riod up to 1988, and one for the period 1988-2003. plot. Within this plot, all trees >7.6 cm diameter at To test the hypothesis that fires were less severe on breast-height, 1.45 m (DBH) were recorded by species, rocky patches, we compared the number of fire scars on DBH, canopy class (dominant, codominant, intermedi- rocky versus matrix cross-sections using the non-para- ate, suppressed), as dead or alive, and presence/absence metric Mann-Whitney U-test (Snedecor & Cochran of visible catface (exposed basal fire scars). To measure 1989). We performed a similar analysis on presence/ fine-scale relationships between tree species, rock cover absence of catfaces (open basal scars of any origin) and shrub cover, percent rock cover (‘bole rock’) and using a t-test for two proportions. shrub cover (‘bole shrub’) was estimated within a 1-m To summarize differences in plant community be- radius of each overstorey tree (cover classes followed tween patch types, densities of major tree species (Acer Daubenmire 1959, with the addition of a 95-100% class). rubrum, Nyssa sylvatica, Sassafras albidum, Betula lenta, Small overstorey trees (2.54 cm > DBH < 7.6 cm) Quercus rubra, Q. montana and Q. coccinea) and per- were tallied by species within the 0.02-ha plot. Tree cent cover of major ground-cover taxa were subjected to regeneration, rock cover and ground cover were meas- Detrended Correspondence Analysis (DCA) using PC- ured within a smaller, nested 0.0004-ha subplot estab- ORD (McCune & Mefford 1999). DCA axes one and lished around the same plot center. Within this subplot, two were interpreted via simple correlation with envi- saplings (DBH < 2.54 cm, > 1 m tall) and seedlings (trees ronmental factors (percent rock cover) and vegetation < 1 m tall) were tallied by species, and percent cover of indices (overstorey richness, ground-cover richness, sap- shrubs and herbs recorded by species or taxonomic ling density). Overstorey richness was defined as the group (same classes described above). Upon comple- total number of tree species in the overstorey of each tion of the rocky plot, a non-rocky paired plot was then plot (excluding seedlings and saplings). Ground-cover established by selecting a random direction and moving richness was the total number of shrub and herbaceous ten meters away from edge of the rocky patch. By taxa present in a plot. This number is not a species count, 678 Signell, S.A. & Abrams, M.D.

Table 1. Fire scar records obtained from cross sections cut from study sites at Fort Indiantown Gap.

pre 1988 1988-2003 Stand Area (ha) # of samples # of major fires* Mean FRI (years) # of major fires* Mean FRI (years)

Burned 1 15 26 4 36.8 4 3.8 Burned 2 10.4 31 3 45.0 4 3.8 Unburned 1 14.3 33 6 25.7 0 - Unburned 2 27.8 31 2 71.0 0 - * > 25% of samples scarred

since some plants were not identified to the species level Results (e.g. Gaylussacia, graminoids, ferns, etc.). We used Two-way ANOVA to test the main and Fire history interaction effects of rock and fire on density of major tree species. Density of all species was square-root Fire scar analysis confirmed that burned and unburned transformed to achieve normality except for N. sylvatica, stands had similar fire histories prior to 1988, but con- which was log-transformed. trasting fire regimes since that time. All stands burned To measure influences on shrub cover, we performed either in 1942 and/or 1953. (Fig. 2, Table 1). Unburned best subsets regressions of bole shrub cover against bole 1 and 2 contained no fire scars during the period 1954- rock cover, DBH, canopy class, plot density, plot basal 2003 (Fig. 2). Burned 1 had major recent fires in 1989, area and bole tree shade tolerance for each fire ‘treat- 1995, 1998 and 2000, and Burned 2 had fires in 1988, ment’ (Hocking 1976). The bole shade tolerance vari- 1992, 1996 and 2002. able was created by assigning each species an ordinal value ranging from one to seven based on increasing Scarring and injury shade tolerance: 1 = Q. coccinea; 2 = S. albidum; 3 = Q. rubra; 4 = Q. montana; 5 = B. lenta; 6 = A. rubrum; 7 = Rocky patches ranged in size from 0.03 to 0.5 ha in N. sylvatica (Burns & Honkala 1990). Although vari- size (mean = 0.15 ha). Initial analysis showed no differ- ables included in the regression were not all normally ence in number of scars between rocky and matrix distributed, regression was deemed valid in this case patches. However, because Quercus had a lower scar- because sample sizes were acceptably large (n = 406 for ring rate than non-Quercus species (median 0 and 2, burned stands, n =706 for unburned stands). respectively, P = 0.002), we ran separate tests for these two groups. These tests showed that Quercus did in fact have fewer scars in rocky patches than in the matrix (median 0 and 1.5, respectively, P = 0.013). Analysis of catfaces showed a similar pattern: when all trees were included, the proportion of trees with catfaces did not differ according to rock cover. However, when analysed separately, Quercus had a smaller proportion of catfaces on rocks than in the matrix (mean = 18% and 35%, respectively, P = 0.012). Quercus also had far fewer catfaces overall than non-Quercus species (mean = 26% vs. 69%, P < 0.001).

Forest composition and structure

DCA axis 1 (eigenvalue = 0.422) was negatively associated with rock cover (P < 0.05, R2 = 0.36), repre- senting a gradient of decreasing rockiness, with rocky plots on the left and matrix plots on the right (Fig. 3). Burned and unburned rocky plots (Fig. 4B, D) were not separated in the ordination, an indication that fires had Fig. 2. Fire scar records obtained from cross sections cut from little influence on rocky patch communities. Although Fort Indiantown Gap study sites. Vertical lines denote years overall tree density was lower in burned vs. unburned containing fire scars. rocky patches, tree composition was similar, with Q. - Influence of interactions between landscape features and fire regime on vegetation dynamics - 679

Fig. 3. Diagram of 80 plots, and major tree and common ground-cover species along axes 1 and 2 of a Detrended Correspondence Analy- sis (DCA. Quru = Q. rubra; Qumo = Q. montana; Quco = Q. coccinea; Nysy = N. sylvatica; Bele = Betula lenta; Acru = Acer rubrum; Saal = Sassafras albidum albidum; frn = ferns; grs = grasses; huk = Gaylussacia spp.; vcc = Vaccinium spp.; whz = Hamamelis virginiana. montana, Q. rubra and B. lenta dominating the overstorey DCA axis 2 (eigenvalue = 0.244) is a fire effects (Table 2). In contrast, fire had a strong impact on matrix axis, with burned matrix plots receiving higher scores communities, with burned matrix plots clearly separated than the other patch types. Ground-cover species rich- from unburned plots, especially along DCA axis 2. Burned ness, percent cover, and sapling density of Quercus and matrix forests consisted of a canopy dominated by Q. S. albidum increased along DCA axis 2, while tree montana and a relatively open understorey (Fig. 4A, species richness decreased (P < 0.05). The ordination Table 2). Unburned matrix forests were also dominated shows that burned stands support two distinct commu- by Q. montana, but supported high densities of A. rubrum, nity types which are clearly separated along both axes of N. sylvatica and B. lenta in the understorey (Fig. 4C, the ordination. In unburned stands, community differ- Table 2). ences between matrix and rocky plots were less distinct,

Table 2. Frequency (% of plots), basal area (m2/ha) and density (stems/ha) for each patch type. Major species shown in bold type.

Burned matrix Burned rocky Unburned matrix Unburned rocky Freq BA Density Freq BA Density Freq BA Density Freq BA Density

Quercus alba 15% 1.7 12 25% 2.6 37 5% 0.1 3 Quercus coccinea 25% 0.5 35 15% 0.2 7 50% 2.2 77 20% 1.3 17 Quercus montana 85% 9.8 193 90% 15.3 247 100% 13.1 269 90% 17.7 235 Quercus rubra 15% 0.8 7 60% 5.3 67 40% 1.4 40 55% 4.5 74 Quercus velutina 25% 0.8 17 5% * 5 10% 0.5 12 5% 0.2 3 Acer rubrum 40% 1.7 77 90% 2.6 175 100% 2.8 368 90% 2.8 353 Amelanchier spp. 5% * 3 15% * 12 Betula lenta 20% 0.1 12 70% 4.8 190 70% 2.1 260 95% 5.7 329 Carya spp. 15% * 3 10% 0.1 7 Castanea dentata 5% 0.1 10 10% * 7 Cornus florida 5% * 5 Liriodendron tulipifera 5% 0.9 3 5% 0.2 3 10% 1.5 12 10% 1.0 15 Nyssa sylvatica 40% 1.1 57 25% 0.2 15 85% 2.1 499 40% 0.4 106 Pinus strobus 5% 1.3 20 Pinus virginiana 5% 0.2 3 Populus grandidentata 5% 0.1 10 5% * 3 Sassafras albidum 30% 0.6 40 45% 0.9 42 80% 1.3 129 55% 0.4 54 Total 18.2 477 29.9 769 29.8 1708 35.6 1241 * BA < 0.05. 680 Signell, S.A. & Abrams, M.D.

Table 3. F-values and significances (P) from the ANOVA especially with respect to DCA axis 2. tables, showing the effects of substrate (rocky/non-rocky), fire The ordination also illustrates the affinity of B. lenta (burned/unburned), and their interaction on the density of and Q. rubra for rocky patches and the preference of Q. selected species. Cells in bold type indicate significant effects coccinea and N. sylvatica for matrix habitats, as deter- of the factor on each species. mined by ANOVA (Table 3). ANOVA also showed that Effect B. lenta, A. rubrum, N. sylvatica and S. albidum were Substrate fire Interaction negatively impacted by fire, while Quercus species

Acer rubrum were not. Interaction terms were significant for A. rubrum, N. sylvatica and S. albidum (Table 3). F 2.22 17.82 7.64 P 0.1407 0.0001 0.0072 Regeneration Betula lenta F 15.45 21.32 1.58 P 0.0002 < 0.0001 0.2120 The burned matrix supported large numbers of saplings, Nyssa sylvatica var. sylvatica primarily S. albidum and Quercus, not found in other patch F 15.24 22.06 5.76 types (Table 4). Seedling density was also highest in the P 0.0002 < 0.0001 0.0188 burned matrix where S. albidum, Q. montana and A. rubrum Sassafras albidum were most abundant. In other patch types, saplings were F 2.13 9.75 4.95 P 0.1486 0.0025 0.0290 rare and A. rubrum was the most abundant seedling. Quercus coccinea F 6.66 2.65 0.88 Ground cover P 0.0118 0.1076 0.3503 Quercus montana The burned matrix was unique among the patch types in F 0.03 1.84 1.31 terms of ground cover, having the greatest number of P 0.8562 0.1790 0.2564 ground-cover species as well as the highest percent Quercus rubra F 9.16 1.65 1.68 ground cover (Table 5). Shrub cover was strongly nega- P 0.0034 0.2024 0.1995 tively correlated with bole rock cover in burned stands (P < 0.001, R2 = 54.7, df = 405), with the matrix support- ing a dense ground-cover layer composed primarily of Gaylussacia and Vaccinium. In unburned stands, the correlation between shrub and rock cover was weak (P < 0.001, R2 = 10.6, df = 705). Overstorey composition also had a weak but significant effect on shrub cover in unburned stands, with less shrub cover beneath the crowns of shade tolerant species (P < 0.001, R2 = 5.0, df = 705).

Table 4. Seedling and sapling regeneration (stems/ha) for each patch type in study stands at Fort Indiantown Gap.

Species BN BR UN UR Seedlings Saplings Seedlings Saplings Seedlings Saplings Seedlings Saplings

Quercus alba 618 0 0 0 124 0 0 0 Quercus coccinea 371 371 0 0 0 0 0 0 Quercus montana 13 837 618 7 660 0 2 347 0 618 0 Quercus rubra 741 0 618 0 1 853 0 1 359 0 Quercus velutina 741 0 0 0 0 0 247 0

Acer rubrum 9 884 0 13 220 0 12 849 0 5 436 0 Carya spp. 0 0 0 124 0 0 0 0 Betula lenta 2 347 0 1 853 0 494 0 1 482 0 Liriodendron tulipifera 618 0 0 0 2 471 0 0 0 Nyssa sylvatica 1 606 0 618 0 741 248 124 124 Pinus strobus 124 0 124 0 247 0 618 124 Populus grandidentata 0 124 0 0 0 0 0 0 Prunus serotina 124 0 0 0 0 0 247 0 Sassafras albidum 27 181 4 695 11 984 0 2 965 0 988 0 Tsuga canadensis 00 124 0 0 0 124 0

Total 58 193 5 807 36 200 124 24 092 248 11 243 248 - Influence of interactions between landscape features and fire regime on vegetation dynamics - 681

Fig. 4. Photographs illustrating the four patch types identified in this study. A. Burned matrix, with few rocks, low tree density and dense ericaceous ground- cover. B. Burned rocky patch. Note lack of groundcover and abrupt border with non-rocky matrix in background. C. Un- burned matrix, with profusion of small trees and sparse shrub cover. D. Unburned rocky patch. Vertically aligned boulders in the foreground are characteristic of periglacial patterned ground.

Discussion

Rocky patches provided refuge for fire-sensitive species during fires. Three of the four non-Quercus species analysed (A. rubrum, S. albidum and N. sylvatica) Table 5. Groundcover summary for study stands at Fort showed both a negative fire effect and a significant Indiantown Gap. interaction term, indicating that rocks and fire together Species Burned Burned Unburned Unburned affected the distribution of these species in burned stands. matrix rocky matrix rocky Our evidence indicates that reduced fire severity on Acer pennsylvanicum * 0.3 0.1 * rocky patches is responsible for this interaction effect. Amelanchier **0.1 * Quercus on rocky patches had fewer fire scars and Azalia 0.3 0.3 0.1 * catfaces than in matrix forests. The close grouping of Ferns 11.6 0.8 2.1 0.8 Gallium 1.5 0.1 * * burned & unburned rocky plots in the DCA ordination Gaultheria procumbens 0.3 * 0.3 0.3 provides further evidence that fire had less impact on Gaylussacia 42.0 5.9 17.8 4.9 rocky patch communities (Fig. 3). Poaceae 6.5 0.1 * * Hamamelis virginiana 1.0 1.0 2.1 10.1 Several factors may contribute to reduced severity on Kalmia latifolia 0.8 * 0.9 0.4 rocky patches. First, by confining fuels to interstices, Parthenocissus cinquefolia * 0.1 * 0.1 rocks create circuitous fuel routes which slow the rate of Pipsis sewa 0.1 0.1 0.3 * Quercus ilicifolia 1.9 * * * spread (Veblen et al. 1994; Johnson & Miyanishi 2001). Rhus copallina 1.5 * * * Second, soil in rock interstices is shadier and cooler than Rubus flagellaris 0.8 * * * in non-rocky areas, which increases moisture content and Rubus 0.1 0.1 * * Smilax 2.0 0.9 2.5 0.6 reduces flammability (Swan 1961; Lauer & Klaus 1975; Solidago 0.3 * * * Pearson & Theimer 2004). Third, by relegating fire to a Urtica 0.1 * * * relatively small percentage of the soil surface, rocks may Vaccinium 23.3 8.3 10.8 2.1 Viburnum * 0.1 * * reduce root mortality in susceptible species such as B. Vitis 0.1 * * 0.1 lenta (Niering et al. 1970). B. lenta was unique in that it was negatively impacted by fire, but did not have a Total shrub & herbaceous cover 79.0 18.6 34.8 12.0 682 Signell, S.A. & Abrams, M.D. significant interaction effect. For this species, therefore, tion. Shrub cover was greater beneath the crowns of fire may merely amplify a preexisting preference for shade intolerant Q. coccinea and S. albidum than under rocky patches; the cool, damp, humus-rich micro-envi- the denser crowns of A. rubrum, B. lenta and N. sylvatica ronments of rock interstices provide ideal germination (Canham et al. 1994). With high tree density and sparse sites for B. lenta (Swan 1961; Lauer & Klaus 1975; ground cover, the unburned matrix was compositionally Middlekauff 1991; Carlton 1993; Pearson & Theimer more similar to burned rocky patches than to the burned 2004). matrix, as seen in the DCA plot (Fig. 3). While rocky patches served as refuges for fire- sensitive trees, the burned matrix was a safe haven for early to mid-successional species such as Quercus and Conclusions Sassafras. Of the major species, Quercus were the best adapted to burned matrix forests. As a result of lower Over time, fire suppression may lead to a loss of scarring rates, none of the Quercus species tested showed patchiness and a subsequent decline in species richness as a significant fire effect. Conditions in the burned matrix disturbance-adapted species such as Quercus are replaced were also favourable for Quercus regeneration, which, by shade tolerant, later successional species (Loucks along with Sassafras dominated the seedling and sap- 1970). Growing concern over the future of Quercus for- ling classes. The shade intolerant shrubs Gaylussacia ests has led many to consider prescribed fire as a means of and Vaccinium also thrived in the burned matrix, form- ecosystem and oak restoration (Abrams 1992; Baker ing a nearly continuous layer. Fire benefits these plants 1994; Ford et al. 1999; Blake & Schuette 2000; Brose et via stimulation of root-suckering and overstorey thin- al. 2001). However, most prescribed fires in this region ning (Matlack et al. 1993). Ericaceous shrubs such as have resulted in little or no benefit to oak regeneration due these are important surface fuels in eastern forests (Brose to low mortality rates of competing species (Wendell & et al. 2004), providing fuel in and of themselves, but Smith 1987; Arthur et al. 1998; Kuddes-Fischer & Arthur also increasing flammability by trapping and suspend- 2002). Most eastern forests have aged to the point where ing leaf litter above the forest floor (Dighton et al. even fire-sensitive species have grown large enough to 2000). survive moderate surface fires (Harmon 1984). Decreases In burned forests, heterogeneity in rock cover cre- in shrub cover, such as those documented in this study, ated a mosaic where fire-adapted species dominate the may also make forests less flammable, decreasing the landscape, but where other species may also persist in efficacy of prescribed fires even further (Mutch 1970). isolated pockets of lower fire severity. This finding In the eastern US Quercus forests, as elsewhere, provides some insight into historical forest dynamics of management prescriptions should be based on an under- the region. For example, the relegation of species like A. standing of current forest conditions in the context of rubrum, B. lenta and N. sylvatica to small but widely historic disturbance regimes (Heinselmann 1973; distributed rocky refugia during presettlement fires may Romme & Despain 1989; Heyerdahl et al. 2001; Motzkin help explain both their low frequency in ridge forests & Foster 2002). Such an understanding would be in- during European colonization, as well as their rapid complete without considering the factors controlling increase during the 20th century fire suppression era spatial variability in those regimes, including the his- (Nowacki & Abrams 1992; Abrams & Ruffner 1995; torical legacies of landscape features such as rocky Abrams & McCay 1996; Steiner et. al. 2004). outcrops. Greater knowledge of such variability may aid Indeed, unburned NGTC-FIG stands contained high land managers whose goal is to restore presettlement densities of relatively shade tolerant A. rubrum, B. lenta forest conditions in landscapes that have been dramati- and N. sylvatica, leaving few opportunities for less cally modified by human land use. tolerant plants to regenerate. No Quercus or Sassafras saplings were found in unburned stands. Some have suggested that rocky patches may provide refuge for Acknowledgements. This research was funded by a grant early successional species in the face of increasing from the Pennsylvania Department of Military and Veterans competition from shade-tolerant competitors (Frelich & Affairs. Reich 2002). We found no evidence of this however, at least at this stage of stand development; Quercus and Sassafras saplings were absent on unburned rocky patches despite lower tree density. Unburned matrix forests had much lower shrub cover than the burned matrix. We found evidence that this lack of shrub cover was due partially to overhead competi- - Influence of interactions between landscape features and fire regime on vegetation dynamics - 683

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