A Meta-Analysis of the Fire-Oak Hypothesis: Does Prescribed Burning Promote Oak Reproduction in Eastern North America?
Patrick H. Brose, Daniel C. Dey, Ross J. Phillips, and Thomas A. Waldrop
Abstract: The fire-oak hypothesis asserts that the current lack of fire is a reason behind the widespread oak (Quercus spp.) regeneration difficulties of eastern North America, and use of prescribed burning can help solve this problem. We performed a meta-analysis on the data from 32 prescribed fire studies conducted in mixed-oak forests to test whether they supported the latter assertion. Overall, the results suggested that prescribed fire can contribute to sustaining oak forests in some situations, and we identified several factors key to its successful use. Prescribed fire reduced midstory stem density, although this reduction was concentrated in the smaller-diameter stems. Prescribed fire preferentially selected for oak reproduction and against mesophytic hardwood reproduction, but this difference did not translate to an increase in the relative abundance of oak in the advance regeneration pool. Fire equalized the height growth rates of the two species groups. Establishment of new oak seedlings tended to be greater in burned areas than in unburned areas. Generally, prescribed burning provided the most benefit to oak reproduction when the fires occurred during the growing season and several years after a substantial reduction in overstory density. Single fires conducted in closed-canopy stands had little impact in
the short term, but multiple burns eventually did benefit oaks in the long term, especially when followed by a canopy disturbance. Finally, we identify several future research needs from our review and synthesis of the fire-oak literature. FOR. SCI. ❚❚(❚):000-000.
Keywords: fire effects, hardwoods, prescribed fire, Quercus spp., shelterwood
THROUGHOUT EASTERN NORTH AMERICA, mixed-oak (Quercus spp.) forests on upland sites are highly valued for many ecological and economic reasons. Generally, these upland forests consist of one or more oak species (black [Quercus velutina Lam.], chestnut [Quercus montana Willd.], northern red [Quercus rubra L.], scarlet [Quercus coccinea Muenchh.], and white [Quercus alba L.]) dominating the canopy with a mix of other hardwood species in the midstory and understory strata. Despite wide- spread abundance and dominance of mixed-oak forests, regenerating them is a chronic challenge for land managers throughout eastern North America and they are slowly being replaced by mesophytic hardwoods such as black birch (Betula lenta L.), black cherry (Prunus serotina Ehrh.), red maple (Acer rubrum L.), sugar maple (Acer saccharum Marsh.), and yellow-poplar (Liriodendron tu- lipifera L.) (Abrams and Downs 1990, Healy et al. 1997, Schuler and Gillespie 2000, Aldrich et al. 2005, Woodall et al. 2008). Many factors contribute to this oak regeneration problem including loss of seed sources, destruction of acorns and seedlings by insects, disease, weather, and wild- life, dense understory shade, competing vegetation, and lack of periodic fire (Crow 1988, Loftis and McGee 1993, John- son et al. 2009). The implication of the lack of periodic fire as a cause to the oak regeneration problem arises from the fact that many of these oak forests exist, in part, due to past fires, and this relationship has led to the creation of the fire-oak hypothesis (Abrams 1992, Lorimer 1993, Brose et al. 2001, Nowacki and Abrams 2008, McEwan et al. 2011). The fire-oak hypothesis consists of four parts: (1) peri odic fire has been an integral disturbance in the mixed-oak forests of eastern North America for millennia; (2) oaks have several physical and physiological characteristics that allow them to survive at higher rates than their competitors in a periodic fire regime; (3) the lack of fire in the latter 20th century is a major reason for the chronic, widespread oak regeneration problem; and (4) reintroducing fire via prescribed burning will promote oak reproduction. The first three parts are supported by the scientific literature to various degrees. For example, paleo-ecological studies and historical documents indicate that American Indian tribes used fire for numerous reasons (Day 1953, Wilkins et al. 1991, Patterson 2006, Ruffner 2006). Many studies reported the differences between oaks and mesophytic hardwood species (Gottschalk 1985, 1987, 1994, Kolb et al. 1990), and the concomitant decline of fire and increase in mesophytic hardwoods during the early 1900s is evident from fire history research (Shumway et al. 2001, Guyette et al. 2006, Hutchinson et al. 2008, Aldrich et al. 2010). It remains hard to verify the fourth part of the fire-oak hypothesis—that
Manuscript received April 3, 2012; accepted July 17, 2012; published online August 16, 2012; http://dx.doi.org/10.5849/forsci.12-039. Patrick H. Brose, USDA Forest Service, Northern Research Station, PO Box 267, Irvine, PA 16329—Phone: (814) 563-1040; Fax: (814) 563-1048; email@example.com. Daniel C. Dey, USDA Forest Service—firstname.lastname@example.org. Ross J. Phillips, USDA Forest Service—email@example.com. Thomas A. Waldrop, USDA Forest Service—firstname.lastname@example.org
Acknowledgments: We thank the many fellow scientists who stimulated our thinking on this subject via engaging conversations as well as by sharing insights on the details of their studies, especially data collection procedures, and pointing us toward publications that had escaped our searches. We thank Alejandro Royo, John Stanovick, and Matthew Trager for guidance with the meta-analysis. In addition, we thank them and three anonymous individuals for reviews of earlier drafts of this article that helped with clarity and conciseness. Funding for this study was provided by the Joint Fire Science Program (Project 10-2-01-1).
This article was written by U.S. Government employees and is therefore in the public domain.
prescribed burning promotes oaks—because the results reported in the literature vary widely. Results range from positive (Brown 1960, Swan 1970, Ward and Stephens 1989, Kruger and Reich 1997) to neutral (Teuke and Van Lear 1982, Merritt and Pope 1991, Hutchinson et al. 2005) to negative (Johnson 1974, Wendel and Smith 1986, Loftis 1990, Collins and Carson 2003). This inconsistency among findings suggests that multiple factors drive fire outcomes and the complex relationships among these factors complicate the development of reliable guidelines for prescribed burning of mixed-oak forests.
Despite the variability in study outcomes and lack of specific guidelines for using fire in oaks, land management agencies throughout eastern North America are increasingly using prescribed fire in mixed-oak forests. For example, the oak-dominated national forests of the Ohio River basin (Allegheny, Daniel Boone, Hoosier, Monongahela, Shaw-nee, and Wayne) all have prescribed fire as part of their respective forest plans and in 2011 conducted 59 burns totaling 7,776 ha (National Interagency Fire Center 2012). The rationale behind these prescribed fires is that they will benefit oaks by increasing the quantity and quality of understory light by reducing midstory stem density, will increase the overall density of oak reproduction, and will improve the relative abundance and height of oak reproduction in the regeneration pool.
This widespread use of prescribed fire in mixed-oak forests without specific guidelines potentially creates problems, i.e., fire may be applied to oak forests not suitable for burning or fire may be withheld from oak forests that would benefit from burning. A meta-analysis of the fire-oak literature would test the final part of the fire-oak hypothesis and provide guidance on how and when prescribed fire is appropriate or is not useful in the regeneration of mixed-oak forests.
Meta-analysis is a systematic review and statistical synthesis of the empirical data contained in the literature on a particular subject (Borenstein et al. 2009, Harrison 2011). In meta-analysis, a common basis or standard for comparing the results of related studies is chosen, the relevant literature is reviewed, and individual publications are selected or rejected based on meeting that predetermined standard. The means, standard deviations, and sample sizes of the selected publications are statistically analyzed; the result is concise findings that are more broadly applicable than the results of the individual publications.
In 2009, we identified a need for a meta-analysis of the fire-oak literature because no large-scale systematic review and synthesis had been done on the subject and there were a sufficient number of published articles, a lack of guidelines specifically for oak forests, and increasing use of prescribed fire in oak forests by land management agencies. For this meta-analysis we posed the following research hypothesis: Fire will disproportionately benefit oak relative to mesophytic tree species. Specifically, we predict the following:
- Fire will reduce the density of midstory trees of all species.
- Oak reproduction will basal sprout after prescribed fires at a higher rate than the reproduction of mesophytic hardwood species.
- The proportion of oak reproduction relative to that of mesophytic hardwood species will increase postfire.
- Oak reproduction will be at least as tall as the reproduction of mesophytic hardwood species postfire.
- Density of new oak seedlings (germinants) will increase postfire.
The first three predictions test direct fire effects, whereas the other two address indirect effects in that they are influenced by other factors (shading, seed production, and adequate seedbed). Prediction 2 is short-term (1 or 2 years postburn), whereas the others are longer, depending on the duration of the study. After testing each prediction, we dissect the result, examining the characteristics of the studies contributing to the outcome of that prediction to comprehend why fire produced that effect. Understanding how and why fire promotes oak reproduction will lay the groundwork for developing prescribed burning guidelines for oak forests.
Data and Methods
For this project, we initially formed a pool of fire-oak publications from our personal files and libraries that we could access directly. This collection was supplemented by Internet searches on Web sites such as Google Scholar and Web of Knowledge for fire-oak publications that we did not possess. Finally, we contacted colleagues involved in fireoak research for unpublished progress reports on active studies and recently accepted manuscripts. These searches resulted in a database of 187 manuscripts from throughout eastern North America.
We then began winnowing the database using three criteria. Our first criterion was whether the publication provided experimental data that addressed at least one of the five test predictions. This step eliminated the fire history and general discussion publications. Our next criterion was whether the publication contained a sufficient replication of fire treatment(s) to permit statistical analysis. Case studies were thereby eliminated. Our last criterion was whether the publication contained a sufficient description of fire behavior (season of burn and fire intensity) and the site (stand density and management history) to help explain the results. Finally, we decided to focus on the prescribed fire projects instead of the individual publications because some of the projects, especially the large, long-term studies, produced multiple publications. Ultimately, we settled on 50 articles/ reports from 32 prescribed fire projects conducted in 15 states for this meta-analysis project (Table 1).
Meta-analysis requires the creation of standards or criteria to compare the results of the studies. These standards may be means, rates, or ratios. For this project, we created the following standards to test the predictions using preburn/postburn or burned/unburned data.
1. Midstory reduction: The mean decrease in the density of stems (2.5-28.0 cm dbh) of all species.
Table 1. Publications of the prescribed fire studies used in this meta-analysis project.
|1||Daniel Boone NF||KY||Alexander et al. 2008||R|
|2||Clemson Forest||SC||Barnes and Van Lear 1998||M|
|3||Horsepen WMA||VA||Brose and Van Lear 1998, 2004, Brose et al. 1999, Brose 2010||R|
|4||State Game Land 29||PA||Brose 2012||R|
|5||Allegheny NF||PA||Brose 2012*||R|
|6||Clear Creek SF||PA||Brose et al. 2007||R|
|7||Westvaco Forest||WV||Collins and Carson 2003||M|
|8||Purdue Forest||IN||Dolan and Parker 2004||R|
|9||Chilton Creek Tract||MO||Sasseen and Muzika 2004, Dey and Hartman 2005, Fan et al. 2012||R|
|10||Land/Lakes NRA||KY||Franklin et al. 2003||R|
|11||Clemson Forest||SC||Geisinger et al. 1989||R|
|12||Moshannon SF||PA||Brose et al. 2007, Gottschalk et al. 2012||R|
|13||Red River Gorge||KY||Arthur et al. 1998, Gilbert et al. 2003, Blankenship and Arthur 2006, Green et al. 2010||R/M|
|14||University of MO Forest||MO||Paulsell 1957, Huddle and Pallardy 1996||M|
|15||Bankhead NF||AL||McGee 1979, 1980, Huntley and McGee 1981, 1983||R|
|16||Vinton Furnace EF||OH||Sutherland and Hutchinson 2003, Hutchinson et al. 2005, 2012||R/M|
|17||Powhatan WMA||VH||Keyser et al. 1996||R|
|18||Jordan Timberlands||WI||Kruger and Reich 1997||R|
|19||Dinsmore Woods||KY||Luken and Shea 2000||R|
|20||Duke Forest||NC||Maslen 1989||R/M|
|21||Broome County||NY||McGee et al. 1995||R|
|22||Morgan SF||IN||Merritt and Pope 1991||R/M|
|23||Schmeeckle Reserve||WI||Reich et al. 1990||R|
|24||Fernow EF||WV||Schuler et al. 2012||R|
|25||Ft. Indiantown Gap||PA||Signell et al. 2005||R/M|
|26||Clemson Forest||SC||Stottlemyer 2011||R|
|27||University of TN Forest||TN||Thor and Nichols 1973, DeSelm et al. 1991, Stratton 2007||R/M|
|28||Sumter NF||SC||Teuke and Van Lear 1982||R|
|29||Green River WMA||NC||Waldrop et al. 2008||R/M|
|30||Zaleski SF||OH||Albrecht and McCarthy 2006, Iverson et al. 2008, Waldrop et al. 2008||R/M|
|31||Zaleski SF||CT||Ward and Brose 2004||R|
|32||Zaleski SF||WI||Will-Wolf 1991||M|
NF, National Forest; WMA, Wildlife Management Area; SF, State Forest; EF, Experimental Forest; NRA, National Recreation Area; R, reproduction; M, midstory.
* Unpublished data on file at the Forestry Sciences Laboratory, Irvine, PA.
- Differential sprouting: The difference in postfire basal sprouting rates between oak reproduction ( 2.5 cm dbh) and those of mesophytic hardwood species.
- Oak relative abundance: The change in the proportion of oak reproduction in the regeneration pool ( 2.5 cm dbh) between the beginning and end of the study.
- Oak relative height: The height of the oak reproduc-tion compared with that of mesophytic hardwood species at the end of the study.
- Oak seedling establishment: The increase in the meannumber of new oak seedlings during the course of the study.
Generally, each project provided data for three or four of the standards. Nine projects provided data for just one of the standards and only three of the projects provided data for all five standards. Sometimes the publications provided the data for the standard in the format we needed for the meta-analysis. For example, the publications containing mean preburn/postburn oak seedling or midstory stem densities generally had these data in a ready-to-use format for andards 1 and 5, but for standards 2, 3, and 4, we had to do some simple grouping and calculations before conducting the meta-analysis. For these three standards, we made two species groups: oak and mesophytic species. Hickory (Carya spp.) was included with oak because these two genera share many silvical characteristics, whereas the mesophytic group included all other hardwoods generally considered to be competitors to oak and potential oak replacements. For the oak sprouting standard (no. 2), we used the preburn and the immediate postburn stem densities to calculate the mean oak basal sprouting rate by dividing the postburn oak stem density by the corresponding preburn density. We did likewise for the mesophytic group and the two basal sprouting rates (oak and mesophytic) were then used in the meta-analysis. For the oak relative abundance standard (no. 3), we divided the preburn oak stem density by the total preburn stem density and did likewise for the oak and total stem densities reported at the end of the study. For the oak height standard (no. 4), we divided the mean oak seedling height at the end of the study by the corresponding height of the mesophytic species.
Once the standards are extracted from the publications or derived from the results, meta-analysis uses them and the corresponding variances and sample sizes to calculate the “effect size,” a measure of the magnitude of the effect of that experiment (Borenstein et al. 2009, Harrison 2011). There are several effect size indices and software programs for calculating them. We chose to use the log response ratio (ln R) as this index because it quantifies the proportionate change that results from experimental manipulation and is commonly used for conducting meta analysis of ecological studies (Osenberg et al. 1997, Hedges et al. 1999) and MetaWin 2.0 software (Rosenberg et al. 1997) for our project. When the effect size (ln R) is positive, then the fire increases the standard, whereas a negative ln R value indicates that fire decreases the standard. An effect size not significantly different from zero indicates that the fire had no discernible effect on the standard. For each standard, once an effect size is calculated, a cumulative effect size (grand mean) is calculated for all studies providing data for that standard.The effects of a fire on hardwood reproduction or midstory trees are a function of several factors (Brose and Van Lear 2004) and we tested the influence of some of these factors with summary analysis. This procedure is similar to analysis of variance in that the effect sizes and variances of the studies applicable to each factor are sorted into categories and tested by comparing resulting P values to a critical threshold indicating a significant difference between or among categories (Borenstein et al. 2009, Harrison 2011).
For our summary analyses, we chose five factors that we considered to be likely influences on the individual and cumulative effect sizes and that were readily available from the literature (Table 2). These factors were status of oak reproduction, season of burn, number of fires, stem size class, and study duration. Each of these factors contained two or three categories, and the studies were assigned to these categories for the summary analyses. Status of oak reproduction was either released or suppressed. Released oak reproduction consisted of oak seedlings or sprouts that were not limited by lack of sunlight. They had been growing in stands treated with a shelterwood release cut or final harvest several years before the prescribed fire. Suppressed oak reproduction was growing in uncut stands. Season of burn was either dormant or growing season. Dormant-season burns occurred between leaf abscission in the autumn and the beginning of leaf expansion of the mesophytic hardwoods the following spring; growing-season fires occurred during the other months. Number of fires referred to how many prescribed burns were conducted during the study (one, two, or more than two). Stem size class was either saplings (2.5-14.0 cm dbh) or poles (15.0-28.0 cm dbh). Study duration was short-term ( 5 years) or long-term ( 5 years). Not
Table 2. Characteristics of the prescribed fire studies used in this meta-analysis project.
|Study||Location||State||Seedling status||Season of burn||No.of fire||Study duration||No.of Replicates|
|1||Daniel Boone NF||KY||Sup||Dor||2||5||3|
|4||State Game Land 29||PA||Rel||Gro||1||3||2|
|6||Clear Creek SF||PA||Sup||Gro||1||3||3|
|9||Chilton Creek Tract||MO||Sup||Dor||1,3,4||5||5|
|13||Red River Gorge||KY||Sup||Dor||2,3||10||3|
|14||University of MO Forest||MO||Sup||Dor||10||10||2|
|16||Vinton Furnace EF||OH||Sup||Dor||2,4||7||4|
|25||Ft. Indiantown Gap||PA||Sup||Dor||3,4||1||4|
|27||University of TN Forest||TN||Rel||Dor||10||10||6|
|29a||Green River WMA||NC||Rel||Dor||2||5||3|
|29b||Green River WMA||NC||Sup||Dor||2||5||3|
NF, National Forest; WMA, Wildlife Management Area; SF, State Forest; EF, Experimental Forest; NRA, National Recreation Area; Rel, Released; Sup, suppressed; Dor, dormant; Gro, growing.
all factors were pertinent to summary analysis of each standard. For our summary analyses, we used random effects models with an value of 0.05 for determining statistical significance.
Of the 32 prescribed fire projects, 14 provided data on the changes in midstory density (Figure 1). Mean preburn midstory densities were 513 115 stems/ha and meanpostfire midstory densities were 234 45 stems/ha, a 54% reduction. Overall, this reduction in stem density was sig-nificant; the grand mean was 0.88 0.61 ln R with the log response ratios of the individual studies ranging from0.06 to 1.94 ln R. Subsequent summary analysis indicated differences in midstory density reduction by size class(P 0.008) and the number of fires (P 0.036). The decrease in stem density was concentrated in the saplings, especially those less than 10 cm dbh, as postburn sapling densities declined by 88% whereas pole densities dropped by only 15%. Of the three fire categories, single fires did not reduce midstory stem density (13% decline), but two fires and more than two fires did, leading to 36 and 71% declines, respectively. It was not possible to test fire season because all 14 projects used dormant-season fires.Twenty-three prescribed fire projects provided appropriate data to examine the postfire basal sprouting rates of oak and mesophytic reproduction (Figure 2). Postfire basal sprouting rates reported in the studies or calculated from their data ranged from 13 to 96% for oak and from 5 to 85% for mesophytic species. Overall, oak reproduction sprouted postfire at a 32% higher rate than the mesophytic species, resulting in a significant grand mean of 0.421 ln R. Sum- mary analysis found significant differences between the two species groups by fire season (P 0.009) and status of thereproduction (P 0.002). For growing-season fires, oak reproduction sprouted at a 58% higher rate than the meso- phytic species, but after dormant-season fires the difference in sprouting rates between the two groups was nearly zero. Similarly, released oak reproduction sprouted at a 56% higher rate than the mesophytic species, whereas suppressed oak reproduction had a 14% greater sprouting rate than the mesophytic species. When these two factors were combined, sprouting rates were 56% higher for released oaks than for the mesophytic species after growing-season fires, 20% higher for released oaks than for the mesophytic species after dormant-season fires, 14% higher for suppressed oaks than for the mesophytic species after dormant season fires, and 65% lower for suppressed oaks than for the mesophytic species after growing-season fires. No significant differences were found for number of fires.
Twenty-three studies provided suitable data for examining the change in the relative abundance of oak reproduction (Figure 3). Overall, prescribed burning did not significantly change the proportion of oak reproduction in the advance regeneration pool. The grand mean was 0.3420.393 ln R. Before burning, mean oak abundance was 25.6% of the seedling pool and after burning it was 26.0%. Summary analysis found only one significant difference: oak relative abundance in studies involving growing-season fire and released reproduction was greater than that with
Figure 1. The reduction of pole and sapling stem density (log response ratio 95% confidence interval) after prescribed fires conducted throughout the eastern United States. Log response ratios significantly less than zero indicate that the number of midstory stems decreased, whereas log response ratios not different from zero indicate that the postburn densities were unchanged. The numbers refer to the prescribed fire projects in Table 2.
Figure 2. The relative sprouting (log response ratio 95% confidence interval) of released (Rel) and suppressed (Sup) oak reproduction in comparison to mesophytic hardwood reproduction following dormant-season (Dor) and growing-season (Gro) prescribed fires conducted throughout the eastern United States. Log response ratios significantly greater than zero indicate that the oak reproduction sprouted postfire at a higher rate than the mesophytic reproduction. Log response ratios significantly less than zero indicate the opposite, and log response ratios not different from zero indicate that the survival rates of the two species groups were equivalent. The numbers refer to the prescribed fire projects in Table 2.
Figure 3. The relative abundance (log response ratio 95% confidence interval) of released (Rel) and suppressed (Sup) oak reproduction after dormant-season (Dor) and growing-season (Gro) prescribed fires conducted throughout the eastern United States. Log response ratios significantly greater than zero indicate that the proportion of oak reproduction increased in the regeneration pool. Log response ratios significantly less than zero indicate the opposite, and log response ratios not different from zero indicate that the proportion of oak did not change. The numbers refer to the prescribed fire projects in Table 2.
dormant-season fire and suppressed reproduction (P0.006). Otherwise, no differences were found among number of fires (P 0.873) or between seasons of burn (P0.62) or by study duration (P 0.982). Only 11 studies provided postburn height data of the oak and mesophytic reproduction (Figure 4). Overall, heights of the oaks were 95% of the heights of the mesophytic species. The grand mean was 0.16 0.18 ln R, indicating no
Figure 4. The relative height (log response ratio 95% confidence interval) of oak reproduction in comparison to mesophytic hardwood reproduction after short-term (<5 years) and long-term (>5 years) prescribed fire studies conducted throughout the eastern United States. Log response ratios significantly greater than zero indicate that the oak reproduction was taller than the mesophytic reproduction postfire. Log response ratios significantly less than zero indicate the opposite, and log response ratios not different from zero indicate that the heights of the two species groups were equivalent. The numbers refer to the prescribed fire projects in Table 2.
difference between the two species groups. Summary analysis also found no differences between the categories by season of burn, seedling status, or study duration because their P values ranged from 0.686 to 0.96.
Fifteen fire projects provided data on the establishment of new oak seedlings (Figure 5). Overall, the number of new oak seedlings increased by an average of 1,315 290 stems/ha during the course of these studies, resulting in agrand mean of 0.33 ln R. This effect size was not different from 0 because of the tremendous variability reported in the studies (individual log response ratios rangedfrom 1.02 to 0.94). Summary analysis showed no differences based on study duration (P 0.334).
Forestry professionals identify periodic fire as a major reason for the historical occurrence of mixed-oak forests in eastern North America and the cessation of that fire regime in the early 20th century as one of the key factors in the current, widespread oak regeneration problem (Abrams 1992, Brose et al. 2001, Nowacki and Abrams 2008). Consequently, researchers have been engaged in trying to determine how to use prescribed fire to help solve this problem, and their efforts have produced dozens of studies and hundreds of publications replete with examples of when prescribed burning benefited oak reproduction, when it hindered forest renewal, and when it had a negligible impact on the regeneration process. Meta-analysis offers a means bywhich these divergent studies can be compared on a common basis to support or refute the notion that prescribed fire can help regenerate mixed-oak forests.
The results of our meta-analysis support the idea that prescribed fire can help regenerate mixed-oak forests in some situations. Prescribed burning reduced the density of midstory stems (prediction 1), oak reproduction sprouted postfire at higher rates than mesophytic reproduction (prediction 2), and postfire height growth of oak reproduction was comparable to that of mesophytic reproduction (prediction 4). In addition, establishment of new oak seedlings showed a trend toward greater density in burned areas relative to unburned control areas (prediction 5). Collectively and individually, all four of these findings indicate that fire moves an oak forest through the regeneration process in a manner consistent with sustaining that forest’s oak component in the future.
Further testing of these predictions and the nonsignificant outcome of prediction 3 (that the postfire proportion of oak reproduction will be greater than that of other hardwood species) illustrate some important caveats on using fire to promote oak regeneration. Reduction of midstory density (prediction 1) was dependent on the diameters of the stems and the number of fires. Single fires, especially those in the dormant season, decreased the number of small saplings, especially those less than 10 cm dbh but had virtually no
Figure 5. The establishment of new oak seedlings (log response ratio 95% confidence interval) after short-term (<5 years) and long-term (>5 years) prescribed fire studies. Log response ratios significantly greater than zero indicate an increase in the density of new oak seedlings, whereas log response ratios significantly less than zero indicate the opposite, and log response ratios not different from zero indicate no change in the density of new oak seedlings. The numbers refer to the prescribed fire projects in Table 2.
effect on larger diameter stems. This outcome is understandable because prescribed fires are conducted under predetermined fuel and weather conditions to minimize the risk of escape and damage to valuable crop trees. Once hardwoods have grown beyond 10 cm dbh, they are large enough and have thick enough bark to survive most prescribed burning, especially single, low-intensity, dormant-season fires. Multiple fires do eventually cause a reduction in the number of larger saplings and poles. Unfortunately, the multifire data came entirely from dormant-season fires so comparing them with growing-season burns was not possible. However, it is likely that growing-season fires would have a faster and greater impact than dormant-season burns on reducing the density of larger diameter stems.
The superior postfire sprouting ability of oak reproduction (prediction 2) was probably a result of their tendency to allocate carbon more to root development than to stem development in contrast with many of the mesophytic hardwood species (Gottschalk 1985, 1987, 1994, Kolb et al. 1990, Brose 2011). Superior oak sprouting was not universally observed, however; the status of the reproduction (released or suppressed) and fire seasonality (dormant or growing season) were major factors in the outcome. Growing-season fires involving released reproduction produced the largest advantage to oaks in postfire sprouting rates. Conversely, growing-season fires involving suppressed reproduction resulted in a postfire oak sprouting rate less than that of the competitors. This was probably the result of the suppressed oak seedlings having smaller roots and depleted carbohydrate reserves relative to the larger, well-established, shade-tolerant mesophytic species. For dormant season burns, the postfire sprouting rates of oaks were slightly but nonsignificantly higher than those of the competing mesophytic species, regardless of whether the oak reproduction was suppressed or had been released. The few dormant-season studies that showed a difference in sprouting rates between the two species groups had extenuating circumstances such as the competitor’s high susceptibility to fire or the use of several burns.
The superior postfire sprouting ability of oak did not translate into an increase in oak’s relative abundance in the regeneration pool (prediction 3). Generally, changes in oak relative abundance tended to follow the previously described patterns of oak sprouting. Prescribed growing season burns involving released oak reproduction resulted in greater oak relative abundance, whereas dormant-season fires or any fires involving suppressed oak reproduction usually showed decreased relative abundance or no appreciable change. The overall lack of change in oak relative
abundance is probably a result of new mesophytic seedlings germinating from the seed stored in the forest floor (Schuler et al. 2010) or disseminated from nearby trees or sprouts arising from root systems.
The equalizing of postfire height growth between oak and mesophytic reproduction (prediction 4) should be interpreted cautiously. First, the mesophytic group contained a wide variety of hardwood species, everything other than oak and hickory, so the mean heights used in the meta-analysis were tempered by the slower growing species. Unfortunately, many of the studies did not differentiate well enough among mesophytic species to allow us to focus on primary competitors such as yellow-poplar. Second, height growth of sprouting hardwoods after fire is a function of their prefire size and vigor, the degree of shading, and site quality. The 11 studies used in the meta-analysis repre-sented a diverse mix of prefire seedling conditions, canopy cover, and sites. Thus, the equal height growth of oak and mesophytic reproduction postfire may be an artifact of the inherent variability among the studies rather than a biological certainty that oak reproduction can match mesophyitc reproduction in height growth postfire.
Prediction 5, that fires facilitate the establishment of new oak seedlings, must also be interpreted cautiously. We intended to use only studies that tallied multiple stems arising from the same rootstock as one stem, but sometimes we could not determine from some of the projects whether this was how the reproduction was inventoried. Moreover, only a few of the publications mentioned the occurrence of an acorn crop, an essential precursor to establishment of new oak seedlings. It is not clear whether fires actually improve the germination success of acorns or whether the reported increases were the result of the inventorying procedures.
In even-aged stand management, the regeneration process for mixed-oak forests can last 10 to 25 years depending on numerous factors (Loftis 2004, Johnson et al. 2009). The process consists of three major phases, production of acorns, establishment of oak seedlings from those acorns, and development of those seedlings into competitive-sized oak reproduction, and an event, an adequate, timely release of that reproduction (Loftis 2004). Two intrinsic factors make the process inevitably slow: sporadic acorn produc tion and root-centered seedling growth. In addition, weather, interfering vegetation, wildlife, dense midstory shade, and other factors can slow or stall any of the three phases.
Based on this meta-analysis, prescribed fire appears to fit into two places in the oak regeneration process. The first is at the beginning of the regeneration process as a site preparation tool. The second is near the end of the regeneration process as a release tool. In either case, the first step in using fire is an inventory of the abundance and size of the oak reproduction, overstory conditions, and potential stand renewal obstacles such as competing and interfering vegetation, browsing pressure by white-tail deer (Odocoileus virginianus), and site limitations. The inventory may be a comprehensive examination as is done with stand prescription programs such as SILVAH (Brose et al. 2008) or less-intensive assessment of stand conditions. However, it must be done to determine whether there is enough oak reproduction to proceed with stand regeneration. The determination of the adequacy of oak reproduction is highly stand-specific; what is sufficient oak reproduction for one stand may be inadequate for another based on several extenuating factors such as site characteristics, composition of the competing species, and impact of white-tail deer.
Mature, closed-canopy oak stands that lack adequate oak reproduction are at the beginning of the regeneration process. Burning can decrease midstory density, thereby increasing understory light and can reduce the thickness of the forest floor, especially the litter layer, which can be a barrierto germination and seedling establishment (Korstian 1927, Barrett 1931, Carvell and Tryon 1961, Wang et al. 2005). Site preparation burning may also have a negative impact on populations of acorn pests such as weevils (Curculio spp.) (Wright 1986, Riccardi et al. 2004) and xerify the upper layers of the soil (Barnes and Van Lear 1998), making it a less hospitable seedbed for mesophytic hardwoods. This approach will probably take a decade or more because the benefits of burning are initially small and multiple burns are needed to create the desired understory conditions. This appears to be especially true with low-intensity fires conducted in the dormant season. In comparing winter and spring burns, Barnes and Van Lear (1998) concluded that three dormant-season fires were needed to equal the impact of one growing-season burn for intermediate-quality sites in the upper Piedmont region of western South Carolina. Regardless of fire seasonality and fire intensity, site preparation burning will probably be a long-term endeavor because oak seedling establishment is dependent on an acorn crop, and masting in oaks can be highly sporadic due to several intrinsic and extrinsic factors. Furthermore, leaf litter re-accumulates within a few years postburn so the benefit of litter reduction is short-lived. Our conclusion is that site preparation is a fair to good use of prescribed fire in oak management, but the time required to achieve satisfactory results may be a major disadvantage. Reducing midstory shade with herbicides (where permitted) may be a more efficient approach with less potential damage to residual canopy trees.
Oak stands with an adequate density of oak reproduction that have received a heavy partial cut or have been completely harvested are well into the regeneration process because the reproduction is no longer limited by shading. In this context, prescribed burning to release the oak reproduction from the competing mesophytic species appears to be an excellent use of fire as long as the competing stems are less than 10 cm dbh. Of the studies included in this meta analysis, those that occurred in stands that had been partly to completely harvested several years before the fires showed consistently strong positive benefits to the oak component. The oak reproduction survived at a higher rate than the mesophytic competitors, oak relative abundance increased postfire, and the oak sprouts grew at a rate comparable to that of the mesophytic hardwoods. In release burning, fire seasonality and fire intensity matter. The strongest benefits to oak were associated with moderate- to high-intensity growing-season fires. In practical application, when an oak stand has adequate oak reproduction to proceed with the regeneration process, we recommend harvesting the overstory via a two-cut shelterwood sequence or a final removal cut and then burning either between the shelterwood harvests or after the overstory is completely removed. The key is to wait several years after the harvest to burn so that the oak reproduction has adequate time to develop its root system and increase its probability of vigorous sprouting after future burns (Brose 2008, 2011).
Our review of fire-oak literature suggested several special circumstances that may alter or curtail burning plans. One is that prescribed fires can damage and kill overstory trees, some of which may be high-value crop trees. Although this negative effect has been known for years (Nelson et al. 1933, Paulsell 1957, Berry 1969, Wendel and Smith 1986), it is especially true for burning during a shelterwood sequence because of the elevated fuel loads (Brose and Van Lear 1999). In such cases, slash management (lopping, scattering, or removal from the bases of crop trees) is essential to prevent unacceptable losses. Another fire damage caveat is when an oak stand is in the stem exclusion stage of development. Sapling- and pole-size oaks are quite susceptible to fire scarring and subsequent value loss with little change in species composition (Carvell and Maxey 1969, Ward and Stephens 1989, Maslen 1989). Acorns appear to be quite susceptible to fire damage (Auchmoody and Smith 1993), so we advise against burning shortly after an acorn crop if the germinants from those acorns are needed to become oak advance reproduction. A closely related caveat pertains to small oak seedlings. Prescribed fires will kill suppressed oak reproduction, especially growing-season burns. Although this meta-analysis did not examine the influence of seedling size on the outcome of the studies, it was apparent from the few studies with detailed height data that sprouting rate was affected by size. Large oak reproduction sprouted postfire at consisently higher rates than small oak reproduction, especially when the fire occurred in the growing season, and initially larger stems grew taller after burning under any given overstory stocking and burn treatment. Initial diameter and size of oak reproduction are good indicators of its ability to survive fire and are good predictors of future competitive capacity (Brose and Van Lear 2004, Dey and Hartman 2005). Consequently, when the oak component of the regeneration pool is mostly small reproduction, land managers should consider using low-intensity dormant-season burns to minimize losses or opt for other silvicultural practices such as a shelterwood preparatory cut or individual stem herbicide treatments to move the oak stand forward in the regeneration process.
Two nonoak caveats are the presence of invasive species and deer browsing. Some plant species such as the native hay-scented fern (Dennstaedtia punctilobula) and the exotic tree of heaven (Ailanthus altissima) can spread rapidly after a fire (Rebbeck et al. 2010, Gottschalk et al. 2012) so their presence in or near the burn unit may require preemptive control measures to prevent their spread. Similarly, whitetail deer will be attracted to burned areas and excessive browsing can quickly turn a potential regeneration success into a failure. Potential deer problems should be identified and mitigated before burning.
Future Research Needs
Our collecting and reviewing of the fire-oak literature and our subsequent meta-analysis identified several Knowledge gaps that merit research. They are the following:
- The relationship between fire intensity and postfires prouting of hardwood reproduction. We had hoped to include fire intensity as one of the contributing factors, but this was not feasible because the studies had widely divergent approaches to measuring this variable. Some simply described fire intensity (cool, hot,or typical for the conditions) or placed it in broad classes (low, moderate, or high) or measured characteristics of the flaming front, but reported them at the stand or treatment level. Despite this variability, it was clear that relationships exist between fire intensity and postfire sprouting of hardwood reproduction. Fire in tensity and postfire sprouting need to be measured at the same scale.
- Fire effects on the establishment of new oak seedlings.Although our meta-analysis suggests that establishment of new oak seedlings increases postfire, wecannot be sure because some studies included in the analysis did not state exactly how the reproductionwas inventoried. Research is needed to determine whether fire promotes establishment of new oak seed-lings and to verify the sensitivity of acorns to fire.
- The impacts of fires on other oak ecosystem compo-nents. The vast majority of the fire-oak publications we found directly address regeneration concerns, but the fire effects on other ecosystem properties may be important indirect influences on oak reproduction and oak forest health. For example, oaks are ectomycorrhizal, whereas most of the mesophytic species are endomycorrhizal, and shoestring fungus (Armillaria mellea) is a common pathogen implicated in oak decline. How does fire affect these fungal communities? In addition, growing-season burns provide excellent control of competing mesophytic hardwoods, but they may adversely affect ground-nesting birds and herpetofauna in the short term via disrupted nesting or direct mortality. Do these short-term losses really occur or do such burns benefit the overall populations in the long-term by creating improved habitat? Knowing the impacts of fire on potentially sensitive species will help managers tailor their burning prescriptions.
- A comparison of fire with other silvicultural treat-ments and the sequencing of fire with other silvicultural treatments. The number of oak forests that could benefit from properly applied prescribed fire far exceeds what can be accomplished, even under the best of circumstances. Knowing the tradeoffs between prescribed fire and a fire surrogate such as herbicide application or mechanical site scarification will help foresters match the right tool with the job. Similarly, the exact sequencing of fire with other silvicultural practices merits more research because the more efficient and streamlined the oak regeneration process is, the more likely it is to succeed. Research on treatment efficiency would help managers make wiser use of their limited budgets.
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