Knapp et al. 2022 (Forest Ecology and Management)
RESEARCH BRIEF #52
A summary based on the following publication:
CLICK HERE TO DOWNLOAD AS A PDF
MANAGEMENT IMPLICATIONS
Frequent burning reduces overstory recruitment across all tree species.
Repeated prescribed fire significantly depletes midstory tree abundance, the source population for overstory recruitment.
A fire-free interval may be eventually necessary to promote the replacement of canopy trees.
This study quantified tree recruitment dynamics over a 20-year period of frequent fire in southeastern Missouri, USA. Repeated fires in temperate broadleaf open forests can create a ‘fire trap’ in which small woody stems remain abundant but are unable to grow into larger size classes due to being top-killed. The study investigated whether frequent fire could contribute to a tree recruitment bottleneck and examined frequent fire’s potential impact on long-term overstory replacement.
Two long-term, landscape-scale projects provided data for this investigation. Study sites at the Missouri Ozark Forest Ecosystem Project (MOFEP) had no fire treatments and served as control, while sites at the Chilton Creek Management Area (CCMA) were burned at randomly assigned 1- to 4-year intervals (a fire frequency commonly used to restore and enhance ground flora diversity in woodlands of the region) with an overall mean fire return interval of 2.4 years over two decades. Forest composition and structure were similar at the two sites when the study began in 1997. Mean overstory basal area was 89.7 ft2/acre at MOFEP and 66.2 ft2/acre at CCMA, with an average of 156.7 overstory trees (i.e., trees at least 4.5 inches in diameter at breast height [DBH], measured 4.5 ft above the ground) per acre at MOFEP and 152.3 at CCMA. Black oak was the dominant overstory species at both sites, followed by scarlet oak and white oak. Other common species at both sites included shortleaf pine, post oak, pignut hickory, and mockernut hickory, as well as black hickory at MOFEP and northern red oak at CCMA.
Most fire treatments occurred between February and April, though fall burns were occasionally implemented. Burn plans specified weather conditions within the following parameters: air temperatures of 34-80°F, relative humidity of 25-70%, and midflame wind speeds of 5-20 mi/hr.
Sampling was conducted across six periods between 1998 and 2017 at both MOFEP and CCMA. Overstory trees were surveyed within 0.5-acre plots, assigning each tree a uniquely numbered aluminum tag and recording species and DBH. Midstory vegetation (1.5 ≤ DBH <4.5 in) was measured within subplots located in each overstory plot.
The authors hypothesized that frequent burning would reduce ingrowth (defined as trees reaching ≥4.5 in DBH) but favor fire-tolerant species, and that resulting patterns would vary across four site types: summits, exposed backslopes, protected backslopes, and waterways. They further expected negative relationships between overstory basal area and both the rate and survival of ingrowth, with fire intensifying these effects. A positive relationship was anticipated between midstory stem abundance and ingrowth, especially with frequent fire. Finally, the authors hypothesized that frequent burning would reduce the source population for tree recruitment (i.e., midstory stems) over time.
Study findings showed that the number of ingrowth trees was consistent through time at MOFEP but decreased at CCMA. The magnitude of recruitment reduction was consistent across ecological site types, suggesting a broad-scale effect of frequent fire (see figure). The tree species composition of the ingrowth population was generally stable through time and similar between the two studies. The only species to decrease in relative abundance at the burned CCMA site (but not the unburned MOFEP site) was scarlet oak, a species that is sensitive to injury from fire due to its relatively thin bark and poor ability to compartmentalize wounds. Frequent burning reduced tree recruitment across all species and did not result in a compositional shift toward fire-tolerant species. Early in the study, stand basal area negatively affected ingrowth at both MOFEP and CCMA. When final measurements were made, it was found that stand basal area continued to negatively affect ingrowth with greater impacts at CCMA than at MOFEP. These findings supported the prediction that ingrowth would be hindered by higher basal area and further restricted by fire.
Over the course of the study, trees that were initially categorized as ingrowth had lower survivorship at the burned CCMA site. This result aligned with the expectation that fire would increase ingrowth mortality.
Midstory tree abundance declined gradually at MOFEP but dropped sharply at CCMA following the onset of frequent prescribed burning. As burning at CCMA reduced the midstory over time through fire-induced mortality or top-kill, tree recruitment declined, highlighting the midstory’s role as a source population for the overstory. Denser overstories were found to limit ingrowth by suppressing growth and extending the period in which trees remain in vulnerable, small size classes.
Overall, the study’s findings indicate that frequent burning affects tree recruitment processes in multiple ways, including reducing the pool of potential recruits by reducing midstory abundance and by increasing mortality or top-kill rates during early establishment. Additionally, these findings suggest that fire free intervals may be necessary for long-term canopy replacement in temperate broadleaf open forests, thereby providing opportunities to escape the ‘fire trap.’
