Varner et al., 2014
RESEARCH BRIEF 18
Fire-adapted oaks are an important component of fire-prone woodlands because they impact fire behavior and species biodiversity through the flammability of their litter.
Frequent fire gives advantage to pyrophytic (fire-adapted) oaks while infrequent fire and fire exclusion gives advantage to mesophytic (fire-sensitive) oaks.
When restoring fire-prone woodlands, planners should recognize the functional role of pyrophytic oaks, thereby targeting mesophysic oaks for removal rather than all oaks equally.
In this study, the authors evaluated 25 life history traits relating to fire adaptation for eight oak (Quercus) species native to the southeastern U.S. Oak species occur throughout many ecosystems in the Southeast, from fire protected forests to annually burned savannas. In fire-prone ecosystems, oaks exhibit several survival strategies and have an important role in promoting animal and plant biodiversity.
Fire-adaptation strategies of woody plants include: investing in protective bark, rapid wound closure, ability to resprout from protected stem buds, and reliance on stored (“banked”) seeds adapted to germinate post-fire. Fire adaption traits analyzed in this study included response to stem injury, rates of bark accumulation, bark thickness, and leaf litter flammability. The study was based upon six published datasets; no field research was conducted.
Leaf litter traits examined included: depth, drying time, moisture-holding capacity, flame height/duration, and smoldering duration. Bark data included: accumulation and wound closure rates, and thickness for both saplings and mature trees. The area of xylem lost due to wounding and decay was examined, as was seed mass. Growth related data included: tree height, radial growth rates, wood density, and seedling growth rates. Leaf traits included: lifespan, persistence on stem, area, mass, chlorophyll, and hydraulic conductance.
Cluster analysis was conducted to segregate the eight oak species into three groups. One cluster, pyrophytes, or fire adapted oaks, shared traits such as highly flammable litter, rapid juvenile bark accumulation, rapid wound closure, and slow growth rates. These included southern red oak (Q. falcata), bluejack oak (Q. incana), turkey oak (Q. laevis), and sand post oak (Q. margaretta).
The second cluster, mesophytes, consisted of fire-sensitive oaks that have long leaf lifespans, low leaf flammability, thin bark, weak wound responses, and rapid growth rates. Included are laurel oak (Q. hemispaerica) and water oak (Q. nigra)
The third cluster, fire avoiders, shared traits between these two extremes. This group included evergreen live oaks with thick bark at maturity, rapid growth rates, and high wood density; they hamper fire by casting year-round shade and dropping moisture-retaining, non-flammable litter. Included are sand live oak (Q. geminata) and live oak (Q. virginiana).
Authors identified a strong correlation between traits that protect oaks against fire, such as thick bark and quick wound closure, and traits that facilitate fire near those trees, such as litter that dries rapidly and burns intensely. This results in a feedback loop between an oak species’ ability to survive fire and its role as an incendiary fuel producer, which effectively kills neighboring competitors.
The four pyrophytic oaks are more persistent through repeated fires due to thick, fast-growing, fire-protecting bark that closes wounds rapidly with minimal tissue decay. These pyrophytic species have deep, porous leaf litter that dries quickly and burns for a short time with tall flames, resulting in high levels of fuel consumption and leading to increased local fire intensity. They have a competitive advantage because these intense fires are likely to kill or injure other tree species that lack a similar bark and wound response.
Producing thick bark is an expensive allocation of resources. Species that invest heavily in bark appear to do so through tradeoffs in growth rates. Sprouts with thick bark are typically shorter than those with less bark. This tradeoff is most advantageous where heating is greatest, usually on the lower stem, as less protective bark is needed higher on the tree bole.
Study authors emphasized fire-adapted oaks’ relationships with the herbaceous understory within woodlands. Compared to mesophytic oaks, pyrophytic oaks better coexist with highly flammable grasses, promoting a fire-prone ecosystem, which in turn limits the presence of fire-sensitive trees. The combustible litter of fire-adapted oaks not only alters local fire behavior but may also influence entire fire regimes.
Oak species not included in this study, in particular post oak (Q. stellata), blackjack oak (Q. marilandica), and white
oak (Q. alba), also have litter flammability and protective traits that are consistent with strategies for the pyrophytic oaks reported here. These species were not included due to lack of data, and authors indicate the need to include these and other species in future studies.
Mesophytic oaks, which grow tall rapidly but lack fire-adapted traits, tend to occupy forests rather than woodlands or savannas. They do well where trees compete aggressively for light in fire protected areas. These oaks benefit from a strategy which impedes rather than promotes fire. In frequently burned landscapes, these species dominate only in protected refugia, where they become established. Their dominance diminishes the site flammability and affects the plant community composition and structure, resulting in a positive feedback loop termed “mesophication” by Nowacki and Abrams (2008). This process begins during fire-free intervals and continues if fire is further excluded.
In fire-adapted forest communities, the consequences of fire exclusion can be severe, including decreases in plant and animal diversity. Novel outcomes can occur when fire is reintroduced, such as mortality among trees that are normally fire-resistant.
Study authors recommend that land managers, in light of the vast diversity among oak species, employ a nuanced approach during ecological restoration activities in fire-prone woodlands. In recognition of the functional role of pyrophytic, fire-adapted oaks, restoration efforts may benefit from removing select mesophytic oaks rather than lumping all oaks together.
J. Morgan Varner, Jeffrey Kane, J. Kevin Hiers, Jesse Kreye, Joseph Veldman (2016) Fire Ecology 12:2
FOR FURTHER READING
Hiers, J.K., J.W. Walters, R.J. Mitchell, J.M. Varner, L.M. Conner, L. Blanc, and J.Stowe. 2014. Ecological value of retaining pyrophytic oaks in longleaf pine ecosystems.Journal of Wildlife Management 78: 383-393.
The Oak Woodlands and Forests Fire Consortium seeks to provide fire science to resource managers, land owners and the public about the use, application, and effects of fire in the region. www.oakfirescience.com
This research brief was funded by The Joint Fire Science Program. www.firescience.gov