Fire and the Serpentine Ecosystem

Summary:The most important role of fire in savannas and grasslands is the regulation of microclimate; i.e., the climate near the ground in which plants and animals grow, reproduce, and evolve.  Dead plant stems and leaves which accumulate between growing seasons have a number of adverse effects on growing conditions.  By removing this dead material, fire improves microclimatic conditions for plant growth immediately.  Light levels increase, the length of the growing season increases, excessive heat and drought conditions improve, along with other more favorable growing conditions.  Even small fires can improve growing conditions substantially; dead biomass can be periodically burned on a small scale with great success.  Microclimate is also important in the evolution of plants and animals.  Over time, species adapt to conditions in which they are growing.  In savannas, species have been/are evolving in microclimate maintained by fires occurring every 4 to 10 years.  Changing this evolutionary fire regime changes growing conditions, and species which cannot adapt to a different microclimate decline and eventually disappear.  Fire effects also differ considerably by season, so knowing the evolutionary fire history of an ecosystem is imperative before designing a prescribed burn program.  Fires in the serpentine ecosystem of Maryland and adjacent Pennsylvania occurred in the “fall” (interpreted to be late fall into early winter) based on historical records and vegetation characteristics.  Spring fire can be used as a temporary tool to manage invasive greenbrier.  Smoke management will continue to limit prescribed burning at Soldiers Delight and other serpentine sites near urban and suburban areas.  Although large volumes of smoke can be managed effectively, the high visibility of smoke columns from several miles away can lead to an overloading of calls at 911 centers.  As a result, only small-scale burns are feasible in non-rural landscapes, limiting the total number of acres burned annually.             

Discussion:

Fire and Microclimate.  Perhaps the most important role of fire in savannas and grasslands is the regulation of microclimate; i.e., the climate near the ground in which plants and animals grow, reproduce, and evolve.  In the 1960s, biologists started measuring weather parameters in burned and unburned prairies; their results are applicable to savannas with prairie-like vegetation such as serpentine barrens.  In brief, growing conditions were found to be more favorable, and occurred earlier in the growing season, in burned prairie than in unburned (Hulbert 1969, Hulbert 1988, Knapp 1984, Knapp and Seastedt 1986, Old 1969, Peet et al. 1975).  Unburned prairie has a large quantity of dead biomass, both standing and on the ground (litter).  Standing dead stems and leaves intercept sunlight and, therefore, reduce photosynthetic light levels resulting in smaller plants and fewer flowers than in burned prairie.  Litter on the ground inhibits the growth of shoots in the spring, by absorbing sunlight and minimizing soil temperature, thus shortening the growing season.  Lower soil temperature also inhibits N and P production by microbes, nutrients which are released to the soil and used by plants.  Burning off the litter layer substantially increases light levels for shoot growth, lengthens the growing season as shoot tips are exposed to light sooner, and increases soil temperature which results in faster plant growth and more N and P released from bacteria and fungi in the soil.  In an Oklahoma tallgrass prairie, optimal soil temperature for growth of dominant prairie plants occurred more than a month sooner in burned prairie than in unburned prairie (Rice and Parenti 1978).  In a Kansas tallgrass prairie (Knapp 1984), early spring soil temperature was as much as 30 F degrees warmer in burned versus unburned prairie, and 16 F degrees warmer six inches aboveground.  Growth of unburned tallgrass was 55 % lower, and photosynthetic light 58 % lower, compared to burned.  In an Illinois tallgrass prairie (Old 1969), a dormant-season spring burn caused a 2- to 3-fold increase in growth of dominant grasses and a 10-fold increase in flowering.    

Standing dead plants also restrict air flow which allows excessive heat to build up around live plants, reducing their photosynthesis and growth (Knapp 1984).  Leaf temperatures in burned prairie in Kansas were optimal for photosynthesis, but much greater than optimal in unburned due to less air flow.  In addition, higher air temperatures cause greater water loss from leaves which limits photosynthesis. When prolonged, heat and water stress also increase mortality particularly during droughts.

Microclimate is also important in the evolution of plants and animals.  Over time, species adapt to conditions in which they are growing.  In savannas, species have been/are evolving in a microclimate maintained by fires occurring 2 to 3 times per decade.  Changing this evolutionary fire regime changes growing conditions, and species which cannot adapt to a new microclimate decline and eventually disappear.  Others may be able to persist, but do not exhibit optimum growth or reproduction.  Fire-free serpentine barrens typically have a lackluster appearance of stunted grasses and scattered diminutive wildflowers.  Removing fire from a fire-dependent ecosystem results in its collapse and eventual replacement, and the end to the evolution of species restricted to that ecosystem.    

Fire Season.  Fire effects differ considerably by season, so knowing the evolutionary fire history of an ecosystem is imperative before designing a prescribed burn program.  Fortunately, Marye (1955) was able to determine that fires in the serpentine ecosystem of Maryland and adjacent Pennsylvania occurred in the “fall” (interpreted to be late fall into early winter).  And that finding is corroborated by the characteristics of serpentine vegetation.  The dominant plants are very robust, competitive, perennial grasses, especially little bluestem and Indian grass.  Less competitive plants, such as annual wildflowers, survive between the clumps and colonies of these grasses.  An important effect of periodic fall fires is keeping the spread of little bluestem, Indian grass, and other warm season grasses in check, leaving habitat for many other species.  Periodic spring fires can have the opposite effect; i.e., growth and spread of warm season perennial grasses is favored, reducing available habitat for annuals and less competitive grasses.  Marye learned that English colonists mimicked fall burning by Native Americans until learning that spring burning would better enhance grass forage for livestock.    

Spring fire can be used as a temporary tool to restore serpentine vegetation if implemented correctly.  The best example might be with spring burning of the non-indigenous greenbrier.  Fire exclusion has led to its invasion, spread, and development of dense thickets.  Being fire intolerant, it can be eradicated with spring burning if the soil is dry and, therefore, rhizomes are exposed to lethal fire temperatures.  Rhizomes in wet soil are buffered from excessive heat of passing flames.  In addition, burning greenbrier in wet soil in the spring can enhance growth through the release of nutrients.  Unfortunately, spring rainfall usually precludes adequate drying of surface soil for spring burning-control of greenbrier.  Winter soils are almost always saturated on mid-Atlantic serpentine.

Fire Size.  Since a fundamental role of fire is on microclimate, even small fires can improve growing conditions substantially.  Dead biomass can be periodically burned on a small scale with great success.  However, small fires tend to be homogeneous and lack the diversity of conditions created by landscape fires.  Large-scale fires result in a landscape of areas ranging from completely burned to unburned thereby leaving a diversity of habitat conditions, and these conditions can fluctuate with subsequent fires.  In addition, landscape fires leave refugia for populations of plants and animals to quickly recolonize high-intensity burned areas.  This is especially important for insect populations which have been persisting in ecosystems subject to fire exclusion. 

Fire Frequency.  Large landscapes under indigenous conditions do not burn completely every year, except during extreme circumstances such as megadroughts.  For example, in a large serpentine oak savanna landscape, south- and west-facing slopes burn more frequently or completely than north- and east facing slopes, and slopes burn more frequently or thoroughly than ravine bottoms.  Areas which burn frequently, every 1-3 years, usually support grasslands, or sparse savanna, since tree seedlings and saplings, including oaks, have insufficient time to become established.  Areas which burn infrequently, every 8 years for example (Peterson and Reich 2001), support woodland or open forest.  

Species diversity increases with fire frequency in oak savanna as well as closed forest communities.  For example, a 40-year fire frequency experiment in Minnesota, involving study areas of open savanna to closed forest, found highest species diversity in the most frequently burned plant communities (Cavender-Bares and Reich 2012).  Fire frequency in the study ranged from nearly annual to none.  High frequency fires favor the development of grasses and forbs, while low frequency fires support woody plant dominance which suppresses grass and forb production (Peterson et al. 2007).   

In a 32-year study of Minnesota oak savanna and woodland stands by Peterson and Reich (2001), low frequency burning, fewer than two fires per decade, produced woodland stands with dense sapling thickets.  Frequent burning, three fires per decade, maintained oak savanna.  The authors concluded that more frequent burning, more than four fires per decade, would result in grassland as too many oak seedlings and saplings would be killed to sustain populations.

Post oak savannas in the Missouri Ozarks were maintained by low intensity ground fires which occurred every 4.3 years on average, during the century before Native Americans emigrated from the region (1710 – 1810) (Guyette and Cutter 1991).  During periods of severe drought, however, fires burned once every 11 years and were more severe.

Fire and Species Diversity.  A universal outcome of fire research in many ecosystems, especially savannas and grasslands, has been an increase in species diversity with increasing fire frequency.  At a long-term ecosystem science reserve in the Midwest, a 40-year prescribed burn experiment was conducted as part of an oak savanna restoration program (Cavender-Bares and Reich 2012).  The total number of plant species, and their distribution across the savanna, were lowest in unburned areas and highest in areas which were burned almost annually.  The same result has been documented repeatedly in prairie studies; annually burned prairies are richest in species diversity.  Although this research has shown that plant species diversity peaks with very frequent fire, staying within the evolutionary fire regime of an ecosystem is important as other ecological components would suffer.  For example, burning savannas annually would kill too many oak seedlings and saplings which are needed to maintain self-sustaining populations.  In addition, excessive burning can have significant adverse effects on certain insect species.

Future of Fire.  Smoke management will continue to limit prescribed burning at Soldiers Delight and other serpentine sites near urban and suburban development.  Although large volumes of smoke can be elevated for dissipation at high altitudes with modern smoke management techniques, the high visibility of smoke columns from several miles away can lead to an overloading of calls at 911 centers.  As a result, only small-scale burns are feasible in non-rural landscapes, limiting the total number of acres burned annually.

References:

Cavender-Bares, J. and P.B. Reich. 2012. Shocks to the system: community assembly of the oak savanna in a 40-year fire frequency experiment. Ecology 93(8) Supplement, pp. S52-S69.

Guyette, R.P. and B.E. Cutter. 1991. Tree-ring analysis of fire history of a post oak savanna in the Missouri Ozarks. Natural Areas Journal 11(2):93-99.

Hulbert, L.C. 1969. Fire and litter effects in undisturbed bluestem prairie in Kansas. Ecology 50:874-877.

Hulbert, L.C. 1988. Causes of fire effects in tallgrass prairie. Ecology 69:46-58.

Knapp, A.K. 1984. Post-burn differences in solar radiation, leaf temperature, and water stress influencing production in a lowland prairie. American Journal of Botany 71:220-227.

Knapp, A.K. and T.R. Seastedt. 1986. Detritus accumulation limits productivity of tallgrass prairie. Bioscience 36:662-668.

Old, S.M. 1969. Microclimate, fire and plant production in an Illinois prairie. Ecological Monographs 39:355-384.

Peet, M., R. Anderson, and M.S. Adams. 1975. Effect of fire on big bluestem production. American Midland Naturalist 94:15-26.

Peterson, D.W. and P.B. Reich. 2001. Prescribed fire in oak savanna: fire frequency effects on stand structure and dynamics. Ecological Applications 11(3): 914-927.

Peterson, D.W., P.B. Reich, and K.J. Wrage. 2007. Plant functional group responses to fire frequency and tree canopy cover gradients in oak savannas and woodlands. Journal of Vegetation Science 18:3-12.

Rice, E.L. and R.L. Parenti. 1978.  Causes of decreases in productivity in undisturbed tallgrass prairie. American Journal of Botany 65(10):1091-1097