Fire Effects on Vegetation and Animals

Savanna oaks and the accompanying understory have physiological adaptations that help them withstand fire, drought and other disturbances (Reich and Hinckley 1989, Peterson and Reich 2001). Species in oak savannas are extremely resistant to fire and repeated prescribed burning. Fire resistant traits that oaks possess are the ability to re-sprout prolifically, maintenance of thick bark, resistance to rotting, compartmentalization of fire wounds, and increased resistance with age (Figure 5) (Johnson et al. 2002). See also: Fire and Oaks.

Peterson and Reich (2001) generalize tree species’ response to fire by placing them into one of three categories defined by life history characteristics. Species characterized as resisters are able to withstand fire due to physical adaptations, which occur in some oak species. Some oaks and other shade tolerants are characterized as endurers because they possess the ability to resprout after fire disturbance. A final category, named avoiders, are those species mostly found in closed canopy fire suppressed forests where they are able to gain dominance in fire free intervals (Peterson and Reich 2001). Oaks that are either resisters or endurers can succumb to avoiders in the absence of disturbance. Declines in overstory trees vary with species based on their physical adaptations. For example, Q. macrocarpa is very resistant to fire induced mortality while Q. ellipsoidalis succumbs quickly to frequent fires (Peterson and Reich 2001). Thus, not all oaks in the savanna community are capable of withstanding short fire return intervals. 

Fire resistance increases with age, and therefore mortality of large overstory trees is low even with fire return intervals of one to three years (Table 3: Mean tree basal area and stem density) (Faber-Langendoen 1995, Peterson and Reich 2001). Following fires, increases in average basal area are evident as the mature trees occupy the growing space vacated by fire sensitive species and smaller oak stems. Fire induced mortality is also loosely correlated with stem diameter of trees, thus susceptibility to fire damage is greatest in smaller DBH classes and decreases as oaks become larger (Table 4: Stem density of saplings ( > 1.5 m height and < 5 cm dbh) by species). Thick bark is often a consequence of increased age in certain oak species allowing them to be even more resistant to fire such as the case with Q. macrocarpa. 

Canopy cover increases with decreased fire frequency and periods of fire suppression can eventually lead to a closed canopy forest (Faber-Langendoen 1995). Rates of canopy change are usually faster in mesic sites where moisture content increases growth. In these areas, succession to closed canopy forests occurs rapidly favoring more shade tolerant species or species that grow more rapidly in the absence of disturbance (Faber-Langendoen 1995). 

The concept of patch dynamics has often been used to describe the space not occupied by trees or what is referred to as gaps in forested systems (Belsky and Canham 1994). Although this same concept can be used to explain differences in microhabitat in oak savannas, the patches refer to the areas where trees are located. Savannas do not rely on gaps for reproduction establishment but rather on those patches where the mature trees persist and affect soil dynamics and resource availability (Belsky and Canham 1994). Survival of reproduction in these patches has been shown to be related to grass cover. As grass cover is reduced by oak shading, regeneration grows unimpeded by competition (Rebertus and Burns 1997). Fire is also a determining factor, varying in frequency and rate, that indirectly influences the formation and suppression of patches. Fire also directly affects regeneration by promoting a mineral seedbed that is beneficial for acorn germination (Johnson et al. 2002). Fire free intervals are needed to allow advance reproduction to attain adequate height and fire resistance. Advance reproduction densities decline with increased fire frequency, however this is dependent on species and age.  Frequent or high intensity fires can prevent development of saplings while low intensity frequency fires can result in dense sapling thickets (Peterson and Reich 2001). This adaptation of prolific sprouting allows oaks to persist despite fire dieback and give the sprouts a competitive advantage with a sufficiently established root system. 

Grimm (1984) suggested that the oak savanna overstory mosaic is a stable system maintained by disturbance by frequent fire. The native savanna understory is also adapted to a fire as both cover and species diversity increase with an invrease in fire frequency. Studies have shown that species richness is highest in plots that are burned every 2 years and native prairie grasses (Andropogon, Schizachyrium, and Sorghastrum) increased 10% and forbs 6% (Tester 1996). Response of shrubs depends on species, with Corylus spp. and Amorpha spp. more common in burned plots while  Spiraea alba, Amelanchier spp, and Rubus spp. thrive in unburned areas with a long fire return interval (Faber-Langendoen 1995). Shrub recruitment is also reduced in frequently burned plots but sprouting increases with fire frequency (Faber-Langendoen 1995). Most species have adapted relatively well to the recently changed fire regime and constant shift in the prairie-oak transition. However, some specialists that require high fire frequency are now restricted to the fringes of oak savannas and openings in oak woodlands and are at high risk as fire suppression moves these communities towards a closed canopy oak forest (Henderson and Epstein 1995). 

Fire frequency and fuel buildup are intricately linked in the maintenance of open oak savanna conditions. Years without a fire result in greater fuel loads leading to more severe fires in subsequent years.  Also, fuel buildup is greater in patches with higher tree density. This results in higher overstory mortality following fires and helps maintain the open complex of oak savannas (Anderson and Brown 1983). In addition, patches of overstory oaks are often dominated by pyrogenic grasses that support frequent high intensity surface fires, thereby perpetuating the sparse canopy of oaks and open grassy understory. Because of stochastic events, there is a constant flux in fire return intervals that shrinks and expands the oak savanna patches and maintains this unique complex. This structure is strikingly different from upland oak sites that have high fuel moisture, low fire occurrence, and species that are well adapted to low light and no disturbance (Streng and Harcombe 1982).

Many vertebrates have managed to adapt to the diminishing amount of oak savannas because of their mobility. Some species however, require the oak savanna edges for some stages in their life cycle (Henderson and Epstein 1995). The avian fauna has been quite resilient with the exception of some of the obligate savanna species (Davis et al. 2000). Research on vertebrate species is limited and invertebrate research is almost nonexistent. The preservation of all native fauna would benefit from more extensive research documenting oak savanna specialists.