Returning fire to Ozark Highland forest ecosystems: Effects on advance regeneration

Abstract

In mature forests of the Ozark Highlands, MO, USA, we evaluated fire effects on the survival and growth of tree seedlings and saplings (i.e., advance regeneration), and used this information to develop species-specific models that predict the probability of survival based on initial tree size and number of times burned. A 1000 ha forest area was divided into five units that were randomly chosen to receive one, three or four dormant season surface fires during the period 1998–2001. A total of 2741 stems of advance regeneration, ranging in size up to 15 cm in basal diameter and 15 m in height, were permanently marked and measured in all the units. One and four years after initiating the burn treatments, height of survivors was measured. Although most stems experienced shoot dieback following the first fire, survival was high (>90%) for all species as most trees produced new shoots from the living rootstock. The probability of surviving one fire was significantly related to initial stem size (basal diameter and height). With additional burning, the probability of survival increased with increasing initial tree size, and decreased as the number of burns increased. For a given initial diameter, black oak and post oak had the highest probability of survival after three or more burns (e.g., 88% for 5 cm stems), followed closely by white oak (80%), and scarlet oak (60%). For similar sized stems, flowering dogwood had low probabilities of survival (e.g., 25%), and blackgum was devastated by frequent burning (2%). Sassafras showed the greatest tolerance to burning, and more than 90% of stems survived three or more fires over a 4-year period. The probability of survival significantly decreased with increasing number of burns for most species. However, frequency of burning had less influence on the probability of survival for larger (e.g., ≥7.6 cm) diameter advance regeneration than it did for smaller stems. One fire significantly altered the height distribution of advance regeneration, concentrating most of the stems in the smallest height class (<1 m tall). Recovery of height was slow even 4 years after a burn due to the suppression of regeneration by the overstory canopy that averaged 18 m 2/ha in basal area (69% stocking). Overall, repeated burning in the dormant season reduces understory structure and favors oak advance regeneration. Survival models can be used to plan for woodland and savanna restoration.