We conducted an experiment designed to study effects of different prescribed burning intensities on 40 year-old longleaf pine at the Savannah River Site near New Ellenton, South Carolina, USA. The randomized complete block study had four blocks containing three burning intensities and an unburned control treatment. Each treatment plot was 2.2 ha. A 100% survey was conducted to mark and note mortality and symptomatic trees immediately prior to treatment initiation and periodically thereafter. Woody roots of symptomatic trees were sampled by methods previously described (Otrosina et al. 1999) prior to treatment initiation and as symptomatic trees appeared during the three years post-treatment. Crown symptoms were evaluated using a rating 5 scale rating system (Otrosina et al. 2002). Fine roots were also evaluated histologically and macroscopically(Otrosina et al. 2002). After the third year post-treatment, selected trees representing each crown symptom class within all treatments were excavated and root systems were evaluated for health, stems were cored at breast height, and cambial samples were analyzed for sucrolytic enzyme activities (Otrosina et al 1996).
Significantly greater mean cumulative mortality occurred in the hot burn treatment (29 stems, P=0.05) compared to the control (5 stems) three years after burning, indicating this effect was not due to acute fire damage, such as severe bole scorching. All three burn treatments had significantly more trees changing from less severe to more severe crown classes (p=0.04, Chi-Square=8.36, 3df). Isolations from woody roots yielded several species of Ophiostomoid fungi such as Leptographium terebrantis, L. procerum, and Sporothrix species. The Leptographium species were widespread throughout the study regardless of crown symptoms or treatment. We found extensive fine-root damage in the upper mineral soil layer and organic layer associated with the hot burn treatment. Also, large woody roots tended to be highly resinous as symptom severity increased. Heterobasidion annosum is widespread in the experimental plots and was associated with root disease of symptomatic trees in addition to Leptographium species. Growth in periodic annual increment (PAI) declined during the four years after burning as crown symptoms severity increased (P=0.03). This decline in PAI was also evident four years prior to treatment (P=0.03) with increasing crown symptoms. We attribute the decline of longleaf pine of these sites to a complex of factors driven by interactions of root diseases, fire damaged fine roots, and soil/site conditions. This study and other studies we have conducted indicate certain Leptographium species may be indicators of ecosystem stress in addition to contributing to tree mortality.