Unanticipated disease problems can emerge in forest ecosystems. These problems may occur later in the life of a stand, despite vigorous growth and development early in stand life, e.g., during the first 30 or so years. One example we studied was in a longleaf pine (Pinus palustris Mill.) ecosystem. Longleaf pine once occupied nearly 30 million has but now its range is reduced to approximately 1.5 million ha. Restoring this species to many sites in its former range is an important goal involving several natural resource organizations. Longleaf pine has evolved with frequent fires and it is dependent upon fire for successful regeneration and for maintenance of stand health. However, increased mortality associated with prescribed fire has been observed in certain 30-40 year-old longleaf pine stands. Preliminary studies indicated several species of root infecting fungi (Leptographium species, Heterobasidion annosum) and certain root colonizing insects are associated with mortality (Otrosina et al. 1995), although longleaf pine is considered highly tolerant to these pathogens. We hypothesized many sites no longer possess specific edaphic and environmental conditions under which the species evolved because of altered fire regimes, changes in soil conditions, or other factors that render threes susceptible to root pathogens.

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.