What kills longleaf pine (Pinus palustris)? 45 years of data says not much

Background/Question/Methods

The original longleaf pine (Pinus palustris) forest was self-perpetuating where seedlings always had to be present. It reproduced itself in openings in the overstory where young stands developed. These openings would have ranged from a few tenths of a hectare due to the loss of a single tree to a lightning strike or wind fall, a few hectares due to insects or a larger scale wind event, or to large openings of several thousands of hectares due to tornados or hurricanes. Regardless of the event size, longleaf pine was able to regenerate these openings.  In 1964, the U.S. Forest Service established the Regional Longleaf Pine Growth Study (RLGS) in the Gulf States. The original objective of the study was to obtain a database for the development of growth and yield predictions for naturally regenerated, even-aged longleaf pine stands.  The study has been expanded over the decades to examine numerous aspects of longleaf pine stand dynamics.  Currently, the RLGS is undergoing its ninth measurement period (45-year measurement).  The completion for this measurement cycle will happen this April.

Results/Conclusions

Landowners who have stands of large longleaf pine have fears they will lose their trees due to some type of mortality before they can harvest them.    For over four decades, the amount of mortality and its cause has been documented for the trees in the RLGS.  One of the major causes of mortality has been suppression which happens among the smallest trees or trees that have been overtopped for several years.  At the other end of the tree size class, larger trees are killed by lightning strikes which will occasionally impact a nearby tree which may be killed by insects attracted to the tree struck by lightning.  These events are low level and happen at low frequencies every year.  The other major cause of mortality has been wind.  These events are episodic in nature, associated with either a hurricane or tornado.  These events, when they do happen, kill a relatively large number of trees.  Surprisingly, despite longleaf pine being among the most fire-adapted tree species there is in nature, fire can kill longleaf pine as evidenced by data from the RLGS.  So what kills longleaf pine and how often does it happen?