Differential survival and damage patterns in planted longleaf and loblolly pine following a catastrophic ice storm

Background/Question/Methods

Concerted efforts to restore longleaf pine (Pinus palustris) across its historic range in the southeastern United States have been undertaken.  In many places, natural regeneration of longleaf pine following the return of frequent surface fires has been sufficient.  However, in locations without a suitable seed source, longleaf pine has been widely planted.  A catastrophic ice storm in early 2014 struck Georgia and the Carolinas, killing and injuring millions of trees.  Planted longleaf pine often suffered high rates of damage, causing some to question the wisdom of restoring longleaf with large-scale plantations.  To help determine if longleaf pine plantations fared worse than planted loblolly pine (Pinus taeda) under similar circumstances, I compared a number of these stands on the Savannah River Site (SRS) for significant differences in survival and damage response after one growing season (in September 2014).  Established adjacent to each other under identical conditions as a part of a mycorrhizae inoculation study, these 22- to 26-year-old longleaf and loblolly plantations were thinned in 2011-2012 and then experienced moderate to extensive damage after ~2.5 cm of ice was deposited by the February 12-13, 2014 storm event.

Results/Conclusions

A contingency table analysis showed that of the 2523 pines (1266 longleaf and 1257 loblolly) alive prior to the ice storm, a significantly higher fraction of longleaf (33.4%) had died as a result compared to loblolly (26.9%) (χ² = 12.57; P < 0.001).  Mortality was not evenly distributed across the range of tree diameters encountered—15-20 cm DBH longleaf experienced the highest rate of mortality (40.9%) for that species, while 10-15 cm DBH stems had the highest loblolly mortality rate (35.5%).  For different reasons, mortality was less for either very small or very large stems.  While both species experienced the same types of non-lethal injuries (branch breakage and stem bending), mid-sized longleaf pine experienced notably higher rates of both branch loss and bent stems.  These results have implications for forest health and longleaf pine restoration in areas frequently struck by catastrophic ice accumulation.  Recent thinning in dense pine stands leads to high levels of glaze damage regardless of species, but the growth pattern and crown architecture of longleaf pine accentuate its losses.  Planting longleaf pine at wider spacing intervals allows for greater bole growth more quickly, thereby adding to the tree’s ability to carry heavier loads of ice and snow.