Sprouting capacity of shortleaf and shortleaf x loblolly pine hybrid seedlings in response to topkill: Is there a relationship between fire suppression and increasing abundance of the hybrids?

Background/Question/Methods

Shortleaf pine (Pinus echinata) has the widest range of any pine in the southeastern U.S.  and is fire-adapted because of its ability to prolifically sprout following topkill as a seedling.  Natural hybrids between shortleaf pine and loblolly pine (P. taeda) have increased in frequency from about 5 to 50% in the last 60 years.  If the current trajectory continues, shortleaf pine is at risk of extinction by introgression.  In the past, fire was a strong selection pressure that favored shortleaf over loblolly pine.  We tested the hypothesis that shortleaf x loblolly pine hybrids do not sprout as prolifically as shortleaf pine and that hybrid pine sprout from higher on the stem which increases susceptibility to fire.  We compared the sprouting ability of artificial F1 shortleaf × loblolly pine hybrids (12 crosses) with their parent’s open-pollinated offspring (6 families each).  Top-cutting, fire (girdling with propane torch), and the combined treatment were applied four times between winter and early summer during the third growing season in Oklahoma, USA.  Sprouting response was compared at the end of the growing season.  

Results/Conclusions

At the end of the growing season sprouting for shortleaf pine in all three treatments across sampling dates was prolific (>95%).  Hybrid pines had sprouts on 87, 91, and 96% of seedlings for the burn, cut, and combined treatments respectively.  Loblolly pine sprouted 40, 63, and 47% of the time for the cut, burn, and combined treatments respectively.  The number of sprouts per seedling was greatest in shortleaf pine and lowest in loblolly pine (31, 22, and 9 sprouts per tree for shortleaf, hybrid, and loblolly pine respectively).  Basal crooks, a suspected fire adaptation that lowers the height of dormant buds, occurred on 84% of shortleaf pine, 34% of hybrid pines, and 4% of loblolly pine.  The height from the soil surface to the lowest sprout averaged 3.5 mm for shortleaf pine, 7.7 mm for hybrid pine, and 21.4 mm for loblolly pine.  These results indicate that not only do shortleaf pine seedlings sprout more prolifically than do the hybrid pines, but that the dormant buds of shortleaf pine are held closer to the ground where they have a better chance of being protected by soil or duff during fire.  Given these findings, prescribed fire probably will reduce hybrid abundance.