Frequent fire protects shortleaf pine from introgression by loblolly pine

Background/Question/Methods

Anthropogenic fire exclusion alters previously fire-maintained ecosystems by allowing more competitive, later successional species to replace fire-adapted plants and their dependent fauna.  In addition to these obvious changes in species composition, fire exclusion may have more subtle yet important effects on the genetic integrity of native plant species.  We tested the hypothesis that prescribed fire can reduce introgression between shortleaf pine (Pinus echinata) and loblolly pine (P. taeda), two widely occurring native tree species in the southeastern United States, that have experienced increased hybridization coincident with wide-scale fire exclusion.  We estimated the genomic proportion of seedlings and saplings in four pairs of adjacent burned every 2 years and non-burned stands (no fire for previous 4-5 decades) growing under a mixed overstory of shortleaf-loblolly pine at Tall Timbers Research Station and Land Conservancy (TTRSLC, Tallahassee, FL USA).  Q_lob (the approximate loblolly pine genomic proportion) was determined for each individual using genotypic data from 25 microsatellite markers from a previous study and Structure 2.3.4.  Individuals were classified as shortleaf pine, loblolly pine, or one of several hybrid types using Q­_lob.

Results/Conclusions

Our results show that recurring prescribed fire over the last 40+ years resulted in a seedling/sapling population dominated by shortleaf pine (82%) with 1% F1 hybrids and an additional 16% first or second backcrosses to shortleaf pine, while fire exclusion over the same time period resulted in a seedling/sapling population that was 48% loblolly pine, 27% shortleaf pine, 8% F1 hybrids, and 17% first or second order backcrosses to loblolly pine.  These results indicate that frequent fire resulted in regeneration dominated by the fire-adapted shortleaf pine, while fire exclusion produced a seedling/sapling population mixed among loblolly pine, shortleaf pine, and hybrids.  Fire exclusion increases survival of hybrids, contributing to the recently measured range-wide increase in hybrids among loblolly pine and shortleaf pine and the apparent introgression of loblolly pine into shortleaf pine where fire is excluded.  Non-hybridized shortleaf pine can be returned to a dominant status through the reintroduction of frequent fire.