Effect of initial seedling size, understory competition, and overstory density on the survival and growth of Pinus echinata seedlings underplanted in hardwood forests for restoration

There is interest in restoring shortleaf pine (Pinus echinata) in pine-oak woodlands where it once was abundant. Because of its shade intolerance and slow initial growth rate, shortleaf pine restoration has remained a challenge because competition from hardwoods exhibits greater initial growth following canopy removal but greater shade tolerance with canopy retention. The study objective was to examine the survival and growth of underplanted shortleaf pine seedlings relative to competing hardwoods as a function of initial seedling size, overstory density, and understory competition. In the Ozark Highlands of southeastern Missouri, USA, 48, 0.4-ha experimental units were each harvested from below to a uniform stocking level from 0 to 90 % and 30, 1-0 improved shortleaf pine seedlings were planted on a 3.7 x 7.3 m spacing. Linear or logistic regression was used to determine how shortleaf pine seedling (1) survival, (2) basal diameter growth, and (3) shoot growth were related to initial seedling size, overstory stocking, and understory competitor height during the first 5 years after underplanting. After five growing seasons, the survival rate of shortleaf pine seedlings was 50 % and was positively related to the initial basal diameter but was not related to overstory stocking or competitor height. Increasing overstory stocking decreased the basal diameter and height growth of shortleaf pine seedlings, explaining > 51 % of the variation in basal diameter and 54 % of the variation in seedling height. Although competing hardwood seedlings were consistently taller than the shortleaf pine seedlings throughout the study, shortleaf pine seedlings maintained similar growth rates as competitors from the second to the fifth growing season. The eventual release of shortleaf pine is essential for recruitment, but releases can be delayed for several years after underplanting.