Longleaf pine (Pinus palustris) needs fire to thrive. But if seedlings burn too late in the growing season, they may not have enough energy to re-grow their scorched leaves and replenish their starch reserves before spring of the next year.
“When seedlings are so short that a prescribed fire is likely to scorch all of their needles, fire should be applied between March and May,” says Mary Anne Sayer, a USDA Forest Service research plant physiologist.
Between March and May, longleaf pine seedlings still have starch reserves. The reserves represent stored energy from late fall of the previous year, as well as the early spring. Young trees rely on this energy to re-grow scorched foliage, as Sayer and her colleagues document.
Starch reserves and photosynthesis are two physiological mechanisms of fire recovery in young longleaf pines, according to a recent study Sayer led. The study was published in the journal Forests. Sayer also led a related study on the timing of foliage re-growth that was published in the journal New Forests.
The studies were conducted on the Palustris Experimental Forest and Winn Ranger District of the Kisatchie National Forest in Louisiana. Sayer and her colleagues measured the stem growth and crown architecture of longleaf pine saplings and seedlings and the starch content of their stems, shoots, and woody roots. The scientists also measured fascicle photosynthetic rates. Fascicles are bundles of two or three pine needles held together by a short sheath attached to young branches.
After fire, fascicle photosynthesis – which signifies food production for the tree – increased temporarily. This suggests needles that remain after fire start working overtime to produce enough food for the rest of the tree. After fire, fewer needles mean extra water for the remaining ones, and the scientists attribute the increases in food production to this extra water.
The researchers also detected starch mobilization – waves of starch moving from the stem, shoot tips, and roots to developing shoots in spring and early summer. The starch helps the young trees re-grow scorched foliage.
And because lower crown branches were partially pruned by repeated prescribed fires, the saplings prioritized their new foliage growth in branches of the upper crown, which are less likely to burn in future fires compared to branches of the lower crown.
Longleaf pine seedlings are short and clumpy for two or more years, in what’s called their grass stage. As seedlings grow out of the grass stage and become saplings, they get tall enough to keep some of their needles above the height of a prescribed fire. These remaining, photosynthesizing needles help re-grow scorched foliage. As starch reserves decrease with progression of the growing season, photosynthesizing needles become the main food source for growth.
However, when fire was applied during a dry period, saplings recovered poorly. “In drought, shortfalls in foliage re-growth after fire were apparent among large saplings,” says Sayer.
Stemwood production for timber is a common landowner objective, along with ecosystem restoration. These objectives are compatible: as they grow, longleaf pine stands can be thinned to provide high-quality timber while ecosystems are being restored. Longleaf pine ecosystems are home to red-cockaded woodpeckers, eastern indigo snakes, and many other animals and plants that have declined along with dwindling longleaf pine ecosystems.
Longleaf pine seedlings and saplings need full sunlight to thrive. Without fire, faster growing plants shade them out, especially while seedlings are in the grass stage.
“Repeated fire keeps woody competitors in check and also promotes a diverse natural ground layer,” says Sayer. Herbicides can help control woody competitors, but unlike fire, they do not promote the native grasses and flowers in the ground layer.
“Our results provide insight into optimal windows for prescribed fire in young stands of longleaf pine. We also raise awareness about how drought may interfere with the growth of young longleaf pines after prescribed fire,” says Sayer. “Additionally, the positive relationship between foliage mass and whole plant growth is universal. Thus, the principles of this research are applicable to all fire-adapted forest systems.”
For more information, email Mary Anne Sword Sayer at firstname.lastname@example.org.