Fire effects on a fire-adapted species: response of grass stage longleaf pine seedlings to experimental burning

Abstract

Background: Longleaf pine (Pinus palustris Mill.) seedlings have a morphological "grass stage" that is considered to be an adaptation to frequent surface fire regimes. However, fire can kill longleaf pine seedlings and thus may play an important role in longleaf pine regeneration dynamics. We used a prescribed burn simulation tool designed to treat individual grass stage longleaf pine seedlings with controlled delivery of fire treatments and then measured survival and growth responses through two growing seasons. Naturally regenerated grass stage longleaf pine seedlings were randomly selected from three size classes and each assigned one of four treatments (Control, no treatment; Clip, mechanical needle removal; LB, a low-temperature burn treatment; or HB, a high-temperature burn treatment) in both the dormant season (January) and the growing season (May). Results: Seedlings greater than 15 mm root collar diameter had greater than 0.5 probability of survival after the first growing season in the HB treatment, regardless of the season of treatment application, and seedlings across all sizes had greater than 0.6 probability of survival in the LB treatment after the first growing season. The growing season treatment application resulted in additional mortality during the second growing season, across all seedling size classes, which was not observed in the dormant season application. Burning reduced root collar growth through two growing seasons, likely due to needle mortality and the subsequent prioritization of growth to needle production rather than to root or stem growth. Conclusions: Our results suggest that the interplay between seedling size and fire intensity likely contributes to the success of longleaf pine natural regeneration and that seedling size should be considered when scheduling the first burn following planting of longleaf pine seedlings.

Physiological Mechanisms of Foliage Recovery after Spring or Fall Crown Scorch in Young Longleaf Pine (Pinus palustris Mill.)

“…For this reason, prescribed fire is an important tool in ongoing longleaf pine restoration efforts. The survival of prescribed burned longleaf pine seedlings is correlated with their size [39], and application of fire is not advocated until after the first growing season following planting [40]. Beyond this developmental stage, survival and vigorous growth among longleaf pine seedlings and saplings that experience severe crown scorch demonstrates the fire-tolerance of this species.…”

“…Application of prescribed fire among vigorously growing longleaf pine seedlings and saplings may be necessary to insure high light availability and reduce establishment of woody competitors [39][40][41]. The stature of longleaf pine seedlings and saplings predisposes them to complete or nearly complete crown scorch.…”

Fire Survival of Longleaf Pine (Pinus palustris) Grass Stage Seedlings: The Role of Seedling Size, Root Collar Position, and Resprouting

“…For seedlings >7.6 mm in root collar diameter, fire-induced mortality was reported at less than 25% (e.g., [14,17,27]). In a recent experimental study, however, Knapp et al [16] reported that mortality could approach 50% for seedlings with root collar diameters >15 mm under high-intensity burns. Seedling size is an important factor affecting the fire survival of grass stage seedlings, but it remains uncertain if fire survival changes continuously with the size or abruptly with size thresholds.…”

“…Despite their well-known fire tolerance, longleaf pine seedlings in the grass stage are far from invulnerable, and high fire-induced mortality has been reported in many studies (e.g., [14][15][16]). During the grass stage, burning often scorches and even consumes the needles around the apical bud.…”

“…As a result, some longleaf pine seedlings are killed by fire even during the grass stage. Previous studies suggested that seedling size [14,[16][17][18][19][20][21] and fire behavior [16,[22][23][24][25] could significantly affect fire-induced mortality of longleaf pine seedlings.…”