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

Songheng Jin,Dapao Yu,Brett Moule,G Geoff Wang

doi:10.3390/f10121070

Abstract

Longleaf pine (Pinus palustris Mill.) forest is a well-known fire-dependent ecosystem. The historical dominance of longleaf pine in the southeast United States has been attributed to its adaptation known as the grass stage, which allows longleaf pine seedlings to survive under a frequent surface fire regime. However, factors affecting post-fire survival of grass stage seedlings are not well understood. In this study, we measured live and dead longleaf pine grass stage seedlings to quantify the role of seedling size, root collar position, and sprouting in seedling survival following a wildfire in the sandhills of South Carolina. We found that fire resulted in almost 50% mortality for longleaf pine grass stage seedlings. Fire survival rate increased with seedling size, but a size threshold for fire tolerance was not supported. Fire survival depended on the position of root collar relative to the mineral soil. Seedlings with protected root collars (i.e., buried in or at the level of mineral soil) experienced <21%, while seedlings with exposed root collars (i.e., elevated above mineral soil) suffered >90% post-fire mortality. Ability to resprout contributed to 45.6% of the total fire survival, with the small seedlings (root collar diameter (RCD) < 7.6 mm) almost exclusively depending on resprouting. Our findings had significant implications for fire management in longleaf pine ecosystems, and the current frequency of prescribed fire in sandhills might need to be lengthened to facilitate longleaf pine natural regeneration.

Highlights

Longleaf pine (Pinus palustris) forest, including woodland and savanna, is a well-known fire-dependent ecosystem [1]
For those seedlings with root collar elevated above mineral soil, their root collar position neither differed among the three seedling size classes (p = 0.287) nor correlated with root collar diameter (r = 0.138, p = 0.230; Figure 1)
Our study indicated that the position of root collar relative to the surface of mineral soil was a critical factor affecting the fire-induced mortality of longleaf pine seedlings during the grass stage

Summary

Introduction

Longleaf pine (Pinus palustris) forest, including woodland and savanna, is a well-known fire-dependent ecosystem [1]. The range of the longleaf pine ecosystems stretched from Virginia to Texas, mainly in Coastal Plain, and found in the Piedmont and mountains of Alabama and Georgia [4]. Most of these longleaf pine forests were consisted of pure stands, with a distinctive structure characterized by an open canopy, sparse mid-story woody vegetation, and a well-developed herbaceous layer often dominated by grasses [5]. The historical dominance of longleaf pine in the southeast United States has been attributed to its superior ability to regenerate under a frequent (1–3 years) surface fire regime that prevailed throughout its native range [1,4,5]. To survive frequent surface burning, longleaf pine seedlings have evolved an adaptation known as the grass stage, defined as a juvenile life-history stage characterized by a Forests 2019, 10, 1070; doi:10.3390/f10121070 www.mdpi.com/journal/forests

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