Natural Regeneration Dynamics of Longleaf Pine Under Frequent, Low-Intensity Prescribed Fires in Southern Alabama, USA

Abstract

Frequent (≤3 y) prescribed burn has been conducted in the Escambia Experimental Forest in southern Alabama, USA, since 2001.The change in the density of new germinants (≤1 y) and older seedlings and saplings (>1 y) of longleaf pine (Pinus palustris Mill.) from stands of varying conditions was quantified using the locally estimated scatterplot smoothing and spatial statistical methods. A relatively longer fire interval or time since last fire (TSLF) > 600 days favored the recruitment of young seedlings. However, a short fire return interval or TSLF≤ 600 days promoted the recruitment of older seedlings and saplings. Large basal area (BA) and site index (SI) tended to diminish natural regeneration, whereas large quadratic mean diameter (QMD) and age were facilitative. The interactive effect of these factors indicated that a ratio of TSLF/ BA of 60 to 120 and QMD/SI of 1.8 to 2.2 would maximize the recruitment of young seedlings. The recruitment of older seedlings and saplings increased linearly with QMD/SI but had no relationship with TSLF/BA. Recruitment was largely absent when QMD/SI was <2.0. As a result, we recommend that QMD/SI be no greater than 2.0 and TSLF/BA range from 60 to 120 to regenerate longleaf pine naturally. Study Implications: Natural regeneration of longleaf pine (Pinus palustris Mill.) is challenging due to its sporadic seed production and sensitivity to fire in the first year after germination and vegetation competition while in the juvenile grass stage. It is widely recommended that a fire should be burned prior to seed fall to prepare the seedbed and not burned for at least 2 years to prevent fire-induced mortality of new germinants and young seedlings in the grass stage. When enough older seedlings accumulate, fire can be prescribed based on site conditions, usually maintaining a fire return interval of 1 to 3 years. At the stand level, two ratios, TSLF (time since last fire, days)/BA (basal area, m2 ha-1) and QMD (quadratic mean diameter, cm)/SI (site index, m with a base age of 50 years) can measure the substrate condition for new germinant recruitment and the realized regeneration potential of a stand, respectively. To naturally generate longleaf pine, fires can be prescribed so that the TSLF/BA maintains a range of 60 to 120 to promote new germinant recruitment. Given the preferred BA range (< 15 m2 ha-1), manipulations of overstory structures to maintain QMD/SI of 1.8 to 2 can facilitate the accumulation of old seedlings and saplings on low or moderate sites (SI < 27 m) in southern Alabama. Considering the data used in this study come inclusively from a single site (the Escambia Experimental Forest) in the center of the historical range of longleaf pine, precautions should be taken regarding the recommended stand conditions and prescribed burn treatments. The appropriate range of these ratios may vary and demands further testing using long-term regeneration data on other sites and regions in the native range of longleaf pine.