PS 10-120 - Patch dynamics of longleaf pine (Pinus palustris): Regeneration in second-growth stands reinforces and expands concepts developed based on old-growth stands

Monday, August 8, 2011
Exhibit Hall 3, Austin Convention Center
Leigh Griffin, Department of Biological Sciences, Louisiana State University, Baton Rouge, LA and William J. Platt, Louisiana State University

Concepts regarding natural regeneration of longleaf pine (Pinus palustris) originated through study of old-growth stands.  Recruitment resulting in entry into tree size classes is proposed to occur over time (10-20 years) after disturbances (e.g., lightning, windstorms) open patches within a matrix of larger uneven aged and sized trees. Frequent lightning-ignited fires facilitated by shed pyrogenic pine needles are hypothesized to preclude tree species other than longleaf pines entering the overstory.  We explored this concept using second-growth longleaf pine stands undergoing restoration at Girl Scout Camp Whispering Pines in eastern Louisiana.  Biennial prescribed fires mimicking lightning-ignited fires initiated in 1990 reversed invasion by hardwood trees during decades of altered fire regimes and facilitated herbaceous groundcover conducive to pine regeneration. In 1994 all trees >1.5m tall were mapped and measured in 10 one-hectare plots; few trees <10 cm dbh were present at this time. We studied recruitment of juvenile pines after 16 years. We examined relationships between location (distance to nearest overstory tree) and sizes (diameter and heights), as well as post-fire survival and growth of recruits. We hypothesized that recruits should be nonrandomly distributed relative to overstory trees, and that larger recruits should be located farther from overstory trees.


Recruits were located farther from overstory trees than random points and aggregated into patches, as predicted by models based on old-growth stands. Taller juveniles, those most likely to enter the overstory, tended to be located in regions of patches farthest from overstory trees. Mean distances to nearest overstory neighbor were significantly greater (t (170) = 1.974; p < 0.001) for recruits >0.5m tall (5.8 + 0.36(se) m) than recruits <0.5m tall (4.4 + 0.20(se) m). Nonetheless, location relative to overstory trees explained only 10% of the variation in recruit height. We hypothesized that height growth should reflect differences in time since initiation of height growth. We used annual rings in a sample of juveniles to obtain age-height relationships for recruits. Results indicated multiple cohorts of different sizes within patches, suggesting that restoration of fire regimes generated conditions facilitating recruitment when overstory trees masted and that recruits in different cohorts entered height growth. We expand the concept of patch regeneration based on old-growth stands to include multiple cohorts comprising local patches of recruits. Management that opens the overstory in second-growth stands, coupled with prescribed fire that mimics natural fire regimes should facilitate restoration of uneven sized/aged longleaf pine stands.

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