Canopy recruitment dynamics in naturally regenerated longleaf pine (pinus palustris) woodlands

Abstract

The ecological and cultural values of the longleaf pine (Pinus palustris) ecosystem, in combination with an approximately 97 percent loss in historic range, has led to increasing interest in ecosystem restoration and management in recent decades. While research has elucidated much about regeneration dynamics, there is a paucity of information regarding the recruitment period between the grass stage and canopy status. The overall goal of this project was to determine the factors influencing canopy recruitment rates and patterns in naturally regenerated longleaf pine woodlands. This research utilized two distinct datasets collected at the Joseph W. Jones Ecological Research Center, a naturally regenerated second-growth landscape in southwest Georgia. First, we utilized data from long-term monitoring plots of tagged natural regeneration to model survival probability. Second, we conducted stem analysis on midstory and overstory trees in order to retrospectively examine rates and patterns of height growth during canopy recruitment. We found that 10-year regeneration mortality was concentrated within the smallest size classes and that survival was strongly driven by individual size (both root collar diameter and height) and relative height within dense regeneration clusters. Individual growth of midstory trees was strongly driven by overstory abundance but minimally related to soil moisture class. In comparing the growth rates of midstory and overstory trees on the same site, we found that midstory trees in open environments displayed comparable or better growth rates than overstory trees. In dense stands, however, midstory trees were generally from the same age cohort as overstory trees and likely stagnated in height and diameter growth after falling behind peers during canopy recruitment. Suppression and release patterns were common in the growth histories of midstory trees within dense stands but less so for overstory trees or midstory trees within open stands. Our results indicate that rapid initial growth and dominant crown position within even-aged cohorts is important for eventual canopy recruitment. Survival probability is low for individuals in lower crown positions, although a given individual may persist in the midstory for decades with minimal height or diameter growth. These individuals may then respond to release and resume growth. To facilitate recruitment of young individuals into the midstory, some level of overstory reduction is necessary in dense stands.