Using diversity measures to identify the health status of longleaf pine stands in the southern USA
The decline of the longleaf pine (Pinus palustris Mill.) ecosystem is one of the worst ecological disasters of the past 150 years. The original 37 million ha of longleaf forests throughout the southeastern USA has been dramatically reduced to a low of 1.5 – 1.7 million ha by cutting, species replacement, and ineffective fire management. Recent efforts to restore these iconic southern pine ecosystems emphasize reclaiming former longleaf sites through planting, which is costly. Rehabilitation of degraded longleaf stands through restoration may be a practical and less expensive alternative. We used data from the USDA Forest Service, Forest Inventory and Analysis (FIA) program, to determine the extent and condition of present-day longleaf stands. Using the premise that natural longleaf stands contain few, or no, associates in the tree stratum, we used tree species richness (S) and the McIntosh Evenness Index (MEI) to identify healthy and degraded longleaf pine stands where an overstory (S) of 1 - 3 along with longleaf as the dominant or co-dominant would indicate a relatively healthy stand and an (S) ≥4 would indicate a more degraded stand condition, especially where longleaf was not dominant or co-dominant. Many of these latter stands might be candidates for restoration applications.
Currently there are 59,588,839 ha (±393,000 C.I.) of forest land across the coastal south, from North Carolina to Texas. Only 3,458,396 (±121,736 C.I.) ha of forest land contained at least 14 longleaf pine trees per ha (TPH) that were ≥2.54 cm dbh (one tree sampled per sample unit [SU]). In these stands, we found 1,467,012 ha (±78,730 C.I.; n=799) where longleaf was established and the overstory structural component appeared adequate. These stands averaged 1.97 (S) SU-1 (±0.06 C.I.), 0.44 MEI SU-1 (±0.02 C.I.), 10.4 m2ha-1basal area (±0.52 C.I.), 201 TPH (±12.9 C.I.), and 24.5 QMD (±0.54 C.I.). In contrast, we found 1,991,384 ha (±63,900 C.I.; n=962) in various stages of degradation (stands with an (S) ≥4). These stands averaged 6.00 (S) SU-1 (±0.14 C.I.), 0.75 MEI SU-1 (±0.02 C.I.), 18.5 m2ha-1 basal area (±0.61 C.I.), 315 TPH (±10.2 C.I.), and 24.9 QMD (±0.36 C.I.). Determining metric thresholds that identify stand degradation can be challenging but studies such as this are important in identifying the range in health of ecosystems. They also point to broader opportunities to rehabilitate (restore) marginally stocked longleaf pine stands, as has been shown in other studies of cutover pines in the south.