Predicting longleaf pine coarse root decomposition in the southeastern US
Storage of belowground carbon (C) is an important component of total forest C. However, belowground C changes temporally due to forest growth and tree mortality (natural and via harvesting) and these fluctuations are critical for modeling C in forests under varying management regimes. To date, little progress has been made in quantifying the rate of decay of southern pines in general, and specifically in longleaf pine (Pinus palustris Mill.) coarse root systems. Decomposition rates of lateral roots and tap roots of longleaf pine were quantified in situ under field conditions across the species’ range to create a model for necromass loss. The roots of 37 longleaf pine stumps were excavated from Florida, Georgia, Louisiana, and North Carolina. The age of the trees when cut ranged from 14 to 260 years, and the time since cut ranged from 5 to 70 years. Remaining lateral roots to a 1m depth plus the entire tap root were removed, dried, weighed and analyzed for C and nitrogen (N) content. Total dry necromass of harvested roots ranged from 8 to 195 kg tree−1. Soil C and N content at 15 cm depth were significantly higher near the stump compared to half-way between and adjacent to the nearest living longleaf pine. A regression model was developed to predict necromass loss. The final model included years since cut, stump diameter, and average minimum monthly air temperature as predictors (R2=0.83). For example, a 100- year-old tree would have a predicted root decomposition rate (k) of −0.120 for lateral roots and −0.038 per year for tap roots. Results suggest that longleaf pine coarse roots persist in the environment longer than the tap roots of loblolly pine.