Modeling mortality of second-rotation loblolly pine plantations in the Piedmont/Upper Coastal Plain and Lower Coastal Plain of the southern United States

Remeasurement data from permanent plots of six loblolly pine studies were used to develop mortality models for second-rotation loblolly pine plantations growing in the Piedmont/Upper Coastal Plain (PUCP) and Lower Coastal Plain (LCP) of the southern United States. The model consists of two complementary parts: a generalized logistic equation predicting the probability of survival for all trees over a period developed using all data with and without occurrence of mortality; and a difference equation predicting tree-number reduction, given that some mortality has occurred. In the PUCP, the survival probability is mainly affected by stand age, tree density, and site index; the probability decreases with increasing values of each of these three variables. In the LCP, the survival probability is related to the stand basal area and site index; the probability decreases with increasing stand basal area, but increases with increasing site index. The relative rates of instantaneous mortality for second-rotation loblolly pine plantations are proportional to stand density, age and site index, but with different functions of age and site index in the PUCP and LCP. Thus, two different forms of the best difference mortality equations were obtained for these two regions. Both survival probability equation and difference mortality equation indicate that site productivity affects mortality in an opposite way in the two regions: mortality increases with increasing productivity in the PUCP, but in the LCP higher mortality is related to lower productivity. The lower quality sites in the PUCP may support higher stocking than the same quality of sites in the LCP, while the higher quality sites in the PUCP may support lower stocking than the same quality of sites in the LCP.