Pine plantations in the southern US are among the most intensively managed forests in the world and their productivity has tripled over natural pine forests through application of intensive plantation establishment and management practices. It is expected that the southeastern US will experience continued decreases in precipitation and increases in temperatures, leading to higher drought severity and drought occurrence. The droughts may reduce southern pine productivity or carbon fixation. We need to develop or modify management systems to increase southern pine productivity, resilience to drought effects and resistance to drought occurrence. However, the understanding of the long-term response of pine plantation ecosystems to the droughts is very limited.
Based on the long-term re-measurement data from two well-designed loblolly pine culture and density studies with a combination of two levels of cultural intensity (operational vs. intensive) and six levels of planting density (740 – 4440 trees/ha) across the southern US, we developed a method to estimate the net primary production (NPP) in total biomass for each plot from ages 4 to 21 years. Then, based on the daily temperature data for each site and the monthly palmer drought severity index (PDSI) for the corresponding climate divisions, we determined the growing season and averaged the PDSI over that growing season for each site by each year. Finally, we investigated the interactions among cultural intensity, planting density, site quality and drought index on the NPP over time.
Results/Conclusions
Under operational treatments, plots on higher quality sites had higher NPP over time, after 8 years old there was no significant difference in the NPP among the plots with higher planting densities (> 740 trees/ha), and severe droughts reduced the NPP of the plots on high-quality sites.
Under intensive treatments, the NPPs were higher on high- and intermediate-quality sites than on low-quality sites. When the severe droughts existed, the increased NPP due to the intensive treatments disappeared during that period.