Pine caterpillar outbreak and stand density impacts on nitrogen and phosphorus dynamics and their stoichiometry in Masson pine (Pinus massoniana) plantations in subtropical China

Masson pine caterpillar (MPC, Dendrolimus punctatus Walker) outbreaks periodically occur in Masson pine (Pinus massoniana Lamb.) plantations in southern China; however, their effects on nutrient cycling remain unclear. In 2011, an MPC outbreak occurred in pine plantations with different stand densities, which facilitated a comparison of nitrogen (N) and phosphorus (P) cycling dynamics. Monitoring of soil and foliar nutrients from pre-to post-outbreak years (i.e., from 2010 to approximately 2012) indicated that soil NO3--N concentration and fluxes were higher after the outbreak in low-density plantations, whereas soil NH4+-N and P availability varied minimally and were not different between the high-and low-density sites. Pine foliage responses to the MPC outbreak were dependent on stand density. At the high-density site, foliar N increased by 40% with an increase of 25% in N resorption efficiency and by approximately 200%-300% in P resorption efficiency, whereas there was an increase of 50% N concentration, a 65% increase in N resorption efficiency, and a >500% increase in P resorption efficiency at the low-density site. Overall, soil nutrients were higher in low-density than high-density plantations, whereas leaf nutrient resorption had the opposite results after an MPC outbreak. The effects of a periodic MPC outbreak could increase plant N: P stoichiometry in these plantations, particularly in the denser stands.